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.
When your mitochondria break down sugars to chemical energy (in the form of ATP) you
are storing potential energy. Your body converts potential energy to kinetic energy when
ATP is used for membrane transport or to move your muscles.
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?
The second law of thermodynamics states the entropy in the universe always increases:
some energy is always 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 distinguishes a reaction that has reached chemical equilibrium?
A reaction that has reached chemical equilibrium has the same rate of forward and
reverse reactions.
4. What are oxidation and reduction, and why are they always linked?
Oxidation is the loss of electrons from a molecule, atom, or ion, while reduction is a gain
of electrons. They always are linked because they occur simultaneously. The oxidized
molecule, atom, or ion will be a source of electrons, while its partner will gain those
electrons in a reduction reaction.
5. 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 is the molecular structure of ATP?
ATP is a nucleotide (adenosine) that has three attached phosphate groups instead of one.
2. How does ATP hydrolysis supply energy for cellular functions?
Hydrolysis of ATP (the combination of ATP and H2O) breaks the bond of ATP’s
endmost phosphate group, releasing energy.
3. Describe the relationship between endergonic reactions, ATP hydrolysis, and cellular
respiration.
The ATP that is produced in cellular respiration can be hydrolyzed to release energy that
can be used to drive endergonic reactions.
4.4
1. What do enzymes do in cells?
Enzymes speed up chemical reactions (catalyze) 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 a chemical catalyst that speeds a chemical reaction. A coenzyme is an
organic molecule that is required for the enzymatic reaction to proceed.
4. What are the roles of negative and positive feedback?
Negative and positive feedback are ways to regulate enzymes. In negative feedback the
product stops the reaction, and in positive feedback the product propagates the reaction.
5. List three conditions that influence enzyme activity.
Three conditions that can influence an enzyme’s activity are pH, temperature, and salt
concentration.
4.5
1. What is diffusion?
Diffusion is the spontaneous movement of molecules from areas of high concentration to
areas of low concentration.
2. What types of substances diffuse freely across a biological membrane?
Lipids and small nonpolar molecules diffuse freely.
3. How do differing concentrations of solutes in neighboring solutions drive osmosis?
Solutions with high concentrations of solutes have relatively low concentrations of water.
Water always moves from a high concentration of water to a low concentration of water.
Therefore, water will move by osmosis 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 it must move
substances against diffusion, which tends to allow gradients to dissipate.
5. Distinguish between simple diffusion, facilitated diffusion, and active transport.
In simple diffusion molecules move across the membrane without the use of a protein or
ATP energy. In facilitated diffusion, ATP energy is still not used, but molecules need a
protein passageway to cross the membrane. In active transport, molecules move from
low concentration to high concentration, requiring both ATP and a transport protein.
6. How do exocytosis and endocytosis use vesicles to transport materials across cell
membranes?
By merging the lipid bilayer that surrounds vesicles with the lipid bilayer of the cell
membrane, exocytosis transports materials toward the outside of a 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 water movement onto the lining of the lungs. The mucus in the lungs is not
thinned, and therefore traps and holds bacteria.
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, resisting cholera would have allowed the individuals to live a healthy
reproductive life, and so natural selection does not remove the faulty copy of the gene.
3. How do you think the results in figure 4.24 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 chemical would prevent the toxin from producing extreme water loss, and so the
graph showing water loss should be evenly low 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 is transformed 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 gazelle then eating the plant, and finally a crocodile eating the gazelle. 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 from
vigorously exercising muscles. Taken together, the laws of thermodynamics describe
how organisms acquire and use energy they need to sustain life and how that energy is
constantly being lost to the environment.
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. State the differences between endergonic and exergonic reactions.
Endergonic reactions require energy input, whereas exergonic reactions release energy.
4. What is chemical equilibrium?
Chemical equilibrium is the point in which a reaction goes in both directions at the same
rate.
5. Why are oxidation and reduction reactions linked?
Oxidation and reduction reactions are linked because they occur simultaneously. If one
molecule is oxidized (loses electrons), then another molecule must be reduced (gain
electrons).
