20.4: Biological Acids and the Henderson-Hasselbalch Equation
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Objectives
After completing this section, you should be able to
- identify the form that carboxylic acids take within living cells.
- use the Henderson‑Hasselbalch equation to calculate the percentages of dissociated and undissociated acids [A − ] and [HA] in a solution, given the p K a value of the acid and the pH of the solution.
- explain why cellular carboxylic acids are always referred to by the name of their anion.
Key Terms
Make certain that you can define, and use in context, the key terms below.
- Henderson‑Hasselbalch equation
- physiological pH
Carboxylic Acids in Buffered Solutions
Due to the acidic nature of carboxylic acids, their level of dissociation is directly related to the pH of the solution. Under acidic conditions (low pH), carboxylic acids are almost completely undissociated and exist primarily in their protonated form (\(\ce{RCOOH}\)). Under, basic conditions (high pH), carboxylic acids become nearly completely dissociated into their deprotonated carboxylate form (\(\ce{RCOO^{-}}\)). Because the environment inside a living cell is an aqueous buffer with a nearly neutral pH = 7.3 (sometimes referred to as the physiological pH ), then in what form do carboxylic acids exist? An important part of answering this question lies in the Henderson-Hasselbalch equation :
\[pH = pK _{ a }+\log \left(\dfrac{[\text { concentration of conjugate base }]}{[\text { concentration of weak acid }]}\right) \label{Henderson-Hasselbalch equation} \]
This equation shows that the amount of dissociation in a carboxylic acid is related to the pH of the solution and the pK a of the acid. In particular, the Henderson-Hasselbalch equation shows that when the concentration of a carboxylic acid is equal to the concentration of carboxylate then the pH of the solution equals the pK a of the acid. Also, the Henderson-Hasselbalch allows for the pH of the solution to be calculated when the pK a of the acid is known along with the concentrations of the carboxylic acid and carboxylate are known.
Example \(\PageIndex{1}\)
What is the pH of an aqueous buffer solution that is 30 mM in acetic acid and 40 mM in sodium acetate? The pK a of acetic acid is 4.8.
Solution
This is a direct application of the Henderson-Hasselbalch equation (Equation \ref{Henderson-Hasselbalch equation}).
\[pH = pK _{ a }+\log \left(\dfrac{[\text { concentration of conjugate base }]}{[\text { concentration of weak acid }]}\right) \nonumber \]
The ratio of base to acid is 40/30, or 1.33. Therefore, substituting these values and the \(pK_a\) results in
\[\begin{align*} pH &= 4.8 +\log \left(\dfrac{40}{30}\right) \\[4pt] &= 4.8 +\log 1.33 \\[4pt] &=4.8 + 0.125 \\[4pt] & = 4.9 \end{align*} \]
The Henderson-Hasselbalch equation is particularly useful when determining the protonation state of different biomolecule functional groups under physiological pH conditions. This is done under the assumption that the concentration of the biomolecule is small compared to the concentration of the buffer components. (The actual composition of a physiological buffer is complex, but it is primarily based on phosphoric and carbonic acids.).
Aspartic acid is an amino acid with a carboxylic acid functional group as part of its side chain. When aspartic acid comprises an amino acid residue of a protein in a human cell, it's side chain is in full contact with the physiological buffer. In what state is the side chain functional group: the protonated state (a carboxylic acid) or the deprotonated state (a carboxylate ion)?
Using the buffer pH of 7.3 and a rough approximation of pK a = 5 for the carboxylic acid side chain, we find that the ratio of carboxylate to carboxylic acid is about 200 to 1. The carboxylic acid is almost completely ionized to the carboxylate inside the cell. This result extends to all other carboxylic acid groups you might find on natural biomolecules or drug molecules. In the physiological environment, carboxylic acids are almost completely deprotonated. Indeed, cellular carboxylic acids are often referred to by the name of their anion. So rather than pyruvic acid, or acetic acid or lactic acid, they are discussed as pyruvate, acetate or lactate.
Using the same process, the Henderson-Hasselbalch equation (Equation \ref{Henderson-Hasselbalch equation}) can also be used to determine the protonation state of carboxylic acids in solutions buffered to other pH levels.
\[\begin{align*} pH &= pK_a + \log \left(\dfrac{[\text{carboxylate}]}{[\text{carboxylic acid}]}\right) \\[4pt] 7.3 &= 5 +\log \left(\dfrac{[\text{carboxylate}]}{[\text{carboxylic acid}]}\right) \\[4pt] 2.3 &= \log \left(\dfrac{[\text{carboxylate}]}{[\text{carboxylic acid}]}\right) \\[4pt] 199 &= \dfrac{[\text{carboxylate}]}{[\text{carboxylic acid}]}\end{align*} \]
Therefore at physiological pH, the carboxylate anion is found with a 199-fold higher concentration than the protonated carboxylic acid. Naturally, changing the pH will alter this ratio.
Exercise \(\PageIndex{1}\)
What is the ratio of acetate ion to neutral acetic acid when a small amount of acetic acid (pK a = 4.8) is dissolved in a buffer of pH 2.8? pH 3.8? pH 4.8? pH 5.8? pH 6.8?
- Answer
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We use the Henderson-Hasselbalch equation (Equation \ref{Henderson-Hasselbalch equation}) and let the base to acid ratio be \(x\).
For pH = 2.8:
\[2.8 = 4.8 + \log x \nonumber \]
\(x = 0.01 to 1\)
- pH 3.8, the ratio is 0.10 to 1
- pH 4.8, the ratio is 1.0 to 1
- pH 5.8, the ratio is 10 to 1
- pH 6.8, the ratio is 100 to 1


1.13.1: The Immune System
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Imagine a number that is ten billion times the number of stars in the universe, this is the estimated number of viruses on Earth [1] . Viruses are just one type of microorganism in our environment. Some microorganisms are detrimental to our health making us sick, others are important for our health (beneficial), and others are inconsequential. The microorganisms that cause diseases in humans are called pathogens . In total, there are about 1,400 known species of human pathogens that fall into five groups known as bacteria, virus, fungi, protozoa, and helminths (worms).
Pathogens are found in the air around us, the food we eat, what we drink, and the people and animals we are around, thus we are constantly exposed pathogens. Thankfully our immune system is in a constant battle working to identify the foreign invader (pathogen) and destroy them. This immune system battle is happening right now while you are reading this book!
The main purpose of the immune system is to protect your body from harmful substances, germs and cell changes that could make you ill. With this purpose, the immune system works to:
- fight disease-causing germs (pathogens) like bacteria, viruses, parasites or fungi, and to remove them from the body,
- recognize and neutralize harmful substances from the environment, and
- fight disease-causing changes in the body, such as cancer cells.
Your immune system is made up of two parts that work together to fight off disease. The innate immune system works to constantly find and destroy any foreign invaders, and the adaptive immune system works by remembering previous pathogens and infections to be able to launch a more efficient battle to destroy pathogens upon re-exposure.
The immune system includes white blood cells, organs, and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes, lymph vessels, and bone marrow. The white blood cells, or immune cells, that help the body fight infections and other diseases include neutrophils, eosinophils, basophils, mast cells, monocytes, macrophages, dendritic cells, natural killer cells, and lymphocytes (B cells and T cells).
The immune system is an amazing system of protection, however sometimes it cannot destroy the pathogen and this is when infection or disease occurs, like the Flu, Strep Throat, or a Sexually Transmitted Disease (STD).
Pathogens and Infectious Diseases
The number of viruses and bacteria on earth is staggering [2] and they occupy essentially every environment. For example, a liter of surface seawater typically contains in excess of ten billion bacteria and 100 billion viruses. The vast majority of viruses and bacteria we are exposed to have no negative effect, some can even be beneficial, however a tiny fraction of these can severely affect our health. It is estimated that only about 1,400 of a trillion microbial species are human pathogens.
The agents that cause disease, or pathogens, fall into five groups: viruses, bacteria, fungi, protozoa, and helminths (worms). Protozoa and worms are usually grouped together as parasites.
| Virus | Bacteria | Fungi | Parasites/Protozoa |
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Common Viruses and Mutations
Every year warnings are shared about the annual flu season. The flu is a contagious respiratory illness caused by the influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death. Although the flu can be caught year round, in the U.S. it is most common in the Fall and Winter seasons. The flu virus is not the same each year due to what is known as antigenic drift and antigenic shift which essentially indicates that the influenza virus has changed and thus the body will likely react differently to the variations.
