72

5C H A P T E R The Working Cell

Cellular respiration

ATP ATP

Membrane Structure and Function

(5.1–5.9)

The phospholipid and proteinstructure of cell membranes

enables their many importantfunctions.

Energy and the Cell(5.10–5.12)

A cell’s metabolic reactionstransform energy, producing

ATP, which drives cellularwork.

B IG IDEAS

How Enzymes Function(5.13–5.16)

Enzymes speed up a cell’schemical reactions and provideprecise control of metabolism.

73

Would you believe that this squid’s glowing blue lightsare a form of camouflage? Ocean predators often hunt

by looking up, searching for a silhouette of their prey abovethem. But can an animal hide its silhouette in the openocean? The answer is yes, if it turns on the lights. The fireflysquid (Watasenia scintillans) shown here has light-producingorgans called photophores, which emit a soft glow thatmatches the light filtering down from above. This counter-illumination masks the squid’s silhouette. It turns out thatmany marine invertebrates and fishes hide from predators by producing such light, a process called bioluminescence.

You may be familiar with bioluminescence if you’ve seen fire-flies. While such light production is fairly rare for land animals, itis quite common in the ocean. An estimated 90% of deep-seamarine life bioluminesce. For example, some microorganisms

light up when attacked, drawing the attention of larger predatorsthat may feed on their attackers. Some squids expel a cloud ofglowing material instead of ink to confuse predators. And a deep-sea anglerfish uses a glowing glob of bacteria on a lure hangingabove its huge mouth to attract both mates and prey.

The light these organisms produce comes from a chemicalreaction that converts chemical energy to visible light. Biolu-minescence is just one example of the multitude of energy con-versions that a cell can perform. Many of a cell’s reactions takeplace in organelles, such as those in the light-producing cells ofa squid. And the enzymes that control these reactions are oftenembedded in the membranes of the organelle. Indeed, every-thing that happens when a squid turns on the lights to hide hassome relation to the topics of this chapter: how working cellsuse membranes, energy, and enzymes.

CYTOPLASM

CYTOPLASM

Fibers of extracellular matrix (ECM)

Attachment to the cytoskeleton and extracellular matrix (ECM)

Signaltransduction

Transport

Intercellularjunctions

Cell-cell recognition

Enzymatic activity

Microfilaments of cytoskeleton

Phospholipid

Cholesterol

GlycoproteinATP

Signalingmolecule

Receptor

CHAPTER 5 The Working Cell74

function as receptors for chemical messengers (signaling mol-ecules) from other cells. The binding of a signaling moleculetriggers a change in the protein, which relays the message intothe cell, activating molecules that perform specific functions.This message-transfer process, called signal transduction, willbe described in more detail in Module 11.10.

Some membrane proteins are enzymes, which may begrouped in a membrane to carry out sequential steps of ametabolic pathway. Membrane glycoproteins may be involvedin cell-cell recognition. Their attached carbohydrates functionas identification tags that are recognized by membrane pro-teins of other cells. This recognition allows cells in an embryoto sort into tissues and enables cells of the immune system torecognize and reject foreign cells, such as infectious bacteria.Membrane proteins also participate in the intercellular junc-tions that attach adjacent cells (see Module 4.20).

A final critical function is in transport of substances acrossthe membrane. Membranes exhibit selective permeability;that is, they allow some substances to cross more easily thanothers. Many essential ions and molecules, such as glucose, re-quire transport proteins to enter or leave the cell.

Review the six different types of functions that proteins in aplasma membrane can perform.

?

The plasma membrane is the edge of life, the boundary thatencloses a living cell. In eukaryotic cells, internal membranespartition the cell into specialized compartments. Recall fromModules 4.2 and 4.19 that membranes are composed of a bi-layer of phospholipids with embedded and attached proteins.Biologists describe such a structure as a fluid mosaic.

In the cell, a membrane remains about as “fluid” as salad oil,with most of its components able to drift about like partygoersmoving through a crowded room. Double bonds in the unsatu-rated fatty acid tails of some phospholipids produce kinks thatprevent phospholipids from packing too tightly (see Module 3.8).In animal cell membranes, the steroid cholesterol helps stabi-lize the membrane at warm temperatures but also helps keepthe membrane fluid at lower temperatures.

A membrane is a “mosaic” in having diverse protein mol-ecules embedded in its fluid framework. The word mosaic canalso refer to the varied functions of these proteins. Different typesof cells have different membrane proteins, and the various mem-branes within a cell each contain a unique collection of proteins.

Figure 5.1, which diagrams the plasma membranes of twoadjacent cells, illustrates six major functions performed bymembrane proteins, represented by the purple oval structures.Some proteins help maintain cell shape and coordinate changesinside and outside the cell through their attachment to thecytoskeleton and extracellular matrix (ECM). Other proteins

Membrane Structure and Function5.1 Membranes are fluid mosaics of lipids and proteins with many functions

!Attachment to the cytoskeleton and ECM, signal transduction, enzymaticactivity, cell-cell recognition, intercellular joining, and transport

! Figure 5.1 Some functions of membrane proteins

75Membrane Structure and Function 75

5.2 Membranes form spontaneously, a critical step in the origin of lifePhospholipids, the key ingredients of biological membranes,were probably among the first organic molecules that formedfrom chemical reactions on early Earth (see Module 15.2). Theselipids could spontaneously self-assemble into simple mem-branes, as we can demonstrate in a test tube. When a mixture ofphospholipids and water is shaken, the phospholipids organizeinto bilayers surrounding water-filled bubbles (Figure 5.2). This

assembly requires neither genes nor other information beyondthe properties of the phospholipids themselves.

The formation of membrane-enclosed collections of mol-ecules was probably a critical step in the evolution of the firstcells. A membrane can enclose a solution that is different incomposition from its surroundings. A plasma membrane thatallows cells to regulate their chemical exchanges with the envi-ronment is a basic requirement for life. Indeed, all cells are en-closed by a plasma membrane that is similar in structure andfunction—illustrating the evolutionary unity of life.

This is a diagram of asection of one of themembrane sacs shown inFigure 5.2. Describe itsstructure.

?

E V O L U T I O NCONNECTION

!The phospholipids form a bilayer.The hydrophobic fatty acid tails clusterin the center, and the hydrophilicphosphate heads face the water onboth sides.

Water-filledbubble made ofphospholipids

Water

Water

5.3 Passive transport is diffusion across a membrane with no energy investmentMolecules vibrate and move randomly as a result of a type ofenergy called thermal motion (heat). One result of this motionis diffusion, the tendency for particles of any kind to spreadout evenly in an available space. How might diffusion affect themovement of substances into or out of a cell?

