The Dynamic Cell

C H A P T E R

5

O U T L I N E

5.1 What Is Energy?

• Energy is the capacity to do work.•70

• Energy cannot be created or destroyed; energy can be changed from one form to another, but energy is always lost in the

process.•71

5.2 ATP: Energy for Cells

• The three phosphate groups of ATP make it a high-energy molecule that easily breaks down.•72

• ATP goes through a cycle: Energy from cellular respiration drives ATP buildup, and then ATP breakdown provides energy for

cellular work.•72

• The breakdown of ATP, which releases energy, can be coupled to reactions that require an input of energy.•73

• Energy flows from the sun through all living things via the cycling of molecules between chloroplasts and mitochondria.•74

5.3 Metabolic Pathways and Enzymes

• Reactions in cells are a part of metabolic pathways, and each is catalyzed by its own specific enzyme.•75

• Enzymes speed reactions because they have an active site where a specific reaction occurs.•76

• The activity of an enzyme is controlled by feedback inhibition.•76

5.4 Cell Transport

• Some solutes move passively across a plasma membrane during a process called diffusion.•77

• Water diffuses across the plasma membrane, and this process, called osmosis, can affect cell size and shape.•77

• During active transport, proteins assist the movement of solutes across the plasma membrane from a higher to a lower

concentration.•78

• During bulk transport, vesicle formation takes large substances into the cell, and vesicle fusion with the plasma membrane

discharges such substances from the cell. Bulk transport requires energy but occurs independently of concentration
gradients.•79

Although we normally think of all the wonderful things water does for our bodies, drinking an excessive amount of it during a short period

of time can be toxic. In 2001, a 4-year-old girl in Utah died after her adoptive parents forced her to drink an extreme amount of fluids.

Drinking too much water has also been reported in people who are under the influence of drugs such as Ecstasy.

In humans, water intake typically equals water output, primarily by the kidneys. When huge amounts of water are consumed very quickly,
the kidneys sometimes can’t keep up, and thus cells swell. This swelling, especially in the brain, causes symptoms similar to those
associated with intoxication, and the condition is called water intoxication.

What makes cells swell? As you will learn in this chapter, the process of osmosis causes water to move from the side of a cell containing

more water to the side containing less water. In the case of water intoxication, first there is much more water outside the cells than inside the

cells. Then, when this water moves into the cells to achieve equilibrium, the cells swell and may literally explode and die.

This chapter also explores how other molecules move in and out of cells, basic properties of energy, and the functions of enzymes. Only

after understanding these principles will you appreciate the processes of photosynthesis and cellular respiration. These processes are vital

for maintaining life and will be described in Chapters 6 and 7.

5.1 What Is Energy?

Energy is defined as the capacity to do work—to make things happen. Without a source of energy, we humans would not be here on Earth—nor would
any other living thing. The biosphere of which we are a part gets its energy from the sun, and thereafter, one form of energy is changed to another form
as life processes take place.

The two basic forms of energy are potential energy and kinetic energy. Potential energy is stored energy, and kinetic energy is the energy of

motion. Potential energy is constantly being converted to kinetic energy, and vice versa. Let’s look at the example in Figure 5.1. The food a
cross-country skier has for breakfast contains chemical energy, which is a form of potential energy. When our skier hits the slopes, she may have to
ascend a hill. During her climb, the potential energy of food is converted to the kinetic energy of motion, a type of mechanical energy. Once she reaches
the hilltop, kinetic energy has been converted to the potential energy of location. As she skis down the hill, this potential energy is converted to the
kinetic energy of motion again. But with each conversion, some energy is lost as heat.

Measuring Energy

Chemists use a unit of measurement called the joule to measure energy, but it is common to measure food energy in terms of calories. A calorie is the
amount of heat required to raise the temperature of 1 gram of water by one degree Celsius. This isn’t much energy, so the caloric value of food is listed
in nutrition labels and in diet charts in terms of kilocalories (1,000 calories). In this text, we will use Calorie to mean 1,000 calories.

Two Energy Laws

Two energy laws govern energy flow and help us understand the principles of energy conversion. The first law, called the law of conservation of energy,
tells us: Energy cannot be created or destroyed, but it can be changed from one form to another. Relating the law to our previous example, we know that
the skier had to acquire energy by eating food before she could climb the hill, and that energy conversions occurred before she reached the bottom of the
hill again.

The second energy law tells us: Energy cannot be changed from one form to another without a loss of usable energy. Many forms of energy are

usable, such as the energy of the sun, food, and ATP. Heat is diffuse energy and the least usable form. Every energy conversion results in a loss of
usable energy in the form of heat. For example, no doubt the skier worked up a sweat and gave off heat as she climbed the hill. This heat represents
a loss of usable energy.

