Biology Test 2 (Ch. 5, 6, & 7) DSM Questions

The overall efficiency of respiration (the percentage of the energy released that is saved in ATP) is approximately __________. a) 0.5% b) 2% c) 94% d) 100% e) 34%

34%

Which of the following would be least likely to pass through a plasma membrane without the help of a transport protein? a) dissolved gases such as oxygen or carbon dioxide b) a large polar molecule c) a large nonpolar molecule d) a small nonpolar molecule e) all of the listed substances would pass easily through the membrane.

A large polar molecule Both its size and the hydrophobic interior of the membrane would restrict it. Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic. They can therefore dissolve in the lipid bilayer of the membrane and cross it easily without the aid of membrane proteins. However, the hydrophobic interior of the membrane impedes the direct passage through the membrane of ions and polar molecules, which are hydrophilic. Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, a very small polar molecule, does not cross rapidly. A charged atom or molecule and its surrounding shell of water are even less likely to penetrate the hydrophobic interior of the membrane.

Which of the following correctly states the relationship between anabolic and catabolic pathways? a) Degradation of organic molecules by anabolic pathways provides the energy to drive catabolic pathways b) Anabolic pathways synthesize more complex organic molecules using the energy derived from catabolic pathways c) Energy derived from catabolic pathways is used to drive the breakdown of organic molecules in anabolic pathways d) Catabolic pathways produce usable cellular energy by synthesizing more complex organic molecules e) The flow of energy between catabolic and anabolic pathways is reversible

Anabolic pathways synthesize more complex organic molecules using the energy derived from catabolic pathways Some metabolic pathways release energy by breaking down complex molecules to simpler compounds. These degradative processes are called catabolic pathways, or breakdown pathways. A major pathway of catabolism is cellular respiration, in which the sugar glucose and other organic fuels are broken down in the presence of oxygen to carbon dioxide and water. Energy that was stored in the organic molecules becomes available to do the work of the cell, such as ciliary beating or membrane transport. Anabolic pathways, in contrast, consume energy to build complicated molecules from simpler ones; they are sometimes called biosynthetic pathways. Examples of anabolism are the synthesis of an amino acid from simpler molecules and the synthesis of a protein from amino acids. Catabolic and anabolic pathways are the "downhill" and "uphill" avenues of the metabolic landscape. Energy released from the downhill reactions of catabolic pathways can be stored and then used to drive the uphill reactions of anabolic pathways

Which of the following metabolic processes can occur without a net influx of energy from some other process? a) C6H12O6 + 6 O2 → 6 CO2 + 6 H2O b) ADP + Pi → ATP + H2O c) 6 CO2 + 6 H2O → C6H12O6 + 6 O2 d) amino acids → protein e) glucose + fructose → sucrose

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O The oxidation of glucose (C6H12O6) to carbon dioxide (CO2) and water (H2O) is a metabolic process that can occur without a net influx of energy from some other process because it is an exergonic chemical reaction that yields energy. In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants. In contrast, endergonic (nonspontaneous) reactions require an input of energy, which is why it takes the input of energy—photosynthesis in living systems—to produce glucose from carbon dioxide and water. The other listed reactions are anabolic pathways, consuming energy to build complicated molecules from simpler ones.

Which of the following molecules is most likely to passively diffuse across the plasma membrane? a) Carbon dioxide b) Glucose c) Sodium ion d) DNA e) Hemoglobin

Carbon dioxide Nonpolar molecules like carbon dioxide, hydrocarbons, and oxygen are hydrophobic and dissolve in the lipid bilayer, crossing the cell membrane easily without the aid of membrane proteins. Ions and polar molecules, however, are hydrophilic and cannot cross the hydrophobic interior of the membrane without a carrier.

In what way do the membranes of eukaryotic cells vary? a) some membranes have hydrophobic surfaces exposed to the cytoplasm, whereas others have hydrophilic surfaces facing the cytoplasm b) only certain membranes are constructed from amphipathic molecules c) only certain membranes of the cell are selectively permeable d) phospholipids are found only in certain membranes e) certain proteins are unique to the membrane of each cell type

Certain proteins are unique to the membrane of each cell type. Somewhat like a tile mosaic, a membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer. More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells, for example. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane's functions. Different types of cells contain different sets of membrane proteins, and the various membranes within a cell each have a unique collection of proteins.