The figures to the right will help you to visualize diffusionacross a membrane. Figure 5.3A shows a solution of green dyeseparated from pure water by a membrane. Assume that thismembrane has microscopic pores through which dye moleculescan move. Thus, we say it is permeable to the dye. Althougheach molecule moves randomly, there will be a net movementfrom the side of the membrane where dye molecules are moreconcentrated to the side where they are less concentrated. Putanother way, the dye diffuses down its concentration gradient.Eventually, the solutions on both sides will have equal concen-trations of dye. At this dynamic equilibrium, molecules stillmove back and forth, but there is no net change in concentra-tion on either side of the membrane.

Figure 5.3B illustrates the important point that two or moresubstances diffuse independently of each other; that is, eachdiffuses down its own concentration gradient.

Because a cell does not have to do work when moleculesdiffuse across its membrane, such movement across a mem-brane is called passive transport. Much of the traffic acrosscell membranes occurs by diffusion. For example, diffusiondown concentration gradients is the sole means by which oxygen (O2), essential for metabolism, enters your cells andcarbon dioxide (CO2), a metabolic waste, passes out of them.

Both O2 and CO2 are small, nonpolar molecules that diffuse easily across the phospholipid bilayer of a membrane.But can ions and polar molecules also diffuse across the

hydrophobic interior of a membrane? They can if they aremoving down their concentration gradients and if they have transport proteins to help them cross.

Why is diffusion across a membrane called passive transport??

!The cell does not expend energy to transport substances that are diffusingdown their concentration gradients.

! Figure 5.3A Passive transport of one type of molecule

Molecules of dye Membrane

Pores

EquilibriumNet diffusionNet diffusion

Equilibrium

Equilibrium

Net diffusion

Net diffusion

Net diffusion

Net diffusion

! Figure 5.3B Passive transport of two types of molecules

! Figure 5.2 Artificial membrane-bounded sacs

CHAPTER 5 The Working Cell76

5.5 Water balance between cells and their surroundings is crucial to organismsBiologists use a special vocabulary to describe the relationshipbetween a cell and its surroundings with regard to the move-ment of water. The term tonicity refers to the ability of a sur-rounding solution to cause a cell to gain or lose water. Thetonicity of a solution mainly depends on its concentration ofsolutes that cannot cross the plasma membrane relative to theconcentration of solutes inside the cell.

Figure 5.5, on the facing page, illustrates how the principlesof osmosis and tonicity apply to cells. The effects of placing ananimal cell in solutions of various tonicities are shown in thetop row of the illustration; the effects of the same solutions on aplant cell are shown in the bottom row.

As shown in the top center of the figure, when an animalcell is immersed in a solution that is isotonic to the cell (iso,same, and tonos, tension), the cell’s volume remains constant.The solute concentration of a cell and its isotonic environmentare essentially equal, and the cell gains water at the same ratethat it loses it. In your body, red blood cells are transported inthe isotonic plasma of the blood. Intravenous (IV) fluids ad-ministered in hospitals must also be isotonic to blood cells. Thebody cells of most animals are bathed in an extracellular fluid

that is isotonic to the cells. And seawater is isotonic to the cellsof many marine animals, such as sea stars and crabs.

The upper left of the figure shows what happens when ananimal cell is placed in a hypotonic solution (hypo, below), asolution with a solute concentration lower than that of the cell.(Can you figure out in which direction osmosis will occur?Where are there more free water molecules available to move?)The cell gains water, swells, and may burst (lyse) like an over-filled balloon. The upper right shows the opposite case—an animal cell placed in a hypertonic solution (hyper, above), asolution with a higher solute concentration. The cell shrivelsand can die from water loss.

For an animal to survive in a hypotonic or hypertonic envi-ronment, it must have a way to prevent excessive uptake or ex-cessive loss of water. The control of water balance is calledosmoregulation. For example, a freshwater fish, which lives ina hypotonic environment, has kidneys and gills that work con-stantly to prevent an excessive buildup of water in the body.(We will discuss osmoregulation further in Module 25.4.)

Water balance issues are somewhat different for the cells of plants, prokaryotes, and fungi because of their cell walls.

5.4 Osmosis is the diffusion of water across a membraneOne of the most important substances that crosses membranesby passive transport is water. In the next module, we considerthe critical balance of water between a cell and its environment.But first let’s explore a physical model of the diffusion of wateracross a selectively permeable membrane, a process calledosmosis. Remember that a selectively permeable membrane al-lows some substances to cross more easily than others.

The top of Figure 5.4 shows what happens if a membranepermeable to water but not to a solute (such as glucose) sepa-rates two solutions with different concentrations of solute.(A solute is a substance that dissolves in a liquid solvent, pro-ducing a solution.) The solution on the right side initially has ahigher concentration of solute than that on the left. As you cansee, water crosses the membrane until the solute concentra-tions are equal on both sides.

In the close-up view at the bottom of Figure 5.4, you can seewhat happens at the molecular level. Polar water moleculescluster around hydrophilic (water-loving) solute molecules.The effect is that on the right side, there are fewer water mol-ecules available to cross the membrane. The less concentratedsolution on the left, with fewer solute molecules, has morewater molecules free to move. There is a net movement ofwater down its own concentration gradient, from the solutionwith more free water molecules (and lower solute concentra-tion) to that with fewer free water molecules (and higher soluteconcentration). The result of this water movement is the differ-ence in water levels you see at the top right of Figure 5.4.

Let’s now apply to living cells what we have learned aboutosmosis in artificial systems.

Indicate the direction of net water movement between two so-lutions—a 0.5% sucrose solution and a 2% sucrose solution—separated by a membrane not permeable to sucrose.

?!From the 0.5% sucrose solution (lower solute concentration) to the 2% sucrosesolution (higher solute concentration)

! Figure 5.4 Osmosis, the diffusion of water across a membrane

Watermolecule

Solute molecule with cluster of water molecules

Higherconcentration

of solute

Equalconcentrations

of solute

Lowerconcentration

of solute

H2O

Selectivelypermeablemembrane

Solutemolecule

Osmosis

77Membrane Structure and Function

5.6 Transport proteins can facilitate diffusion across membranes

As shown in the bottom center ofFigure 5.5, a plant cell immersed inan isotonic solution is flaccid(limp). In contrast, a plant cell isturgid (very firm), which is thehealthy state for most plant cells, ina hypotonic environment (bottomleft). To become turgid, a plant cellneeds a net inflow of water. Al-though the somewhat elastic cellwall expands a bit, the pressure itexerts prevents the cell from takingin too much water and bursting, asan animal cell would in a hypo-tonic environment. Plants that arenot woody, such as most house-plants, depend on their turgid cellsfor mechanical support.

In a hypertonic environment(bottom right), a plant cell is nobetter off than an animal cell. Asa plant cell loses water, it shrivels, and its plasma membranepulls away from the cell wall. This process, called plasmoly-sis, causes the plant to wilt and can be lethal to the cell andthe plant. The walled cells of bacteria and fungi alsoplasmolyze in hypertonic environments. Thus, meats andother foods can be preserved with concentrated salt solu-tions because the cells of food-spoiling bacteria or fungibecome plasmolyzed and eventually die.