Entropy

The second energy law can be stated another way: Every energy transformation leads to an increase in the amount of disorganization or disorder. The term
entropy refers to the relative amount of disorganization. The only way to maintain or bring about order is to add more energy to a system. To take an
example from your own experience, you know that a tidy room is more organized and less stable than a messy room, which is disorganized and more stable
(Fig. 5.2a). In other words, your room is much more likely to stay messy than it is to stay tidy. Why? Unless you continually add energy to keep your room
organized and neat, it will inevitably become less organized and messy.

Because our universe is a closed system in which energy cannot be created, all energy transformations, including those in cells, increase the total

entropy of the universe. Figure 5.2b shows a process that occurs in cells because it proceeds from a more ordered state to a more disordered state. Just as a
tidy room tends to become messy, hydrogen ions (H

1

) that have accumulated on one side of a membrane tend to move to the other side unless they are

prevented from doing so by the addition of energy. Why? Because when H

•

ions are distributed randomly on either side of the membrane, no additional

energy is needed to keep them that way, and the entropy, or disorder, of their arrangement has increased. The result is a more stable arrangement of H

•

ions.

What about reactions in cells that apparently proceed from disorder to order? For example, we know that plant cells can make glucose out of

carbon dioxide and water. How do they do it? Energy provided by the sun allows plants to make glucose, which is a highly organized molecule. Even this
process, however, involves a loss of some potential energy. When light energy is converted to chemical energy in plant cells, some of the sun’s energy is
always lost as heat. In other words, the organization of a cell has a constant energy cost that results in an increase in the entropy of the universe.

5.2 ATP: Energy for Cells

ATP (adenosine triphosphate) is the energy currency of cells. Just as you use coins to purchase all sorts of products, a cell uses ATP to carry out nearly
all of its activities, including synthesizing proteins, transporting ions across the plasma membranes, and causing organelles and cilia to move.

Structure of ATP

ATP is a nucleotide, the type of molecule that serves as a monomer for the construction of DNA and RNA. ATP’s name, adenosine triphosphate, means
that it contains the sugar ribose, the nitrogen-containing base adenine, and three phosphate groups (Fig. 5.3). The three phosphate groups are negatively
charged and repel one another. It takes energy to overcome their repulsion, and thus these phosphate groups make the molecule unstable. ATP easily
loses the last phosphate group because the breakdown products, ADP (adenosine diphosphate) and a separate phosphate group symbolized as

s

P , are

more stable than ATP. This reaction is written as: ATP

ADP •

s

P . ADP can also lose a phosphate group to become AMP (adenosine

monophosphate).

Use and Production of ATP

The continual breakdown and regeneration of ATP is known as the ATP cycle (Fig. 5.4). ATP stores energy for only a short period of time before it is
used in a reaction that requires energy. Then ATP is rebuilt from ADP •

s

P . Each ATP molecule undergoes about 10,000 cycles of synthesis and

breakdown every day. Our bodies use some 40 kg (about 88 lb) of ATP daily, and the amount on hand at any one moment is sufficiently high to meet
only current metabolic needs.

ATP’s instability, the very feature that makes it an effective energy donor, keeps it from being an energy storage molecule. Instead, the many H±C

bonds of carbohydrates and fats make them the energy storage molecules of choice. Their energy is extracted during cellular respiration and used to
rebuild ATP, mostly within mitochondria. You will learn in Chapter 7 that the breakdown of one molecule of glucose permits the buildup of some 38
molecules of ATP. During cellular respiration, only 39% of the potential energy of glucose is converted to the potential energy of ATP; the rest is lost as
heat.

The production of ATP is still worthwhile for the cell for the following reasons:

1. ATP is suitable for use in many different types of cellular reactions.
2. When ATP becomes ADP •

s

P , the amount of energy released is more than the amount needed for a biological purpose, but not overly wasteful.

3. The structure of ATP allows its breakdown to be coupled to an energy-requiring reaction, as described next.

Coupled Reactions

Coupled reactions are reactions that occur in the same place, at the same time, and in such a way that an energy-releasing reaction can drive an
energy-requiring reaction. Usually the energy-releasing reaction is ATP breakdown. Because the cleavage of ATP’s phosphate group releases more
energy than the amount consumed by the energy-requiring reaction, entropy has increased and both reactions will proceed. The simplest way to represent
a coupled reaction is like this:

This reaction tells you that coupling occurs, but it does not show how coupling is achieved. A cell has two main ways to couple ATP breakdown

to an energy-requiring reaction: ATP is used to energize a reactant or to change its shape. Both are often achieved by transferring a phosphate group to
the reactant. For example, when polypeptide synthesis occurs at a ribosome, an enzyme transfers a phosphate group from ATP to each amino acid in
turn, and this transfer activates the amino acid, causing it to bond with another amino acid.

