4.2: Price Elasticity of Demand and Price Elasticity of Supply
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Learning Objectives
- Calculate the price elasticity of demand
- Calculate the price elasticity of supply
Both the demand and supply curve show the relationship between price and the number of units demanded or supplied. Price elasticity is the ratio between the percentage change in the quantity demanded (\(Q_d\)) or supplied (\(Q_s\)) and the corresponding percent change in price. The price elasticity of demand is the percentage change in the quantity demanded of a good or service divided by the percentage change in the price. The price elasticity of supply is the percentage change in quantity supplied divided by the percentage change in price.
Elasticities can be usefully divided into three broad categories: elastic, inelastic, and unitary. An elastic demand or elastic supply is one in which the elasticity is greater than one, indicating a high responsiveness to changes in price. Elasticities that are less than one indicate low responsiveness to price changes and correspond to inelastic demand or inelastic supply . Unitary elasticities indicate proportional responsiveness of either demand or supply, as summarized in Table \(\PageIndex{1}\).
| If . . . | Then . . . | And It Is Called . . . |
|---|---|---|
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\(\frac{\text{% change in quantity}}{\text{% change in price}} > 1\) | Elastic |
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\(\frac{\text{% change in quantity}}{\text{% change in price}} = 1\) | Unitary |
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\(\frac{\text{% change in quantity}}{\text{% change in price}} < 1\) | Inelastic |
To calculate elasticity, instead of using simple percentage changes in quantity and price, economists use the average percent change in both quantity and price. This is called the Midpoint Method for Elasticity, and is represented in the following equations:
\[\text{% change in quantity} = \frac{Q_2-Q_1}{(Q_2+Q_1)/2} \times 100\]
\[\text{% change in price} = \frac{P_2-P_1}{(P_2+P_1)/2} \times 100\]
The advantage of the is Midpoint Method is that one obtains the same elasticity between two price points whether there is a price increase or decrease. This is because the formula uses the same base for both cases.
Calculating Price Elasticity of Demand
Let’s calculate the elasticity between points \(A\) and \(B\) and between points \(G\) and \(H\) shown in Figure \(\PageIndex{1}\).
First, apply the formula to calculate the elasticity as price decreases from \(\$70\) at point \(B\) to \(\$60\) at point \(A\):
\[\begin{align*} \text{% change in quantity} &= \frac{3,000-2,800}{(3,000+2,800)/2} \times 100\\ & = \frac{200}{2,900} \times 100\\ & = 6.9 \end{align*}\]
\[\begin{align*} \text{% change in price} &= \frac{60-70}{(60+70)/2} \times 100\\ & = \frac{-10}{65} \times 100\\ & = -15.4 \end{align*}\]
\[\begin{align*} \text{Price Elasticity of Demand} &= \frac{6.9\%}{-15.4\%} \\ & = 0.45 \end{align*}\]
Therefore, the elasticity of demand between these two points is \(\frac{6.9\%}{-15.4\%}\)
This means that, along the demand curve between point \(B\) and \(A\), if the price changes by \(1\%\), the quantity demanded will change by \(0.45\%\). A change in the price will result in a smaller percentage change in the quantity demanded. For example, a \(10\%\) increase in the price will result in only a \(4.5\%\) decrease in quantity demanded. A \(10\%\) decrease in the price will result in only a \(4.5\%\) increase in the quantity demanded. Price elasticities of demand are negative numbers indicating that the demand curve is downward sloping, but are read as absolute values. The following Work It Out feature will walk you through calculating the price elasticity of demand.
Example \(\PageIndex{1}\): Finding the Price Elasticity of Demand
Calculate the price elasticity of demand using the data in Figure \(\PageIndex{1}\) for an increase in price from \(G\) to \(H\). Has the elasticity increased or decreased?
