Mastering Concepts
4.1
1. What are some examples of the “work” of a cell?
Examples of the “work” of a cell include cell division, biochemical reactions (such as
dehydration synthesis and hydrolysis), and membrane transport.
2. Give an example of how your body has both potential and kinetic energy.
Stored fuels such as glucose, glycogen, and fat contain potential energy. The body’s
muscle movements illustrate kinetic energy, as does the movement of substances across
membranes and within cells.
3. What are the first and second laws of thermodynamics?
The first law of thermodynamics says that energy cannot be destroyed or created, only
changed in form. The second law of thermodynamics says that each time energy is
converted from one form to another, some energy is always lost as heat.
4. Why does the amount of entropy in the universe always increase?
According to the second law of thermodynamics, the entropy in the universe always
increases because in every reaction some energy is lost as heat, and heat is a disordered
form of energy.
4.2
1. What is metabolism on a cellular level?
Metabolism is the sum of all chemical reactions in a cell.
2. Distinguish between endergonic and exergonic reactions.
Endergonic reactions require an input of energy to proceed to completion; exergonic
reactions release energy.
3. What are oxidation and reduction, and why are they always linked?
Oxidation is the loss of electrons from a molecule, atom, or ion; reduction is a gain of
electrons. Oxidation and reduction are always linked because they occur simultaneously.
If a molecule, atom, or ion loses electrons (is oxidized), then another must gain those
electrons (be reduced).
4. What is an electron transport chain?
Electron transport chains are groups of proteins that accept and then pass on electrons,
releasing small amounts of energy along the way.
4.3
1. What are the main parts of an ATP molecule?
ATP is a nucleotide consisting of ribose, a nitrogenous base, and three phosphate groups.
2. How does ATP hydrolysis supply energy for cellular functions?
Hydrolysis of ATP breaks the bond to the third phosphate group, releasing energy. This
reaction is coupled to cellular reactions that require energy input.
3. Describe the relationships among endergonic reactions, ATP hydrolysis, and cellular
respiration.
The ATP that is produced in cellular respiration can undergo hydrolysis, releasing energy
that can be used to drive endergonic reactions.
4.4
1. What do enzymes do in cells?
Enzymes speed up chemical reactions without being consumed in the process.
2. How does an enzyme lower a reaction’s activation energy?
Enzymes bring reactants into contact with each other so that less energy is required to
start the reaction.
3. Distinguish between an enzyme and a coenzyme.
An enzyme is an organic molecule (typically a protein) that speeds a chemical reaction. A
coenzyme is an organic molecule, such as a vitamin, that is required for the enzyme to
function.
4. What are the roles of negative and positive feedback?
Negative feedback and positive feedback are ways to regulate the rates of enzymecatalyzed reactions. In negative feedback, the accumulation of the reaction’s product
causes the reaction to slow down. In positive feedback, the accumulation of the reaction’s
product makes the reaction go faster.
5. List three conditions that influence enzyme activity.
Three conditions that can influence an enzyme’s activity are pH, temperature, and salt
concentration. Noncompetitive and competitive inhibitors can also affect enzyme
activity.
4.5
1. What is diffusion?
Diffusion is the spontaneous movement of a substance from an area of high concentration
to an area of low concentration. It is a consequence of random movement.
2. What types of substances diffuse freely across a biological membrane?
Lipids and small nonpolar molecules diffuse freely across a membrane.
3. How do differing concentrations of solutes in neighboring solutions drive osmosis?
Solutions with high concentrations of solutes have relatively low concentrations of water.
In osmosis, water diffuses from a low concentration of solutes (high concentration of
water) to a high concentration of solutes (low concentration of water).
4. Why does it cost energy for a cell to maintain a concentration gradient?
It costs energy for a cell to maintain a concentration gradient because random molecular
movement (diffusion) tends to dissipate gradients.