Figure 5.5 shows how ATP breakdown provides the necessary energy for muscular contraction. During muscle contraction, myosin filaments pull

actin filaments to the center of the cell, and the muscle shortens. First, myosin combines with ATP, and only then does ATP break down to ADP •

s

P

. The release of ADP •

s

P from the molecule causes myosin to change shape and pull on the actin filament.

The Flow of Energy

In the biosphere, the activities of chloroplasts and mitochondria enable energy to flow from the sun through all living things. During photosynthesis, the
chloroplasts in plants capture solar energy and use it to convert water and carbon dioxide into carbohydrates, which serve as food for themselves and for
other organisms. During cellular respiration, mitochondria complete the breakdown of carbohydrates and use the released energy to build ATP
molecules.

Notice in Figure 5.6 that cellular respiration requires oxygen and produces carbon dioxide and water, the very molecules taken up by chloroplasts.

It is actually the cycling of molecules between chloroplasts and mitochondria that allows a flow of energy from the sun through all living things. This
flow of energy maintains the levels of biological organization from molecules to organisms to ecosystems. In keeping with the energy laws, useful
energy is lost with each chemical transformation, and eventually the solar energy captured by plants is lost in the form of heat. In this way, living things
are dependent upon an input of solar energy.

Human beings are also involved in the cycling of molecules between plants and animals and in the flow of energy from the sun. We inhale oxygen

and eat plants and their stored carbohydrates, or we eat other animals that have eaten plants. Oxygen and nutrient molecules enter our mitochondria,
which produce ATP and release carbon dioxide and water. Without a supply of energy-rich foods, we could not produce the ATP molecules needed to
maintain our bodies and carry on activities (Fig. 5.7).

5.3 Metabolic Pathways and Enzymes

Reactions do not occur haphazardly in cells; they are usually part of a metabolic pathway, a series of linked reactions. Metabolic pathways begin with
a particular reactant and terminate with an end product. In the pathway, one reaction leads to the next reaction, which leads to the next reaction, and so
forth in an organized, highly structured manner. This arrangement makes it possible for one pathway to lead to others, and for metabolic energy to be
captured and utilized in small increments rather than all at once.

A metabolic pathway can be represented by the following diagram:

E

1

E

2

E

3

E

4

E

5

E

6

A• •B• •C• •D• •E• • F• •G

In this diagram, the letters A–F are reactants and the letters B–G are products. In other words, the product from the previous reaction becomes the
reactant of the next reaction. The letters E

1

–E

6

represent enzymes.

An enzyme is a protein molecule that functions as an organic catalyst to speed a chemical reaction. Enzymes can only speed reactions that are

possible to begin with. In the cell, an enzyme is analogous to a mutual friend who causes two people to meet and interact, because an enzyme brings
together particular molecules and causes them to react with one another.

The reactants in an enzymatic reaction are called the substrates for that enzyme. In the first reaction, A is the substrate for E

1

, and B is the

product. Now B becomes the substrate for E

2

, and C is the product. This process continues until the final product (G) forms.

Energy of Activation

Molecules frequently do not react with one another unless they are activated in some way. In t he lab, for example, in the absence of an enzyme,
activation is very often achieved by heating a mixture to increase the number of effective collisions between molecules. The energy needed to
cause molecules to react with one another is called the energy of activation (E

a

). Figure 5.8 compares the E

a

without an enzyme to the E

a

with an

enzyme, illustrating that enzymes lower the amount of energy required for a reaction to occur. Enzymes lower the energy of activation by bringing
the substrates into contact and even by participating in the reaction at times.

An Enzyme’s Active Site

In most instances, only one small part of the enzyme, called the active site, accommodates the substrate(s). At the active site, the substrate fits into the
enzyme seemingly like a key fits a lock. However, the active site undergoes a slight change in shape in order to accommodate the substrate(s). This is
called the induced fit model because the enzyme is induced to undergo a slight alteration to achieve optimum fit (Fig. 5.9).

The change in the shape of the active site facilitates the reaction that next occurs. After the reaction has been completed, the product(s) is released,

and the active site returns to its original state, ready to bind to another substrate molecule. Only a small amount of each enzyme is actually needed in a
cell because enzymes are not used up by the reaction.

Enzyme Inhibition

Enzyme inhibition occurs when an active enzyme is prevented from combining with its substrate. Enzyme inhibitors are often poisonous to certain
organisms. Cyanide, for example, is an inhibitor of the enzyme cytochrome c oxidase, which performs a vital function in all cells because it is involved in
making ATP. Penicillin, in contrast, blocks the active site of an enzyme unique to bacteria. Therefore, penicillin is a poison for bacteria but not for
humans.