Solution
Step 1: We know that:
\[\text{Price Elasticity of Demand} = \frac{\text{% change in quantity}}{\text{% change in price}}\]
Step 2: From the Midpoint Formula we know that:
\[\text{% change in quantity} = \frac{Q_2-Q_1}{(Q_2+Q_1)/2} \times 100\]
\[\text{% change in price} = \frac{P_2-P_1}{(P_2+P_1)/2} \times 100\]
Step 3: So we can use the values provided in the figure in each equation:
\[\begin{align*} \text{% change in quantity} &= \frac{1,600-1,800}{(1,600+1,800)/2} \times 100\\ & = \frac{-200}{1,700} \times 100\\ & = -11.76 \end{align*}\]
\[\begin{align*} \text{% change in price} &= \frac{130-120}{(130+120)/2} \times 100\\ & = \frac{10}{125} \times 100\\ & = 8.0 \end{align*}\]
Step 4: Then, those values can be used to determine the price elasticity of demand:
\[\begin{align*} \text{Price Elasticity of Demand} &= \frac{\text{% change in quantity}}{\text{% change in price}} \\ &= \frac{-11.76}{8.0}\\ &= 1.47 \end{align*}\]
Therefore, the elasticity of demand from \(G\) to \(H\) is \(1.47\). The magnitude of the elasticity has increased (in absolute value) as we moved up along the demand curve from points \(A\) to \(B\). Recall that the elasticity between these two points was \(0.45\). Demand was inelastic between points \(A\) and \(B\) and elastic between points \(G\) and \(H\). This shows us that price elasticity of demand changes at different points along a straight-line demand curve .
Calculating the Price Elasticity of Supply
Assume that an apartment rents for \(\$650\) per month and at that price \(10,000\) units are rented as shown in Figure \(\PageIndex{2}\). When the price increases to \(\$700\) per month, \(13,000\) units are supplied into the market. By what percentage does apartment supply increase? What is the price sensitivity?
Price Elasticity of Supply
Using the Midpoint Method ,
\[\begin{align*} \text{% change in quantity} &= \frac{13,000-10,000}{(13,000+10,000)/2} \times 100\\ & = \frac{3,000}{11,500} \times 100\\ & = 26.1 \end{align*}\]
\[\begin{align*} \text{% change in price} &= \frac{700-650}{(700+650)/2} \times 100\\ & = \frac{50}{675} \times 100\\ & = 7.4 \end{align*}\]
\[\begin{align*} \text{Price Elasticity of Supply} &= \frac{\text{% change in quantity}}{\text{% change in price}} \\ &= \frac{26.1}{7.4}\\ &= 3.53 \end{align*}\]
Again, as with the elasticity of demand, the elasticity of supply is not followed by any units. Elasticity is a ratio of one percentage change to another percentage change—nothing more—and is read as an absolute value. In this case, a \(1\%\) rise in price causes an increase in quantity supplied of \(3.5\%\). The greater than one elasticity of supply means that the percentage change in quantity supplied will be greater than a one percent price change. If you're starting to wonder if the concept of slope fits into this calculation, read the following Clear It Up box.
Is the elasticity the slope?
It is a common mistake to confuse the slope of either the supply or demand curve with its elasticity. The slope is the rate of change in units along the curve, or the rise/run (change in \(y\) over the change in \(x\)). For example, in Figure \(\PageIndex{1}\), each point shown on the demand curve, price drops by \(\$10\) and the number of units demanded increases by \(200\). So the slope is \(-10/200\) along the entire demand curve and does not change. The price elasticity, however, changes along the curve. Elasticity between points \(A\) and \(B\) was \(0.45\) and increased to \(1.47\) between points \(G\) and \(H\). Elasticity is the percentage change, which is a different calculation from the slope and has a different meaning.
When we are at the upper end of a demand curve, where price is high and the quantity demanded is low, a small change in the quantity demanded, even in, say, one unit, is pretty big in percentage terms. A change in price of, say, a dollar, is going to be much less important in percentage terms than it would have been at the bottom of the demand curve. Likewise, at the bottom of the demand curve, that one unit change when the quantity demanded is high will be small as a percentage.
So, at one end of the demand curve, where we have a large percentage change in quantity demanded over a small percentage change in price, the elasticity value would be high, or demand would be relatively elastic. Even with the same change in the price and the same change in the quantity demanded, at the other end of the demand curve the quantity is much higher, and the price is much lower, so the percentage change in quantity demanded is smaller and the percentage change in price is much higher. That means at the bottom of the curve we'd have a small numerator over a large denominator, so the elasticity measure would be much lower, or inelastic.
