14.14: Channels of Business Communication
- Differentiate between face-to-face, written, oral, web-based, and other typical channels of business communication.
- Explain the importance of tailoring the message to the audience.
Business communication is held to a higher standard than everyday communication. The consequences of misunderstandings are usually higher and the chances to recognize and correct a mistake are lower. The barriers to communication and technique for improving communication are the same regardless of where the conversation takes place.
Business Communication Channels
A communication channel is the medium, mean, manner or method through which a message is sent to its intended receiver. The basic channels are written (hard copy print or digital formats), oral or spoken, and electronic and multimedia. Within those channels, business communications can be formal, informal, or unofficial. Finally, communications can be rich or lean.
Channel richness refers to the amount and immediacy of information that can be transmitted. Face-to-face communication is very high in richness because it allows information to be transmitted with immediate feedback. A tweet is very low in richness because it allows only 140 characters to be transmitted with no feedback. On the other hand, face-to-face communication is limited to one person communicating with a few other people in close proximity, whereas a tweet can go out to thousands of followers around the world. The following diagram shows the richness of different types of communication.
Oral Communications
Oral channels depend on the spoken word. They are the richest mediums and include face-to-face, in-person presentations, mobile phone conferences, group presentations, telephone, video meetings, conferences, speeches, and lectures. These channels deliver low-distortion messages because body language and voice intonation also provide meaning for the receiver. They allow for immediate feedback of the communication to the sender. They are also the most labor-intensive channels in terms of the number of people involved in the transaction. Oral channels are generally used in organizations when there is a high likelihood of the message creating anxiety, confusion, or an emotional response in the audience. For instance, a senior manager should address rumors about layoffs or downsizing in face-to-face meetings with management staff. This allows the receiver (audience) to get immediate clarification and explanations, even if the explanation is a simple but direct: “At this time, I just don’t know.”
Oral communications are also useful when the organization wants to introduce a key official or change a long-established policy, followed up with a written detailed explanation. Senior managers with high credibility usually deliver complex or disturbing messages. For example, a senior manager will usually announce plans to downsize in person so that everyone gets the same message at the same time. This will often include a schedule so people know when to expect more details.
Written Communications
Written communications include e-mails, texts, memos, letters, documents, reports, newsletters, spreadsheets, etc. (Even though e-mails are electronic, they are basically digital versions of written memos.) They are among the leaner business communications. With written communications, the writer must provide enough context that the words can be interpreted easily. The receiver should be alert for ambiguity and ask for clarification if needed. An e-mail sender cannot take receipt for granted. Most people receive too much e-mail and sort and filter it quickly, sometimes incorrectly.
Written messages are effective in transmitting large messages. Humans are limited in the amount of data they can absorb at one time. Written information can be studied over time. Reports can include supporting data and detailed explanations when it is important to persuade the receiver about a course of action. Written communications can be carefully crafted to say exactly what the sender means. Formal business communications, such as job offer letters, contracts and budgets, proposals and quotes, should always be written.
Electronic (Multimedia) Communications
Television broadcasts, web-based communications such as social media, interactive blogs, public and intranet company web pages, Facebook, and Twitter belong in this growing category of communication channels. Electronic communications allow messages to be sent instantaneously and globally. People can talk face-to-face across enormous distances. Marketing and advertising can be targeted to many different types of customers, and business units can easily communicate in real time. This is especially important when customers must be advised of product recalls or security issues.
Although extremely effective, the widespread utilization of electronic communications for business purposes can also be risky. In recent years, the private communications and customer files of many large corporations have been hacked and their data stolen. In 2016, New Jersey Horizon Blue Cross Blue Shield was fined $1.1 million for failing to safeguard the personal information of medical patients. The company stored unencrypted sensitive data including birth dates and Social Security numbers on laptops that were stolen out of their main offices.
| TYPE OF CHANNEL | ADVANTAGES | DISADVANTAGES |
|---|---|---|
| Oral communications | Build relationships and trust; accelerate decision making due to immediate feedback | Spontaneous nature may lead to unwise statements; people are unable to refer to the communication once it is said unless a record is made. |
| Written communications | Message can be revised for exactness; can be archived for reference; can be studied. Appropriate for legal and formal business functions. | Message is static; sender does not receive immediate feedback. Hard for the sender to gauge if the receiver has understood. |
| Multimedia | Instant, global, and adaptable to multiple targets. | Technical difficulties and hack attacks threaten the security of organizations and their customers/clients. |
Which Channel Is Best?
Quite simply, the best channel is the one that most effectively delivers the message so that it is understood as the sender intended. Nuanced or emotionally charged messages require a rich medium; simple, routine messages don’t need the personal touch. If you want to advise your department that at 2 p.m. you want to have a five-minute stand-up meeting in the hallway outside of your office to congratulate them on meeting a goal, then send a quick e-mail. You really don’t want people to reply with questions. E-mail is a lean medium but works very well when the content of the message is neither complex nor emotionally charged. On the other hand, a telephone call is a more appropriate channel to apologize for having to cancel a lunch date. The speaker can hear the sincerity in your voice and can express their disappointment or offer to reschedule.
A good rule of thumb is the more emotional the context of the message, the richer the medium should be to deliver the message. But remember—even face-to-face business meetings can be followed up with a written note to ensure that both parties are truly on the same page. If a meeting results in assignments or agreements, then a written note will be useful documentation for future reference.
Tailoring a Message to an Audience
Have you ever had to converse with a young child and really needed him to listen to you? You probably knelt down so that you were at eye level. You may have even put your hand gently on his arm to focus his attention on you. Speaking clearly and slowly in words that you knew he understood, you made exaggerated facial gestures of fear or surprise or happiness to emphasize your points. In brief, you tailored your message to your audience so that it would be effective.
Although the channel of communication you select to transmit your message is important, so also is knowing the intended audience. Effective communicators seem to instinctively adjust their styles to their audience, but it is a skill that can also be learned. The way the information is conveyed should complement the audience, whether it is one person or a roomful of people. Three factors are involved in tailoring a message: the sender, the message itself, and the audience.
The Sender
For the majority of everyday business communications, a manager needs only to focus on presenting a professional image. On different occasions, she may want to present herself as a serious and responsible leader, a technical expert, or informally casual. But when uncertain what a situation calls for, always err on the side of professionalism. Studies have shown that an audience reacts initially to the demeanor of the speaker and not to what is being said. If you appear nervous, your audience will pick up on that and become nervous as well.
The Message
The first step in tailoring a message, of course, is to choose the right communication channel. But even then, there are questions to ask about the message itself that will influence how it is presented. Some of these questions are listed below:
- Which channel suits the content best? A critical or urgent message may demand the fastest channel, whether that is in-person, text-message, or e-mail. A scripted presentation requires planning and preparation. Businesses can use journalists or Twitter for brief announcements. If you want to demonstrate how your organization does something, clarify values, or construct common meanings for people across an organization, you might tell a memorable story in either an oral or written presentation. Decide which channel best suits the content.
- What do you want to achieve by sending the message? Is it intended to persuade or to inform? Are you just reassuring your boss that everything is on track? A face-to-face communication is much more effective than an e-mail or report in trying to convince someone to change a belief or a behavior.
- Does the message require interaction from the audience or is it more of an “information dump”? Don’t try to deliver a rationale for a $1 million investment in a phone call, especially if you need supporting evidence. Allow enough time to answer all questions.
- Will visual aids help the message or will they just distract from it? Arguments that a new computer software program will increase your department’s productivity will need to be backed by solid evidence presented in charts or graphs. But excessive or unfocused visuals may irritate senior-level managers who want just the concise summation and the triple bottom line (economic, social and environmental impacts).
- Do you have to establish your credibility? Does the audience know you? Are you speaking to people who know more than you about the topic? The answers to these questions will influence how you design your message.