The activity of almost every enzyme in a cell is regulated by feedback inhibition. In the simplest case, when a product is in abundance, it

competes with the substrate for the enzyme’s active site. As the product is used up, inhibition is reduced, and more product can be produced. In
this way, the concentration of the product always stays within a certain range.

Most metabolic pathways are regulated by more complex types of feedback inhibition (Fig. 5.10). In these instances, when the end product is

plentiful, it binds to a site other than the active site of the first enzyme in the pathway. This binding changes the shape of the active site, preventing the
enzyme from binding to its substrate. Without the activity of the first enzyme, the entire pathway shuts down.

5.4 Cell Transport

The plasma membrane regulates the passage of molecules into and out of the cell. This function is crucial because the life of the cell depends on
maintenance of its normal composition. The plasma membrane can carry out this function because it is differentially permeable, meaning that certain
substances can freely pass through the membrane while others are transported across. Basically, substances enter a cell in one of three ways: passive
transport, active transport, and bulk transport. Although there are different types of passive transport, in all cases substances move from a higher to a

lower concentration, and no energy is required. Active transport moves substances against a concentration gradient and requires energy. Bulk transport
requires energy, but movement of the large substances involved is independent of concentration gradients.

Passive Transport: No Energy Required

Actually, no membrane is required for simple diffusion. During simple diffusion, molecules move down their concentration gradient until
equilibrium is achieved and they are distributed equally. Simple diffusion occurs because molecules are in motion, but i t is a passive form of
transport because a cell does not need to expend energy for it to happen. Small, noncharged molecules, such as oxygen, carbon dioxide, glycerol,
alcohol, and water, are able to slip between the phospholipid molecules making up the plasma membrane. Therefore, these molecules can diffuse
across the membrane.

Figure 5.11 demonstrates simple diffusion. Water is present on two sides of a membrane, and dye is added to one side. The dye particles move

in various directions, but the net movement is toward the opposite side of the membrane. Eventually, the dye is dispersed, with no net movement of dye
in either direction. A solution contains both a solute and a solvent. In this case, the dye is called the solute, and the water is called the solvent. Solutes
are usually solids, and solvents are usually liquids.

Dissolved gases can diffuse through the phospholipid bilayer, and indeed this is the mechanism by which oxygen enters cells and carbon dioxide

exits them. Also, oxygen enters blood from the air sacs of the lungs, and carbon dioxide moves in the opposite direction by diffusion.

Ions and polar molecules, such as glucose and amino acids, are often assisted across the plasma membrane by transport proteins. This process is

called facilitated diffusion. Even those proteins that simply provide a channel for passage are usually specific to the solute. In these cases, the transport
proteins most likely undergo a change in shape as the solute enters the cell.

Osmosis

Diffusion of water across a differentially permeable membrane is called osmosis. To illustrate osmosis, a tube containing a 5% salt solution and covered
at one end by a membrane is placed in a beaker that contains water only (Fig. 5.12a). The beaker has a higher concentration of water molecules than the
tube does because the tube also contains a solute. Water can cross the membrane, but the solute cannot. Therefore, there will be a net movement of water
across the membrane from the beaker to the inside of the tube. Theoretically, the solution inside the tube will rise until there is an equal concentration of
water on both sides of the membrane (Fig. 5.12b,c). What would happen if the beaker contained a 2% salt solution? Water would still diffuse into the
tube, because the tube at 5% salt would still contain a lower concentration of water molecules than the beaker at 2%.

The Effect of Osmosis on Cells

Osmosis can affect the size and shape of cells, as shown in Figure 5.13. In the laboratory, cells are normally placed in isotonic solutions (iso, same as)
in which the cell neither gains nor loses water—that is, the concentration of water is the same on both sides of the membrane. In medical settings, a 0.9%
solution of the salt sodium chloride (NaCl) is known to be isotonic to red blood cells; therefore, intravenous solutions usually have this concentration.•

Cells placed in a hypotonic solution (hypo, less than) gain water. Outside the cell, the concentration of solute is less, and the concentration

of water is greater, than inside the cell. Animal cells placed in a hypotonic solution expand and sometimes burst. The term lysis refers to disrupted
cells; hemolysis, then, is disrupted red blood cells.

When a plant cell is placed in a hypotonic solution, the large central vacuole gains water, and the plasma membrane pushes against the rigid cell

wall as the plant cell becomes turgid. The plant cell does not burst because the cell wall does not give way. Turgor pressure in plant cells is extremely
important in maintaining their erect position.

Cells placed in a hypertonic -solution (hyper, more than) lose water. Outside the cell, the concentration of solute is more, and the

concentration of water is less, than inside the cell. Animal cells placed in a hypertonic solution shrink. For example, meats are sometimes preserved
by being salted. Bacteria are killed not by the salt, but by the lack of water in the meat.