Viruses can only replicate if they are absorbed by cells in the body, thus viruses survive based on the ability to continue to infect hosts. In order to continue to infect humans, the virus has to overcome the humans immune systems. Since the immune system can remember past viral infections, viruses mutate and change over time to be able to adapt to their surroundings and more effectively move from host to host. When viruses mutate they evolve into new variants. Virus variants are similar to a family tree with multiple lineages (or closely related groups of variants) and sublineages. Each variant starts with a parent lineage followed by descendant lineages. For example, with COVID-19, the BA.1.1.529 variant is from the Omicron variant.
Pandemics
There is a difference between an endemic, outbreak, epidemic, and pandemic [3] . An endemic condition occurs at a predictable rate among a population. An outbreak corresponds to an unpredicted increase in the number of people presenting a health condition or in the occurrence of cases in a new area. An epidemic is an outbreak that spreads to larger geographic areas. A pandemic is an epidemic that spreads globally.
Pandemics have occurred throughout history impacting the entire world, some of these include:
- Three plagues during the years: 541-543, 1347-1351, and 1885-ongoing.
- Six pandemics of cholera during the years: 1817-1824, 1827-1835, 1839-1856, 1863-1875, 1881-1886, and 1899-1893.
- And numerous flu pandemics: Russian flu from 1889-1893, Spanish flu from 1918-1919, Asian flu from 1957-1959, Hong Kong flu from 1968-1970, SARS from 2002-2003, Swine flu from 2009-2010, MERS from 2015-ongoing, and more recent COVID-19 from 2019-ongoing.
Table \(\PageIndex{2}\) List of some of the pandemics that occurred throughout history.
| Pandemic | Timeline | Area of emergence | Pathogen | Vector | Death toll |
|---|---|---|---|---|---|
| Athenian Plague | 430-26 B.C. | Ethiopia | Unknown | Unknown | Unknown |
| Antonine Plague | 165-180 | Iraq | Variola virus | Humans | 5 million |
| Justinian Plague | 541-543 | Egypt | Yersinia pestis | Rodents’ associated fleas | 30-50 million |
| Black Death | 1347-1351 | Central Asia | Yersinia pestis | Rodents’ associated fleas | 200 million |
| The Seven Cholera Pandemics | 1817-present | India | Vibrio cholerae | Contaminated water | 40 million |
| Spanish Flu | 1918-1919 | USA | Influenza A (H1N1) | 50 million | |
| Asian Flu | 1957-1958 | China | Influenza A (H2N2) | >1 million | |
| Hong Kong Flu | 1968 | China | Influenza A (H3N2) | 1-4 million | |
| HIV/AIDS | 1981-present | Central Africa | HIV | 36 million | |
| Severe acute respiratory syndrome coronavirus | 2002-2003 | China | Severe acute respiratory syndrome coronavirus | Bats | 774 |
| Swine Flu | 2009-2010 | Mexico | Influenza A (H1N1) |<PHONE_NUMBER>00 | |
| Ebola | 2014-2016 | Central Africa | Ebola virus | Unknown | 11000 |
| COVID-19 | 2019- July 2021 (ongoing) | China | SARS-Cov-2 | Unknown | >4 million (ongoing) |
The COVID-19 pandemic was caused by a virus called SARS-CoV-2. Viruses constantly change through mutation and sometimes these mutations result in a new variant of the virus. COVID variations include the Omicron variant and Delta variant, with new variants of the virus expected to occur. The COVOD-19 pandemic is the first global pandemic requiring large-scale responses since the 1918 Spanish flu. As of 2022, COVID-19 is still considered a pandemic and there is still concern of continued spread, sickness, and death.
Pandemics: The Chain of Infection
The way in which people become infected with a pathogen is referred to as the Chain of Infection.
The chain of infection begins with the “Infectious Agent.” The infectious agent is a pathogen, a microorganism that is capable of causing disease. There are about 1,400 different pathogens that fall within five main categories of bacteria, virus, fungi, protozoa, and worms.
The infectious agent begins in a “ Reservoir .” The reservoir could be another person, an animal, in water, or in food; it is the habitat in which the infectious agent normally lives, grows, and multiplies. The reservoir may or may not be the source from which an agent is transferred to a host. For example, the reservoir of Clostridium botulinum is soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores. Human reservoirs may or may not show the effects of illness. A carrier is a person with inapparent infection who is capable of transmitting the pathogen to others. Asymptomatic or passive or healthy carriers are those who never experience symptoms despite being infected. Carriers commonly transmit disease because they do not realize they are infected, and consequently take no special precautions to prevent transmission.
The infectious agent needs a way to leave the reservoir through a “Portal of Exit.
The infectious agent then needs a “Mode of Transmission ” to the new host. The mode of transmission could be direct or indirect. Direct contact and direct spreading of droplets often occur when two people are close together or touching, this could be through hugging, sexual intercourse, or droplets spreading when talking or sneezing when someone is close by. Indirect transmission could occur through the air, on a vehicle such as food or drinks, or transmitted by an insect like a mosquito, flee, or tick.
The infectious agent then needs to a “Portal of Entry ” into the new susceptible new host, these are ways the infectious agent could enter the body. The portal of entry must provide access to tissues in which the pathogen can multiply. This might include inhaling the infectious agent into the respiratory system, consuming it through food or drink therefore entering the digestive system, or entering directly into blood or tissues through breaks in the skin.
The final step in the Chain of Infection is when the infectious agent has successfully entered the Susceptible Host . If a pathogen gets past a host’s defenses, it will attempt to infect the host and begin replicating itself; The pathogens must infect a host in order to grow or replicate. However, just because the pathogen has successfully entered the host does not mean they will get infected! This is where the power of the immune system steps in to fight off the infection destroying the infectious agent.
The survival of human pathogens, like viruses, bacteria, and parasites, is dependent upon quickly invading a human, replicating, and efficiently transmitting to others. Pathogens depend on the chain of infection for survival. You can take precautions to break the chain of infection at each step in the chain by doing the following:
- Recognize signs and symptoms of disease in order to get treatment and isolate when needed.
- Wash your hands and/or use hand sanitizer
- Cleaning and disinfect surfaces
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Get treatment from a doctor
- If you have a bacterial infection you need antibiotics. Antibiotics are medicines that fight bacterial infections.
- Avoid touching your face
- Do not cough into your hand, cough into your arm or elbow.
- Use pest control
- Wear personal protective equipment to put a barrier between the portal of exit or entry.
- Use air purifiers to reduce airborne pathogens.
- Properly dispose of waste.
- Drink clean drinking water.
- Follow food safety guidelines.
- Immunizations/vaccines to reduce the susceptibility of the new host.
The Immune System Battle
When an infectious agent leaves a reservoir through a portal of exit and finds a mode of transmission, it is then up to the defenses of the new host to try to reduce the chances of infection. This is when the immune system battle commences which includes two main subsystems of the immune system, called the innate immune system and the adaptive immune system. The innate immune system is what you are born with and the adaptive immune system is developed overtime as your body is exposed to pathogens. The innate immune response is always present and attempts to defend against all pathogens rather than focusing on specific ones. Conversely, the adaptive immune response stores information about past infections and mounts pathogen-specific defenses.
The innate immune system includes the bodies physical and chemical barriers along with white blood cells that are always present in the blood and tissues ready to destroy any and all invading pathogens. The adaptive immune system also includes white blood cells, but they are more specialized and ready to destroy specific pathogens.
Immune System: Physical and Chemical Barriers
The first step in the battle against pathogens is to put up a physical barrier against the portal of entry. The largest physical barrier against pathogens is our skin. Skin provides provides both a barrier of entry and a means to destroy pathogens through skin acidity and dryness. The areas of the body that are not covered with skin have alternative methods of defenses using mucus and secretions, like tears in the eyes, wax in the ears, and cilia and mucus in the nose, mouth, and lungs. Despite these barriers, pathogens may enter the body through skin abrasions or punctures, or by collecting on mucosal surfaces in large numbers that overcome the mucus or cilia. Some pathogens have evolved specific mechanisms that allow them to overcome physical and chemical barriers. When pathogens do enter the body, the innate and adaptive immune systems responds.
The Innate and Adaptive Immune System
The main job of the innate immune system is to fight harmful substances and germs that enter the body, for instance through the skin or digestive system. The innate immune system is the rapid response to any invading pathogen, the response is non-specific meaning the same response for any and all pathogens. The adaptive immune system makes antibodies and uses them to specifically fight certain germs that the body has previously come into contact with. This is also known as an “acquired” (learned) or specific immune response. Because the adaptive immune system is constantly learning and adapting, the body can also fight bacteria or viruses that change over time.