- Most importantly, before deciding on a communication, ask yourself why the audience should care about it. When you know, then tailor the communication so that it answers that question. The story of United Airlines Flight 4311 is example of a CEO who failed to identify his primary audience and ask himself what they cared about before he spoke. In June of 2017, a seated passenger was informed by the flight attendants that his seat was needed and that he would have to leave the plane. He refused to give up his seat and was eventually dragged, literally kicking and screaming, off the plane. The video immediately went viral. In an effort to prevent a public relations nightmare, the CEO initially blamed the passenger and defended the actions of the United Airlines employees and security guards. When public outrage grew and customers threatened a boycott, the CEO finally realized that the first communication should have focused not on the employees and the company but on the passenger, who was, after all, a paying customer.
 Here are some questions to consider when getting to know your audience.
- How big is your audience? Is it one person or a large group? One individual means getting “up close and personal” as opposed to a conference room. If a large group, is it diverse or fairly homogenous? Do you need language translators or signers for the hearing impaired?
- What is the status of your audience? Is the audience mostly senior-level managers, peers, or subordinates? Your body language, presentation, and formality will vary depending upon the status of the group. Although subordinates may be more easily put at ease with an informal, relaxed approach, high-level managers usually want to get straight to the point and move on to the next thing.
- Can you establish empathy with the audience? The more the audience trusts you, the easier it will be to retain their attention. Notice the audience—are people shifting around, avoiding eye contact, yawning, or are they nodding in agreement and focused? You may have to adapt your message to meet the mood of the audience.
Contributors and Attributions
- Channels of Business Communication. Authored by : John/Lynn Bruton and Lumen Learning. License : CC BY: Attribution
- Image: Business meeting. Authored by : 089photoshootings. Located at : https://pixabay.com/en/men-employees-suit-work-greeting-1979261/ . License : CC0: No Rights Reserved
- Image: Media Richness Theory Diagram. Authored by : Tntdj. Located at : https://en.Wikipedia.org/wiki/File:Media_Richness_Theory_Diagram_PNG.png . License : CC BY: Attribution
- Image: Steve Jobs. Authored by : waldryano. Located at : https://pixabay.com/en/steve-jobs-technology-illustration-1249665/ . License : CC0: No Rights Reserved


4.5: Weathering
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2.5.1 Weathering Introduction
Weathering is the term used for the chemical decomposition and physical disintegration of bedrock at and just below the earth’s surface . Weathering acts upon all bedrock near the surface, although with greatly varying nature and rate depending upon a number of factors.
Rock at depth tends to be brought upward toward the surface, by gradual unroofing by weathering at the surface and removal of the weathering products. It’s a matter of relative motion: the bedrock at depth may be either motionless (relative to, say, the center of the earth) or moving slowly upward by uplift or downward by subsidence, but in any case it’s moving upward relative to the earth’s surface if material at the surface is being stripped away . Astherock nears the surface, it tends to be subjected to what’s called near-surface alteration : it finds itself at much lower pressures and temperatures than at greater depth, and surface-derived waters, which tend to be oxygenated and carry dissolved CO2, have the opportunity to cause certain chemical reactions in many constituent minerals, like ferromagnesian silicates and feldspars. In a sense, you can think of such near-surface alteration as a precursor or forerunner to weathering in the stricter sense. But despite such near-surface alteration, the rock is still solidly bedrock, and nowhere near a condition that could be described as regolith.
Weathering, in the usually accepted sense, acts upon rock that is close enough to the surface to be affected not only by active circulation of surface- derived waters but also by such things as temperature changes and plant growth. But in thinking about weathering you don’t have to restrict yourself to the actual rock surface, upon which the sun shines!
Weathering is traditionally subdivided into two sets of processes:
- chemical weathering , the chemical decomposition of some or all of the constituent minerals or the bedrock.
- physical weathering , the mechanical breakdown or disintegration of the bedrock itself into particles large and small.
This is a useful distinction, but here’s a very important fact to understand: chemical weathering and physical weathering tend to act simultaneously and reinforce one another .
2.5.2 Chemical Weathering
Chemically, the earth’s surface is a very reactive place . To begin with, in most places and at least at certain times the surface is an aqueous environment , and water is not only a very effective solvent in itself but also a necessary medium for a great many chemical reactions.
Owing to the high content of oxygen in the atmosphere, in most places the earth’s surface is a well oxygenated environment , as in the shallow subsurface wherever the concentration of organic matter is not so great and the permeability so low (as is common in bogs and swamps) that the oxygen is consumed in oxidation of organic matter before it passes downward to deeper depths. (If you’re unsure about the nature of oxidation, there will be some background material on it later.) Because so many of the common rock-forming minerals contain ferrous iron (that is, iron in the reduced state, and thus susceptible to oxidation to the insoluble ferric state), oxidation reactions are an important aspect of chemical weathering .
In the same way, because the atmosphere contains a substantial percentage of carbon dioxide (CO 2 ), surface waters invariably contain appreciable dissolved CO 2 . You know already that CO 2 readily dissolves in water. That dissolved CO 2 reacts with the water itself to form a weak acid, carbonic acid , according to the reaction
\[\ce{CO2 + H2O <=> H2CO3} \label{1}\]
At equilibrium, about one percent of the dissolved CO 2 is in the form of carbonic acid. Most minerals are less stable in an acidic medium than in a neutral medium, so the dissolved CO 2 enhances the weatherability of many minerals.
Specifically, what’s going on with Reaction 1? A molecule of carbon dioxide, which consists of a carbon atom tightly bound up with two oxygen atoms, combines with a molecule of water, which consists of an oxygen atom tightly bound up with two hydrogen atoms, in such a way that a single, new, larger molecule, with the chemical formula H 2 CO 3 , is produced. In the aqueous solution, this process is going on all the time, and at the same time the H 2 CO 3 molecules are being changed back into CO 2 and H 2 O molecules. Where the balance is struck depends on the imposed concentration of CO 2 , which in this case comes either from the atmosphere or from the chemical breakdown of organic matter. (You probably know that, by photosynthesis, plants take up water and carbon dioxide and convert it into complex carbohydrate molecules, and when the plant dies and the plant tissues are exposed to an oxygenated aqueous environment the reverse happens and the organic matter is broken back down into carbon dioxide and water.)
Carbonic acid is not the only acid common in natural near-surface waters: in most land areas, organisms (both plants and animals) living on or in the soil (that is, the uppermost part of the layer of regolith) use carbon dioxide and various nutrients to produce organic matter, which in its decay back to the constituent compounds tends to produce a variety of weak organic acids .
In the rest of this section, several kinds of chemical reaction will show the range of weathering reactions that are important in chemical decomposition of bedrock minerals. They involve solution, oxidation, and carbonation , in various ways. Keep in mind that although these particular reactions are important, they are just representative reactions : details vary depending upon the assemblage of minerals that are being weathered, and upon their particular compositions (which, remember, vary widely because of the wide range of ionic substitutions in silicate minerals).