When a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water.

This is an example of -plasmolysis, shrinking of the cytoplasm due to osmosis. The dead plants you may see along a roadside could have died due to
exposure to a hypertonic solution during the winter, when salt was used on the road.

Active Transport: Energy Required

During active transport, molecules or ions move through the plasma membrane, accumulating on one side of the cell (Fig. 5.14). For example, iodine
collects in the cells of the thyroid gland; glucose is completely absorbed from the digestive tract by the cells lining the digestive tract; and sodium can be
almost completely withdrawn from urine by cells lining the kidney tubules. In these instances, molecules have moved against their concentration
gradients, a situation that requires both a transport protein and ATP. Therefore, cells involved in active transport, such as kidney cells, have a large
number of mitochondria near their plasma membranes to generate the ATP.

The passage of salt (NaCl) across a plasma membrane is of primary importance in cells because the salt causes water to move to that side of the

plasma membrane. First, sodium ions are actively transported across a membrane, and then chloride ions simply diffuse through channels that allow
their passage. Chloride ion channels malfunction in persons with cystic fibrosis, leading to the symptoms of this inherited disorder. Proteins engaged in

active transport are often called pumps. The sodium-potassium pump, vitally important to nerve conduction, undergoes a change in shape that allows it
to combine alternately with sodium ions and potassium ions.

Bulk Transport

Macromolecules, such as polypeptides, polysaccharides, or polynucleotides, are too large to be moved by transport proteins. Instead, vesicle formation
takes them into or out of a cell. For example, digestive enzymes and hormones use molecules transported out of the cell by exocytosis (Fig. 5.15a). In
cells that synthesize these products, secretory vesicles accumulate near the plasma membrane. These vesicles release their co ntents only when the
cell is stimulated by a signal received at the plasma membrane, a process called regulated secretion.

When cells take in substances by vesicle formation, the process is known as endocytosis (Fig. 5.15b). If the material taken in is large, such as

a food particle or another cell, the process is called phagocytosis. Phagocytosis is common in unicellular organisms, such as amoebas. It also occurs
in humans. Certain types of human white blood cells are amoeboid—that is, they are mobile like an amoeba, and are able to engulf debris such as
worn-out red blood cells or bacteria. When an endocytic vesicle fuses with a lysosome, digestion occurs. In Chapter 26, we will see that this process
is a necessary and preliminary step toward the development of immunity to bacterial diseases.

Pinocytosis occurs when vesicles form around a liquid or around very small particles. White blood cells, cells that line the kidney tubules and

the intestinal wall, and plant root cells all use pinocytosis to ingest substances.

During receptor-mediated endocytosis, receptors for particular substances are found at one location in the plasma membrane. This location

is called a coated pit because there is a layer of protein on its intracellular side (Fig. 5.16). Receptor -mediated endocytosis is selective and much
more efficient than ordinary pinocytosis. It is involved when substances move from maternal blood into fetal blood at the placenta, for example.

T H E T H E C H A P T E R I N R E V I E W

Summary

5.1

What Is Energy?

Potential energy can be converted to kinetic energy, and vice versa. Solar and chemical energy are forms of potential energy; mechanical energy is a
form of kinetic energy. Two energy laws hold true universally:

• Energy cannot be created or destroyed, but can be transferred or transformed.

• One form of energy cannot be completely converted into another form without a loss of usable energy. Therefore, the entropy of the universe is

increasing, and only a constant input of energy maintains the organization of living things.

5.2

ATP: Energy for Cells

Energy flows from the sun through chloroplasts and mitochondria, which produce ATP. Because ATP has three adjoining negative phosphate groups, it
is a high-energy molecule that tends to break down to ADP •

s

P

. ATP breakdown is coupled to various energy-requiring cellular reactions, including

protein synthesis, active transport, and muscle contraction. Cellular respiration provides the energy for the production of ATP. The following diagram
summarizes the ATP cycle:

5.3

Metabolic Pathways and Enzymes

A metabolic pathway is a series of reactions that proceed in an orderly, step-by-step manner. Each reaction requires an enzyme that is specific to its
substrate. Enzymes bring substrates together at an enzyme’s active site, and speed reactions by lowering the energy of activation. The activity of most
enzymes and metabolic pathways is regulated by feedback inhibition.

5.4

Cell Transport

The plasma membrane is differentially permeable; some substances can freely cross the membrane, and some must be assisted across if they are to
enter the cell.

Passive transport requires no metabolic energy and moves substances from a higher to a lower concentration.

• In simple diffusion, molecules move from an area of higher concentration to the area of lower concentration. Some molecules cross plasma

membranes by simple diffusion.

• In facilitated diffusion, molecules diffuse across a plasma membrane with the assistance of transport proteins.