Isotonic solution

Animalcell

Plantcell

Hypotonic solution Hypertonic solution

H2O

H2O

H2O

H2O

H2OH2O

H2O Plasmamembrane

Normal Lysed Shriveled

FlaccidTurgid(normal)

Shriveled(plasmolyzed)

Recall that nonpolar, hydrophobic molecules can dissolve inthe lipid bilayer of a membrane and cross it with ease. Polaror charged substances, meanwhile, can move across a mem-brane with the help of specific transport proteins in a processcalled facilitated diffusion. Without the transport protein,the substance cannot cross the membrane or it diffuses acrossit too slowly to be useful to the cell. Facilitated diffusion is atype of passive transport because it does not require energy.As in all passive transport, the driving force is the concentra-tion gradient.

Figure 5.6 shows a common type of transport protein,which provides a hydrophilic channel that some molecules orions use as a tunnel through the membrane. Another type oftransport protein binds its passenger, changes shape, and re-leases its passenger on the other side. In both cases, the trans-port protein is specific for the substance it helps move acrossthe membrane. The greater the number of transport proteinsfor a particular solute in a membrane, the faster the solute’s rateof diffusion across the membrane.

Substances that use facilitated diffusion for crossing cellmembranes include a number of sugars, amino acids, ions—and even water. The water molecule is very small, but becauseit is polar (see Module 2.6), its diffusion through a mem-brane’s hydrophobic interior is relatively slow. The very rapid

diffusion of water into and out of certain cells, such as plantcells, kidney cells, and red blood cells, is made possible by aprotein channel called an aquaporin. A single aquaporinallows the entry or exit of up to 3 billion water molecules persecond—a tremendous increase in water transport oversimple diffusion.

How do transport proteins contribute to a membrane’sselective permeability?

?

!Because they are specific for the solutes they transport, the numbers and kindsof transport proteins affect a membrane’s permeability to various solutes.

! Figure 5.6Transport proteinproviding a channelfor the diffusion of aspecific soluteacross a membrane

Solutemolecule

Transportprotein

In the next module, we explore how water and other polarsolutes move across cell membranes.

Explain the function of the contractile vacuoles in the fresh-water Paramecium shown in Figure 4.11A in terms of whatyou have just learned about water balance in cells.

?

!The pond water in which Parameciumlives is hypotonic to the cell. Thecontractile vacuoles expel the water that constantly enters the cell by osmosis.

" Figure 5.5 How animal and plant cells react to changes in tonicity. (Deepening shades of blue reflectincreasing concentrations of solutes in the surrounding solutions.)

S C I E N T I F I CD I S C O V E R Y

CHAPTER 5 The Working Cell78

5.7 Research on another membrane protein led to the discovery of aquaporins

Peter Agre received the 2003 Nobel Prize in Chemistry for his discovery of aquaporins. In a recent interview, Dr. Agre described his research that led to this discovery:

I’m a blood specialist (hematologist), and my particular interest hasbeen proteins found in the plasma membrane of red blood cells.When I joined the faculty at the John Hopkins School of Medicine,I began to study the Rh blood antigens. Rh is of medical importancebecause of Rh incompatibility, which occurs when Rh-negativemothers have Rh-positive babies. Membrane-spanningproteins are really messy to work with. But weworked out a method to isolate the Rh protein.Our sample seemed to consist of two pro-teins, but we were sure that the smallerone was just a breakdown product of thelarger one. We were completely wrong.

Using antibodies we made to thesmaller protein, we showed it to be one ofthe most abundant proteins in red cell mem-branes—200,000 copies per cell!—and evenmore abundant in certain kidney cells.

We asked Dr. Agre why cells have aquaporins.

Not all cells do. Before our discovery, however, manyphysiologists thought that diffusion was enoughfor getting water into and out of all cells. Others

said this couldn’t be enough, especially for cells whose water perme-ability needs to be very high or regulated. For example, our kidneysmust filter and reabsorb many liters of water every day. . . . Peoplewhose kidney cells have defective aquaporin molecules need to drink20 liters of water a day to prevent dehydration. In addition, some pa-tients make too much aquaporin, causing them to retain too muchfluid. Fluid retention in pregnant women is caused by the synthesis oftoo much aquaporin. Knowledge of aquaporins may in the futurecontribute to the solution of medical problems.

Figure 5.7 is an image taken from asimulation produced by computationalbiophysicists at the University of Illinois,

Urbana. Their model included four aqua-porin channels spanning a membrane.You can see a line of blue water mol-ecules flipping their way single filethrough the gold aquaporin. Thesimulation of this flipping movementallowed researchers to discover how

aquaporins selectively allow only watermolecules to pass through them.

5.8 Cells expend energy in the active transport of a soluteIn active transport, a cell must expend energy to move a soluteagainst its concentration gradient—that is, across a membranetoward the side where the solute is more concentrated. The en-ergy molecule ATP (described in more detail in Module 5.12)supplies the energy for most active transport.

Figure 5.8 shows a simple model of an active transport sys-tem that pumps a solute out of the cell against its concentrationgradient. ! The process begins when solute molecules on thecytoplasmic side of the plasma membrane attach to specificbinding sites on the transport protein. " ATP then transfers aphosphate group to the transport protein,# causing the protein to change shape insuch a way that the solute is released onthe other side of the membrane. $ Thephosphate group detaches, and the trans-port protein returns to its original shape.

Active transport allows a cell to main-tain internal concentrations of smallmolecules and ions that are different fromconcentrations in its surroundings. Forexample, the inside of an animal cell has ahigher concentration of potassium ions(K+) and a lower concentration of sodium

ions (Na+) than the solution outside the cell. The generation ofnerve signals depends on these concentration differences,which a transport protein called the sodium-potassium pumphelps maintain by shuttling Na+ and K+ against their concentra-tion gradients.

Cells actively transport Ca2+ out of the cell. Is calcium moreconcentrated inside or outside of the cell? Explain.

?

!Outside: Active transport moves calcium against its concentration gradient.

1 2 3 4Solute binding Phosphate attaching Transport Protein reversion

ADP

PATPPPProtein

changes shape.Solute

Transportprotein

Phosphatedetaches.

"! # $

!Kidney cells must reabsorb a large amount of water when producing urine.

Why are aquaporins important in kidney cells?

?

! Figure 5.8 Active transport of a solute across a membrane

! Figure 5.7 Aquaporin in action

79Membrane Structure and Function

!Exocytosis: When a transport vesicle fuses with the plasma membrane, itscontents are released and the vesicle membrane adds to the plasma membrane.

5.9 Exocytosis and endocytosis transport large molecules across membranesSo far, we’ve focused on how water and small solutes enter andleave cells. The story is different for large molecules.