Solution
Some minerals simply dissolve in water . Halite (sodium chloride) and gypsum (hydrated calcium sulfate) are good examples:
\[\ce{NaCl (s) <=> Na^{+} + Cl^{-}} \label{2}\]
\[\ce{CaSO4 \cdot H2O <=> Ca^{2+} + SO4^{2-} + H2O} \label{3}\]
This is the simplest kind of weathering reaction to understand, but not the most important. A more important, and more complicated, kind of solution involves calcite, the constituent of limestone:
\[\ce{CaCO3 (s) + CO2 +H2O <=> Ca^{2+} + 2H^{+} + CO3^{2-}} \label{4}\]
The reason this reaction is more complicated is that it involves the atmospheric carbon dioxide dissolved in the water. The CO 2 has to be involved because of the CO 3 2 - ion both in the structure of the mineral and as a product of the solution and then dissociation of CO 2 in water. (Incidental note: this reaction can go in either direction, depending upon the concentration of CO 2 : if CO 2 is added to the system, as in weathering environments, calcite is dissolved; if it’s extracted from the system, as in certain shallow warm ocean environments, calcium carbonate sediment is precipitated. Think about that the next time you’re lying on a white carbonate-sand beach in the Caribbean.)
carbonation of silicate minerals that don’t contain aluminum or iron:
Most silicate minerals that don’t contain aluminum contain both Fe 2 + and Mg 2 + ions in various proportions. Rather than writing a single weathering reaction for such minerals, it’s neater to look at separate reactions for those with only magnesium and those with only iron. Such minerals, although they exist, are not common, but their weathering reactions are instructive. First look at the weathering of olivine, which is a common mineral in igneous rocks, and one that’s very susceptible to chemical weathering. Most olivines in igneous rocks contain some iron in addition to magnesium, but let’s assume, for simplicity, that we are dealing with a pure magnesium olivine:
\[\ce{Mg2SiO4 + 4CO2 + 4H2O → 2Mg2+ + 4HCO3- + H4SiO4} \label{5}\]
(I’ve used the single arrow to the right, rather than the double arrow, here and below because the reverse of these reactions is not actually how the minerals get built under deep Earth conditions.)
Note that “silica” (that is, SiO 2 in some form or other) is extracted from the original silicate mineral. In most natural waters, the dominant species of silica in solution is H 4 SiO 4 , a weak acid called silicic acid . It’s in the form of a molecule that consists of four hydrogen atoms, four oxygen atoms, and one silicon atom, all tightly bonded together.
carbonation of silicate minerals that don’t contain aluminum but do contain iron:
Now look at a ferromagnesian mineral with all Fe and no Mg. (We’ll assume that the mineral is a pyroxene mineral; no details here.) In our oxygen- rich near-surface environment, it’s a two-step process. The first step is like that expressed in Reaction 5 above, but the numerical values of the coefficients in front of the reactants and products are somewhat different, because we started out with a mineral with a different chemical formula. Qualitatively, the result is the same: the iron-bearing silicate mineral is broken down in the presence of water and carbon dioxide to put iron and silica in solution . The dissolved iron is in the ferrous (“plus-two”) form, which is freely soluble in pure water. It is converted in the presence of oxygen to ferric iron oxide and silica in solution:
\[\ce{FeSiO3 + 2CO2 + 3H2O → Fe2+ + 2HCO3- + H4SiO4} \label{6}\]
But that’s not the end of the story, because the ferrous iron in solution reacts with
oxygen dissolved in the water, to precipitate an iron oxide mineral, Fe 2 O 3 :
\[\ce{2Fe2+ + 3O2 → 2Fe2O3} \label{7}\]
The iron in the Fe 2 O 3 mineral is in the ferric (“plus-three”) form, which is almost totally insoluble in pure water.
The representation on Reaction 7 is still a bit of a fake, however, because ferric oxides produced at the earth’s surface are commonly hydrous or hydrated: one could describe them as hydrous ferric oxides. There are several closely related minerals of that kind; the most common are goethite, FeO(OH), and ferrihydrite, 5Fe2O3.9H2O). They all have colors that range from yellow through orange to brown; that’s why weathered surfaces on bedrock are so commonly of those colors. In a sense, ferrous-iron-bearing rocks rust when exposed at the earth’s surface.
If we had started with a real pyroxene, with a combination of Fe 2 + ions, Mg 2 + ions, and Ca 2 + ions, the latter two would have been freed to solution while the first would have found its way into insoluble oxides; that’s because magnesium and calcium are readily soluble in water but ferric iron is almost completely insoluble.
weathering of aluminosilicates:
Now to the biggest show of all in chemical weathering. The example here will be what is certainly the single most important weathering reaction on earth: hydrolysis and carbonation of feldspar . We’ll use potassium feldspar as the example; weathering of plagioclase feldspar is similar.
\[\ce{2KAlSi3O8 + 2H2CO3 + 9H2O → Al2Si2O25(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-} \label{8}\]
Comments:
- The first product on the right is the clay mineral kaolinite . (There will be more material on clay minerals in a later section.) Other kinds of clay minerals are possible as solid products of weathering of feldspar as well, depending upon the details of the chemical environment.
- The products in solution are silicic acid, as before, and potassium ions.
- If the feldspar had been plagioclase, Na + and Ca 2 + ions would have gone into solution rather than K+ ions.
- The reaction could have been rewritten slightly to start with CO 2 rather than with H 2 CO 3 . The differences are immaterial. Those of you who are well versed in chemistry can try to rewrite the reaction by adding to it the following reaction: 2CO 2 + 2H 2 O ⇔ 2H 2 CO 3 .
In conclusion, if you don’t remember anything else about chemical weathering, remember this:
when feldspar is weathered, by reaction with water and carbon dioxide, the result is a clay mineral, positive ions (K + and/or Na + and/or Ca 2 + ) in solution, and silica, mostly in the form of H 4 SiO 4 , in solution .
In intense weathering situations, in warm and humid environments, weathering of aluminosilicate minerals like feldspar can go a step further, to the ultimate weathering product:
\[\ce{2Al2Si2O25(OH) 4 → 4Al(OH) 3 + 4H4SiO4 + 5H2O} \label{9}\]
The kaolinite, which itself was the weathering product of potassium feldspar, is stripped further of all of its silica, to produce a totally insoluble aluminum hydroxide mineral and more silica in solution. This is the ultimate weathering residue! There are several hydrous aluminum oxide minerals that come out of a reaction like this; the mineral gibbsite is the most common. Surficial materials that are especially rich in such insoluble hydrous aluminum oxides are called bauxite . As you probably know, such weathering-residue mixtures of hydrous aluminum oxides are the world’s supply of aluminum ore. Depending upon the composition of the original rocks, such ultimate weathering residues are commonly mixed with insoluble hydrous iron oxides as well.
One final point, which will be important for our later consideration of sediments and soils: all of the solid weathering products in the reactions above— clay minerals and various oxides of aluminum and ferric iron—are commonly of finer particle size than the minerals from which they are produced during weathering .
What are the controls on chemical weathering? Viewed on the large scale, there are four major controls on chemical weathering:
- climate: The intensity of chemical weathering increases with precipitation and temperature. The reason it increases with precipitation is that weathering requires an aqueous medium for the reactions. The greater the percentage of time a given mass of weatherable rock is saturated with surface water, the more chemical weathering it is likely to experience. The reason that the intensity of chemical weathering increases with temperature is that, as with all chemical reactions, the rate of the reaction increases with temperature. That’s basically because the vigor of the thermal agitation of the atoms and molecules increases with temperature.
- rock type: Some kinds of rocks are more susceptible to chemical weathering than others. Rocks that contain minerals that were formed under conditions of the high temperatures and pressures of the deep crust or upper mantle are much more weatherable than rocks that contain abundant minerals like quartz, calcite, or muscovite, which are stable, or at least only slowly weatherable, under most Earth-surface conditions. For that reason, most igneous and metamorphic rocks are generally more susdceptible to weathering than most sedimentary rocks.
- relief: Regions with high relief usually also have steep slopes. Gravity-driven mass movements (more on that in a later chapter) then are effective at stripping away newly weathered material, thereby exposing fresher bedrock to continued chemical weathering. In regions with low relief, however, the layer of regolith thickens to the point where the underlying bedrock is largely sealed off from further contact with the surface environment of chemical weathering.
- vegetation: As mentioned earlier, when plant materials decay they release carbon dioxide and weak acids into the regolith environment. These substances acidify the surficial material, thereby tending to drive the various chemical weathering reactions discussed above to the right.
