Answers to Mastering Concepts Questions

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 examples of potential and kinetic energy in your body.

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 some energy conversions that occur in cells?

Photosynthesis and cellular respiration are two processes that convert energy in cells.  Photosynthesis converts light energy into glucose, and cellular respiration converts glucose to chemical energy.

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. Which reactions require energy input and which release energy?

If a reaction requires an input of energy, the products contain more energy than the reactants. In contrast, if a reaction releases energy, the products contain less energy than the reactants.

3. 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.

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. 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.

2. Describe the relationship between energy-requiring 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 energy requiring 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. What is the role of negative feedback in enzyme production?

In negative feedback the product stops the reaction.

4. List three conditions that influence enzyme activity.

Temperature, pH, and salt concentration influence enzyme activity.

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 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 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 role of CFTR in 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 would the results in figure 4.20 have been different if, before adding cholera toxin, the researchers 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.

 

Answers to Write It Out Questions

1. Some people claim that life’s high degree of organization defies the physical law that says entropy always increases. 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.

2. 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.

3. 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.

4. 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.

5. Figure 4.12 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.

6. 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. 

7. 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.

8. 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). 

9. 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.

10. 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.

 

Answers to Pull It Together Questions

1.  What types of organic molecules are ATP and enzymes?

ATP is a nucleotide that carries chemical bond energy, which is used as energy (i.e. “fuel”) for the reactions in cells.  Enzymes are protein catalysts that lower the activation energy required for chemical reactions in cells.

2. What are some examples of potential energy and kinetic energy other than those included on the concept map?

A concentration gradient across a membrane and the chemical bond energy in ATP are examples of potential energy.  Examples of kinetic energy include pushing molecules across membranes or a rock falling down a mountain.

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”.

5. Explain the differences among diffusion, facilitated diffusion, active transport, endocytosis, and exocytosis.

Diffusion and facilitated diffusion are passive forms of movement for molecules.  They do not require energy and molecules will flow from an area of high concentration to low concentration.  Facilitated diffusion requires a carrier protein in the membrane because the molecules cannot flow through the membrane itself.  Endocytosis and exocytosis are forms of active transport.  Active transport requires energy in order for molecules to move across the membrane.  Endocytosis brings molecules into the cell, whereas exocytosis is the process that moves molecules out of the cell.