If the entropy of a living organism is decreasing, which of the following is also occurring? a) The entropy of the organism's environment must also be decreasing b) Heat is being used by the organism as a source of energy c) Energy is being supplied to the organism d) The first law of thermodynamics is being violated e) In this situation, the second law of thermodynamics must not apply

Energy is being supplied to the organism If the entropy of a living organism is decreasing, then energy is being supplied to the organism. A logical consequence of the loss of usable energy during energy transfer or transformation is that each such event makes the universe more disordered. Scientists use a quantity called entropy as a measure of disorder, or randomness. The more randomly arranged a collection of matter is, the greater its entropy. Order can increase locally, although there is an unstoppable trend toward randomization of the universe as a whole. Living cells create ordered structures from less organized starting materials. For example, simpler molecules are ordered into the more complex structure of an amino acid, and amino acids are ordered into polypeptide chains. These are endergonic reactions that require an input of energy.

Which of the following statements about enzyme function is correct? a) Enzymes can greatly speed up reactions, but they cannot change the net energy output because they cannot change the activation energy b) Enzymes can change the equilibrium point of reactions, but they cannot speed up reactions because they cannot change the net energy output. c) Enzymes can greatly speed up reactions, but they cannot change the activation energy because they cannot change the net energy output d) Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point because they cannot change the net energy output e) None of the listed responses is correct.

Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point because they cannot change the net energy output The initial investment of energy for starting a reaction—the energy required to contort the reactant molecules so the bonds can break—is known as the free energy of activation, or activation energy, An enzyme catalyzes a reaction by lowering the activation energy (EA) barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. An enzyme cannot alter the net change in free energy for a reaction; it cannot make an endergonic reaction exergonic. Enzymes can only hasten reactions that would eventually occur anyway. Enzymes, therefore, cannot alter the equilibrium, the point at which the forward and reverse reactions occur at the same rate, and there is no further net change in the relative concentration of products and reactants

Which of these statements describes some aspect of facilitated diffusion? a) there is only one kind of protein pore for facilitated diffusion b) facilitated diffusion of solutes may occur through transport proteins in the membrane c) facilitated diffusion is another name for osmosis d) facilitated diffusion of solutes occurs through phospholipid pores in the membrane e) facilitated diffusion requires energy to drive a concentration gradient

Facilitated diffusion of solutes may occur through transport proteins in the membrane Specific ions and a variety of polar molecules can't move through cell membranes on their own. However, these hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. Specific transport proteins called channel proteins function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. For example, the passage of water molecules through the plasma membrane of certain cells is greatly facilitated by channel proteins called aquaporins.

Molecular oxygen (O2) has what role in aerobic cellular respiration? a) It oxidizes pyruvate at the end of glycolysis. b) It is necessary in the regeneration of citric acid in the citric acid cycle c) It is the final electron acceptor at the end of the electron transport chain. d) It removes a carbon only from pyruvate, producing CO2 e) It is the source of oxygen in all of the CO2 molecules produced during cellular respiration

It is the final electron acceptor at the end of the electron transport chain.

Which of the following is an endergonic reaction? a) HCl → H+ + Cl b) Glucose + fructose → sucrose c) C6H12O6 + 6 O2 → 6 CO2 + 6 H2O d) ATP → ADP + inorganic phosphate e) All of the listed responses are correct

Glucose + fructose → sucrose This reaction would be endergonic. An endergonic reaction is one that absorbs free energy from its surroundings. Because this kind of reaction essentially stores free energy in molecules (G increases), ΔG is positive. Such reactions are nonspontaneous, and the magnitude of ΔG in the equation ΔG = ΔH - TΔS is the quantity of energy required to drive the reaction. Combining glucose and fructose to produce sucrose is an example of an energy-storing endergonic reaction with the product more complex (lower entropy) than the reactants (glucose and fructose).

Testosterone, a lipid-soluble signaling molecule, crosses the membranes of cells throughout the body but affects only target cells because _________. a) intracellular receptors for testosterone are present only in target cells b) only target cells retain the appropriate DNA segments c) most cells lack the Y chromosome d) only target cells possess the cytosolic enzymes that transduce signals e) only in target cells is testosterone able to initiate the phosphorylation cascade leading to activated transcription factor

Intracellular receptors for testosterone are present only in target cells

When a poison such as cyanide blocks the electron transport chain, glycolysis and the citric acid cycle also eventually stop working. Which of the following is the best explanation for this? a) They run out of ADP b) A high level of NADH is present in the cell c) The uptake of oxygen stops because electron transport was inhibited. d) NAD+ and FAD are not available for glycolysis and the citric acid cycle to continue e) Electrons are no longer available from the electron transport chain to power glycolysis and the citric acid cycle

NAD+ and FAD are not available for glycolysis and the citric acid cycle to continue Unless the electron transport molecules (NADH and FADH2) can recycle back to their oxidized states (NAD+ and FAD), they will be unable to receive electrons in the other steps of cellular respiration and these will not proceed.