• Osmosis is the simple diffusion of water across a membrane toward the area of lower concentration of water. Cells in an isotonic solution neither

gain nor lose water; cells in a hypotonic solution gain water; and cells in a hypertonic solution lose water.

• Some molecules diffuse with the assistance of membrane proteins. This process is termed facilitated diffusion.

Active transport requires metabolic energy (ATP) and moves substances from a lower to a higher concentration across a membrane.

• A transport protein acts as a pump that causes a substance to move against its concentration gradient. For example, the sodium-potassium pump

carries Na

1

to the outside of the cell and K

1

to the inside of the cell.

Bulk transport requires vesicle formation and metabolic energy. It occurs independent of concentration gradients.

• Exocytosis transports macromolecules out of a cell via vesicle formation and often results in secretion.

• Endocytosis transports macromolecules into a cell via vesicle formation.

• Phagocytosis is a type of endocytosis that transports large particles; pinocytosis transports liquids or small particles; and receptor-mediated

endocytosis is a form of pinocytosis, which makes use of receptor proteins in the plasma membrane.

The following diagram illustrates the different types of passive and active transport:

Thinking Scientifically

1. Every energy conversion results in the loss of energy as heat. However, some organisms also carry out reactions that give off energy as light. The

production of light by living organisms is called bioluminescence. Some marine algae, including dinoflagellates, bioluminesce when the water
surrounding them is agitated. One hypothesis suggests that when predators begin to feed on a population of dinoflagellates, the bioluminescence
attracts predators of the dinoflagellate-eaters. How would you test this hypothesis?

2. Cystic fibrosis is a genetic disorder caused by a defective membrane transport protein. The defective protein closes chloride channels in

membranes, preventing chloride from being exported out of cells. This results in the development of a thick mucus on the outer surfaces of cells.
This mucus clogs the ducts that carry digestive enzymes from the pancreas to the small intestine, clogs the airways in the lungs, and promotes lung
infections. How do you think the defective protein results in a thick, sticky mucus outside the cells, instead of a loose, fluid covering?

People with one copy of the gene for cystic fibrosis are less susceptible to cholera, an infection that causes severe diarrhea, than those with

normal channel proteins. Why might this be?

Testing Yourself

Choose the best answer for each question.

1. Which of the following is not a fundamental law of energy?

a. Energy cannot be created or destroyed.

b. Energy can be changed from one form to another.

c. Energy is lost when it is converted from one form to another.

d. Potential energy cannot be converted to kinetic energy.

2. As a result of energy transformation,

a. entropy increases.

b. entropy decreases.

c. heat energy is gained.

d. energy is lost in the form of heat.

e. Both a and d are correct.

3. Which of the following is not a component of ATP?

a. adenine

c. phosphate

b. glucose

d. ribose

4. ATP is a good source of energy for a cell because

a. it is versatile

—able to be used in many types of reactions.

b.

its breakdown is easily coupled with energy-requiring reactions.

c.

it provides just the right amount of energy for cellular reactions.

d. All of these are correct.

5. Compared to carbon dioxide and water, glucose

a. is less organized.

c. is less stable.

b. has less potential energy. d. exhibits higher entropy.

For questions 6

–9, match the items to those in the key. Each answer may include more than one item.

Key:

a. oxygen

b. carbon dioxide

c. water

d. carbohydrates

6. Consumed by cellular respiration.

7. Produced by cellular respiration.

8. Consumed by photosynthesis.

9. Produced by photosynthesis.

10. In the following figure, one curve illustrates the energy of activation required for a reaction in the presence of an enzyme, while the other illustrates

the energy of activation in the absence of an enzyme. Label each curve.

11. The current model for enzyme action is called the

a. induced fit model.

c. lock-and-key model.

b. activation model.

d. active substrate model.

For questions 12

–18, match the items to those in the key. Each answer may include more than one item.

Key:

a. simple diffusion

b. facilitated diffusion

c. osmosis

d. active transport

12. Movement of molecules from high concentration to low concentration.

13. Requires a membrane.

14. Requires energy input.

15. Requires a protein pump.

16. Mechanism by which oxygen enters cells.

17. Mechanism by which glucose enters cells.

18. Movement of water across a membrane.

19. Cells involved in active transport have a large number of _______ near their plasma membrane.

a. vacuoles

c. actin filaments

b. mitochondria

d. lysosomes

20. The movement of substances from maternal blood to fetal blood at the placenta occurs as a result of

a. receptor-mediated endocytosis.

b. substrate-mediated endocytosis.

c. receptor-mediated diffusion.

d. substrate-mediated diffusion.

21. Energy is used for

a. simple diffusion.

b. osmosis.

c. active transport.

d. None of these are correct.