There’s another way of looking at the controls on chemical weathering: from the standpoint of what a small mass of weatherable material actually senses . This is the small-scale view, in contrast to the foregoing largescale view. It boils down, largely, to four more specific physicochemical factors:
- mineral composition of the rock
- chemical composition of the water
- rate of passage of water through the rock • temperature
2.5.3 Physical Weathering
Most rocks are strong: you need to pound on them with a hammer to break them. The strongest rocks, like tough quartzites, yield only reluctantly to even the most vigorous hammer blows. Other rocks, like soft sandstones, you can break with your own hands. (In fact, there is a continuous gradation between loose and unconsolidated sediment and strongly lithified sedimentary rock, depending in large part upon depth of burial.) Rock strength is a complicated topic, which is important for the construction industry, as well as for geoscientists who try to understand rock fracture that causes large earthquakes.
Let me a bit more specific about the nature of the strength of solids like rocks. There are three ways of breaking solids: by compression , by tension , and by shear (Figure 2-5). In rocks, the compressive strength and the shear strength are both large, but the tensile strength is much less: generally, it’s much easier to break a rock by pulling it apart than by squeezing it. (That’s true of concrete, an artificial rock, as well.)
The easiest way to break a rock is to wedge open a preexisting crack . That’s partly because of the low tensile strength, but also because of an effect known as stress concentration at the tip of the propagating crack (for reasons that are beyond the scope of this course). A simple analogy can serve well here: it’s much easier to “break” a sheet of paper in tension by starting a cut and then pulling the edges of the cut apart than by grasping the sheet at opposite edges and pulling on it uniformly. When a rock fractures in tension, either rapidly as a result of a blow or an explosion, or slowly, by a variety of natural weathering processes, the crack enlarges as the tip of the crack propagates into the non-fractured rock.
Breakage of polycrystalline aggregates like rocks (or metals) is more complicated than breakage of homogeneous materials like glass. That’s because the crystal grains of the rock are joined together at the grain boundaries , so the strength of the grain contacts needs to be taken into consideration as well as the strength of the mineral grains themselves. When a rock is stressed, extremely fine cracks tend to develop first along the grain boundaries.
Rocks at the surface tend to be broken down (disaggregated, disintegrated) by a variety of processes, whose importance varies greatly from place to place and from time to time. These physical processes (you might also call them mechanical processes) have traditionally been cited as important in physical weathering, but definitive quantitative studies of the various effects are discouragingly scarce. I’ll set them out for you here without making much of an attempt to rank them in importance, except for some concluding comments at the end of this section.
sheeting: Sheeting is a style of rock fracturing that develops sets of fractures just below the bedrock surface and concordant with it . The reason such fractures tend to form is that as rock is brought up to the surface it is unloaded : the weight of overlying rock is reduced. The rock tries to expand slightly (rock has small but definite compressibility), and it can expand only upward, not sideways . This leads to development of fractures that are roughly parallel with the rock surface. Sheeting is most common in otherwise massive and non-fractured bedrock. The sheeting joints decreases rapidly in spacing downward from the surface, usually disappearing within several meters. The next time you see a photo of the mountain slopes in Yosemite, think in terms of sheeting fractures.
exfoliation: Exfoliation (Figure 2-6) is the separation, during weathering, of successive shells, a few millimeters to some tens of centimeters thick, from massive bedrock . The mechanical effect is somewhat like that of sheeting: as the thin near-surface layer of bedrock undergoes chemical weathering, the weathering-product minerals tend to be of lower density that the original minerals, which means larger volume, so the thin uppermost zone of the rock tries to expand. It can expand only upward, so fractures parallel to the rock surface are formed. In this situation, chemical weathering and mechanical weathering act together to disintegrate the bedrock. A related process leads to what’s called spheroidal weathering : the exfoliation and spalling is slightly more effective at edges and corners than on rock faces, so in an outcrop cut by sets of planar fractures in various orientations (such fractures are called joints ; they are ubiquitous in bedrock outcrops) the edges and corners are rounded off to produce a very spheroidal or “blobby” look to the outcrop. In advanced cases the spheroids are completely freed from the outcrop, and lie around on the surface or roll downslope.
granular disintegration: Granular disintegration is another process that is in part a mechanical effect of chemical weathering. Many if not most rocks consist of a mixture of minerals, some susceptible to chemical weathering and others not. As the weatherable minerals expand upon incipient chemical weathering, while the non-weatherable minerals retain their original size, intergranular forces are set up, which tend to break the bonds between mineral grains. The mineral grains fall from the rock surface, one by one, to leave an overall smoothly rounded but granular-rough surface. The mass of loose grains at the base of the outcrop is called grus . This kind of weathering is most common in crystalline rocks like granite.
thermal expansion and contraction: Expansion and contraction in response to changes in temperature can by itself aid in propagation of fractures in bedrock ranging in scale from intergranular fractures to widespread joints. Such a process is most effective in arid regions where the day-to-night temperature differences are great. It’s unclear whether the process is effective in thoroughly dry rocks, or whether the presence of water is an important element in the process. Experiments on heating and cooling of rock samples, over a great many cycles to simulate many decades or even centuries of the natural environment, have not been successful in producing non-negligible fracturing, but maybe the conditions of the simulation have not been representative. It’s well known, however, that fire causes breakdown of rock by sudden expansion of the surface layer exposed to the extreme heating.
frost wedging: Frost wedging (Figure 2-7) is important in climates where surface water is abundant and the day-to-night temperature range spans the freezing point many times through the year, as in New England in the late fall and especially the early spring. Water expands upon freezing, to the tune of about 9% in volume, an extremely large change. When water confined deeply in a thin crack in bedrock freezes, it can exert enormous side forces on the walls of the crack. The maximum possible force, which has been computed to be over 2000 kilograms per square centimeter (!), is not anywhere close to being achieved, however, because the ice does not seal the crack as strongly as the rock walls themselves, but the process is nonetheless very effective. Mountaintops with expanses of bedrock subjected to freeze–thaw cycles tend to be a jumble of frost-shattered rock; hikers in the Presidential Range in New Hampshire know that well.
hydrofracturing: It’s thought by some investigators that the frost-wedging effect is more complicated than just simple expansion upon freezing. Water as thin films in very small cracks may stay liquid even at very low temperatures, for subtle thermodynamic reasons. As the freezing front progresses inward from the rock surface, water can be forced under high pressures into microscopic cracks, causing propagation of fractures. The phenomenon is known as hydrofracturing.
“boulder lifting”: It’s often said that boulders “grow” in New England fields. Frost action provides a good explanation for the effect (Figure 2-8). As the water-saturated ground freezes from the top down, freezing reaches the base of a large boulder sooner than it reaches the surrounding soil, because the rock of the boulder has a much lower heat capacity than the water-rich soil around it and is therefore cooled faster. Freezing of the water beneath the boulder lifts or pushes the boulder upward a short distance. Upon thawing of the ice beneath the boulder, the boulder does not fall as far as it was raised, because of the tendency for collapse and flowage of soil and finer rock fragments into the space beneath the boulder. Given enough cycles, the boulder makes its appearance at the surface.
root growth: Root growth can be very effective at widening preexisting cracks in bedrock near the surface. All of you have seen plants growing heroically in cracks in rock. In many cases the plant just occupies an already large crack, but in other cases it’s clear that the growing root has opened the crack.
Which of these various processes are the more important, and which are the less important? I can do no more than make an educated guess. (It depends, of course, on the particular setting of bedrock and climate, in any case.) In the right circumstances, frost wedging is clearly of great importance, but granular disintegration , and perhaps exfoliation , are probably of more general importance. Thermal expansion and contraction alone is generally considered to be less important than the other processes, except perhaps in particularly favored environments.