Both the innate and the adaptive immune system rely on markers that are on body cells. These markers tell the immune system whether the cell is a human cell that belongs in the body, or whether the cell is foreign. Pathogens have two markers. One marker is non-specific called a Pathogen-Associated Molecular Patterns (PAMPs), the other markers is specific called an Antigen. Many pathogens share the same PAMPs, however have unique and distinctive antigens; Every invader’s antigenic pattern is unique. The innate immune cells have receptors called Pattern Recognizing Molecules (PRMs) that recognize the PAMP. When the innate cells recognize the PAMP they begin the battle to destroy the foreign invaders. Adaptive immune system cells recognize the specific antigens on the pathogen and launch a battle specifically to destroy that pathogen.
The innate immune system cells are the first to respond, these include the following types of leukocytes: mast cells, macrophages, natural killer cells, dendritic cells, monocytes, neutrophils, basophils, and eosinophils (described in Figure 11.?). Cells of the adaptive immune system are classified as lymphocytes (a type of leukocyte) and include two main types: T-cells and B-cells (the letters denote ‘thymus’ and ‘bone marrow’, the tissues where each of these leukocytes mature).
Steps to the The Immune System Battle:
- The cellular immune response begins when a pathogen gets through the bodies physical and chemical barriers.
- If a pathogen gets past the hosts defenses it will attempt to infect the host and begin replicating itself. The subsequent battle between the germs and the body’s immune system will cause the symptoms of illness.
- The first immune cells to respond to the invading pathogen are the innate immune cells: mast cells, macrophages, natural killer cells, dendritic cells, monocytes, neutrophils, basophils, and eosinophils. These cells are always ready to fight off pathogens and work hard to destroy the foreign invaders.
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Actions of the innate immune system cells
- Dendritic Cells: Ingest pathogen and help activate the adaptive immune system by presenting their antigen to Helping T-Cells and killer T-Cells.
- Neutrophils: Migrate from the bloodstream to ingest and kill bacteria and fungi and recruit more immune cells.
- Macrophages: Known as the “big eaters”, they ingest pathogens and dead cells and help activate the adaptive immune system by presenting their antigen. They also recruit more immune cells.
- Mast Cells: launch inflammatory response and recruit macrophages and neutrophils.

- Basophils: Defend against parasites and contribute to inflammatory response.
- Eosinophils: Kill bacteria and parasites.
- The innate immune cells alert the whole body that there is a problem by activating the inflammatory response and initiating the adaptive immune system.
- The inflammatory response brings swelling, pain and higher temperature, which attracts more cells to the site of infection. This means more immune cells join the fight to destroy the pathogen.
- The innate immune cells provide important information to the adaptive immune cells to “train” the adaptive immune cells how to destroy the pathogen.
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Training the adaptive immune cells is done in two ways, by releasing cytokines and displaying the pathogens antigens.
- Cytokines are signals that tell cells what to do and where to go. Your body responds to threats in different ways depending on the cytokine signal released by your immune cells.
- The antigen is displayed when the innate immune cells capture and breakdown the invader.
- Together, the cytokines and antigens train individual adaptive immune cells to recognize and destroy specific patterns of each foreign invader.
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The Adaptive Immune System Cells
- Helper and killer T-Cells are activated after the Dendritic cells engulf the pathogen.
- When the helper T-Cells create B-Cells they are initiating the adaptive immune system.
- Antibodies, B Cells, are then produced that attach to the pathogen.
- The B Cells divide to produce plasma and memory cells.
- If the same pathogen invades again, the memory cells help the immune system activate much quicker.
- When the adaptive cells are “trained” how to fight a specific antigen, they remember how to fight it, thus the next time that this pathogen tries to infect you your adaptive immune cells will remember it and destroy it quickly.
- The ability of the immune system to remember previous pathogens is commonly called adaptive or acquired immunity and is the basis for vaccines.
- However, pathogens really want to live and they can be tricky. Some pathogens, including those that cause flu, strep throat, and malaria, can mutate and change the way they look to your immune system over time by changing their antigen markers thus disguising themselves marking it harder for your immune system to recognize the mutated germs even though you’ve been exposed to them before. This is why you can get sick from flu, strep throat, or COVID multiple times.
Vaccines
It is possible to acquire adaptive immunity naturally or artificially. Natural immunity results from natural exposure to an antigen. For example, someone sneezes the influenza virus into the air and then it is breathed into another person’s nasal passageway. However, artificial immunity refers to deliberately introducing an antigen into an individual to stimulate an immune response, known as a vaccine. Whether you have been introduced to a pathogen naturally or artificially the end result is the same, to develop memory cells to acquire immunity against the pathogen.
If the immune system did not have the ability to remember pathogens then vaccines would not work; vaccines are effective because of memory cells. The mechanism underlying vaccination is that exposure of the adaptive immune system to a small dose of an antigen will produce an initial immune response. More importantly, this small dose of antigen establishes a population of memory T and B cells that will live long-term in the body. When that antigen is encountered again, the immune system is already primed. Upon a subsequent exposure, a more robust and faster response is established and the pathogen is destroyed faster. Vaccines can either prevent or decrease the severity of infections.
There are several different types of vaccines and even more types of vaccines in development. Each type is designed to teach your immune system how to fight off certain kinds of germs—and the serious diseases they cause. When scientists create vaccines, they consider how your immune system responds to the germ, who needs to be vaccinated against the germ, and the best technology or approach to create the vaccine.
| Vaccine Type | Characteristics | Vaccine Protects against |
| Inactivated vaccines |
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| Live-attenuated vaccines |
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| Messenger RNA (mRNA) vaccines |
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| Subunit, recombinant, polysaccharide, and conjugate vaccines |
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| Toxoid vaccines |
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| Viral vector vaccines |
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| DNA vaccines |
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In development. No vaccines available yet. |
| Recombinant vector vaccines (platform-based vaccines) |
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In development. No vaccines available yet. |
There is another way to get temporary immunity and that is called passive immunity, which is when antibodies are provided to the person from a donor recipient, for example from a mother to child as antibodies move across the placenta before birth. Passive immunity is not long-lasting because the individual does not produce memory cells.


16.11: Eukaryotic Gene Regulation - Regulating Protein Activity and Longevity
- Explain how chemical modifications affect protein activity and longevity
Chemical Modifications, Protein Activity, and Longevity
Proteins can be chemically modified with the addition of methyl, phosphate, acetyl, and ubiquitin groups. The addition or removal of these groups from proteins regulates their activity or the length of time they exist in the cell. Sometimes these modifications can regulate where a protein is found in the cell; for example, in the nucleus, the cytoplasm, or attached to the plasma membrane.
Chemical modifications occur in response to external stimuli such as stress, the lack of nutrients, heat, or ultraviolet light exposure. These changes can alter protein function, epigenetic accessibility, transcription, mRNA stability, or translation; all resulting in changes in expression of various genes. This is an efficient way for the cell to rapidly change the abundance levels of specific proteins in response to the environment. Because proteins are involved in every stage of gene regulation, the phosphorylation of a protein (depending on the protein that is modified) can alter accessibility to the chromosome, can alter translation (by altering transcription factor binding or function), can change nuclear shuttling (by influencing modifications to the nuclear pore complex), can alter RNA stability (by binding or not binding to the RNA to regulate its stability), can modify translation (increase or decrease), or can change post-translational modifications (add or remove phosphates or other chemical modifications). All of these protein activities are affected by the phosphorylation process. The enzymes which are responsible for phosphorylation are known as protein kinases. The addition of a phosphate group to a protein can result in either activation or deactivation; it is protein dependent.
Another example of chemical modifications affecting protein activity include the addition or removal of methyl groups. Methyl groups are added to proteins via the process of methylation; this is the most common form of post-translational modification. The addition of methyl groups to a protein can result in protein-protein interactions that allows for transcriptional regulation, response to stress, protein repair, nuclear transport, and even differentiation processes. Methylation on side chain nitrogens is considered largely irreversible while methylation of the carboxyl groups is potentially reversible. Methylation in the proteins negates the negative charge on it and increases the hydrophobicity of the protein. Methylation on carboxylate side chains covers up a negative charge and adds hydrophobicity. The addition of this chemical group changes the property of the protein and, thus, affects it activity.
The addition of an ubiquitin group to a protein marks that protein for degradation. Ubiquitin acts like a flag indicating that the protein lifespan is complete. These proteins are moved to the proteasome, an organelle that functions to remove proteins to be degraded. One way to control gene expression is to alter the longevity of the protein: ubiquitination shortens a protein’s lifespan.