5. What are the characteristics of active transport? Give an example of active transport.
Active transport requires the expenditure of energy to move a substance against its
concentration gradient. An example of active transport is the movement of sodium ions
from an area where they are less concentrated to an area where they are more
concentrated.
6. How do exocytosis and endocytosis use vesicles to transport materials across cell
membranes?
In exocytosis, a vesicle carries materials to be transported out of the cell. The lipid
bilayer surrounding the vesicle merges with the lipid bilayer of the cell membrane,
releasing the substance outside the cell. Conversely, in endocytosis, a vesicle forms as the
cell membrane pinches inward, bringing substances into the cell.
4.6
1. What is the normal role of CFTR in humans, and how can faulty CFTR proteins cause
cystic fibrosis?
CFTR is a membrane protein that sends Cl- out of the cell by active transport. This
accumulation of Cl- outside the cell draws water by osmosis. A faulty CFTR protein leads
to less Cl- movement and, consequently, less water movement. The resulting thick mucus
causes severe problems in the lungs and digestive system.
2. Summarize the question Gabriel and his colleagues asked, and explain how their
experiment helped answer the question.
The researchers were interested in understanding why such a harmful mutation to CFTR
has not been removed by natural selection from the population. They hypothesized that
faulty CFTR proteins give increased resistance to cholera. Their experiment showed that
mice with just one faulty copy of the CFTR gene had some resistance to cholera. In the
human population, individuals with cholera resistance are more likely to survive to
reproduce, so natural selection does not remove the faulty copy of the gene.
3. How do you think the results in figure 4.22 would have been different if, before adding
cholera toxin, the researcher had added a chemical that blocked the site at which the toxin
binds to CFTR?
The toxin overstimulates CFTR, causing extreme water loss. Blocking the toxin would
prevent this effect. The graph would therefore show equally low water loss for all three
groups.
Write It Out
1. Cite everyday illustrations of the first and second laws of thermodynamics. How do the
laws of thermodynamics underlie every organism’s ability to function?
The first law of thermodynamics says that energy changes from one form to another, but
it is never created or destroyed. An illustration of the first law of thermodynamics would
be a plant cell using light energy to assemble glucose molecules (a form of chemical
energy). The second law of thermodynamics says that in any energy transformation, some
energy is always lost as heat. An illustration of the second law of thermodynamics would
be the loss of heat energy from vigorously exercising muscles. Taken together, the laws
of thermodynamics describe the limits of how organisms can acquire and use energy and
explain why every ecosystem requires continuous energy input.
2. Some people claim that life’s high degree of organization defies the second law of
thermodynamics. What makes this statement false?
The second law of thermodynamics states that all reactions result in the loss of energy as
heat. Life’s high degree of organization defies this statement only if life is considered as
a closed system. In fact, life is not a closed system; energy in sunlight powers most life
on Earth. The sun is constantly decreasing in complexity as it releases energy, and
entropy overall increases even as life remains highly organized.
3. List some examples of endergonic and exergonic reactions that have been introduced
in previous chapters.
Dehydration synthesis and photosynthesis are examples of endergonic reactions, as are
most processes involved in the production of milk (see figure 3.13); hydrolysis and
respiration are examples of exergonic reactions.
4. Describe where oxidation and reduction occur along an electron transport chain.
The initial electron donor molecule is oxidized when it donates electrons to protein #1 in
the chain; this first protein therefore becomes reduced. But protein #1 is oxidized when it
passes the electrons to protein #2, which is reduced. Similar oxidation-reduction reactions
occur between each pair of proteins in the chain until the electrons reach the end of the
chain. At that point, the final electron acceptor molecule is reduced.
5. Classify oxidation reactions and reduction reactions as exergonic or endergonic
processes. Explain your answer.
Electrons carry energy. An oxidation reaction (the loss of electrons) is therefore
exergonic, and a reduction reaction (the gain of electrons) is endergonic.
6. Provide an example of an appliance that uses coupled reactions.
In a coupled reaction, energy released in one process is used to fuel some other process.