22. Which of these are methods of endocytosis?

a. phagocytosis

b. pinocytosis

c. receptor-mediated endocytosis

d. All of these are correct.

23. Isotonic solutions have _______________ concentration(s) of water and _______________ concentration(s) of solute.

a. the same, the same

b. the same, different

c. different, the same

d. different, different

24. A coated pit is associated with

a. simple diffusion.

b. osmosis.

c. receptor-mediated endocytosis.

d. pinocytosis.

25. Pumps used in active transport are made of

a. sugars.

c. lipids.

b. proteins.

d. cholesterol.

26. Which of these is correct?

a.

Energy can be transformed into another type as shown by the production of ATP in mitochondria.

b.

Energy can be created as shown by the production of ATP in mitochondria.

c.

Energy transformations cannot occur; therefore, ecosystems need a constant input of solar energy.

d. Both a and c are correct.

27. ATP is a modified

a. protein.

c. nucleotide.

b. amino acid.

d. fat.

28. The amount of energy needed to get a chemical reaction started is known as the

a. starter energy.

b. energy of activation.

c. reaction energy.

d. product energy.

29. Entropy is a term used to indicate the relative amount of

a. organization.

b. disorganization.

c. enzyme action.

d. None of these are correct.

30. Enzymes catalyze reactions by

a. bringing the reactants together.

b. lowering the activation energy.

c. Both a and b are correct.

d. Neither a nor b is correct.

31. The active site of an enzyme

a. is identical to that of any other enzyme.

b. is the part of the enzyme where its substrate can fit.

c. can be used over and over again.

d.

is not affected by environmental factors such as pH and temperature.

e. Both b and c are correct.

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical Issue

Some organophosphates are agricultural pesticides that inhibit the enzyme acetylcholinesterase. This enzyme breaks down the nerve-excitation
chemical acetylcholine. When the enzyme is inhibited by organophosphates, the nervous system becomes overstimulated. Exposure to low levels of
organophosphates is believed to cause sleeplessness, anxiety, and depression. Because organophosphates interfere with the nervous system of
humans and other animals in agricultural areas, some people believe that their use should be banned. Others argue that they should not be eliminated
from use against insects because they would be replaced by pesticides for which modes of action are not as well characterized, making them even more
harmful. Currently, large-scale agricultural systems in the United States require the use of pesticides, so the complete elimination of pesticides is not an
option.

Do you believe organophosphates should be banned from use in agriculture? Or is the risk of exposure to organophosphates worth the benefits of

an abundant, inexpensive food supply?

Understanding the Terms

active site•76
active transport•78
calorie•70
coupled reaction•73
endocytosis•79
energy•70
energy laws•70
energy of activation•75
entropy•71
enzyme•75
enzyme inhibition•76
exocytosis•79

facilitated diffusion•77
feedback inhibition•76
heat•71
hypertonic solution•78
hypotonic solution•78
induced fit model•76
isotonic solution•78
kilocalorie•70
kinetic energy•70
metabolic pathway•75
osmosis•77
phagocytosis•79
pinocytosis•79
plasmolysis•78
potential energy•70
receptor-mediated
•endocytosis•79
simple diffusion•77
sodium-potassium pump•79
solute•77
solution•77
solvent•77
substrate•75

Match the terms to these definitions:

a. _______________

The tendency toward disorder.

b. _______________

Shrinking of the cytoplasm due to osmosis.

c. _______________

A series of linked reactions in a cell.

d. _______________

A protein that acts as an organic catalyst.

e. _______________

The reactant in an enzymatic pathway.

Body temperature is maintained by energy conversions in cells.
Drinking excessive amounts of water can have a toxic effect on cells.
Penicillin, a life-saving antibiotic, works by inhibiting the activity of certain enzymes.

Figure 5.1•Potential energy versus kinetic energy.
Food contains potential energy that a skier can convert to kinetic energy in order to climb a hill. Height is potential energy due to location, which the skier converts to kinetic energy as she descends the hill. With every conversion to
kinetic energy, some potential energy is lost as heat.

Figure 5.2•Cells and entropy.
The second energy law tells us that entropy (disorder) always increases. Therefore (a) a tidy room tends to become messy and disorganized, and (b) hydrogen ions (H

1

) on one side of a membrane tend to move to the other side so that

the ions are randomly distributed. Both processes result in a loss of potential energy and an increase in entropy.

Figure 5.3•ATP.

ATP, the universal energy currency of cells, is composed of adenosine and three phosphate groups (called a triphosphate).

Figure 5.4•The ATP cycle.
When ATP is used as an energy source, a phosphate group is removed by hydrolysis. ATP is regenerated in mitochondria as cellular respiration occurs.