A cell uses the process of exocytosis (from the Greek exo,outside, and kytos, cell) to export bulky materials such as pro-teins or polysaccharides. As you saw in Figure 4.12, a transportvesicle filled with macromolecules buds from the Golgi appara-tus and moves to the plasma membrane. Once there, the vesiclefuses with the plasma membrane, and the vesicle’s contentsspill out of the cell when the vesicle membrane becomes part ofthe plasma membrane. When we weep, for instance, cells inour tear glands use exocytosis to export a salty solution con-taining proteins. In another example, certain cells in the pan-creas manufacture the hormone insulin and secrete it into thebloodstream by exocytosis.

Endocytosis (endo, inside) is a transportprocess that is the opposite of exocytosis.In endocytosis, a cell takes in largemolecules. A depression in the plasmamembrane pinches in and forms a vesicleenclosing material that had been outsidethe cell.

Figure 5.9 shows three kinds of endocyto-sis. The top diagram illustrates phagocytosis,or “cellular eating.” A cell engulfs a particle bywrapping extensions called pseudopodiaaround it and packaging it within a membrane-enclosed sac large enough to becalled a vacuole. As described in Module4.10, the vacuole then fuses with a lysosome,whose hydrolytic enzymes digest the contentsof the vacuole. The micrograph on the topright shows an amoeba taking in a food par-ticle via phagocytosis.

The center diagram shows pinocytosis,or “cellular drinking.” The cell “gulps”droplets of fluid into tiny vesicles. Pinocyto-sis is not specific; it takes in any and allsolutes dissolved in the droplets. The micro-graph in the middle shows pinocytosis vesi-cles forming (arrows) in a cell lining a smallblood vessel.

In contrast to pinocytosis, receptor-mediated endocytosis is highly selective.Receptor proteins for specific molecules are embedded in regions of the membranethat are lined by a layer of coat proteins. The bottom diagram shows that the plasmamembrane has indented to form a coatedpit, whose receptor proteins have picked up particular molecules from the surround-ings. The coated pit then pinches closed toform a vesicle that carries the molecules intothe cytoplasm. The micrograph shows mate-rial bound to receptor proteins inside acoated pit.

Your cells use receptor-mediated endocytosis to take incholesterol from the blood for synthesis of membranes and as aprecursor for other steroids. Cholesterol circulates in the bloodin particles called low-density lipoproteins (LDLs). LDLs bindto receptor proteins and then enter cells by endocytosis. In hu-mans with the inherited disease familial hypercholesterolemia,LDL receptor proteins are defective and cholesterol accumu-lates to high levels in the blood, leading to atherosclerosis (see Modules 9.11 and 23.6).

As a cell grows, its plasma membrane expands. Does this in-volve endocytosis or exocytosis? Explain.

?

EXTRACELLULARFLUID

CYTOPLASM

“Food” or other particle

Pseudopodium

Foodvacuole

Plasmamembrane

Vesicle

ReceptorCoat protein

Coatedvesicle

CoatedpitSpecific

molecule

LM 1

00!

TEM

62,

000!

TEM

109,

000!

Coatedpit

Plasma membrane

Phagocytosis

Pinocytosis

Receptor-mediated endocytosis

Material boundto receptor proteins

Foodbeingingested

! Figure 5.9 Three kinds of endocytosis

CHAPTER 5 The Working Cell80

If energy cannot be destroyed, then why can’t organismssimply recycle their energy? It turns out that during every trans-fer or transformation, some energy becomes unusable—un-available to do work. In most energy transformations, someenergy is converted to heat, a disordered form of energy. Scien-tists use a quantity called entropy as a measure of disorder, orrandomness. The more randomly arranged a collection of mat-ter is, the greater its entropy. According to the second law ofthermodynamics, energy conversions increase the entropy(disorder) of the universe.

Figure 5.10 compares a car and a cell to show how energycan be transformed and how entropy increases as a result. Au-tomobile engines and living cells use the same basic process tomake the chemical energy of their fuel available for work. Theengine mixes oxygen with gasoline in an explosive chemical re-action that pushes the pistons, which eventually move thewheels. The waste products emitted from the exhaust pipe aremostly carbon dioxide and water, energy-poor, simple mol-ecules. Only about 25% of the chemical energy stored in gaso-line is converted to the kinetic energy of the car’s movement;the rest is lost as heat.

Cells also use oxygen in reactions that release energy fromfuel molecules. In the process called cellular respiration, the

The title of this chapter is “The Working Cell.” But just whattype of work does a cell do? You just learned that a cell activelytransports substances across membranes. The cell also buildsthose membranes and the proteins embedded in them. A cell isa miniature chemical factory in which thousands of reactionsoccur within a microscopic space. Some of these reactions re-lease energy; others require energy. To understand how the cellworks, you must have a basic knowledge of energy.

Forms of Energy We can define energy as the capacity tocause change or to perform work. There are two basic forms ofenergy: kinetic energy and potential energy. Kinetic energy isthe energy of motion. Moving objects can perform work bytransferring motion to other matter. For example, the movementof your legs can push bicycle pedals, turning the wheels andmoving you and your bike up a hill. Heat, or thermal energy, is atype of kinetic energy associated with the random movement ofatoms or molecules. Light, also a type of kinetic energy, can beharnessed to power photosynthesis.

Potential energy, the second main form of energy, is energythat matter possesses as a result of its location or structure. Waterbehind a dam and you on your bicycle at the top of a hill possesspotential energy. Molecules possess potential energy because ofthe arrangement of electrons in the bondsbetween their atoms. Chemical energy is thepotential energy available for release in achemical reaction. Chemical energy is themost important type of energy for living or-ganisms; it is the energy that can be trans-formed to power the work of the cell.

Energy Transformations Thermodynamicsis the study of energy transformations thatoccur in a collection of matter. Scientists usethe word system for the matter under studyand refer to the rest of the universe—every-thing outside the system—as the surroundings.A system can be an electric power plant, a sin-gle cell, or the entire planet. An organism is anopen system; that is, it exchanges both energyand matter with its surroundings.

The first law of thermodynamics, alsoknown as the law of energy conservation,states that the energy in the universe is con-stant. Energy can be transferred and trans-formed, but it cannot be created ordestroyed. A power plant does not create en-ergy; it merely converts it from one form(such as the energy stored in coal) to themore convenient form of electricity. A plantcell converts light energy to chemical en-ergy; it, too, is an energy transformer, not anenergy producer.

Energy and the Cell5.10 Cells transform energy as they perform work

Oxygen

Carbon dioxide

Water

Cellular respiration

Heatenergy

Energy for cellular work

!

Carbon dioxide

Water

!

Glucose

Oxygen

Gasoline

!

!

Waste productsFuel

Heatenergy

Kinetic energyof movement

Energy conversion

Combustion

Energy conversion in a car

Energy conversion in a cell

ATP ATP

! Figure 5.10 Energy transformations (with an increase in entropy) in a car and a cell

81Energy and the Cell

5.11 Chemical reactions either release or store energyChemical reactions are of two types: Either they release energyor they require an input of energy and store energy.