After completion of the citric acid cycle, most of the usable energy from the original glucose molecule is in the form of __________. a) acetyl CoA b) NADH c) ATP d) CO2 e) FADH2

NADH The citric acid cycle produces three NADH per turn of the cycle. For each acetyl group entering the cycle, 3 NAD+ are reduced to NADH. However, in one step electrons are transferred not to NAD+ but to FAD, which accepts 2 electrons and 2 protons to become FADH2. Energy is passed from the citric acid cycle to the remaining steps of aerobic cellular respiration in the form of electrons passed from NADH and FADH2 to the electron transport chain associated with oxidative phosphorylation and chemiosmosis.

What do the sign and magnitude of the ΔG of a reaction tell us about the speed of the reaction? a) The more negative the ΔG, the faster the reaction is b) The sign does not matter, but the smaller the magnitude of ΔG, the faster the reaction c) The sign does not matter, but the larger the magnitude of ΔG, the faster the reaction d) Neither the sign nor the magnitude of ΔG has anything to do with the speed of a reaction e) The sign determines whether the reaction is spontaneous, and the magnitude determines the speed.

Neither the sign nor the magnitude of ΔG has anything to do with the speed of a reaction Neither the sign nor the magnitude of ΔG has anything to do with the speed of a reaction. The speed of a reaction is determined by the activation energy (EA) barrier of the reaction and the temperature. Enzymes can be used to speed a reaction, without affecting ΔG, the free-energy change for a reaction, by reducing its activation energy.

Steroid hormones enter a cell by simple diffusion. In doing so, these steroids _________. a) move up a concentration gradient and are nonpolar b) move up a concentration and are polar c) move down a concentration gradient and are polar d) move through a channel and down a gradient and are nonpolar e) none of the listed responses is correct

None of the listed responses is correct. Steroid hormones are signaling molecules that, because of their hydrophobic characteristics, readily cross cell membranes and bind to receptor proteins in the cytoplasm (or nucleus), activating the receptor. Simple diffusion must occur down a concentration gradient, and because the molecule moving must pass through the plasma membrane, that molecule must be hydrophobic, or nonpolar; no membrane protein is required for passive diffusion of nonpolar molecules.

Organisms are described as thermodynamically open systems. Which of the following statements is consistent with this description? a) The metabolism of an organism is isolated from its surroundings b) Organisms acquire energy from, and lose energy to, their surroundings c) Because energy must be conserved, organisms constantly recycle energy and thus need no input of energy d) Heat produced by the organism is conserved in the organism and not lost to the environment. e) All of the listed responses are correct

Organisms acquire energy from, and lose energy to, their surroundings Organisms are described as thermodynamically open systems because organisms acquire energy from, and lose energy to, their surroundings. In an open system, energy and matter can be transferred between the system and its surroundings. Organisms are open systems. They absorb energy—for instance, light energy or chemical energy in the form of organic molecules—and release heat and metabolic waste products, such as carbon dioxide, to the surroundings.

Which of the following is a correct description and sequence of events in cellular respiration? a) Oxidation of glucose to pyruvate; reduction of pyruvate; TCA cycle; oxidative phosphorylation b) Glycolysis; reduction of pyruvate; TCA cycle; oxidative phosphorylation c) Oxidation of glucose to pyruvate; oxidation of pyruvate; oxidation of acetyl-coA; oxidative phosphorylation d) Glycolysis; oxidative phosphorylation; TCA cycle; oxidation of pyruvate e) Oxidation of pyruvate; TCA cycle; oxidation of glucose to pyruvate; oxidative phosphorylation

Oxidation of glucose to pyruvate; oxidation of pyruvate; oxidation of acetyl-coA; oxidative phosphorylation Aerobic respiration occurs in three stages: (1) glycolysis, (2) pyruvate oxidation and the citric acid cycle, and (3) oxidative phosphorylation (electron transport and chemiosmosis). In glycolysis, glucose, a six-carbon sugar, is split into two three-carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. If molecular oxygen is present, the pyruvate enters a mitochondrion (in eukaryotic cells) via active transport and it is first converted to a compound called acetyl coenzyme A, or acetyl CoA, which is further oxidized to CO2 in the citric acid cycle. NADH and a similar electron carrier, a coenzyme called FADH2, transfer electrons derived from glucose to electron transport chains, which are built into the inner mitochondrial membrane. (In prokaryotes, the electron transport chains are located in the plasma membrane.) During oxidative phosphorylation, electron transport chains convert the chemical energy to a form used for ATP synthesis in the process called chemiosmosis.