6. Why is ATP called the cell’s “energy currency”?
ATP is called a cell’s “energy currency” because it provides energy for many chemical
reactions in the cell. Just as you can spend money in many ways, a cell can spend ATP
on many essential processes that require energy input.
7. How does an enzyme speed a chemical reaction?
Enzymes speed chemical reactions by lowering the energy of activation, the amount of
energy required to start a reaction.
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?
To build a road over the mountain peak requires a very large energy expenditure to go up
over the top and come back down the other side. Building a tunnel avoids the peak and
keeps a fairly level road. Without enzymes the hill of activation energy that a reaction
requires is quite high, but in the presence of the enzyme the energy requirement is so low
as to be nearly level.
9. Why would a cell’s fat-digesting enzymes not be able to digest an artificial fat such as
Olestra (see chapter 2)?
The active site of an enzyme has a specific shape; an artificial fat such as Olestra has a
different shape than the shape of a normal fat, so the digestive enzymes would not
recognize Olestra.
10. Figure 4.14 shows the effect of temperature on enzyme activity. Draw similar curves
that show the optimal pH for trypsin (an enzyme in the small intestine, pH 10), amylase
(an enzyme in the saliva, pH 6.5) and pepsin (an enzyme in the stomach, pH 2).
Replace temperature with pH on the X axis, and draw inverted curves centered over the
optimal pH for each of the enzymes.
11. 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.
12. Why does poking a hole in a cell’s membrane kill the cell?
The cell membrane maintains the cell’s internal environment, which is essential to life. A
hole in the cell membrane would allow important cell substances (including organelles
and dissolved chemicals) to leave, and the cell would die.
13. 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 polar 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.
14. 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).
15. In the kidneys, water moves by osmosis from tubules called nephrons to the
bloodstream. Do you expect a nephron to be hypotonic, isotonic, or hypertonic to the
blood? Explain.
The nephron would by hypotonic to the blood, making the blood hypertonic since water
moves by osmosis from areas of low solute to high solute.
16. A drop of a 5% salt (NaCl) solution is added to a leaf of the aquatic plant Elodea.
When the leaf is viewed under a microscope, colorless regions appear at the edges of
each cell as the cell membranes shrink from the cell walls. What is happening to these
cells?
The Elodea cells are shrinking because water is leaving the cells by osmosis. Water
moves to the higher concentration of solutes on the outside of the cells.
17. Seawater contains about 35 grams of salt per liter, whereas a liter of fresh water
contains 0.5 g of salt or less. The blood of a fish has about 10 g of dissolved salt per liter.
Cells in a fish’s gills have transport proteins that pump salts across their membranes. In
what direction would a saltwater fish pump ions? What about a freshwater fish?
A freshwater fish maintains a higher concentration of ions than the surrounding water; it
therefore must pump additional ions from water into its cells. Conversely, a saltwater fish
maintains a lower concentration of ions than the surrounding seawater. It therefore must
pump excess ions out of its cells and into the water.
18. 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 accept more glucose molecules.
Pull It Together
1. What types of molecules are ATP and enzymes?
ATP is a modified nucleotide that carries chemical bond energy, which is used as energy
(i.e. “fuel”) in cellular processes. Enzymes are protein molecules that speed up
biochemical reactions by lowering the energy of activation.
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 a form of potential energy. Examples of kinetic energy
include walking, a rock falling down a mountain, and changing the shape of a molecule.
3. Add the terms substrate, active site, and activation energy to this concept map.
“Substrate” could be connected to “Enzymes” by the phrase “act on”. “Active site” could
be connected to “substrate” by the phrase “binds to the”. “Activation energy” could be
connected to “Enzymes” by the phrase “lower the”. “Activation energy” would then lead
to “Chemical reactions” with the word “of.”
4.Where does passive transport fit on this concept map?
“Facilitated diffusion” and “Simple diffusion” could lead to “Passive transport” with the
phrase “are forms of”. “Passive transport” could then lead to “ATP” with the phrase
“does not require”.