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Key Points
- Proteins can be chemically modified by adding methyl, phosphate, acetyl, and ubiquitin groups.
- Protein longevity can be affected by altering stages of gene regulation, including but not limited to altering: accessibility to chromosomal DNA for transcription, rate of translation, nuclear shuttling, RNA stability, and post-translational modifications.
- Ubiquitin is added to a protein to mark it for degradation by the proteasome.
Key Terms
- ubiquitin : a small polypeptide present in the cells of all eukaryotes; it plays a part in modifying and degrading proteins
- proteasome : a complex protein, found in bacterial, archaeal and eukaryotic cells, that breaks down other proteins via proteolysis


7.2: Overview of Photosynthesis
By the end of this section, you will be able to do the following:
- Explain the significance of photosynthesis to other living organisms
- Describe the main structures involved in photosynthesis
- Identify the substrates and products of photosynthesis
Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates from sunlight and converts it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. It is also a source of oxygen necessary for many living organisms. In brief, the energy of sunlight is “captured” to energize electrons, whose energy is then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis 350 to 200 million years ago during the Carboniferous Period.
Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (Figure \(\PageIndex{1}\)). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds. For this reason, they are referred to as chemoautotrophs .
The importance of photosynthesis is not just that it can capture sunlight’s energy. After all, a lizard sunning itself on a cold day can use the sun’s energy to warm up in a process called behavioral thermoregulation . In contrast, photosynthesis is vital because it evolved as a way to store the energy from solar radiation (the “photo-” part) to energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (Figure \(\PageIndex{2}\)), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to visible light, to photosynthesis, to vegetation, to deer, and finally to the wolf.
Main Structures and Summary of Photosynthesis
Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure \(\PageIndex{3}\)). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (G3P), as well as simple carbohydrate molecules (high in energy) that can then be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.
The following is the chemical equation for photosynthesis (Figure \(\PageIndex{4}\)):
Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.
Basic Photosynthetic Structures
In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll . The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss due to high temperatures on the upper surface of the leaf. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.
In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast . For plants, chloroplast-containing cells exist mostly in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane), and are ancestrally derived from ancient free-living cyanobacteria. Within the chloroplast are stacked, disc-shaped structures called thylakoids . Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen . As shown in Figure \(\PageIndex{5}\), a stack of thylakoids is called a granum , and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).
Visual Connection
Visual Connection
On a hot, dry day, the guard cells of plants close their stomata to conserve water. What impact will this have on photosynthesis?
The Two Parts of Photosynthesis
Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions. In the light-dependent reactions , energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions , the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, however, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers . The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure \(\PageIndex{6}\) illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.
Link to Learning
Everyday Connection
Everyday Connection
Photosynthesis at the Grocery Store
Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle (Figure \(\PageIndex{7}\)) contains hundreds, if not thousands, of different products for customers to buy and consume.
Although there is a large variety, each item ultimately can be linked back to photosynthesis. Meats and dairy link, because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: For instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) are derived from “algae” (unicellular plant-like organisms, and cyanobacteria). Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.
Contributors and Attributions
Remixed and/or curated from the following works:
Clark, M. A., Douglas, M., & Choi, J. (2018). 2.5.2 Overview of Photosythesis . In Biology 2e . OpenStax (CC BY 4.0; Access for free at https://openstax.org/books/biology-2e/pages/1-introduction ).


11.3: Electrolytes
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Learning Objectives
By the end of this section, you will be able to:
- Define and give examples of electrolytes
- Distinguish between the physical and chemical changes that accompany dissolution of ionic and covalent electrolytes
- Relate electrolyte strength to solute-solvent attractive forces
When some substances are dissolved in water, they undergo either a physical or a chemical change that yields ions in solution. These substances constitute an important class of compounds called electrolytes . Substances that do not yield ions when dissolved are called nonelectrolytes . If the physical or chemical process that generates the ions is essentially 100% efficient (all of the dissolved compound yields ions), then the substance is known as a strong electrolyte . If only a relatively small fraction of the dissolved substance undergoes the ion-producing process, it is called a weak electrolyte .
Substances may be identified as strong, weak, or nonelectrolytes by measuring the electrical conductance of an aqueous solution containing the substance. To conduct electricity, a substance must contain freely mobile, charged species. Most familiar is the conduction of electricity through metallic wires, in which case the mobile, charged entities are electrons. Solutions may also conduct electricity if they contain dissolved ions, with conductivity increasing as ion concentration increases. Applying a voltage to electrodes immersed in a solution permits assessment of the relative concentration of dissolved ions, either quantitatively, by measuring the electrical current flow, or qualitatively, by observing the brightness of a light bulb included in the circuit (Figure \(\PageIndex{1}\)).
Ionic Electrolytes
Water and other polar molecules are attracted to ions, as shown in Figure \(\PageIndex{2}\). The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction . These attractions play an important role in the dissolution of ionic compounds in water.
When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them. This process represents a physical change known as dissociation . Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes. Even sparingly, soluble ionic compounds are strong electrolytes, since the small amount that does dissolve will dissociate completely.
Consider what happens at the microscopic level when solid KCl is added to water. Ion-dipole forces attract the positive (hydrogen) end of the polar water molecules to the negative chloride ions at the surface of the solid, and they attract the negative (oxygen) ends to the positive potassium ions. The water molecules surround individual K + and Cl − ions, reducing the strong interionic forces that bind the ions together and letting them move off into solution as solvated ions, as Figure \(\PageIndex{2}\) shows. Overcoming the electrostatic attraction permits the independent motion of each hydrated ion in a dilute solution as the ions transition from fixed positions in the undissolved compound to widely dispersed, solvated ions in solution.
Covalent Electrolytes
Pure water is an extremely poor conductor of electricity because it is only very slightly ionized—only about two out of every 1 billion molecules ionize at 25 °C. Water ionizes when one molecule of water gives up a proton (H + ion) to another molecule of water, yielding hydronium and hydroxide ions.
\[\ce{H2O(l) + H2O(l) \rightleftharpoons H3O^{+}(aq) + OH^{-}(aq)} \nonumber \]
In some cases, solutions prepared from covalent compounds conduct electricity because the solute molecules react chemically with the solvent to produce ions. For example, pure hydrogen chloride is a gas consisting of covalent HCl molecules. This gas contains no ions. However, an aqueous solution of HCl is a very good conductor, indicating that an appreciable concentration of ions exists within the solution.
Because HCl is an acid , its molecules react with water, transferring H + ions to form hydronium ions (H 3 O + ) and chloride ions (Cl − ):
This reaction is essentially 100% complete for HCl (i.e., it is a strong acid and, consequently, a strong electrolyte). Likewise, weak acids and bases that only react partially generate relatively low concentrations of ions when dissolved in water and are classified as weak electrolytes. The reader may wish to review the discussion of strong and weak acids provided in the earlier chapter of this text on reaction classes and stoichiometry.


5.6: Near-field Scanning Optical Microscopy (NSOM)
Near-field scanning optical microscopy (NSOM), also called scanning near-field optical microscopy (SNOM), is a scanning probe technique that overcomes the diffraction barrier in traditional far-field optical microscopy. Conventional optical microscopy techniques are limited by the diffraction of light and the resolution is limited to roughly 250 nm, which make it very difficult to resolve the domains or clusters in the cellular membranes[1]. The basic idea in NSOM is to confine the illuminating light to nanometric dimensions to break the diffraction limit that cannot be achieved by traditional far-field optical microscopy[2]. Thus, NSOM can produce high-resolution topographical and optical images to study biological membranes. Figure \(\PageIndex{1}\) shows the diffraction of light and the near-field in the NSOM fiber probe.
Near-field Theory
In traditional far-field optical microscopy, the illumination source is a monochromatic plane wave[3]. The lens collecting the scattering light is placed several wavelengths of the illumination light far away from the sample surface. This causes the commonly known diffraction limit, that is far-field optical techniques are limited to resolve features approximately on the order of half of the wavelength of the illuminating light due to the diffraction of light. However, the classical NSOM uses a tapered optical fiber probe and an aperture that is much smaller than the wavelength of the light. As shown in Figure \(\PageIndex{2}\), after light passes through the cantilever tip with nano-aparture, a optical near-field (or evanescent field) on the far side of this tip can be created that is not diffraction limited[4]. Then the resolution in near-field microscopy is directly affected by the size of the aperture and independent of the wavelength of the light.