Most of the ATP produced in aerobic cellular respiration comes from which of the following processes? a) Glycolysis b) Oxidative phosphorylation c) Reduction of NAD+ d) Substrate-level phosphorylation e) The citric acid cycle

Oxidative phosphorylation Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration.

The general name for an enzyme that transfers phosphate groups from ATP to a protein is ________. a) protein kinase b) protein dehydrogenase c) protein phosphatase d) peptidase e) protein cyclase

Protein kinase The phosphorylation and dephosphorylation of proteins is a widespread cellular mechanism for regulating protein activity. Most protein kinases attach phosphate groups to proteins different from themselves. However, some protein kinases phosphorylate other protein kinases of the same type.

Which of these statements about enzyme inhibitors is true? a) The action of competitive inhibitors may be reversible or irreversible b) A competitive inhibitor binds to the enzyme at a place that is separate from the active site c) A noncompetitive inhibitor does not change the shape of the active site d) When the product of an enzyme or an enzyme sequence acts as its inhibitor, this is known as positive feedback e) Inhibition of enzyme function by compounds that are not substrates is something that only occurs under controlled conditions in the laboratory

The action of competitive inhibitors may be reversible or irreversible The action of competitive inhibitors may be reversible or irreversible. Certain chemicals selectively inhibit the action of specific enzymes, and we have learned a lot about enzyme function by studying the effects of these molecules. If the inhibitor attaches to the enzyme by covalent bonds, inhibition is usually irreversible. Many enzyme inhibitors, however, bind to the enzyme by weak interactions, in which case inhibition is reversible. Some reversible inhibitors resemble the normal substrate molecule and compete for admission into the active site. These mimics, called competitive inhibitors, reduce the productivity of enzymes by blocking substrates from entering active sites. This kind of inhibition can be overcome by increasing the concentration of substrate so that as active sites become available, more substrate molecules than inhibitor molecules are around to gain entry to the sites.

Which of the following is changed in a reaction by the action of an enzyme? a) The magnitude of ΔG b) G of the reactants c) The activation energy d) The sign of ΔG e) G of the products

The activation energy The activation energy is changed in a reaction by the action of an enzyme. An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up a reaction without being consumed by the reaction. An enzyme catalyzes a reaction by lowering the activation energy (EA) barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. An enzyme cannot alter the change in free energy, ΔG, for a reaction; it cannot make an endergonic reaction exergonic. Enzymes have no effect on G of the reactants or products. Enzymes can only hasten reactions that would eventually occur anyway, but this function makes it possible for the cell to have a dynamic metabolism, routing chemicals smoothly through the cell's metabolic pathways. And because enzymes are very specific for the reactions they catalyze, they determine which chemical processes will be going on in the cell at any particular time.

Which of the following statements about the active site of an enzyme is correct? a) The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits b) The active site has a fixed structure (shape) c) Coenzymes are rarely found in the active site of an enzyme d) The structure of the active site is not affected by changes in temperature e) The active site allows the reaction to occur under the same environmental conditions as the reaction without the enzyme

The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits Usually, the active site is formed by only a few of the enzyme's amino acids, with the rest of the protein molecule providing a framework that determines the configuration of the active site. The specificity of an enzyme, and therefore the ability of it to catalyze a specific reaction, is attributed to a compatible fit between the shape of its active site and the shape of the substrate. An enzyme is not a stiff structure locked into a given shape. In fact, recent work by biochemists has shown clearly that enzymes (and other proteins as well) seem to "dance" between subtly different shapes in a dynamic equilibrium, with slight differences in free energy for each "pose." Environmental conditions, such as temperature and pH, can also influence enzyme shape and function.

The binding of an allosteric inhibitor to an enzyme causes the rate of product formation by the enzyme to decrease. Which of the following best explains why this decrease occurs? a) The allosteric inhibitor binds to the active site, preventing the substrate from binding. b) The allosteric inhibitor causes free energy change of the reaction to increase. c) The allosteric inhibitor lowers the temperature of the active site. d) The allosteric inhibitor binds to the substrate and prevents it from binding at the active site. e) The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site.

The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site. Most enzymes known to be allosterically regulated are constructed from two or more subunits, each composed of a polypeptide chain with its own active site. The entire complex oscillates between two different shapes, one catalytically active and the other inactive. In the simplest kind of allosteric regulation, an activating or inhibiting regulatory molecule binds to a regulatory site (sometimes called an allosteric site), often located where subunits join. The binding of an activator to a regulatory site stabilizes the shape that has functional active sites, whereas the binding of an inhibitor stabilizes the inactive form of the enzyme. The subunits of an allosteric enzyme fit together in such a way that a shape change in one subunit is transmitted to all others. Through this interaction of subunits, a single activator or inhibitor molecule that binds to one regulatory site will affect the active sites of all subunits.