An exergonic reaction is a chemical reaction that releasesenergy (exergonic means “energy outward”). As shown inFigure 5.11A, an exergonic reaction begins with reactantswhose covalent bonds contain more energy than those in theproducts. The reaction releases to the surroundings an amountof energy equal to the difference in potential energy betweenthe reactants and the products.

As an example of an exergonic reaction, consider what hap-pens when wood burns. One of the major components ofwood is cellulose, a large energy-rich carbohydrate composedof many glucose monomers. The burning of wood releases theenergy of glucose as heat and light. Carbon dioxide and waterare the products of the reaction.

As you learned in Module 5.10, cells release energy fromfuel molecules in the process called cellular respiration. Burn-ing and cellular respiration are alike in being exergonic. Theydiffer in that burning is essentially a one-step process that re-leases all of a substance’s energy at once. Cellular respiration,on the other hand, involves many steps, each a separate chemi-cal reaction; you can think of it as a “slow burn.” Some of theenergy released from glucose by cellular respiration escapes asheat, but a substantial amount is converted to the chemical en-ergy of ATP. Cells use ATP as an immediate source of energy.

The other type of chemical reaction requires a net input ofenergy. Endergonic reactions yield products that are rich in po-tential energy (endergonic means “energy inward”). As shown inFigure 5.11B, an endergonic reaction starts out with reactantmolecules that contain relatively little potential energy. Energy isabsorbed from the surroundings as the reaction occurs, so theproducts of an endergonic reaction contain more chemical en-ergy than the reactants did. And as the graph shows, the amountof additional energy stored in the products equals the differencein potential energy between the reactants and the products.

Photosynthesis, the process by which plant cells make sugar,is an example of an endergonic process. Photosynthesis startswith energy-poor reactants (carbon dioxide and water mol-ecules) and, using energy absorbed from sunlight, produces energy-rich sugar molecules.

Every working cell in every organism carries out thousands ofexergonic and endergonic reactions. The total of an organism’s

chemical reactions is called metabolism (from the Greekmetabole, change). We can picture a cell’s metabolism as a roadmap of thousands of chemical reactions arranged as intersectingmetabolic pathways. A metabolic pathway is a series of chemi-cal reactions that either builds a complex molecule or breaksdown a complex molecule into simpler compounds. The “slowburn” of cellular respiration is an example of a metabolic path-way in which a sequence of reactions slowly releases the poten-tial energy stored in sugar.

All of an organism’s activities require energy, which is obtainedfrom sugar and other molecules by the exergonic reactions of cel-lular respiration. Cells then use that energy in endergonic reac-tions to make molecules and do the work of the cell. Energycoupling—the use of energy released from exergonic reactions todrive essential endergonic reactions—is a crucial ability of allcells. ATP molecules are the key to energy coupling. In the nextmodule, we explore the structure and function of ATP.

Cellular respiration is an exergonic process. Rememberingthat energy must be conserved, what becomes of the energyextracted from food during cellular respiration?

?

!Some of it is stored in ATP molecules; the rest is released as heat.

!Diffusion across a membrane results in equal concentrations of solute,which is a more disordered arrangement (higher entropy) than a highconcentration on one side and a low concentration on the other.

! Figure 5.11AExergonic reaction,energy released

Energy

Pote

ntia

l ene

rgy

of m

olec

ules

Amount ofenergy

released

Reactants

Products

Amount ofenergy

required

Reactants

Products

Energy

Pote

ntia

l ene

rgy

of m

olec

ules

! Figure 5.11BEndergonic reaction,energy required

chemical energy stored in organic molecules is converted to aform that the cell can use to perform work. Just like for the car,the waste products are mostly carbon dioxide and water. Cellsare more efficient than car engines, however, converting about34% of the chemical energy in their fuel to energy for cellularwork. The other 66% generates heat, which explains why vigor-ous exercise makes you so warm.

According to the second law of thermodynamics, energytransformations result in the universe becoming more disor-dered. How, then, can we account for biological order? A cellcreates intricate structures from less organized materials. Although this increase in order corresponds to a decrease in

entropy, it is accomplished at the expense of ordered forms of matter and energy taken in from the surroundings. As shownin Figure 5.10, cells extract the chemical energy of glucose andreturn disordered heat and lower-energy carbon dioxide andwater to the surroundings. In a thermodynamic sense, a cell isan island of low entropy in an increasingly random universe.

How does the second law of thermodynamics explain the dif-fusion of a solute across a membrane?

?

CHAPTER 5 The Working Cell82

!Exergonic processes phosphorylate ADP to form ATP. ATP transfers energyto endergonic processes by phosphorylating other molecules.

! Figure 5.12A The structure and hydrolysis of ATP.The reaction of ATP and water yields ADP, a phosphategroup, and energy.

5.12 ATP drives cellular work by coupling exergonic and endergonic reactionsATP powers nearly all forms of cellular work. The struc-ture of ATP, or adenosine triphosphate, is shown below inFigure 5.12A. The adenosine part of ATP consists of ade-nine, a nitrogenous base (see Module 3.15), and ribose, afive-carbon sugar. The triphosphate part is a chain of threephosphate groups (each symbolized by ). All three phos-phate groups are negatively charged (see Table 3.2). Theselike charges are crowded together, and their mutual repulsionmakes the triphosphate chain of ATP the chemical equiva-lent of a compressed spring.

As a result, the bonds connecting the phosphate groupsare unstable and can readily be broken by hydrolysis, theaddition of water. Notice in Figure 5.12A that when thebond to the third group breaks, a phosphate group leavesATP—which becomes ADP (adenosine diphosphate)—andenergy is released.

Thus, the hydrolysis of ATP is exergonic—it releasesenergy. How does the cell couple this reaction to an en-dergonic one? It usually does so by transferring a phos-phate group from ATP to some other molecule. Thisphosphate transfer is called phosphorylation, and mostcellular work depends on ATP energizing molecules by phosphorylating them.

There are three main types of cellular work: chemical,mechanical, and transport. As Figure 5.12B shows, ATPdrives all three types of work. In chemical work, the phospho-rylation of reactants provides energy to drive the endergonicsynthesis of products. In an example of mechanical work,the transfer of phosphate groups to special motor proteins inmuscle cells causes the proteins to change shape and pull onprotein filaments, in turn causing the cells to contract. Intransport work, as discussed in Module 5.8, ATP drives the ac-tive transport of solutes across a membrane against their con-centration gradient by phosphorylating transport proteins.

P

PPP

Energy!!

Adenine

PPP

Adenosine

Ribose

Phosphategroup

Adenosine TriphosphateATP:

ADP:

Hydrolysis H2O

Diphosphate

!