Figure 5.5•Coupled reaction.
Muscle contraction occurs only when it is coupled to ATP breakdown. Myosin combines with ATP prior to its breakdown. Release of ADP •

s

P

causes myosin to change position and pull on an actin filament.

Figure 5.6•Flow of energy.
Chloroplasts convert solar energy to the chemical energy of nutrient molecules. Mitochondria convert this chemical energy to ATP molecules, which cells use to perform chemical, transport, and mechanical work.

Figure 5.7•Energy for life.
Our food provides all the energy our bodies need. The energy in food is measured in Calories. To drive a car requires about 61 Calories per hour. To run at a speed of 1 mile in 7 minutes takes about 865 Calories per hour. If you take in
more Calories than you burn, you will gain weight.

Figure 5.8•Energy of activation (E

a

).

Enzymes speed the rate of reactions because they lower the amount of energy required for the reactants to react. Even reactions like this one, in which the energy of the product is less than the energy of the reactant, speed up when an
enzyme is present.

Figure 5.9•Enzymatic action.
An enzyme has an active site where the substrates and enzyme fit together in such a way that the substrates are oriented to react. Following the reaction, the products are released, and the enzyme is free to act again.

Figure 5.10•Feedback inhibition.
This type of feedback inhibition occurs when the end product (P) of an active enzyme pathway is plentiful and binds to the first enzyme (E

1

) of the pathway at a site other than the active site. This changes the shape of the active site so

that the substrate (S) can no longer bind to the enzyme. Now the entire pathway becomes inactive.

Figure 5.11•Simple diffusion demonstration.
Simple diffusion is spontaneous, and no energy is required to bring it about. a. Red dye is added to water separated by a membrane. The dye molecules can pass through the membrane. The dye molecules move randomly about, but over
time the net movement of dye is toward the region of lower concentration. b. Eventually, the dye molecules are equally distributed throughout the container and there is no net movement of dye in either direction.

Figure 5.12•Osmosis demonstration.
a. A tube covered at the broad end by a differentially permeable membrane contains a 5% salt solution, and the beaker contains only water. The salt ions are unable to pass through freely, but the water molecules can pass through the
membrane. b. Therefore, there is a net movement of water toward the inside of the tube, where the percentage of water molecules is lower than the outside. c. The level of the solution rises in the tube because of the incoming water.
Figure 5.13•Osmosis in animal and plant cells.
In an isotonic solution, cells neither gain nor lose water. In a hypotonic solution, cells gain water. Red blood cells swell to bursting, and plant cells become turgid. In a hypertonic solution, cells lose water. Red blood cells shrivel, and plant
cell cytoplasm shrinks away from the cell wall.

Figure 5.14•Active transport.
During active transport, a transport protein uses energy to move a solute across the plasma membrane toward a higher concentration. Note that the transport protein changes shape during the process.

Figure 5.15•Bulk transport.
During exocytosis (a) and endocytosis (b), vesicle formation transports substances out of or into a cell, respectively.

Figure 5.16•Receptor-mediated endocytosis.
During receptor-mediated endocytosis, molecules first bind to specific receptor proteins that are in a coated pit.The vesicle that forms contains the molecules and their receptors.

Check Your Progress

1. Contrast potential energy with kinetic energy.
2. Explain how the second energy law is related to entropy.

Answers:•1. Potential energy is stored energy, while kinetic energy is energy of motion.•2. One way to express the second energy law is to say that every energy transformation increases disorder. Entropy is the tendency toward disorder.

Check Your Progress

1. Explain why ATP is a good short-term energy storage molecule.
2. 
 Briefly explain the function of ATP in coupled reactions.

Answers:•1. ATP holds energy but easily gives it up because the last phosphate group is easily lost, releasing energy.•2. ATP can donate a phosphate to energize a compound for a reaction. Alternatively, it causes a molecule to change its
shape.

Check Your Progress

Explain why reactions in a cell are usually part of a metabolic pathway.

Answer:•Each reaction produces a product that can be used for another reaction. Therefore, each reaction can lead to others, and energy is used in small increments.

Check Your Progress

Explain how an enzyme facilitates a reaction.

Answer:•The active site of the enzyme undergoes a slight change in shape, allowing it to fit together with the substrate. That change in shape allows the reaction to occur with a lower energy of activation.

Check Your Progress

1. Compare and contrast simple diffusion with facilitated diffusion.
2. 
 Describe the relationship between a solute, a solvent, and a solution.

Answers:•1. Both types of diffusion move molecules from high to low concentration. Simple diffusion does not require a membrane or transport proteins, while facilitated diffusion does.•2. A solution is composed of a solute dissolved in
a solvent.

Check Your Progress

Compare and contrast exocytosis and endocytosis.

Answer:•Both use vesicles to transport materials across the plasma membrane. Molecules are transported out by exocytosis and in by endocytosis.