Metabolic pathways in cells are typically far from equilibrium. Which of the following processes tend(s) to keep these pathways away from equilibrium? a) The continuous removal of the products of a pathway to be used in other reactions and the input of free energy from outside the pathway b) The continuous removal of the products of a pathway to be used in other reactions c) An input of free energy from outside the pathway d) An input of heat from the environment e) All of the listed choices are correct

The continuous removal of the products of a pathway to be used in other reactions and the input of free energy from outside the pathway Metabolic pathways in cells are typically far from equilibrium because the constant flow of materials in and out of the cell keeps the metabolic pathways from ever reaching equilibrium, and the cell continues to do work throughout its life. Reactions in an isolated system eventually reach equilibrium and can then do no work. The chemical reactions of metabolism are reversible, and they, too, would reach equilibrium if they occurred in the isolation of a test tube. Because systems at equilibrium are at a minimum of free energy (G) and can do no work, a cell that has reached metabolic equilibrium is dead! The fact that metabolism as a whole is never at equilibrium is one of the defining features of life. Input of energy—typically in the form of chemical energy, not heat—maintains metabolic activity.

What is meant by the "induced fit" of an enzyme? a) The substrate can be altered so that it is induced to fit into the enzyme's active site b) The enzyme structure is altered so that it can be induced to fit many different types of substrate c) The presence of the substrate in solution induces the enzyme to slightly change its structure d) The shape of the active site is nearly perfect for specifically binding the enzyme's substrate(s) e) The enzyme changes its shape slightly as it binds the substrate

The enzyme changes its shape slightly as it binds the substrate Induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction. It has been known for more than 50 years that the active site itself is not a rigid receptacle for the substrate. As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate's chemical groups and chemical groups on the side chains of the amino acids that form the active site. This shape change makes the active site fit even more snugly around the substrate. This induced fit of the enzyme is like a clasping handshake

Which of the following determines the sign of ΔG for a reaction? a) The affinity of the enzyme for the reactants b) The enzyme concentration c) The free energy of the products d) The free energy of the reactants e) The free energy of the reactants and the free energy of the products

The free energy of the reactants and the free energy of the products For example, if ΔG = -686 kcal/mol for respiration, which converts glucose and oxygen to carbon dioxide and water, then the reverse process—the conversion of carbon dioxide and water to glucose and oxygen—must be strongly endergonic, with ΔG = +686 kcal/mol. Because the chemical mixture loses free energy (G decreases), ΔG is negative for an exergonic reaction. An endergonic reaction is one that absorbs free energy from its surroundings. Because this kind of reaction essentially stores free energy in molecules (G increases), ΔG is positive.

Which of the following best characterizes the functional role of ATP in cellular metabolism? a) The free energy released by ATP hydrolysis is coupled to endergonic processes via the formation of a phosphorylated intermediate b) The release of free energy during the hydrolysis of ATP heats the surrounding environment c) ATP is catabolized to carbon dioxide and water d) The ΔG associated with ATP hydrolysis is positive e) Hydrolysis of ATP releases a large amount of free energy because the nonpolar phosphate groups are repelled by water.

The free energy released by ATP hydrolysis is coupled to endergonic processes via the formation of a phosphorylated intermediate. The functional role of ATP in cellular metabolism is that the free energy released by ATP hydrolysis is coupled to endergonic processes via the formation of a phosphorylated intermediate. For example, with the help of specific enzymes, the cell is able to use the energy released by ATP hydrolysis directly to drive chemical reactions that, by themselves, are endergonic. If the ΔG, the change in free energy, of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that, overall, the coupled reactions are exergonic. This usually involves the transfer of a phosphate group from ATP to some other molecule, such as the reactant. The recipient with the phosphate group covalently bonded to it is then called a phosphorylated intermediate. The key to coupling exergonic and endergonic reactions is the formation of this phosphorylated intermediate, which is more reactive (less stable) than the original unphosphorylated molecule.