2.5.4 Weathering Profiles and Weathering Fronts
You’ve probably had the experience of picking up a broken stone and noticing that the outer part is colored reddish or brownish while the inner part looks a fresh gray. What’s going on is that the stone, having been derived from some outcrop as a fresh piece and then perhaps rounded at least somewhat by transport, rested somewhere at the Earth’s surface for a time long enough for the outer zone to have experienced some chemical weathering. The boundary between the outer, weathered zone and the inner, non-weathered zone is called the weathering front . There’s commonly a very abrupt change in the degree of weathering at the weathering front. In many cases the characteristic difference in color arises from the conversion of ferrous iron in minerals of the fresh rock to ferric iron oxides in the weathered zone.
The same effect tends to be present on a much larger scale when a bedrock landscape in an area of low relief is subjected to protracted weathering and the weathering products remain where they were formed. To see the effect, you need to have a vertical or steeply dipping cut down through the weathered rock, all the way down into fresh bedrock, which may be many meters or even tens of meters below the surface. Above the weathering front is a gradation from the most weathered material, at and near the land surface, down to the least weathered rock, at the weathering front. The succession of zones of degrees of weathering, from the surface down to the weathering front, is called a weathering profile .
Weathering profiles vary greatly in their nature, depending upon the factors listed in an earlier section (largely a matter of rock composition and climate). Figure 2-9 shows a common weathering profile, of the kind that tends to develop on deeply in-place-weathered coarse-grained igneous rocks like granite. The material just above the weathering front, called saprock , consists of partly chemically weathered minerals together with yet-unweathered minerals. The saprock preserves all of the original textures and structures of the parent rock, but its composition has been somewhat changed and its strength has been reduced. The saprock grades upward into material called saprolite , which is more altered chemically than the saprock below but still retains much of the textures and structures of the parent rock. You might describe such rock as “rotten rock” (and that is in fact the etymology of the term saprolite). Above the saprolite is even more strongly altered material, which has lost even more of the coherence of the parent and shows little of the original textures and structures. Above this, where the material has lost its coherence entirely and could be described as “loosed”, it is called regolith (see later section). The uppermost layer is the soil (see later section).
Note in Figure 2-9 that the geometry of the weathering front is strongly controlled by the presence of joints in the fresh rock . The weathering can make better progress along the joints than in the non-jointed intervening rock. In the process, there’s a tendency for non-weathered masses to be left behind as the weathering front progresses downward. These rounded, irregular masses are called corestones . They are similar to the weathering spheroids mentioned earlier, but they differ in being formed well below the land surface, whereas weathering spheroids develop at the rock surface as weathering proceeds at the same time that the weathering products are removed from the outcrop.


10.3: Muscle Fiber Contraction and Relaxation
Learning Objectives
- Describe the components involved in a muscle contraction
- Explain how muscles contract and relax
- Describe the sliding filament model of muscle contraction
The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na + ) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the T-tubules. This triggers the release of calcium ions (Ca ++ ) from storage in the sarcoplasmic reticulum (SR). The Ca ++ then initiates contraction, which is sustained by ATP (Figure \(\PageIndex{1}\)). As long as Ca ++ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.
Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca ++ ions are then pumped back into the SR, which causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued (Figure \(\PageIndex{2}\)).
The molecular events of muscle fiber shortening occur within the fiber’s sarcomeres (see Figure \(\PageIndex{3}\)). The contraction of a striated muscle fiber occurs as the sarcomeres, linearly arranged within myofibrils, shorten as myosin heads pull on the actin filaments.
The region where thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. This zone where thin and thick filaments overlap is very important to muscle contraction, as it is the site where filament movement starts. Thin filaments, anchored at their ends by the Z-discs, do not extend completely into the central region that only contains thick filaments, anchored at their bases at a spot called the M-line. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.
The Sliding Filament Model of Contraction
When signaled by a motor neuron, a skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres. This process is known as the sliding filament model of muscle contraction (Figure \(\PageIndex{3}\)). The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca ++ entry into the sarcoplasm.
Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin “heads” from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Ca ++ ions.
To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament to allow cross-bridge formation between the actin and myosin microfilaments. The first step in the process of contraction is for Ca ++ to bind to troponin so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges. The thin filaments are then pulled by the myosin heads to slide past the thick filaments toward the center of the sarcomere. But each head can only pull a very short distance before it has reached its limit and must be “re-cocked” before it can pull again, a step that requires ATP.
ATP and Muscle Contraction
For thin filaments to continue to slide past thick filaments during muscle contraction, myosin heads must pull the actin at the binding sites, detach, re-cock, attach to more binding sites, pull, detach, re-cock, etc. This repeated movement is known as the cross-bridge cycle. This motion of the myosin heads is similar to the oars when an individual rows a boat: The paddle of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked) and then immersed again to pull (Figure \(\PageIndex{4}\)). Each cycle requires energy, and the action of the myosin heads in the sarcomeres repetitively pulling on the thin filaments also requires energy, which is provided by ATP.
Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (P i ) are still bound to myosin (Figure10.3.4.a,b.). P i is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. As actin is pulled, the filaments move approximately 10 nm toward the M-line. This movement is called the power stroke , as movement of the thin filament occurs at this step (Figure \(\PageIndex{4}\).c.). In the absence of ATP, the myosin head will not detach from actin.
One part of the myosin head attaches to the binding site on the actin, but the head has another binding site for ATP. ATP binding causes the myosin head to detach from the actin (Figure \(\PageIndex{4}\).d). After this occurs, ATP is converted to ADP and P i by the intrinsic ATPase activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position (Figure \(\PageIndex{4}\).e). The myosin head is now in position for further movement.
When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke, and at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together. As long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.
Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much energy (ATP) is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the actin-binding sites, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.
Sources of ATP
ATP supplies the energy for muscle contraction to take place. In addition to its direct role in the cross-bridge cycle, ATP also provides the energy for the active-transport Ca ++ pumps in the SR. Muscle contraction does not occur without sufficient amounts of ATP. The amount of ATP stored in muscle is very low, only sufficient to power a few seconds worth of contractions. As it is broken down, ATP must therefore be regenerated and replaced quickly to allow for sustained contraction. There are three mechanisms by which ATP can be regenerated: creatine phosphate metabolism, anaerobic glycolysis, fermentation and aerobic respiration.
Creatine phosphate is a molecule that can store energy in its phosphate bonds. In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction. However, creatine phosphate can only provide approximately 15 seconds worth of energy, at which point another energy source has to be used (Figure \(\PageIndex{5}\)).
As the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as an ATP source. Glycolysis is an anaerobic (non-oxygen-dependent) process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate. Thus, the switch to glycolysis results in a slower rate of ATP availability to the muscle. The sugar used in glycolysis can be provided by blood glucose or by metabolizing glycogen that is stored in the muscle. The breakdown of one glucose molecule produces two ATP and two molecules of pyruvic acid , which can be used in aerobic respiration or when oxygen levels are low, converted to lactic acid (Figure \(\PageIndex{5}\).b).
If oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to lactic acid , which may contribute to muscle fatigue. This conversion allows the recycling of the enzyme NAD + from NADH, which is needed for glycolysis to continue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. Glycolysis itself cannot be sustained for very long (approximately 1 minute of muscle activity), but it is useful in facilitating short bursts of high-intensity output. This is because glycolysis does not utilize glucose very efficiently, producing a net gain of two ATPs per molecule of glucose, and the end product of lactic acid, which may contribute to muscle fatigue as it accumulates.
Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen (O 2 ) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O 2 to the skeletal muscle and is much slower (Figure \(\PageIndex{5}\).c). To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue. Aerobic training also increases the efficiency of the circulatory system so that O 2 can be supplied to the muscles for longer periods of time.
Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, although certain factors have been correlated with the decreased muscle contraction that occurs during fatigue. ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. This may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts. Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity. Imbalances in Na + and K + levels as a result of membrane depolarization may disrupt Ca ++ flow out of the SR. Long periods of sustained exercise may damage the SR and the sarcolemma, resulting in impaired Ca ++ regulation.