As we move along the demand curve, the values for quantity and price go up or down, depending on which way we are moving, so the percentages for, say, a \(\$1\) difference in price or a one unit difference in quantity, will change as well, which means the ratios of those percentages will change.
Key Concepts and Summary
Price elasticity measures the responsiveness of the quantity demanded or supplied of a good to a change in its price. It is computed as the percentage change in quantity demanded (or supplied) divided by the percentage change in price. Elasticity can be described as elastic (or very responsive), unit elastic, or inelastic (not very responsive). Elastic demand or supply curves indicate that quantity demanded or supplied respond to price changes in a greater than proportional manner. An inelastic demand or supply curve is one where a given percentage change in price will cause a smaller percentage change in quantity demanded or supplied. A unitary elasticity means that a given percentage change in price leads to an equal percentage change in quantity demanded or supplied.
Glossary
- elastic demand
- when the elasticity of demand is greater than one, indicating a high responsiveness of quantity demanded or supplied to changes in price
- elastic supply
- when the elasticity of either supply is greater than one, indicating a high responsiveness of quantity demanded or supplied to changes in price
- elasticity
- an economics concept that measures responsiveness of one variable to changes in another variable
- inelastic demand
- when the elasticity of demand is less than one, indicating that a 1 percent increase in price paid by the consumer leads to less than a 1 percent change in purchases (and vice versa); this indicates a low responsiveness by consumers to price changes
- inelastic supply
- when the elasticity of supply is less than one, indicating that a 1 percent increase in price paid to the firm will result in a less than 1 percent increase in production by the firm; this indicates a low responsiveness of the firm to price increases (and vice versa if prices drop)
- price elasticity
- the relationship between the percent change in price resulting in a corresponding percentage change in the quantity demanded or supplied
- price elasticity of demand
- percentage change in the quantity demanded of a good or service divided the percentage change in price
- price elasticity of supply
- percentage change in the quantity supplied divided by the percentage change in price
- unitary elasticity
- when the calculated elasticity is equal to one indicating that a change in the price of the good or service results in a proportional change in the quantity demanded or supplied


4: Line
Line
This is where we begin when we learn to draw. But what is a line? If we consider a dot as the simplest of marks we make in drawing, then a line becomes the second simplest mark since a line is defined in geometry as the shortest distance between two points and can be thought of as a series of dots placed side by side and viewed as a line. Another way of looking at it is a mark that is longer than its breadth (width).
The figure below defines seven primary functions of line:
https://www.joshuanava.biz/creative-illustration/the-form-principle-as-a-basis-of-approach.html
The figure below shows aesthetic qualities of line such as thick and thin lines, implied lines, "loose and find" lines, and lines suggesting a shadow at the bottom of forms. These types of lines are quite sensual and often used to draw organic forms. The term for these lines is sensual lines . Prior to the Modern Art movement of the early 20th Century, academic drawings and paintings were dominated by the use of sensual lines.
Lines, as we see in the figure below, can be used to create formal designs. Symmetric designs such as these are visually balanced and stable by nature. They also lack the fluidity of the lines in the above figure. These lines appear more mechanical and are referred to as impersonal lines . Several 20th Century Modern Art movements looked to the impersonal line as a means of expressing new ways art concepts.
Historical Uses of Line
Drawing and mark-making using line played a role in human development leading to civilized cultures and societies. From the earliest rock carvings in Africa, to the Pre-historic cave paintings and drawings, we see art in the form of drawing telling stories, visually communicating, and being a part of these ancient ceremonies and traditions of peoples across the globe. Evolution of line drawings coincides with human evolution leading to pictographs . Line drawings in ancient societies representing words and eventually lead to the creation of alphabets and written languages. With documents in translatable languages comes documented history. By reading cuneiform we can better understand the daily, administrative, military, religious and political lives of Sumerians. Although knowledge of their spoken language is lost, we still have the library of recovered cunieform writings to study this most ancient culture. Egyptians also leave an invaluable legacy of hieroglyphs which also document the lives and culture of this ancient empire.