The formation of glucose-6-phosphate from glucose is an endergonic reaction and would, therefore, be coupled to which of the following reactions or pathways? a) The conversion of glucose + fructose to sucrose b) The contraction of a muscle cell c) The hydrolysis of ATP d) The active transport of a phosphate ion into the cell e) The formation of ATP from ADP and inorganic phosphate

The hydrolysis of ATP An endergonic reaction is one that requires a net input of free energy (the portion of a system's energy that can perform work when temperature and pressure are uniform throughout the system). A key feature in the way cells manage their energy resources to do work is energy coupling, the use of an exergonic process to drive an endergonic one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy that powers cellular work. For example, with the help of specific enzymes, the cell is able to use the energy released by ATP hydrolysis directly to drive chemical reactions that, by themselves, are endergonic. If the change in free energy of an endergonic reaction is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that, overall, the coupled reactions are exergonic. This usually involves the transfer of a phosphate group from ATP to some other molecule, such as the glucose-6-phosphate, which is a phosphorylated intermediate. The key to coupling exergonic and endergonic reactions is the formation of this phosphorylated intermediate, which is more reactive (less stable) than the original unphosphorylated molecule.

Where do the reactions of the citric acid cycle occur in eukaryotic cells? a) The cytosol b) The cristae of the mitochondrion c) The intermembrane space of the mitochondrion d) The matrix of the mitochondrion e) Across the inner membrane of the mitochondrion

The matrix of the mitochondrion

Which of the following would tend to increase membrane fluidity? a) a greater proportion of phospholipids with unsaturated hydrocarbon tails b) a greater proportion of phospholipids with saturated hydrocarbon tails c) a lower temperature d) a relatively high protein content in the membrane e) a greater proportion large glycolipids relative to lipids having smaller molecular masses

a greater proportion of phospholipids with unsaturated hydrocarbon tails A greater proportion of phospholipids with unsaturated hydrocarbon tails would tend to increase membrane fluidity. Unsaturated hydrocarbons are those that contain double bonds between adjacent carbon atoms. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and this looseness makes the membrane more fluid.

The concentration of solutes in a particular cell is about 2%, but the cell contains almost no sucrose or urea. Sucrose cannot pass through the membrane of this cell, but water and urea can. Osmosis would cause this cell to shrink the most when immersed in which of the following solutions? a) pure water b) a hypotonic urea solution c) a hypertonic urea solution d) a hypertonic sucrose solution e) a hypotonic sucrose solution

a hypertonic sucrose solution

Which of the following substances is/are involved in oxidative phosphorylation? a) ATP b) ADP c) Oxygen d) none of the listed substances are involved in oxidative phosphorylation e) all of the listed responses are involved in oxidative phosphorylation

all of the listed substances are involved in oxidative phosphorylation In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, the processes that together constitute oxidative phosphorylation. Electrons cascade down the electron transport chain from one carrier molecule to the next in a series of redox reactions, losing a small amount of energy with each step until they finally reach oxygen, the terminal electron acceptor, which has a very great affinity for electrons. Populating the inner membrane of the mitochondrion are many copies of a protein complex called ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate. ATP synthase works like an ion pump running in reverse. At certain steps along the electron transport chain, electron transfer causes protein complexes to move H+ from the mitochondrial matrix (in eukaryotes) to the intermembrane space, storing energy as a proton-motive force (H+ gradient). As H+ diffuses back into the matrix through ATP synthase, its passage drives the phosphorylation of ADP, a process called chemiosmosis.

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule is an example of __________. a) feedback inhibition b) competitive inhibition c) noncompetitive inhibition d) cooperativity e) allosteric regulation

allosteric regulation Intrinsic to life's processes is a cell's ability to tightly regulate its metabolic pathways by controlling when and where its various enzymes are active. It does this either by switching on and off the genes that encode specific enzymes or by regulating the activity of enzymes once they are made. In many cases, the molecules that naturally regulate enzyme activity in a cell behave something like reversible noncompetitive inhibitors: These regulatory molecules change an enzyme's shape and the functioning of its active site by binding to a site elsewhere on the molecule, via noncovalent interactions. Allosteric regulation includes any case in which a protein's function at one site is affected by the binding of a regulatory molecule to a separate site. It may result in either inhibition or stimulation of an enzyme's activity. Most enzymes known to be allosterically regulated are constructed from two or more subunits, each composed of a polypeptide chain with its own active site. The entire complex oscillates between two different shapes, one catalytically active and the other inactive. In the simplest kind of allosteric regulation, an activating or inhibiting regulatory molecule binds to a regulatory site (sometimes called an allosteric site), often located where subunits join. The binding of an activator to a regulatory site stabilizes the shape that has functional active sites, whereas the binding of an inhibitor stabilizes the inactive form of the enzyme. The subunits of an allosteric enzyme fit together in such a way that a shape change in one subunit is transmitted to all others. Through this interaction of subunits, a single activator or inhibitor molecule that binds to one regulatory site will affect the active sites of all subunits.