The burner of a gas stove, for example, releases heat that is harnessed for cooking food.
7. Name at least four ways that the cell uses ATP.
Many answers are possible, but here are four examples. A cell uses ATP to move
muscles, to move some substances across membranes, to move vesicles along the
cytoskeleton, and to build critical molecules such as proteins and DNA.
8. In what ways is an enzyme’s function similar to engineers digging a tunnel through a
mountain rather than building a road over the peak?
Driving up a mountain requires a large amount of energy to counteract the force of
gravity; building a tunnel saves cars from having to climb the peak to get to the other
side. Many chemical reactions are exergonic but have a relatively high activation energy.
The enzyme reduces this activation energy so the reaction proceeds more rapidly.
9. Considering that enzymes are essential to all cells, including microbes, why might
refrigeration and freezing help preserve food?
The warmer the temperature, the faster a chemical reaction proceeds (unless the
temperature is so high that the enzyme is denatured). Food spoilage reactions happen
rapidly at room temperature, but if food is refrigerated or frozen, the same chemical
reactions occur much more slowly or not at all.
10. When a person eats a fatty diet, excess cholesterol accumulates in the bloodstream.
Cells then temporarily stop producing cholesterol. What phenomenon described in the
chapter does this control illustrate?
The control of cholesterol level illustrates negative feedback.
11. Explain the differences among diffusion, facilitated diffusion, active transport, and
endocytosis.
Diffusion is the spontaneous movement of a substance from a region where it is more
concentrated to a region where it is less concentrated. Facilitated diffusion is a type of
diffusion that requires proteins to assist the movement of a solute along its concentration
gradient. Active transport means that a cell uses a transport protein to move a solute
against its concentration gradient (from a region where it is less concentrated to an area
that is more concentrated). Endocytosis allows a cell to engulf large molecules and fluids
and bring them into a cell.
12. Diffusion is an efficient means of transport only over small distances. How does this
relate to a cell’s surface-area-to-volume ratio (see chapter 3)?
Cells acquire nutrients and dispose of wastes by diffusion. Diffusion is efficient only
over small distances, but that is sufficient as long as all parts of the cell’s interior are
close to the surface of the cell (which they will be if the surface area is high relative to
the cell’s volume).
13. Liver cells are packed with glucose. If the concentration of glucose in a liver cell is
higher than in the surrounding fluid, what mechanism could the cell use to import even
more glucose? Why would only this mode of transport work?
The cell would use active transport to move the glucose against its concentration
gradient. A concentration gradient is a form of potential energy, so the liver cell would
need to expend some type of energy in order to import more glucose molecules.
14. List three ways the content in this chapter relates to an organism’s ability to maintain
homeostasis.
Maintaining homeostasis means maintaining internal conditions within a certain range
even if the environment changes. (1) Maintaining homeostasis requires chemical
reactions to build DNA, proteins, lipids, and many other substances that make up the cell.
Enzymes catalyze these reactions. (2) Many reactions that enable a cell to maintain
homeostasis require energy in the form of ATP. (3) Membrane transport relates to
homeostasis by enabling cells to maintain optimal concentrations of solutes in cells and
body fluids.
Pull It Together
1. Add competitive inhibitor and noncompetitive inhibitor to this concept map.
Both “Competitive inhibitor” and “Noncompetitive inhibitor” could connect with the
phrase “are substances that reduce the activity of an” to “Enzyme”
2. What are some examples of potential energy and kinetic energy?
An example of potential energy is a concentration gradient across a membrane. The
chemical bond energy in ATP is also a form of potential energy. Examples of kinetic
energy include walking, a rock falling down a mountain, and the motion of a molecule.
3. Add the terms substrate, active site, and activation energy to this concept map.
“Enzymes” could connect with the phrase “act on” to “Substrate.” In turn, “Substrate”
could connect with the phrase “binds to the” to “Active site.” “Enzymes” could connect
with the phrase “speed chemical reactions by lowering the” to “Activation energy.”