Intense muscle activity results in an oxygen debt , which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction. Oxygen is required to restore ATP and creatine phosphate levels, convert lactic acid to pyruvic acid, and, in the liver, to convert lactic acid into glucose or glycogen. Other systems used during exercise also require oxygen, and all of these combined processes result in the increased breathing rate that occurs after exercise. Until the oxygen debt has been met, oxygen intake is elevated, even after exercise has stopped.
Relaxation of a Skeletal Muscle
Relaxing skeletal muscle fibers, and ultimately, the skeletal muscle, begins with the motor neuron, which stops releasing its chemical signal, ACh, into the synapse at the NMJ. The muscle fiber will repolarize, which closes the gates in the SR where Ca ++ was being released. ATP-driven pumps will move Ca ++ out of the sarcoplasm back into the SR. This results in the “reshielding” of the actin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes.
Muscle Strength
The number of skeletal muscle fibers in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress (and artificial anabolic steroids), acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers, a change called hypertrophy, which results in the increased mass and bulk in a skeletal muscle. Likewise, decreased use of a skeletal muscle results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers). It is common for a limb in a cast to show atrophied muscles when the cast is removed, and certain diseases, such as polio, show atrophied muscles.
DISORDERS OF THE...
Muscular System
Duchenne muscular dystrophy (DMD) is a progressive weakening of the skeletal muscles. It is one of several diseases collectively referred to as “muscular dystrophy.” DMD is caused by a lack of the protein dystrophin, which helps the thin filaments of myofibrils bind to the sarcolemma. Without sufficient dystrophin, muscle contractions cause the sarcolemma to tear, causing an influx of Ca++, leading to cellular damage and muscle fiber degradation. Over time, as muscle damage accumulates, muscle mass is lost, and greater functional impairments develop.
DMD is an inherited disorder caused by an abnormal X chromosome. It primarily affects males, and it is usually diagnosed in early childhood. DMD usually first appears as difficulty with balance and motion, and then progresses to an inability to walk. It continues progressing upward in the body from the lower extremities to the upper body, where it affects the muscles responsible for breathing and circulation. It ultimately causes death due to respiratory failure, and those afflicted do not usually live past their 20s.
Because DMD is caused by a mutation in the gene that codes for dystrophin, it was thought that introducing healthy myoblasts into patients might be an effective treatment. Myoblasts are the embryonic cells responsible for muscle development, and ideally, they would carry healthy genes that could produce the dystrophin needed for normal muscle contraction. This approach has been largely unsuccessful in humans. A recent approach has involved attempting to boost the muscle’s production of utrophin, a protein similar to dystrophin that may be able to assume the role of dystrophin and prevent cellular damage from occurring.
Chapter Review
A sarcomere is the smallest contractile portion of a muscle. Myofibrils are composed of thick and thin filaments. Thick filaments are composed of the protein myosin; thin filaments are composed of the protein actin. Troponin and tropomyosin are regulatory proteins.
Muscle contraction is described by the sliding filament model of contraction. ACh is the neurotransmitter that binds at the neuromuscular junction (NMJ) to trigger depolarization, and an action potential travels along the sarcolemma to trigger calcium release from SR. The actin sites are exposed after Ca ++ enters the sarcoplasm from its SR storage to activate the troponin-tropomyosin complex so that the tropomyosin shifts away from the sites. The cross-bridging of myposin heads docking into actin-binding sites is followed by the “power stroke”—the sliding of the thin filaments by thick filaments. The power strokes are powered by ATP. Ultimately, the sarcomeres, myofibrils, and muscle fibers shorten to produce movement.
Interactive Link Questions
The release of calcium ions initiates muscle contractions. Watch this video to learn more about the role of calcium. (a) What are “T-tubules” and what is their role? (b) Please also describe how actin-binding sites are made available for cross-bridging with myosin heads during contraction.
Answer: (a) The T-tubules are inward extensions of the sarcolemma that trigger the release of Ca ++ from SR during an Action Potential. (b) Ca ++ binds to tropomyosin, and this slides the tropomyosin rods away from the binding sites.
Review Questions
Q. In relaxed muscle, the myosin-binding site on actin is blocked by ________.
A. titin
B. troponin
C. myoglobin
D. tropomyosin
Answer: D
Q. According to the sliding filament model, binding sites on actin open when ________.
A. creatine phosphate levels rise
B. ATP levels rise
C. acetylcholine levels rise
D. calcium ion levels rise
Answer: D
Q. The cell membrane of a muscle fiber is called ________.
A. myofibril
B. sarcolemma
C. sarcoplasm
D. myofilament
Answer: B
Q. Muscle relaxation occurs when ________.
A. calcium ions are actively transported out of the sarcoplasmic reticulum
B. calcium ions diffuse out of the sarcoplasmic reticulum
C. calcium ions are actively transported into the sarcoplasmic reticulum
D. calcium ions diffuse into the sarcoplasmic reticulum
Answer: C
Q. During muscle contraction, the cross-bridge detaches when ________.
A. the myosin head binds to an ADP molecule
B. the myosin head binds to an ATP molecule
C. calcium ions bind to troponin
D. calcium ions bind to actin
Answer: C
Q. Thin and thick filaments are organized into functional units called ________.
A. myofibrils
B. myofilaments
C. T-tubules
D. sarcomeres
Answer: D
Critical Thinking Questions
Q. How would muscle contractions be affected if skeletal muscle fibers did not have T-tubules?
A. Without T-tubules, action potential conduction into the interior of the cell would happen much more slowly, causing delays between neural stimulation and muscle contraction, resulting in slower, weaker contractions.
Q. What causes the striated appearance of skeletal muscle tissue?
A. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell cause the entire cell to appear striated.
Q. How would muscle contractions be affected if ATP was completely depleted in a muscle fiber?
A. Without ATP, the myosin heads cannot detach from the actin-binding sites. All of the “stuck” cross-bridges result in muscle stiffness. In a live person, this can cause a condition like “writer’s cramps.” In a recently dead person, it results in rigor mortis.
Glossary
- aerobic respiration
- production of ATP in the presence of oxygen
- ATPase
- enzyme that hydrolyzes ATP to ADP
- creatine phosphate
- phosphagen used to store energy from ATP and transfer it to muscle
- glycolysis
- anaerobic breakdown of glucose to ATP
- lactic acid
- product of anaerobic glycolysis
- oxygen debt
- amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction
- power stroke
- action of myosin pulling actin inward (toward the M line)
- pyruvic acid
- product of glycolysis that can be used in aerobic respiration or converted to lactic acid


3.12: The Future of Large Telescopes
Learning Objectives
By the end of this section, you will be able to:
- Describe the next generation of ground- and space-based observatories
- Explain some of the challenges involved in building these observatories
If you’ve ever gone on a hike, you have probably been eager to see what lies just around the next bend in the path. Researchers are no different, and astronomers and engineers are working on the technologies that will allow us to explore even more distant parts of the universe and to see them more clearly.
The premier space facility planned for the next decade is the James Webb Space Telescope (Figure \(\PageIndex{1}\)), which (in a departure from tradition) is named after one of the early administrators of NASA instead of a scientist. This telescope will have a mirror 6 meters in diameter, made up, like the Keck telescopes, of 36 small hexagons. These will have to unfold into place once the telescope reaches its stable orbit point, some 1.5 million kilometers from Earth (where no astronauts can currently travel if it needs repair.) The telescope is scheduled for launch in 2021 and should have the sensitivity needed to detect the very first generation of stars, formed when the universe was only a few hundred million years old. With the ability to measure both visible and infrared wavelengths, it will serve as the successor to both HST and the Spitzer Space Telescope.
Watch this video to learn more about the James Webb Space Telescope and how it will build upon the work that Hubble has allowed us to begin in exploring the universe.