NSOM Instrumentation
Standard NSOM normally contains three parts, the illumination unit, the collection and redistribution unit and the detection module[3]. The illuminating light comes from the probe, goes through the aperture at the tip and interacts with the sample surface. As light passes through the sample, the absorption or the fluorescence produced by the labeled molecules on the sample surface can be collected. During the scan, the probe can be brought to the sample at a very small distance(<10 nm)[5]. The topographic and optical images showing the high spatial resolution can be generated simultaneously. The lateral resolution can be reached down to tens of nanometers, which is determined by the size of the aperture and sample-probe distance[6]. The probe tip movement is monitored and controlled by the feedback system and the x-y-z scanner (usually piezoelectric) to keep the tip within the near-field.
Figure \(\PageIndex{4}\) shows three common modes of NSOM measurements, which are illumination mode, collection mode and scattering mode for apertureless NSOM. In illumination mode, the evanescent field can be generated at the tip end when the illuminated light passes through the probe near the sample. Then the scattered light from the probe-sample system is collected to generate the NSOM image. In collection mode, the evanescent wave near sample is acquired by the local probe within near-field of sample. The dashed line means the critical angle. Besides aperture NSOM, the NSOM with apertureless probe was also developed, which use STM-like tip or AFM cantilever tip. The probe tip is also brought very close to the sample, and the scattering light is induced near sample.
NSOM Fluorescence Imaging of Membranes
The combination of NSOM and the simultaneous fluorescence has been used to investigate the membrane systems such as lipid bilayers, lipid rafts, membrane receptor, clustering and other small domains on the membrane[2]. The near-field technique makes it possible to detect the small domains with high resolution that cannot be achieved by the traditional optical microscopes. Further, the specific assignments of detected domains can be made by comparing the simultaneous fluorescence mapping and the surface topography. Thus, the high-resolution images of NSOM combined with fluorescence can provide abundant information such as the size and specific features of different domains on the membrane, which is very helpful for us to understand the membrane organization and important function.
Supported lipid bilayers (SLB) studied by NSOM
Supported lipid bilayers (SLB) are phospholipid bilayers that contain different lipids, cholesterol or proteins. SLBs are significant model membranes that have been studied for a long time. NSOM help researchers to better understand the crucial factors in the small domains and structures in SLBs with high-resolution. NSOM images of SLBs can identify different domains of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC) on membranes[5].
Lipid rafts studied by NSOM
For the heterogeneous lipid domains in cell membranes, also called lipid rafts, NSOM has been used to visualize the nanolandscape of ganglioside GM1 after tightening by its ligand cholera toxin (CTxB). Figure \(\PageIndex{4}\) demonstrates the resolution of NSOM being able to resolve nanodomains that were not possible with the conventional optical microscopes. NSOM was able to show the nanodomains were smaller than 120 nm.
Membrane protein receptors studied by NSOM
The formation of receptors nanoclusters on membrane could also be investigated by NSOM. For example, the distribution of receptor DC-SIGN, a transmembrane protein in immature dendritic cells (DC) has been studied by NSOM. As shown in Figure1.5, about 80% of the immature dendritic cells' protein receptors are approximately 185 nm in diameter. The "finger" domains are the main topographic feature of dendritic cells, where the colorful fluorescent areas indicate nanocluster distributions of DC-SIGN receptors.
Summary
NSOM has great advantages in studying biological membranes over conventional optical microscopy. The combination of near-field technique and scanning probe techniques utilized by NSOM can generate high spatial resolution images beyond the diffraction limit, which is not possible in conventional optical microscopy. Combined with fluorescence technique, NSOM can show both topographic image and the simultaneous optical fluorescence image that can detect the small domains and the specific features in membrane systems.
References
- Sezgin, E., Super-resolution Optical Microscopy for Studying Membrane Structure and Dynamics. J. Phys.: Condens. Matter 2017, 29, 27
- Huckabay, H. A.; Armendariz, K. P.; Newhart, W. H.; Wildgen, S. M.; Dunn, R. C., Near-Field Scanning Optical Microscopy for High-Resolution Membrane Studies. Methods in molecular biology (Clifton, N.J.) 2013, 950 , 373-394.
- Hecht, B.; Sick, B.; Wild, U. P.; Deckert, V.; Zenobi, R.; Martin, O. J. F.; Pohl, D. W., Scanning near-field optical microscopy with aperture probes: Fundamentals and applications. The Journal of Chemical Physics 2000, 112 (18), 7761-7774.
- Germany, WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm. https://www.witec.de/techniques/snom/ . Retrieved 2017-04-06.
- Raigoza, A. F.; Dugger, J. W.; Webb, L. J., Review: Recent Advances and Current Challenges in Scanning Probe Microscopy of Biomolecular Surfaces and Interfaces. ACS Applied Materials & Interfaces 2013, 5 (19), 9249-9261.
- Hinterdorfer, P.; Garcia-Parajo, M. F.; Dufrêne, Y. F., Single-Molecule Imaging of Cell Surfaces Using Near-Field Nanoscopy. Accounts of Chemical Research 2012, 45 (3), 327-3Near-Field Nanoscopy


4.3: Hess's Law
The calculation of a standard reaction enthalpy can be performed using the following cycle:
\[\begin{equation} \begin{aligned} \text{reactants} & \quad \xrightarrow{\Delta_{\text{rxn}} H^{\; -\kern-5pt{\ominus}\kern-5pt-}} \quad \text{products} \\ \scriptstyle{-\Delta_{\text{f}} H_{\text{reactants}}^{\; -\kern-5pt{\ominus}\kern-5pt-}} \quad \bigg\downarrow \quad & \qquad \qquad \qquad \qquad \scriptstyle{\bigg\uparrow \; \Delta_{\text{f}} H_{\text{products}}^{\; -\kern-5pt{\ominus}\kern-5pt-}} \\ \text{"elements in } & \text{their standard reference state"} \end{aligned} \end{equation} \label{4.3.1} \]
This process is summarized by the simple formula:
\[ \Delta_{\text{rxn}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}= \Delta_{\mathrm{f}} H_{\text{products}}^{\; -\kern-7pt{\ominus}\kern-7pt-}- \Delta_{\mathrm{f}} H_{\text{reactants}}^{\; -\kern-7pt{\ominus}\kern-7pt-}. \label{4.3.2} \]
Notice how there is a negative sign in front of the enthalpy of formation of the reactants because they are normally defined for the reactions that go from the elements to the reactants and not vice-versa. To close the cycle in Equation \ref{4.3.1}, however, we should go from the reactants to the elements, and therefore we must invert the sign in front of the formation enthalpies of the reactants. Equation \ref{4.3.2} can be generalized using the same technique used to derive eq. 4.1.4 , resulting in:
\[ \Delta_{\text{rxn}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}= \sum_i \nu_i \Delta_{\mathrm{f}} H_i^{\; -\kern-7pt{\ominus}\kern-7pt-}, \label{4.3.3} \]
which is a mathematical expression of the law that is known as Hess’s Law . Hess’s law is valid at constant pressure because, at those conditions, the heat of reaction—a path function—is equal to the enthalpy of reaction—a state function. Therefore, the enthalpy of a reaction depends exclusively on the initial and final state, and it can be obtained via the pathway that passes through the elements in their standard state (the formation pathway).
Calculate the standard enthalpy of formation at 298 K for the combustion of 1 mol of methane, using the data in eq. 4.2.1 .