Protein filament moved

Reactants

Product

ADP ! P ADP ! P ADP ! P

P

P

P

Solute transportedMolecule formed

Chemical work Mechanical work Transport work

Membraneprotein

Solute

Motorprotein P

P

P

ATP ATP ATP

Phosph

oryl

atio

n Hydrolysis

ADP + P

Energy fromexergonicreactions

Energy for endergonicreactions

ATP

! Figure 5.12B How ATP powers cellular work

! Figure 5.12C The ATP cycle

Work can be sustained because ATP is a renewable resourcethat cells regenerate. Figure 5.12C, below, shows the ATP cycle.Each side of this cycle illustrates energy coupling. Energyreleased in exergonic reactions, such as the breakdown of glu-cose during cellular respiration, is used to regenerate ATP fromADP. In this endergonic (energy-storing) process, a phosphategroup is bonded to ADP. The hydrolysis of ATP releases energythat drives endergonic reactions. A cell at work uses ATPcontinuously, and the ATP cycle runs at an astonishing pace.In fact, a working muscle cell may consume and regenerate10 million ATP molecules each second.

But even with a constant supply of energy, few metabolicreactions would occur without the assistance of enzymes. Weexplore these biological catalysts next.

Explain how ATP transfers energy from exergonic to ender-gonic processes in the cell.

?

83How Enzymes Function

The answer to our dilemma lies in enzymes—molecules thatfunction as biological catalysts, increasing the rate of a reactionwithout being consumed by the reaction. Almost all enzymesare proteins, although some RNA molecules can also functionas enzymes. An enzyme speeds up a reaction by lowering the EAneeded for a reaction to begin. Figure 5.13 compares a reactionwithout (left) and with (right) an enzyme. Notice how mucheasier it is for the reactant to get over the activation energy bar-rier when an enzyme is involved. In the next module, we ex-plore how the structure of an enzyme enables it to lower theactivation energy, allowing a reaction to proceed.

The graph below illustrates the course of a reaction with andwithout an enzyme. Which curve represents the enzyme-catalyzed reaction? What energy changes are represented bythe lines labeled a, b, and c?

?

Your room gets messier; water flows downhill; sugar crystalsdissolve in your coffee. Ordered structures tend toward disor-der, and high-energy systems tend to change toward a morestable state of low energy. Proteins, DNA, carbohydrates,lipids—most of the complex molecules of your cells are rich inpotential energy. Why don’t these high-energy, ordered mol-ecules spontaneously break down into less ordered, lower-energy molecules? They remain intact for the same reasonthat wood doesn’t normally burst into flames or the gas in anautomobile’s gas tank doesn’t spontaneously explode.

There is an energy barrier that must be overcome before a chemical reaction can begin. Energy must be absorbed to contort or weaken bonds in reactant molecules so that they can break and new bonds can form. We call this the activation energy (abbreviated EA for energy of activation).We can think of EA as the amount of energy needed for reac-tant molecules to move “uphill” to a higher-energy, unstablestate so that the “downhill” part of a reaction can begin.

The energy barrier of EA protects the highly orderedmolecules of your cells from spontaneously breaking down.But now we have a dilemma. Life depends on countlesschemical reactions that constantly change a cell’s molecularmakeup. Most of the essential reactions of metabolismmust occur quickly and precisely for a cell to survive. Howcan the specific reactions that a cell requires get over thatenergy barrier?

One way to speed reactions is to add heat. Heat speeds upmolecules and agitates atoms so that bonds break more easilyand reactions can proceed. Certainly, adding a match to kin-dling will start a fire, and the firing of a spark plug ignitesgasoline in an engine. But heating a cell would speed up allchemical reactions, not just the necessary ones, and too muchheat would kill the cell.

How Enzymes Function5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers

!The red (lower) curve is the enzyme-catalyzed reaction. Line a is EAwithoutenzyme; b is EAwith enzyme; c is the change in energy between reactants andproducts, which is the same for both the catalyzed and uncatalyzed reactions.

! Figure 5.13 The effect of an enzyme in lowering EA

Reactant

Enzyme

Activationenergybarrierreduced byenzyme

Activationenergy barrier

Without enzyme With enzyme

Ener

gy

Products

Reactant

Products

Ener

gy

Ener

gy

Progress of the reaction

Reactants

Products

a

c

b

CHAPTER 5 The Working Cell84

! Figure 5.14 The catalytic cycle of an enzyme

5.14 A specific enzyme catalyzes each cellular reactionYou just learned that an enzyme catalyzes a reaction bylowering the EA barrier. How does it do that? With the aid ofan enzyme, the bonds in a reactant are contorted into thehigher-energy, unstable state from which the reaction canproceed. Without an enzyme, the energy of activation mightnever be reached. For example, a solution of sucrose (tablesugar) can sit for years at room temperature with no appre-ciable hydrolysis into its components glucose and fructose.But if we add a small amount of an enzyme to the solution, allthe sucrose will be hydrolyzed within seconds.

An enzyme is very selective in the reaction it catalyzes. Asa protein, an enzyme has a unique three-dimensional shape,and that shape determines the enzyme’s specificity. The spe-cific reactant that an enzyme acts on is called the enzyme’ssubstrate. A substrate fits into a region of the enzyme calledan active site. An active site is typically a pocket or groove onthe surface of the enzyme formed by only a few of the en-zyme’s amino acids. The rest of the protein maintains theshape of the active site. Enzymes are specific because theiractive sites fit only specific substrate molecules.

The Catalytic Cycle Figure 5.14 illustrates the catalytic cycleof an enzyme. Our example is the enzyme sucrase, which cat-alyzes the hydrolysis of sucrose to glucose and fructose. (Mostenzymes have names that end in -ase, and many are named fortheir substrate.) ! The enzyme starts with an empty activesite. " Sucrose enters the active site, attaching by weak bonds.The active site changes shape slightly, embracing the substratemore snugly, like a firm handshake. This induced fit may con-tort substrate bonds or place chemical groups of the aminoacids making up the active site in position to catalyze the reac-tion. (In reactions involving two or more reactants, the active

site holds the substrates in the proper orientation for a reactionto occur.) # The strained bond of sucrose reacts with water,and the substrate is converted (hydrolyzed) to the productsglucose and fructose. $ The enzyme releases the products andemerges unchanged from the reaction. Its active site is nowavailable for another substrate molecule, and another round ofthe cycle can begin. A single enzyme molecule may act onthousands or even millions of substrate molecules per second.

Optimal Conditions for Enzymes As with all proteins, an enzyme’s shape is central to its function, and this three-dimensional shape is affected by the environment. For everyenzyme, there are optimal conditions under which it is mosteffective. Temperature, for instance, affects molecular motion,and an enzyme’s optimal temperature produces the highestrate of contact between reactant molecules and the enzyme’sactive site. Higher temperatures denature the enzyme, alteringits specific shape and destroying its function. Most human en-zymes work best at 35–40°C (95–104°F), close to our normalbody temperature of 37°C. Prokaryotes that live in hot springs,however, contain enzymes with optimal temperatures of 70°C(158°F) or higher. You will learn in Module 12.12 how the en-zymes of these bacteria are used in a technique that rapidlyreplicates DNA sequences from small samples.