On the ground, astronomers have started building the Large Synoptic Survey Telescope (LSST), an 8.4-meter telescope with a significantly larger field of view than any existing telescopes. It will rapidly scan the sky to find transients , phenomena that change quickly, such as exploding stars and chunks of rock that orbit near Earth. The LSST is expected to see first light in 2021.
The international gamma-ray community is planning the Cherenkov Telescope Array (CTA), two arrays of telescopes, one in each hemisphere, which will indirectly measure gamma rays from the ground. The CTA will measure gamma-ray energies a thousand times as great as the Fermi telescope can detect.
Several groups of astronomers around the globe interested in studying visible light and infrared are exploring the feasibility of building ground-based telescopes with mirrors larger than 30 meters across. Stop and think what this means: 30 meters is one-third the length of a football field. It is technically impossible to build and transport a single astronomical mirror that is 30 meters or larger in diameter. The primary mirror of these giant telescopes will consist of smaller mirrors, all aligned so that they act as a very large mirror in combination. These include the Thirty-Meter Telescope for which construction has begun at the top of Mauna Kea in Hawaii.
The most ambitious of these projects is the European Extremely Large Telescope (E-ELT) (Figure \(\PageIndex{2}\)). (Astronomers try to outdo each other not only with the size of these telescopes, but also their names!) The design of the E-ELT calls for a 39.3-meter primary mirror, which will follow the Keck design and be made up of 798 hexagonal mirrors, each 1.4 meters in diameter and all held precisely in position so that they form a continuous surface.
Construction on the site in the Atacama Desert in Northern Chile started in 2014. The E-ELT, along with the Thirty Meter Telescope and the Giant Magellan Telescope, which are being built by international consortia led by US astronomers, will combine light-gathering power with high-resolution imaging. These powerful new instruments will enable astronomers to tackle many important astronomical problems. For example, they should be able to tell us when, where, and how often planets form around other stars. They should even be able to provide us images and spectra of such planets and thus, perhaps, give us the first real evidence (from the chemistry of these planets’ atmospheres) that life exists elsewhere.
Check out this fun diagram comparing the sizes of the largest planned and existing telescopes to a regulation basketball and tennis court.
Summary
New and even larger telescopes are on the drawing boards. The James Webb Space Telescope, a 6-meter successor to Hubble, is currently scheduled for launch in 2018. Gamma-ray astronomers are planning to build the CTA to measure very energetic gamma rays. Astronomers are building the LSST to observe with an unprecedented field of view and a new generation of visible-light/infrared telescopes with apertures of 24.5 to 39 meters in diameter.
Contributors and Attributions
-
Andrew Fraknoi (Foothill College), David Morrison (NASA Ames Research Center), Sidney C. Wolff (National Optical Astronomy Observatory) with many contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at https://openstax.org/details/books/astronomy ).


8.5: Which Species are at Risk of Extinction?
The IUCN has formalised the evaluation of threatened species using an internationally accepted standard of conservation categories describing a taxon’s risk of extinction.
An important task for conservation biologists is to identify and prioritise those species in greatest danger of extinction. Accomplishing this task requires biologists to collect and review all the information we have on each species. To facilitate this major undertaking, the IUCN has formalised the evaluation and reporting of threatened species assessments using an internationally accepted standard of conservation categories to reflect a taxon’s risk of extinction. These nine categories (Figure 8.7), known as Red List Assessments (IUCN, 2017), are:
- Extinct (EX). These species are no longer known to exist. As of mid-2019, the IUCN has listed 84 Sub-Saharan African species as Extinct.
- Extinct in the Wild (EW). These species exist only in cultivation, in captivity, or other human-managed situations. As of mid-2019, the IUCN has listed nine Sub-Saharan African species as Extinct in the Wild.
- Critically Endangered (CR). These species have an extremely high risk of going extinct in the wild. As of mid-2019, the IUCN has listed 880 Sub-Saharan African species as Critically Endangered. Also included in this category are the 202 Sub-Saharan African species that the IUCN considered possibly Extinct.
- Endangered (EN). These species have a very high risk of extinction in the wild. As of mid-2019, the IUCN has listed 1,600 Sub-Saharan African species as Endangered.
- Vulnerable (VU). These species have a moderately high risk of extinction in the wild. As of mid-2019, the IUCN has listed 2,153 Sub-Saharan African species as Vulnerable.
- Near Threatened (NT). These species are close to qualifying for a threatened category but are not currently considered threatened. As of mid-2019, the IUCN has listed 1,034 Sub-Saharan African species as Near Threatened.
- Data Deficient (DD). Inadequate information exists to determine the risk of extinction for these species. As of mid-2019, the IUCN has listed 2,441 Sub-Saharan African species as Data Deficient.
- Least Concern (LC). These species are not considered Near Threatened or threatened. (Widespread and abundant species are included in this category.) As of mid-2019, the IUCN has listed 11,776 Sub-Saharan African species as Least Concern.
- Not Evaluated (NE). Species that have not yet been evaluated. Most species fall in this category.
These categories, and the Red List Criteria (Table 8.1) used to classify each species, are broadly based on population viability analysis (Section 9.2), and consider population size, population trends, and habitat availability. Species that are Extinct in the Wild, Critically Endangered, Endangered, and Vulnerable categories are officially considered “threatened with extinction”. The advantage of this system is that it provides a standard protocol by which decisions can be reviewed and evaluated according to widely accepted yet flexible criteria. Consequently, species, subspecies, varieties, populations, and subpopulations can be assessed on a global or regional level, all under a unified set of standards. The resultant threat status assessment forms the basis of Red Data Books and Red Lists: detailed lists of threatened wildlife by group and/or by region compiled by the IUCN and its affiliate organizations. All global (and many regional) Red List assessments are freely available at http://www.iucnredlist.org , with feedback links provided from which anyone can alert the IUCN if they find errors or have suggestions for improvements.
|
Red List criteria A–E |
Summary criteria used to evaluate a taxon as Critically Endangered a |
|---|---|
|
A. Population size declining |
The population size has declined by 90% (or more) over last 10 years or 3 generations (whichever is longer). |
|
B. Geographical range declining |
The species is restricted to < 100 km 2 and it occurs at a single location and its distribution range is observed/expected to decline. |
|
C. Small and declining populations |
There are less than 250 mature individuals left and population has declined by 25% (or more) over last 3 years or 1 generation (whichever is longer). |
|
D. Small populations |
There are less than 50 mature individuals left. |
|
E. Population viability analysis |
There is a 50% (or greater) chance of extinction within 10 years or 3 generations. |
a Additional criteria for Critically Endangered, as well as criteria for Endangered and Vulnerable listings can be found at http://www.iucnredlist.org .
While nearly 20,000 Sub-Saharan African species have been evaluated as of mid-2019 (IUCN, 2019), these assessments only cover a small proportion of the region’s overall biodiversity. Consider for example that as of mid-2019, just over 4,900 Sub-Saharan Africa’s plants have been listed on the IUCN Red List website. Yet, the Cape Floristic Region alone hosts over 6,200 endemic plant species. The assessment gaps are even more conspicuous for lesser-known taxa; for example, only seven species of bryophytes (a group of non-vascular plants that includes mosses) have been assessed as of mid-2019; some readers of this textbook will have more bryophyte species in their gardens. The reasons for such assessment gaps are many, but most boil down to manpower and funding limitations, which restrict our ability to obtain the data needed for comprehensive assessments. It is thus important to understand that the lack of information on these and other poorly known groups does not mean there is no threat. For example, as of mid-2019, no African abalone (Haliotis spp.) have been assessed, even though these highly valued molluscs are some of Africa’s most heavily exploited (and heavily poached) marine organisms (Minnaar et al., 2018). A lack of information about a species’ threats and populations trends is thus a good argument that more studies are needed, sometimes urgently. Similarly, continued monitoring of species thought to be common is also important, as it can shed light on how new threats may emerge or escalate over time.