Choose the pair of terms that correctly completes this sentence: Catabolism is to anabolism as __________ is to __________. a) exergonic; endergonic b) exergonic; spontaneous c) free energy; entropy d) work; energy e) entropy; enthalpy

exergonic; endergonic Catabolism is to anabolism as exergonic is to endergonic. Enzymes catalyze reactions in intersecting metabolic pathways, which may be catabolic (breaking down molecules and releasing energy) or anabolic (building molecules and consuming energy). In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants, which means that energy has been released, as in a catabolic reaction. Endergonic (nonspontaneous) reactions require an input of energy, which means that energy has been consumed, as in an anabolic reaction.

Which of the following processes and organelle(s) account for the replacement of lipids and proteins lost from the plasma membrane? a) endocytosis and Golgi b) active transport and the rough ER c) receptor-mediated endocytosis and smooth ER and Golgi d) exocytosis and smooth and rough ER e) flip-flop of phospholipids from one side of the plasma membrane to the other and the Golgi

exocytosis and smooth and rough ER In exocytosis, the cell secretes certain biological molecules by the fusion of vesicles with the plasma membrane. The smooth ER produces lipids destined for the membrane, whereas the rough ER produces proteins destined for the plasma membrane. These products are transported to the Golgi apparatus. A transport vesicle that buds from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle membrane and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse. The contents of the vesicle then spill to the outside of the cell, and the vesicle membrane becomes part of the plasma membrane.

Enzyme activity is affected by pH because __________. a) most substrates don't function well at high or low pH b) high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the enzyme's active site c) low pH will denature all enzymes d) changes in pH cause loss of all cofactors from enzymes e) the binding of hydrogen ions to the enzyme absorbs energy and thus there may not be enough energy to overcome the activation energy barrier

high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the enzyme's active site Just as each enzyme has an optimal temperature, it also has a pH at which it is most active. The optimal pH values for most enzymes fall in the range of pH 6-8, but there are exceptions. For example, pepsin, a digestive enzyme in the human stomach, works best at pH 2. Such an acidic environment denatures most enzymes, but pepsin is adapted to maintain its functional three-dimensional structure in the acidic environment of the stomach. In contrast, trypsin, a digestive enzyme residing in the alkaline environment of the human intestine, has an optimal pH of 8 and would be denatured in the stomach.

Consider the currently accepted fluid mosaic model of the plasma membrane. Where in the plasma membrane would cholesterol most likely be found? a) on the outside (external) surface of the membrane b) on the inside (cytoplasmic) surface c) in the interior of the membrane d) in the interior and on the inside surface, but not on the outside surface e) on either surface of the membrane, but not in the interior of the membrane

in the interior of the membrane Cholesterol would most likely be found in the interior of the membrane. Cholesterol in the cell membrane is found wedged between the hydrophobic portions of the phospholipid molecules in animal cells. One of its jobs is to act as a "fluidity buffer," keeping membrane fluidity normal in the face of changing temperatures. At relatively high temperatures—at 37°C, the body temperature of humans, for example—cholesterol makes the membrane less fluid by restraining phospholipid movement. However, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify.

Nitric oxide is unusual among animal signal molecules in that it __________. a) is a gas b) enters the cell via a protein channel c) acts by directly binding to DNA d) binds to membrane receptors and cytoplasm receptors e) activates proteins by removing phosphate

is a gas A number of important signaling molecules can enter cells through the plasma membrane because they are either hydrophobic enough or small enough to cross the hydrophobic interior of the membrane. These types of signaling molecules do not bind to membrane receptors. Such hydrophobic chemical messengers include the steroid hormones and the small gaseous molecule nitric oxide, which is a local regulator. After diffusing through the plasma membrane, nitric oxide binds to an intracellular receptor, causing a cellular response. Nitric oxide does not bind to DNA nor does it act as a phosphatase.

A molecule becomes oxidized when it __________. a) changes shape b) gains a hydrogen (H+) ion c) loses a hydrogen (H+) ion d) loses an electron e) gains an electron

loses an electron

Receptors for signal molecules __________. a) are only found associated with the plasma membrane b) all work by opening ion channels c) are never found in the nucleus of a cell d) may be found embedded in the plasma membrane or found within the cytoplasm or nucleus e) all work via protein kinases

may be found embedded in the plasma membrane or found within the cytoplasm or nucleus Receptors for signal molecules may be found embedded in the plasma membrane or found within the cytoplasm or nucleus. Most water-soluble signaling molecules bind to specific sites on receptor proteins that span the cell's plasma membrane. Such a transmembrane receptor transmits information from the extracellular environment to the inside of the cell by changing shape or aggregating when a specific ligand binds to it. There are three major types of cell surface transmembrane receptors: G protein-coupled receptors, receptor tyrosine kinases, and ion channel receptors. Additional signal receptors involved in other signal transduction pathways are located in the cytoplasm or nucleus of target cells. For example, steroids bind to receptors in the cytoplasm and the activated hormone-receptor complex is transported into the nucleus where it regulates gene expression.