- Answer
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The reaction that is under consideration is: \[ \mathrm{CH}_{4(g)} + 2 \mathrm{O}_{2(g)} \rightarrow \mathrm{CO}_{2(g)} + 2 \mathrm{H}_2 \mathrm{O}_{(l)} \qquad \Delta_{\mathrm{f}} H_{\mathrm{CH}_{4(g)}}^{\; -\kern-7pt{\ominus}\kern-7pt-}= ? \nonumber \]
Using Hess’s Law, Equation \ref{4.3.3}, the enthalpy of formation for methane is:
\[ \Delta_{\text{rxn}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}= \Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{CO}_{2(g)}} + 2 \Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_{2}O_{(l)}} - \Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{CH}_{4(g)}} - 2 \underbrace{\Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{O}_{2(g)}}}_{=0} \nonumber \]
whose values are reported in eq. 4.2.1 . Notice that the formation enthalpy of \(O_{2(g)}\) is zero, since it is an element in its standard state. The final result is:
\[ \Delta_{\text{rxn}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}= \overbrace{-394}^{\Delta_{\text{f}} H^{\; -\kern-5pt{\ominus}\kern-5pt-}_{\mathrm{CO}_{2(g)}}} +2 \overbrace{(-286)}^{\Delta_{\text{f}} H^{\; -\kern-5pt{\ominus}\kern-5pt-}_{\mathrm{H}_{2}O_{(l)}}} - \overbrace{(-75)}^{\Delta_{\text{f}} H^{\; -\kern-5pt{\ominus}\kern-5pt-}_{\mathrm{CH}_{4(g)}}} = -891 \mathrm{kJ/mol}. \nonumber \]
where the negative sign indicates that the reaction is exothermic (see eq. 4.1.1 ), as we should expect. The cycle that we used to solve this exercise can be summarized with :
\[ \begin{aligned} \mathrm{CH}_{4(g)} + & 2 \mathrm{O}_{2(g)} \quad \xrightarrow{\Delta_{\text{rxn}} H^{\; -\kern-5pt{\ominus}\kern-5pt-}} \quad \mathrm{CO}_{2(g)} + 2 \mathrm{H}_2 \mathrm{O}_{(l)} \\ \scriptstyle{-\Delta_{\text{f}} H_{\mathrm{CH}_{4(g)},\mathrm{O}_{2(g)}}^{\; -\kern-5pt{\ominus}\kern-5pt-}} & \searrow \qquad \qquad \qquad \qquad \qquad \nearrow \; \scriptstyle{\Delta_{\text{f}} H_{\text{CO}_{2(g)},\mathrm{H}_{2(g)}}^{\; -\kern-5pt{\ominus}\kern-5pt-}}\\ & \qquad \mathrm{H}_{2(g)}, \mathrm{C}_{(s,\text{graphite})}, \mathrm{O}_{2(g)} \end{aligned} \nonumber \]
Notice that at standard pressure and \(T = 298 \; \mathrm{K}\) water is in liquid form. However, when we burn methane, the heat associated with the exothermic reaction immediately vaporize the water. Substances in different states of matter have different formation enthalpies, and \(\Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_{2}O_{(l)}} = -242 \ \mathrm{kJ/mol}\). The difference between the formation enthalpies of the same substance in different states represents the latent heat that separates them. For example, for water:
\[ \begin{aligned} \Delta_{\text{vap}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_2O} & = \Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_{2}O_{(g)}} - \Delta_{\text{f}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_{2}O_{(l)}} \\ & = (-242) - (-286) = + 44 \; \text{kJ/mol} \end{aligned} \nonumber \]
which is the latent heat of vaporization for water, \(\Delta_{\text{vap}} H^{\; -\kern-7pt{\ominus}\kern-7pt-}_{\mathrm{H}_2O}\). The latent heat is positive to indicate that the system absorbs energy in going from the liquid to the gaseous state (and it will release energy when going the opposite direction from gas to liquid).


6.2: Normal Random Variables (2 of 6)
Learning Objectives
- Use a normal probability distribution to estimate probabilities and identify unusual events.
Example
Beyond One Standard Deviation from the Mean
Earlier we stated that for all normal curves, the area within 1 standard deviation of the mean will equal 0.68. From this fact, we can see that the area outside of this region equals 1 − 0.68 = 0.32. And since normal curves are symmetric, this outside area of 0.32 is evenly divided between the two outer tails . So the area of each tail = 0.16.
The outer tail areas allow us to answer related probability questions:
- Question: What is the probability that a normal random variable is more than 1 standard deviation from its mean?
- Answer 0.32
- Question: What is the probability that a normal random variable is more than 1 standard deviation larger than its mean?
- Answer 0.16
Before leaving this example, we highlight one more geometric fact about normal curves. Look at the arrows pointing at the normal curve in the following figure.
At these points, the curve changes the direction of its bend and goes from bending upward to bending downward, or vice versa. A point like this on a curve is called an inflection point . Every normal curve has inflection points at exactly 1 standard deviation on each side of the mean.
With the following simulation, you can look at a variety of normal curves. Use the slider to change the standard deviation. As you change the standard deviation, you will of course get different normal curves. Observe that the two properties we discussed in the examples remain true for any standard deviation you select:
- The probability that a value is within 1 standard deviation of the mean is 68%.
- The x -values of the inflection points correspond to 1 standard deviation above and below the mean.
Click here to open this simulation in its own window.
Now we extend this idea to look at the probability of a value falling within 2 standard deviations of the mean or 3 standard deviations of the mean.
If X is a normal random variable with mean and standard deviation , then
- The probability that X is within 1 standard deviation of the mean equals approximately 0.68 .
- The probability that X is within 2 standard deviations of the mean equals approximately 0.95 .
- The probability that X is within 3 standard deviations of the mean equals approximately 0.997 .
To summarize using probability notation:
These three facts together are called the empirical rule for normal curves.
Comment
Let’s take a moment to look a bit deeper at what the empirical rule tells us.
- The first statement of the empirical rule really defines a range of likely values of X . It gives us an interval – within 1 standard deviation of the mean – that contains the central 68% of the values. This statement is very similar to statements about the interquartile range (IQR) that we saw back in the module Summarizing Data Graphically and Numerically . The IQR is the width of the interval that captures the central 50% of the data points of a quantitative distribution.
-
The second and third statements in the empirical rule help us identify values that are unlikely to occur. Compare this to the discussion in
Summarizing Data Graphically and Numerically
where we defined an outlier to be a value that is either more than 1.5 IQRs above quartile 3 or more than 1.5 IQRs below quartile 1. Here we can make the following characterizations of extreme values in normal distributions.
- 95% of values fall within 2 standard deviations of the mean. It is therefore unlikely for a value to fall more than 2 standard deviations away from the mean. Values more than 2 standard deviations away from the mean in a normal distribution are often called outliers .
- 99.7% of values fall within 3 standard deviations of the mean. It is therefore extremely unlikely for a value to fall more than 3 standard deviations away from the mean. Values more than 3 standard deviations away from the mean are often called extreme outliers .
Contributors and Attributions
- Concepts in Statistics. Provided by : Open Learning Initiative. Located at : http://oli.cmu.edu . License : CC BY: Attribution


4.4: Stop
The first step in SIFT is also the most simple: stop reading and consider why you’re engaging with this source instead of another. What drew you to this source, and how is it applicable for your needs? Specifically, you’ll want to stop and check these two things:
Check yourself
Everyone brings their own expectations to their research. These expectations can affect every step of your research process, from the search terms you use to the sources you choose to cite. For example, you may have a preconceived notion of what you think the research should show, but this may not be what the research actually shows.
If a source confirms or denies what you believe to be true, you may have an emotional reaction (e.g. anger, sadness, vindication, excitement). Use your reaction as a reminder to check yourself and make sure your emotions are not affecting the sources you use and how you use them.
Make it a habit to check your emotions and ensure that they aren’t affecting the choices you make in your research or life. After you check yourself, you can move on to evaluate your source using the other steps in SIFT. Verifying the truth of a source you agree with or finding additional sources that present a different perspective (even if it may upset you or contradict your beliefs) can strengthen your case. [1]
Checking yourself in everyday life
Checking yourself is particularly important in your everyday life as well. You’ve likely had friends or family share an article on social media and had the comments explode. How many of the commenters do you think actually read the article? It’s possible that even the person who shared it didn’t read the whole article before reacting to the headline. When you know the person sharing something, it is natural to rely on their judgment. This can lead to embarrassment if you share or react to something that isn’t saying what you thought it was.
Checking your emotions and reading beyond the headlines and summary information for an article can help you avoid sharing false or inaccurate information, whether it’s for a class project or in conversations with friends on social media.
Check that you’re on task
Stop is also a reminder to stay on task . It is easy to get lost exploring interesting tangents while researching. As soon as you find yourself getting distracted, stop and remember your original goal. For a paper or project, this means focusing on your research question and sources that are directly related to it.
Let’s say you’re looking for examples of accessible interior design for a class project. As you’re searching, you stumble upon this article: “ 12 Incredible Dog Houses for Your Best Friend. ” From here, you might decide to start exploring more examples of dog house design, or you can stop and ask whether this article is really adding anything to your project. Yes, it’s a fun distraction, but you still need to finish your research. By cultivating a habit of checking yourself regularly, you can avoid situations where a fun distraction becomes a lost afternoon.
In addition to checking that you’re on task through what you read, you can also check that you’re on task by using the right search terms. Think about the purpose of your research and whether the keywords you are using closely align with your research question. You can use the tips you learned in Chapter 1 to make sure you’re using the best keywords for your research question.