• Earliest rock art drawings and engravings are found in Africa around 100,000 BCE
http://www.bradshawfoundation.com/africa/oldest_art/index.php
• Cave paintings are the most impressive pre-historic images and are dated between 40,000 - 12,000 BCE
https://www.smithsonianmag.com/history/journey-oldest-cave-paintings-world-180957685/
• Egyptian pictographs evolve into hieroglyphics around 3100 BCE
https://www.ancient.eu/image/3912/egyptian-hieroglyphics/
• Sumerian pictographs are dated around 3000 BCE and evolve into cuneiform writing
https://www.history.com/news/sumerians-inventions-mesopotamia
• Code of Hammurabi dated 1800 BCE (example of cuneiform writing in Babylon)
https://sheg.stanford.edu/history-lessons/hammurabis-code
https://www.history.com/topics/ancient-history/hammurabi
https://www.louvre.fr/en/oeuvre-notices/law-code-hammurabi-king-babylon
Drawing with a Sensual Line
Drawing with a sensual line is an expressive approach to art. Sensuality is linked to things which are pleasing to the senses such as emotions, sensitivity, close observation of nature, the beauty of nature, and the appreciation of organic forms. The sensual line is a dynamic line which may curve in expressive ways denoting the play of light on forms, shadows, movement and gesture. Contrast in the line is a key component of creating this sensuality. Contrast may be in the form of light and dark; thick or thin; "loose and find," and implied lines. Imagine you are preparing a meal from scratch. Chances are you'll add salt or possibly spices to the dish. Contrast is just that. It is the salt or spice in your drawing. Without contrast, often the drawing will not have desired the visual impact.
Drawing Exercise #1 - Blind Contour Drawing
The first drawing in the course is one that forces you to feel your senses and experience drawing in a new way. It is called a blind contour drawing . Using an organic form like a branch of a small tree, or house plant with interesting shapes to the leaves, you draw the contour (outside edge) of the plant carefully without looking at your paper once you begin the drawing. It is sometimes best to start at the top of the plant and the top of the paper. So you look to place the pencil and then look at the top of the plant and not at the paper again until you complete the exercise. Recommended pencil grip with this exercise is Thumb and index finger grip and the pencil choice is an HB. Do not press too hard but feel the pencil move across the surface of the paper. Create a comfortable rhythm with your continuous contour line. Do not lift your pencil during the exercise. You are not sketching, you are also not concerned about composition. Your goal is to slow down and carefully observe the contours of the leaves, stems and branches of your chosen plant. If it is in a planter or pot, focus just on the plant, not the container. You will be taught how to draw ellipses and geometric shapes later on the course.
The above drawing is an example of a blind contour drawing
Drawing Exercise #2 - Contour Drawing
This example below is the second drawing assignment titled Contour Drawing . It is also a continuous line drawing but with added elements of thick and thin lines and contrast. Use the entire sheet of paper and draw while closely observing the plant and occasionally looking at your paper. Apply what you learned from the first drawing assignment. Try to bring the same sensitivity needed to concentrate on drawing the subtle contours of the leaves, stems and branches while not looking at the paper that you used in drawing exercise #1. Do not sketch, but practice keeping your pencil to the paper and creating a continuous, organic line. Darker lines should be placed on the shadow sides of the leaves, stems and branches. Leaves closer to you may be rendered a bit darker to bring them visually closer. Note that overlapping forms also create a depth cue demonstrating that one is closer than the other. Do not press too hard while using the HB pencil for this drawing. Hold the pencil the same as Exercise #1 and please do not erase. Just draw.
The above drawing is an example of a contour drawing (looking at plant and paper)
Drawing Lesson:
Drawing with an Impersonal Line
This will be covered in the Perspective chapter.