The optimal pH for most enzymes is near neutrality, in therange of 6–8. There are exceptions, however. Pepsin, a diges-tive enzyme in the stomach, works best at pH 2. Such an envi-ronment would denature most enzymes, but the structure ofpepsin is most stable and active in the acidic environment ofthe stomach.

Cofactors Many enzymes require nonprotein helpers calledcofactors, which bind to the active site and function in catal-ysis. The cofactors of some enzymes are inorganic, such asthe ions of zinc, iron, and copper. If the cofactor is an organicmolecule, it is called a coenzyme. Most vitamins are impor-tant in nutrition because they function as coenzymes or rawmaterials from which coenzymes are made. For example, folicacid is a coenzyme for a number of enzymes involved in thesynthesis of nucleic acids. And in Chapter 6, you will learnabout the roles of riboflavin and niacin as coenzymes of im-portant enzymes involved in cellular respiration.

Chemical chaos would result if all of a cell’s metabolicpathways were operating simultaneously. A cell must tightlycontrol when and where its various enzymes are active. Itdoes this either by switching on or off the genes that encodespecific enzymes (as you will learn in Chapter 11) or by regu-lating the activity of enzymes once they are made. We explorethis second mechanism in the next module.

Explain how an enzyme speeds up a specific reaction.?

"

!

#

$

Enzyme availablewith empty activesite

Products arereleased

Substrate isconverted toproducts

Substrate bindsto enzyme withinduced fit

H2O

Enzyme(sucrase)

Substrate(sucrose)

Glucose

Fructose

Active site

!An enzyme lowers the activation energy needed for a reaction when itsspecific substrate enters its active site. With an induced fit, the enzyme strainsbonds that need to break or positions substrates in an orientation that aids theconversion of reactants to products.

85How Enzymes Function

5.16 Many drugs, pesticides, and poisons are enzyme inhibitorsMany beneficial drugs act as enzyme inhibitors. Ibuprofen(Figure 5.16) is a common drug that inhibits an enzyme in-

volved in the production of prostaglandins—messenger molecules that increase thesensation of pain and inflammation. Otherdrugs that function as enzyme inhibitorsinclude some blood pressure medicinesand antidepressants. Many antibiotics workby inhibiting enzymes of disease-causing

bacteria. Penicillin, for example, blocksthe active site of an enzyme that manybacteria use in making cell walls. Pro-tease inhibitors are HIV drugs that tar-

get a key viral enzyme. And many cancerdrugs are inhibitors of enzymes that pro-mote cell division.

Humans have developed enzyme inhibitors as pesticides,and occasionally as deadly poisons for use in warfare. Poisonsoften attach to an enzyme by covalent bonds, making the inhi-bition irreversible. Poisons called nerve gases bind in the activesite of an enzyme vital to the transmission of nerve impulses.The inhibition of this enzyme leads to rapid paralysis of vitalfunctions and death. Pesticides such as malathion andparathion are toxic to insects (and dangerous to the people whoapply them) because they also irreversibly inhibit this enzyme.Interestingly, some drugs reversibly inhibit this same enzymeand are used in anesthesia and treatment of certain diseases.

What determines whether enzyme inhibition is reversible orirreversible?

?

CONNECTION

5.15 Enzyme inhibitors can regulate enzyme activity in a cellA chemical that interferes with an enzyme’s activity is calledan inhibitor. Scientists have learned a great deal about enzymefunction by studying the effects of these chemicals. Some in-hibitors resemble the enzyme’s normal substrate and competefor entry into the active site. As shown in the lower left ofFigure 5.15A, such a competitive inhibitor reduces anenzyme’s productivity by blocking substrate molecules fromentering the active site. Competitive inhibition can be over-come by increasing the concentration of the substrate, makingit more likely that a substrate molecule rather than an in-hibitor will be nearby when an active site becomes vacant.

In contrast, a noncompetitive inhibitor does not enter theactive site. Instead, it binds to the enzyme somewhere else, aplace called an allosteric site, and its binding changes the shapeof the enzyme so that the active site no longer fits the substrate(lower right of Figure 5.15A).

Although enzyme inhibition sounds harmful, cells useinhibitors as important regulators of cellular metabolism.Many of a cell’s chemical reactions are organized into meta-bolic pathways in which a molecule is altered in a series ofsteps, each catalyzed by a specific enzyme, to form a finalproduct. If a cell is producing more of that product than itneeds, the product may act as an inhibitor of one of the en-zymes early in the pathway. Figure 5.15B illustrates this sortof inhibition, called feedback inhibition. Because only weakinteractions bind inhibitor and enzyme, this inhibition is re-versible. When the product is used up by the cell, the enzymeis no longer inhibited and the pathway functions again.

In the next module, we explore some uses that people makeof enzyme inhibitors.

Explain an advantage of feedback inhibition to a cell.?

!It prevents the cell from wasting valuable resources by synthesizing more ofa particular product than is needed.

! Figure 5.15A How inhibitors interfere with substrate binding

Noncompetitiveinhibitor

Competitiveinhibitor

SubstrateActive site

Enzyme

Allosteric site

Normal binding of substrate

Enzyme inhibition

A B C DEnzyme 1 Enzyme 2 Enzyme 3

Reaction 1 Reaction 2 Reaction 3ProductStarting

molecule

Feedback inhibition!

! Figure 5.15B Feedback inhibition of a biosynthetic pathway in whichproduct D acts as an inhibitor of enzyme 1

!If the inhibitor binds to the enzyme with covalent bonds, the inhibition isusually irreversible. When weak chemical interactions bind inhibitor andenzyme, the inhibition is reversible.! Figure 5.16

Ibuprofen, anenzyme inhibitor

CHAPTER 5 The Working Cell86

Reviewing the ConceptsMembrane Structure and Function (5.1–5.9)5.1 Membranes are fluid mosaics of lipids and proteins withmany functions. The proteins embedded in a membrane’s phos-pholipid bilayer perform various functions.5.2 Membranes form spontaneously, a critical step in the originof life.

5.3 Passive transport is diffusion across a membrane with noenergy investment. Solutes diffuse across membranes down theirconcentration gradients.5.4 Osmosis is the diffusion of water across a membrane.

5.5 Water balance between cells and their surroundings is cru-cial to organisms. Cells shrink in a hypertonic solution and swellin a hypotonic solution. In isotonic solutions, animal cells are nor-mal, but plant cells are flaccid. 5.6 Transport proteins can facilitate diffusion across membranes.

5.7 Research on another membrane protein led to the discoveryof aquaporins. Aquaporins are water channels in cells with highwater transport needs.5.8 Cells expend energy in the active transport of a solute.

5.12 ATP drives cellularwork by coupling exergonicand endergonic reactions.The transfer of a phosphategroup from ATP is involvedin chemical, mechanical, andtransport work.