You’ll also want to determine whether the search tool you are using can help you find the right kind of sources for your research. Do you need in-depth research articles, information on current events, or other types of sources? You may find a lot of interesting sources in a newspaper index, but if you need to use peer-reviewed journal articles for your project, you should use the article indexes and databases rather than spending a lot of time on sources you won’t be able to use, no matter how interesting they seem. You can refer back to Chapter 2 for help navigating that process.
- Building A Fact-Checking Habit By Checking Your Emotions: https://webliteracy.pressbooks.com/chapter/building-a-habit-by-checking-your-emotions/ ↵


6.5B: Stable Isotopes
- Demonstrate how isotopes are used in bacterial identification
The term isotope refers to the number of neutrons a certain element contains. Elements of the same name (for example, oxygen) must always have the same number of protons, but the number of neutrons can change. Adding or subtracting neutrons from an atom does not change the elemental properties, but it can alter some of its features (like making it more radioactive).
Stable Isotopes
While the number of neutrons in a particular atom can change, there is a certain threshold where the atom is given more neutrons that its nuclear force can hold. At this point, the neutrons start to be released. The release is also known as decay. During this time, the atom is deemed “unstable. ” The atom will continue to lose neutrons until it become stable again. A stable isotope is a chemical isotope that is not radioactive. There are some cases where atoms have no stable isotopes so they continue to lose neutrons, and later protons and electrons, until they become another element entirely. Research has shown that there are 80 elements with one or more stable isotopes. Of these, 26 have only one stable isotope which is also known as being monoisotopic. The element with the most stable isotopes is tin which as 10 stable isotopes.
Extensions
Knowledge about stable isotopes is important in a variety of fields. Scientists have used information on these topics in botanical and plant biological investigations as well as ecological and biological studies. Additionally, some scientists have used oxygen isotope ratios to reconstruct historical atmospheric temperatures. This work is especially important due to our current concern with climate change/global warming.
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, SPECIFIC ATTRIBUTION
- Assay. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Assay. License : CC BY-SA: Attribution-ShareAlike
- DOE: HSS - redirectredirect. Provided by : www.hss.doe.gov/healthsafety/...intro_9_4.html. Located at : www.hss.doe.gov/healthsafety/...intro_9_4.html. License : Public Domain: No Known Copyright
- Biosensor. Provided by : Wikipedia. Located at : http://en.Wikipedia.org/wiki/Biosensor . License : CC BY-SA: Attribution-ShareAlike
- Gram staining. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Gram_staining. License : CC BY-SA: Attribution-ShareAlike
- Hydrolysis. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Hydrolysis. License : CC BY-SA: Attribution-ShareAlike
- Oxidative/fermentation glucose test. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Oxidative/fermentation_glucose_test. License : CC BY-SA: Attribution-ShareAlike
- Electrode. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Electrode. License : CC BY-SA: Attribution-ShareAlike
- Gentamicin protection assay. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Gentamicin_protection_assay. License : CC BY-SA: Attribution-ShareAlike
- peptidoglycan. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/peptidoglycan. License : CC BY-SA: Attribution-ShareAlike
- Gram stain 01. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/File:Gram_stain_01.jpg. License : Public Domain: No Known Copyright
- Introduction to Physical Science/Isotopes. Provided by : Wikibooks. Located at : en.wikibooks.org/wiki/Introduction_to_Physical_Science/Isotopes. License : CC BY-SA: Attribution-ShareAlike
- Stable isotopes. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Stable_isotopes. License : CC BY-SA: Attribution-ShareAlike
- Radioactive. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/Radioactive. License : CC BY-SA: Attribution-ShareAlike
- isotope. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/isotope. License : CC BY-SA: Attribution-ShareAlike
- atom. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/atom. License : CC BY-SA: Attribution-ShareAlike
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Key Points
- The term isotope refers to the number of neutrons a certain element contains.
- Elements of the same name must always have the same number of protons, but the number of neutrons can change.
- Adding or subtracting neutrons from an atom does not change the elemental properties, but it can alter some of its features (like making it more radioactive ).
- While the number of neutrons in a particular atom can change, there is a certain threshold where the atom is given more neutrons that its nuclear force can hold. At this point, the atom is deemed unstable.
- The atom will continue to lose neutrons, or decay, until it become stable again. A stable isotope is a chemical isotope that is not radioactive.
Key Terms
- isotope : Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. As a consequence, atoms for the same isotope will have the same atomic number but a different mass number (atomic weight).
- atom : The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
- Radioactive : A particle that has spontaneous emission of ionizing radiation as a consequence of a nuclear reaction, or directly from the breakdown of an unstable nucleus.


2.4: Fitting Linear Models to Data
In the real world, rarely do things follow trends perfectly. When we expect the trend to behave linearly, or when inspection suggests the trend is behaving linearly, it is often desirable to find an equation to approximate the data. Finding an equation to approximate the data helps us understand the behavior of the data and allows us to use the linear model to make predictions about the data, inside and outside of the data range.
The table below shows the number of cricket chirps in 15 seconds, and the air temperature, in degrees Fahrenheit (Selected data from classic.globe.gov/fsl/scientistsblog/2007/10/. Retrieved Aug 3, 2010). Plot this data, and determine whether the data appears to be linearly related.
| chirps | 44 | 35 | 20.4 | 33 | 31 | 35 | 18.5 | 37 | 26 |
| Temp | 80.5 | 70.5 | 57 | 66 | 68 | 72 | 52 | 73.5 | 53 |
Solution
Plotting this data, it appears there may be a trend, and that the trend appears roughly linear, though certainly not perfectly so.
The simplest way to find an equation to approximate this data is to try to “eyeball” a line that seems to fit the data pretty well, then find an equation for that line based on the slope and intercept.
You can see from the trend in the data that the number of chirps increases as the temperature increases. As you consider a function for this data you should know that you are looking at an increasing function or a function with a positive slope.
- What descriptive variables would you choose to represent Temperature & Chirps?
- Which variable is the independent variable and which is the dependent variable?
- Based on this data and the graph, what is a reasonable domain & range?
- Based on the data alone, is this function one-to-one, explain?
- Answer
-
1. a. T = Temperature, C = Chirps (answers may vary)
b. Independent (Chirps) , Dependent (Temperature)
c. Reasonable Domain (18.5, 44) , Reasonable Range (52, 80.5) (answers may vary)
d. NO, it is not one-to-one, there are two different output values for 35 chirps.
Using the table of values from the previous example, find a linear function that fits the data by “eyeballing” a line that seems to fit.
Solution
On a graph, we could try sketching in a line. Note the scale on the axes have been adjusted to start at zero to include the vertical axis and vertical intercept in the graph.
Using the starting and ending points of our “hand drawn” line, points (0, 30) and (50, 90), this graph has a slope of \(m=\dfrac{60}{50} =1.2\) and a vertical intercept at 30, giving an equation of
\[T(c)=30+1.2c\nonumber \]
where \(c\) is the number of chirps in 15 seconds, and \(T(c)\) is the temperature in degrees Fahrenheit.
This linear equation can then be used to approximate the solution to various questions we might ask about the trend. While the data does not perfectly fall on the linear equation, the equation is our best guess as to how the relationship will behave outside of the values we have data for. There is a difference, though, between making predictions inside the domain and range of values we have data for, and outside that domain and range.
Interpolation: When we predict a value inside the domain and range of the data
Extrapolation: When we predict a value outside the domain and range of the data
For the Temperature as a function of chirps in our hand drawn model above,
- Interpolation would occur if we used our model to predict temperature when the values for chirps are between 18.5 and 44.
- Extrapolation would occur if we used our model to predict temperature when the values for chirps are less than 18.5 or greater than 44.
- Would predicting the temperature when crickets are chirping 30 times in 15 seconds be interpolation or extrapolation? Make the prediction, and discuss if it is reasonable.
- Would predicting the number of chirps crickets will make at 40 degrees be interpolation or extrapolation? Make the prediction, and discuss if it is reasonable.
Solution
With our cricket data, our number of chirps in the data provided varied from 18.5 to 44. A prediction at 30 chirps per 15 seconds is inside the domain of our data, so would be interpolation. Using our model:
\[T(3)=30+1.2(30)=66\text{ degrees}\nonumber \]
Based on the data we have, this value seems reasonable.