Line Drawing Examples
illustration by Michael E. Leonard
drawing by Michael E. Leonard illustration by Michael E. Leonard


16.8: Eukaryotic Gene Regulation - Epigenetic Control- Regulating Access to Genes within the Chromosome
- Discuss how eukaryotic gene regulation occurs at the epigenetic level and the various epigenetic changes that can be made to DNA
Epigenetic Control: Regulating Access to Genes within the Chromosome
The human genome encodes over 20,000 genes; each of the 23 pairs of human chromosomes encodes thousands of genes. The DNA in the nucleus is precisely wound, folded, and compacted into chromosomes so that it will fit into the nucleus. It is also organized so that specific segments can be accessed as needed by a specific cell type.
The first level of organization, or packing, is the winding of DNA strands around histone proteins. Histones package and order DNA into structural units called nucleosome complexes, which can control the access of proteins to the DNA regions. Under the electron microscope, this winding of DNA around histone proteins to form nucleosomes looks like small beads on a string. These beads (histone proteins) can move along the string (DNA) and change the structure of the molecule.
If DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery ( RNA polymerase ) to initiate transcription. Nucleosomes can move to open the chromosome structure to expose a segment of DNA, but do so in a very controlled manner.
How the histone proteins move is dependent on signals found on both the histone proteins and on the DNA. These signals are tags, or modifications, added to histone proteins and DNA that tell the histones if a chromosomal region should be open or closed. These tags are not permanent, but may be added or removed as needed. They are chemical modifications (phosphate, methyl, or acetyl groups) that are attached to specific amino acids in the protein or to the nucleotides of the DNA. The tags do not alter the DNA base sequence, but they do alter how tightly wound the DNA is around the histone proteins. DNA is a negatively-charged molecule; therefore, changes in the charge of the histone will change how tightly wound the DNA molecule will be. When unmodified, the histone proteins have a large positive charge; by adding chemical modifications, such as acetyl groups, the charge becomes less positive.
The DNA molecule itself can also be modified. This occurs within very specific regions called CpG islands. These are stretches with a high frequency of cytosine and guanine dinucleotide DNA pairs (CG) found in the promoter regions of genes. When this configuration exists, the cytosine member of the pair can be methylated (a methyl group is added). This modification changes how the DNA interacts with proteins, including the histone proteins that control access to the region. Highly-methylated (hypermethylated) DNA regions with deacetylated histones are tightly coiled and transcriptionally inactive. These changes to DNA are inherited from parent to offspring, such that while the DNA sequence is not altered, the pattern of gene expression is passed to the next generation.
This type of gene regulation is called epigenetic regulation. Epigenetics means “above genetics.” The changes that occur to the histone proteins and DNA do not alter the nucleotide sequence and are not permanent. Instead, these changes are temporary (although they often persist through multiple rounds of cell division) and alter the chromosomal structure (open or closed) as needed. A gene can be turned on or off depending upon the location and modifications to the histone proteins and DNA. If a gene is to be transcribed, the histone proteins and DNA are modified surrounding the chromosomal region encoding that gene. This opens the chromosomal region to allow access for RNA polymerase and other proteins, called transcription factors, to bind to the promoter region, located just upstream of the gene, and initiate transcription. If a gene is to remain turned off, or silenced, the histone proteins and DNA have different modifications that signal a closed chromosomal configuration. In this closed configuration, the RNA polymerase and transcription factors do not have access to the DNA and transcription cannot occur.
Key Points
- DNA is packaged by wrapping around histone proteins into structures called nucleosomes, which resemble beads on a string.
- When DNA is to be transcribed, the nucleosomes can slide away from that region of DNA, opening it up to the transcription machinery of the cell.
- Chemical modifications to either the histone proteins or the DNA itself signals whether or not a particular region of the genome should be “open” or “closed” to the transcription machinery.
- Modifications such as acetylation or methylation of the histones can alter how tightly DNA is wrapped around them, while methylation of DNA changes how the DNA interacts with proteins, including the histone proteins that control access to the region.
- This type of genetic regulation is called epigenetic regulation (“above genetics”) as it does not change the nucleotide sequence of the DNA.