How Enzymes Function (5.13–5.16)5.13 Enzymes speed up the cell’s chemical reactions by loweringenergy barriers. Enzymes are protein catalysts that decrease theactivation energy (EA) needed to begin a reaction. 5.14 A specific enzyme catalyzes each cellular reaction. An en-zyme’s substrate binds specifically to its active site.5.15 Enzyme inhibitors can regulate enzyme activity in a cell. Acompetitive inhibitor competes with the substrate for the activesite. A noncompetitive inhibitor alters an enzyme’s function bychanging its shape. Feedback inhibition helps regulate metabolism. 5.16 Many drugs, pesticides, and poisons are enzyme inhibitors.

Connecting the Concepts1. Fill in the following concept map to review the processes by

which molecules move across membranes.

2. Label the parts of the following diagram illustrating thecatalytic cycle of an enzyme.

of

Molecules crosscell membranes

(a)

(b)

(d)(e)

passivetransport

by by

movingdown

movingagainst

diffusion

may be

of

uses

uses

polar moleculesand ions

(c)

requires

ATP

P

For Practice Quizzes, BioFlix, MP3 Tutors,and Activities, go to www.masteringbiology.com.

Higher solute concentration

Higher solute concentration

Diffusion Facilitateddiffusion

Osmosis

Higher free waterconcentration

Active transport(requires energy)

Lower free waterconcentration

Solute

Passive transport(requires no energy)

Lower solute concentrationLower solute concentration

ATPWater

ADP +

ATP cycle

P

Energy fromexergonicreactions

Energy for endergonicreactions

ATP

5.9 Exocytosis and endocytosis transport large molecules acrossmembranes. A vesicle may fuse with the membrane and expel itscontents (exocytosis), or the membrane may fold inward, enclos-ing material from the outside (endocytosis).

Energy and the Cell (5.10–5.12)5.10 Cells transform energy as they perform work. Kinetic energyis the energy of motion. Potential energy is energy stored in the lo-cation or structure of matter. Chemical energy is potential energyavailable for release in a chemical reaction. According to the lawsof thermodynamics, energy can change form but cannot be cre-ated or destroyed, and energy transformations increase disorder,or entropy, with some energy being lost as heat.5.11 Chemical reactions either release or store energy. Exergonicreactions release energy. Endergonic reactions require energy andyield products rich in potential energy. Metabolism encompassesall of a cell’s chemical reactions.

c.

e.

d.

b.

a.

f.

C H A P T E R 5 R E V I E W

87Chapter 5 Review

Testing Your KnowledgeMultiple Choice

3. Which best describes the structure of a cell membrane?a. proteins between two bilayers of phospholipidsb. proteins embedded in a bilayer of phospholipidsc. a bilayer of protein coating a layer of phospholipidsd. phospholipids between two layers of proteine. cholesterol embedded in a bilayer of phospholipids

4. Consider the following: chemical bonds in the gasoline in acar’s gas tank and the movement of the car along the road; abiker at the top of a hill and the ride he took to get there. Thefirst parts of these situations illustrate ______, and the secondparts illustrate ______.a. the first law of thermodynamics ... the second lawb. kinetic energy ... potential energyc. an exergonic reaction ... an endergonic reactiond. potential energy ... kinetic energye. the second law of thermodynamics ... the first law

5. A plant cell placed in distilled water will ______; an animal cellplaced in distilled water will ______.a. burst ... burstb. become flaccid ... shrivelc. become flaccid ... be normal in shaped. become turgid ... be normal in shapee. become turgid ... burst

6. The sodium concentration in a cell is 10 times less than theconcentration in the surrounding fluid. How can the cell movesodium out of the cell? (Explain.)a. passive transport d. osmosisb. diffusion e. any of these processesc. active transport

7. The synthesis of ATP from ADP and a. is an exergonic process.b. involves the hydrolysis of a phosphate bond.c. transfers a phosphate, priming a protein to do work.d. stores energy in a form that can drive cellular work.e. releases energy.

8. Facilitated diffusion across a membrane requires ______ andmoves a solute ______ its concentration gradient.a. transport proteins ... up (against)b. transport proteins ... downc. energy ... upd. energy and transport proteins ... upe. energy and transport proteins ... down

Describing, Comparing, and Explaining9. What are aquaporins? Where would you expect to find

them?10. How do the two laws of thermodynamics apply to living

organisms?11. What are the main types of cellular work? How does ATP

provide the energy for this work?12. Why is the barrier of the activation energy beneficial for

organic molecules? Explain how enzymes lower EA.13. How do the components and structure of cell membranes

relate to the functions of membranes?14. Sometimes inhibitors can be harmful to a cell; often they are

beneficial. Explain.

P

Applying the Concepts15. Explain how each of the following food preservation methods

would interfere with a microbe’s enzyme activity and ability tobreak down food: canning (heating), freezing, pickling (soak-ing in acetic acid), salting.

16. A biologist performed two series of experiments on lactase, theenzyme that hydrolyzes lactose to glucose and galactose. First,she made up 10% lactose solutions containing different con-centrations of enzyme and measured the rate at which galac-tose was produced (grams of galactose per minute). Results ofthese experiments are shown in Table A below. In the secondseries of experiments (Table B), she prepared 2% enzyme solu-tions containing different concentrations of lactose and againmeasured the rate of galactose production.

Table A: Rate and Enzyme ConcentrationLactose concentration 10% 10% 10% 10% 10%

Enzyme concentration 0% 1% 2% 4% 8%

Reaction rate 0 25 50 100 200

Table B: Rate and Substrate ConcentrationLactose concentration 0% 5% 10% 20% 30%

Enzyme concentration 2% 2% 2% 2% 2%

Reaction rate 0 25 50 65 65

a. Graph and explain the relationship between the reactionrate and the enzyme concentration.

b. Graph and explain the relationship between the reactionrate and the substrate concentration. How and why did theresults of the two experiments differ?

17. The following graph shows the rate of reaction for two differentenzymes: One is pepsin, a digestive enzyme found in the stom-ach; the other is trypsin, a digestive enzyme found in the intes-tine. As you may know, gastric juice in the stomach containshydrochloric acid. Which curve belongs to which enzyme?

A lysosome, a digestive organelle in a cell, has an internal pH ofaround 4.5. Draw a curve on the graph that you would predictfor a lysosomal enzyme, labeling its optimal pH.

18. Organophosphates (organic compounds containing phosphategroups) are commonly used as insecticides to improve cropyield. Organophosphates typically interfere with nerve signaltransmission by inhibiting the enzymes that degrade transmit-ter molecules. They affect humans and other vertebrates as wellas insects. Thus, the use of organophosphate pesticides posessome health risks. On the other hand, these molecules breakdown rapidly upon exposure to air and sunlight. As a con-sumer, what level of risk are you willing to accept in exchangefor an abundant and affordable food supply?

Answers to all questions can be found in Appendix 4.

pH0 1 2 3 4 5 6 7 8 9 10

Rate

of r

eact

ion