The temperature values varied from 52 to 80.5. Predicting the number of chirps at 40 degrees is extrapolation since 40 is outside the range of our data. Using our model:
\[\begin{array} {rcl} {40} &= & {30 + 1.2c} \\ {10} &= & {1.2c} \\ {c} &\approx & {8.33} \end{array}\nonumber \]
Our model predicts the crickets would chirp 8.33 times in 15 seconds. While this might be possible, we have no reason to believe our model is valid outside the domain and range. In fact, generally crickets stop chirping altogether below around 50 degrees.
When our model no longer applies after some point, it is sometimes called model breakdown .
What temperature would you predict if you counted 20 chirps in 15 seconds?
- Answer
-
Add answer text here and it will automatically be hidden if you have a "AutoNum" template active on the page.
Fitting Lines with Technology
While eyeballing a line works reasonably well, there are statistical techniques for fitting a line to data that minimize the differences between the line and data values (Technically, the method minimizes the sum of the squared differences in the vertical direction between the line and the data values.). This technique is called least-square regression , and can be computed by many graphing calculators, spreadsheet software like Excel or Google Docs, statistical software, and many web-based calculators(For example, http://www.shodor.org/unchem/math/lls/leastsq.html ).
Find the least-squares regression line using the cricket chirp data from above.
Solution
Using the cricket chirp data from earlier, with technology we obtain the equation:
\[T(c)=30.281+1.143c\nonumber \]
Notice that this line is quite similar to the equation we “eyeballed”, but should fit the data better. Notice also that using this equation would change our prediction for the temperature when hearing 30 chirps in 15 seconds from 66 degrees to:
\[T(30) =30.281+1.143(30)=64.571 \approx 64.6\text{ degrees}\nonumber \]
Most calculators and computer software will also provide you with the correlation coefficient , a measure of how closely the line fits the data.
The correlation coefficient is a value, \(r\), between -1 and 1.
\(r > 0\) suggests a positive (increasing) relationship
\(r < 0\) suggests a negative (decreasing) relationship
The closer the value is to 0, the more scattered the data
The closer the value is to 1 or -1, the less scattered the data is
The correlation coefficient provides an easy way to get some idea of how close to a line the data falls.
We should only compute the correlation coefficient for data that follows a linear pattern; if the data exhibits a non-linear pattern, the correlation coefficient is meaningless. To get a sense for the relationship between the value of \(r\) and the graph of the data, here are some large data sets with their correlation coefficients:
Examples of Correlation Coefficient Values
( http://en.Wikipedia.org/wiki/File:Co...n_examples.png )
Calculate the correlation coefficient for our cricket data.
Solution
Because the data appears to follow a linear pattern, we can use technology to calculate r = 0.9509. Since this value is very close to 1, it suggests a strong increasing linear relationship.
Gasoline consumption in the US has been increasing steadily. Consumption data from 1994 to 2004 is shown below.(www.bts.gov/publications/nati...ble_04_10.html) Determine if the trend is linear, and if so, find a model for the data. Use the model to predict the consumption in 2008.
| Year | '94 | '95 | '96 | '97 | '98 | '99 | '00 | '01 | '02 | '03 | '04 |
| Consumption (billions of gallons) | 113 | 116 | 118 | 119 | 123 | 125 | 126 | 128 | 131 | 133 | 136 |
Solution
To make things simpler, a new input variable is introduced, \(t\), representing years since 1994.
Using technology, the correlation coefficient was calculated to be 0.9965, suggesting a very strong increasing linear trend.
The least-squares regression equation is:
\[C(t)=113.318+2.209t\nonumber \]
Using this to predict consumption in 2008 (t = 14),
\[C(14)=113.318+2.209(14)=144.244\text{ billions of gallons}\nonumber \]
The model predicts 144.244 billion gallons of gasoline will be consumed in 2008.
Use the model created by technology in example 6 to predict the gas consumption in 2011. Is this an interpolation or an extrapolation?
- Answer
-
1. 54 degrees Fahrenheit
2. 150.871 billion gallons; extrapolation
Important Topics of this Section
- Fitting linear models to data by hand
- Fitting linear models to data using technology
- Interpolation
- Extrapolation
- Correlation coefficient


2.41: Introduction to Vocal Music in the Baroque
This section contains materials that will focus on the major genres of vocal music you’ll encounter in your study of the Baroque period: namely opera, cantata, and oratorio, and some of the composers who developed those genres. Because Johann Sebastian Bach and George Frideric Handel are generally viewed as the towering masters of the late Baroque, readings focusing on their lives and works of vocal music will be in a separate section.
This section includes the following pages:
- Slideshow: Vocal Music in the Baroque
- Opera
- Monody
- Monteverdi’s L’Orfeo
- Henry Purcell
- Dido and Aeneas
- Cantata
- Oratorio
Contributors and Attributions
CC licensed content, Original
- Provided by : Lumen Learning. License : CC BY: Attribution
- Authored by : Elliott Jones. Provided by : Santa Ana College. Located at : http://www.sac.edu . License : CC BY: Attribution


37.2: Types of Hormones - Lipid-Derived, Amino Acid-Derived, and Peptide Hormones
- Recognize characteristics associated with lipid-derived, amino acid-derived, and peptide hormones
Types of Hormones
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones (which includes peptides and proteins). One of the key, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.
Lipid-Derived Hormones (or Lipid-soluble Hormones)
Most lipid hormones are derived from cholesterol, so they are structurally similar to it. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols; their chemical names will end in “-ol” for alcohols or “-one” for ketones.  These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water; they are carried by transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.
Amino Acid-Derived Hormones
The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan. If a hormone is amino acid-derived, its chemical name will end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.
Peptide Hormones
The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, such as growth hormones produced by the pituitary, and large glycoproteins, such as follicle-stimulating hormone produced by the pituitary.
Secreted peptides, such as insulin, are stored within vesicles in the cells which synthesize them. They are then released in response to stimuli (e.g., as high blood glucose levels in the case of insulin). Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells; therefore, their receptors are found on the surface of the target cells.
Contributions and Attributions
- endocrine system. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/endocrine_system . License : CC BY-SA: Attribution-ShareAlike
- OpenStax College, Biology. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44765/latest/?collection=col11448/latest . License : CC BY: Attribution
- Principles of Biochemistry/Hormones. Provided by : Wikibooks. Located at : en.wikibooks.org/wiki/Principles_of_Biochemistry/Hormones . License : CC BY-SA: Attribution-ShareAlike
- OpenStax College, Biology. November 30, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/?collection=col11448/latest . License : CC BY: Attribution
- hormone. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/hormone . License : CC BY-SA: Attribution-ShareAlike
- target cell. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/target_cell . License : CC BY-SA: Attribution-ShareAlike
- OpenStax College, Introduction. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44765/latest/Figure_37_00_01abc.jpg . License : CC BY: Attribution
- OpenStax College, Biology. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/?collection=col11448/latest . License : CC BY: Attribution
- OpenStax College, Biology. October 23, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/?collection=col11448/latest . License : CC BY: Attribution
- OpenStax College, Biology. November 30, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/?collection=col11448/latest . License : CC BY: Attribution
- oxytocin. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/oxytocin . License : CC BY-SA: Attribution-ShareAlike
- estrogen. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/estrogen . License : CC BY-SA: Attribution-ShareAlike
- epinephrine. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/epinephrine . License : CC BY-SA: Attribution-ShareAlike
- OpenStax College, Introduction. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44765/latest/Figure_37_00_01abc.jpg . License : CC BY: Attribution
- OpenStax College, Types of Hormones. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/Figure_37_01_03abc.jpg . License : CC BY: Attribution
- OpenStax College, Types of Hormones. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/Figure_37_01_02ab.jpg . License : CC BY: Attribution
- OpenStax College, Types of Hormones. October 17, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44766/latest/Figure_37_01_01abc.jpg . License : CC BY: Attribution
Key Points
- Most lipid hormones are steroid hormones, which are usually ketones or alcohols and are insoluble in water.
- Steroid hormones (ending in ‘-ol’ or ‘-one’) include estradiol, testosterone, aldosterone, and cortisol.
- The amino acid – derived hormones (ending in ‘-ine’) are derived from tyrosine and tryptophan and include epinephrine and norepinephrine (produced by the adrenal medulla).
- Amino acid-derived hormones also include thyroxine (produced by the thryoid gland) and melatonin (produced by the pineal gland).
- Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ).
- Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids.
Key Terms
- oxytocin : a hormone that stimulates contractions during labor, and then the production of milk
- epinephrine : (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
- estrogen : any of a group of steroids (lipid-hormones) that are secreted by the ovaries and function as female sex hormones