Key Terms
- nucleosome : any of the subunits that repeat in chromatin; a coil of DNA surrounding a histone core
- epigenetics : the study of heritable changes caused by the activation and deactivation of genes without any change in DNA sequence
- histone : any of various simple water-soluble proteins that are rich in the basic amino acids lysine and arginine and are complexed with DNA in the nucleosomes of eukaryotic chromatin


12.1: Lipids
39
Harper College Chemistry Department
- Describe the four major types of lipids
- Explain the role of fats in storing energy
- Differentiate between saturated and unsaturated fatty acids
- Describe phospholipids and their role in cells
- Define the basic structure of a steroid and some steroid functions
- Explain how cholesterol helps maintain the plasma membrane’s fluid nature
Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals ( Figure 3.12 ). For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellent hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.
Fats and Oils
A fat molecule consists of two main moieties—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12–18 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule’s three carbons with an ester bond through an oxygen atom ( Figure 3.13 ).
During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. We also call a fat a triacylglycerol or triglyceride because of its chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid , is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Saturated fatty acids are saturated with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid ( Figure 3.14 ).
When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated . Oleic acid is an example of an unsaturated fatty acid ( Figure 3.15 ).
Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).
When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain’s carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells, or adipocytes, where fat globules occupy most of the cell’s volume. Plants store fat or oil in many seeds and use them as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present on the same side of the double bond, it is a cis fat. If the hydrogen atoms are on two different sides, it is a trans fat . The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature ( Figure 3.16 ). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels; whereas, saturated fats contribute to plaque formation in the arteries. A trans double bond removes the ‘kink’ from the fatty acid chain, so the fat behaves like a saturated fat.
Trans Fats
The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis – conformation in the hydrocarbon chain may convert to double bonds in the trans – conformation.
Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned using trans fats, and food labels are required to display the trans fat content.
Omega Fatty Acids
Essential fatty acids are those that the human body requires but does not synthesize. Consequently, they have to be supplemented through ingestion via the diet. Omega -3 fatty acids (like those in Figure 3.17 ) fall into this category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain’s end to the fourth carbon.
The farthest carbon away from the carboxyl group is numbered as the omega ( ω ) carbon, and if the double bond is between the third and fourth carbon from that end, it is an omega-3 fatty acid. These are nutritionally important because the body does not make them; omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help lower the risk of some cancers in animals.
Like carbohydrates, fats have received considerable bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, we should consume “healthy” fats in moderate amounts on a regular basis.
Waxes
Wax covers some aquatic birds’ feathers and some plants’ leaf surfaces. Because of waxes’ hydrophobic nature, they prevent water from sticking on the surface ( Figure 3.18 ). Long fatty acid chains esterified to long-chain alcohols comprise waxes.
Phospholipids
Phospholipids are major plasma membrane constituents that comprise cells’ outermost layer. Like fats, they are comprised of fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate group occupies the glycerol backbone’s third carbon ( Figure 3.19 ). A phosphate group alone attached to a diacylglycerol does not qualify as a phospholipid. It is a phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. An alcohol modifies the phosphate group. The modifying group contains a nitrogen and has a positive charge. Two such groups are choline and serine. When added to the phospatidate, a phospholipid forms. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are in plasma membranes.
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic and interacts with water ( Figure 3.20 ).
The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the structure’s matrix, phospholipids’ fatty acid tails face inside, away from water; whereas, the phosphate group faces the outside, aqueous side ( Figure 3.20 ).
Phospholipids are responsible for the plasma membrane’s dynamic nature. If a drop of phospholipids is placed in water, it spontaneously forms a structure that scientists call a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the structure’s interior.
Steroids
Unlike the phospholipids and fats that we discussed earlier, steroids have a fused ring structure. Although they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail ( Figure 3.21 ). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).
Cholesterol is the most common steroid. The liver synthesizes cholesterol and is the precursor to many steroid hormones such as testosterone and estradiol, which gonads and endocrine glands secrete. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help emulsifying fats and their subsequent absorption by cells. Although lay people often speak negatively about cholesterol, it is necessary for the body’s proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the plasma membrane of cells and are found within the phospholipid bilayer.
LINK TO LEARNING
For an additional perspective on lipids, explore the interactive animation “Biomolecules: The Lipids” .