Course-filter assessments
To fill Red List species assessment gaps, conservation biologists are increasingly relying on broader metrics, or coarse-filter assessments , to identify groups of species that are threatened with extinction. One such method, which reduces the need to evaluate every individual species, is to identify ecosystems that are threatened. This premise rests on the assumption that any threatened ecosystem will contain many threatened species. Hence, protecting and restoring threatened ecosystems will simultaneously allow many populations living in those ecosystems to recover. To facilitate this type of coarse-filter assessment, the IUCN recently established a Red List of Ecosystems (RLE, http://iucnrle.org ). The RLE assesses ecosystem status against five criteria: (1) distribution declines, (2) distribution restrictions, (3) environmental degradation, (4) disruption of ecological processes and interactions, and (5) quantitative estimates for risk of ecosystem collapse (Keith et al., 2013). While the ecosystem assessment protocol was only recently developed—only three African ecosystems have been assessed as of mid-2019—its holistic strategy promises a more comprehensive accounting of local biodiversity which could be more informative than an accumulation of single species assessments.


3.5: Characteristics of Ionic Compounds-Electrolytes
- To describe the physical properties of ionic compounds.
The figure below shows just a few examples of the color and brilliance of naturally occurring ionic crystals. The regular and orderly arrangement of ions in the crystal lattice is responsible for the various shapes of these crystals, while transition metal ions give rise to the colors.
Physical Properties of Ionic Compounds
Melting Points
Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about 800 o C. As a comparison, the molecular compound water melts at 0 °C.
Shattering
Ionic compounds are generally hard, but brittle. Why? It takes a large amount of mechanical force, such as striking a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens, it brings ions of the same charge next to each other (see below). The repulsive forces between like-charged ions cause the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions.
Conductivity
Another characteristic property of ionic compounds is their electrical conductivity . The figure below shows three experiments in which two electrodes that are connected to a light bulb are placed in beakers containing three different substances.
In the first beaker, distilled water does not conduct a current because water is a molecular compound. In the second beaker, solid sodium chloride also does not conduct a current. Despite being ionic and thus composed of charged particles, the solid crystal lattice does not allow the ions to move between the electrodes.  In the third beaker, the NaCl has been dissolved into the distilled water. Now the crystal lattice has been broken apart and the individual positive and negative ions can move. Cations move to one electrode, while anions move to the other, allowing electricity to flow (see figure below). Melting an ionic compound also frees the ions to conduct a current. Ionic compounds conduct an electric current when melted or dissolved in water.
Write the dissociation equation of solid NaCl in water.
Solution
NaCl( s ) → Na + ( aq ) + Cl – ( aq )
Exercise \(\PageIndex{1}\)
Write the dissociation equation of solid NH 4 NO 3 in water.
- Answer
-
NH 4 NO 3 ( s ) → NH 4 + ( aq ) + NO 3 – ( aq )
Key Takeaways
- Ionic compounds have high melting points.
- Ionic compounds are hard and brittle.
- Ionic compounds dissociate into ions when dissolved in water.
- Solutions of ionic compounds and melted ionic compounds conduct electricity, but solid materials do not.
- An ionic compound can be identified by its chemical formula: cation+anion.
Contributors and Attributions
- CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.
Exercises
1. Explain how the bonding in an ionic solid explains some of the properties of these solids.
2. Which type(s) of solid conduct(s) electricity in their liquid state but not in their solid state?
3. Based on chemical formula, identify which of the following is an ionic solid?
a. Hg
b. PH 3
c. Ba(NO 3 ) 2


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accelerate
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to change velocity; to speed up, slow down, or change direction.
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apparent magnitude
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a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us
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astrology
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the pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets
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celestial equator
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a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator
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celestial poles
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points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis
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celestial sphere
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the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere
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circumpolar zone
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those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon
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cosmology
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the study of the organization and evolution of the universe
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ecliptic
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the apparent annual path of the Sun on the celestial sphere
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epicycle
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the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)
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geocentric
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centered on Earth
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heliocentric
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centered on the Sun
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horizon (astronomical)
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a great circle on the celestial sphere 90° from the zenith; more popularly, the circle around us where the dome of the sky meets Earth
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horoscope
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a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth
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parallax
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the apparent displacement of a nearby star that results from the motion of Earth around the Sun
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planet
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today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars
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precession (of Earth)
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the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge
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retrograde motion
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the apparent westward motion of a planet on the celestial sphere or with respect to the stars
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year
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the period of revolution of Earth around the Sun
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zenith
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the point on the celestial sphere opposite the direction of gravity; point directly above the observer
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zodiac
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a belt around the sky about 18° wide centered on the ecliptic


4.7: Evaluating Sources
- Discuss the importance of evaluating sources and understanding biases
Perhaps never before in mainstream discussion has the importance of evaluating sources been more important. The effect of “fake news,” and contested information is now a common feature of contemporary life. When evaluating sources, for which a variety of good techniques and rules of thumb exist, we argue that doing so is an essential part of critical thinking, which is the bedrock for good communication and report writing in any field.
To illustrate the importance of evaluating sources, consider our case study with Martha of The Human Fund. Recall that she read a variety of secondary source materials after developing her research question. In addition to ensuring her sources were scoped properly—that they addressed elements of her research question—she would need to evaluate their authorship, determine how recent and reliable the information is, and understand any bias.
Evaluating Websites
As our world becomes more and more connected by technology, our ability to evaluate and use information has become more difficult, but not impossible. It is essential to understand how these technologies work and how people use them.
Websites, broadly speaking, are perhaps the most difficult sources to evaluate; however, the following tips can act as basic guidelines:
- Consider the URL: generally speaking, .com, .org, .ac.uk, .edu and other more common domains are a bit more likely to have reliable and good content.
- What type of website is it? If it is a blog, social media site, or other tool for personal expression, proceed with caution. Much of the “fake news” problem is driven by sharing questionable material on social media.
- What is the main purpose or claim of the website? Be careful with websites interested in selling downloadable information sources, such as “How to Conduct Research” or “Make Money in Real Estate.” The content may be accurate and useful; however, the sheer abundance of poor sources means you should only use these types of data/sources with caution.
Learn More
While we’ve boiled the evaluation of websites down to a few key tips, it is actually a complex topic that could fill books. For more information, check out Mike Caulfield’s Web Literacy for Student Fact-Checkers .
This book is (as stated in its own introduction) “an unabashedly practical guide for the student fact-checker. It supplements generic information literacy with the specific web-based techniques that can get you closer to the truth on the web more quickly.” [1]
Understanding Bias in Your Sources
A word on bias: some consider bias to be a problem. However, we might argue here that bias is a normal part of life and human interaction. We are all biased by our upbringing, our experiences, and our perspectives. While any attempt to be objective in your analysis is a good thing, it can be just as useful to acknowledge your biases in your research and arm the reader or consumer of your material accordingly. In a way, this is a form of respect to your readership; you acknowledge their critical thinking role in consuming your material and also acknowledge that ruling out all bias—no matter how professional or scientific one’s research approach might be—is ultimately impossible.
The following video from Chris Flipp illustrates one way to acknowledge and represent bias. It uses a term called Bracketing, that comes from a qualitative research method/idea known as Phenomenology. Bracketing is the act of reviewing and gaining awareness around your preconceived notions of a given topic before pursuing further study. This awareness should help your evaluation of sources and keep you mentally engaged in the review of your own sentiment towards your data.
The following video from Practical Psychology illustrates various biases. Note how these biases could affect your thinking, and consider ways you might apply this awareness to evaluating sources:
Caulfield, Mike. " Why This Book? " Web Literacy for Student Fact-Checkers . Web. 30 June 2018.