Which of the following statements concerning carbohydrates associated with the plasma membrane is correct? a) carbohydrates are only found associated with the membranes of prokaryotic cells b) membrane carbohydrates have a primary role in cell-cell recognition c) the carbohydrate composition of most eukaryotic plasma membranes is quite similar d) carbohydrates on the plasma membrane are typically short chains of between two and five monosaccharides e) carbohydrates associated with the plasma membrane are located on both surfaces of the membrane

membrane carbohydrates have a primary role in cell-cell recognition Cell-cell recognition, a cell's ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. It is important, for example, in the sorting of cells into tissues and organs in an animal embryo. It is also the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals. Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane.

Which of the following environments or actions would not affect the rate of an enzyme reaction? a) pH b) substrate concentration c) cooling the enzyme d) heating the enzyme e) none of the listed responses is correct

none of the listed responses is correct Changes to the temperature, substrate concentration, and pH are all likely to affect enzyme activity. The rate at which a particular amount of enzyme converts substrate to product is partly a function of the initial concentration of the substrate: The more substrate molecules that are available, the more frequently they access the active sites of the enzyme molecules. Up to a point, the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the molecules move rapidly. Above that temperature, however, the speed of the enzymatic reaction drops sharply. The thermal agitation of the enzyme molecule disrupts the hydrogen bonds, ionic bonds, and other weak interactions that stabilize the active shape of the enzyme, and the protein molecule eventually denatures. Just as each enzyme has an optimal temperature, it also has a pH at which it is most active. The optimal pH values for most enzymes fall in the range of pH 6-8, but there are exceptions. For example, pepsin, a digestive enzyme in the human stomach, works best at pH 2. Such an acidic environment denatures most enzymes, but pepsin is adapted to maintain its functional three-dimensional structure in the acidic environment of the stomach. In contrast, trypsin, a digestive enzyme residing in the alkaline environment of the human intestine, has an optimal pH of 8 and would be denatured in the stomach.

Which of the following processes includes all the others? a) diffusion of a solute across a membrane b) osmosis c) passive transport d) facilitated diffusion e) transport of an ion down its electrochemical gradient

passive transport

Which of the following statements about passive transport is correct? a) passive transport operates independently of diffusion b) passive transport involves movement of solute in both directions across a membrane, but with a net movement of solute down its concentration gradient c) passive transport operates independently of the concentrations of the moving solute d) in passive transport, solute movement stops when the solute concentration is the same on both sides of a membrane e) passive transport does not occur in the human body

passive transport involves movement of solute in both directions across a membrane, but with a net movement of solute down its concentration gradient In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient, the region along which the density of a substance increases or decreases (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy.

Which of the following pairs correctly matches a membrane transport process to its primary function? a) receptor-mediated endocytosis; engulfing a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole b) osmosis; passive diffusion of water and small solutes across a membrane c) exocytosis; the movement of water and solutes out of the cell by vesicle fusion with the plasma membrane d) phagocytosis; secretion of large particles from the cell by fusion of vesicles with the plasma membrane e) pinocytosis; the uptake of water and small solutes into the cell by formation of vesicles at the plasma membrane

pinocytosis; the uptake of water and small solutes into the cell by formation of vesicles at the plasma membrane

The molecule cAMP directly activates ________. a) adenylyl cyclase b) G proteins c) receptor tyrosine kinases d) phosphodiesterase e) protein kinase A

protein kinase A Cyclic AMP (cAMP) is one of the most widely used second messengers. The relay molecule immediately after cAMP in a signaling pathway is usually protein kinase A, a serine/threonine kinase, which is activated by cAMP.

Second messengers tend to be water-soluble and small. This accounts for their ability to __________. a) rapidly move throughout the cell by diffusion b) rapidly cross the plasma membrane c) pass quickly from cell to cell d) move from substrate to substrate during a phosphorylation cascade e) cross the nuclear membrane and interact with DNA

rapidly move throughout the cell by diffusion Second messengers tend to be water-soluble and small. This accounts for their ability to rapidly move throughout the cell by diffusion. A second messenger links an extracellular signaling molecule that binds to a membrane receptor (the signal pathway's "first messenger") to the cellular response. The two most common second messengers are cyclic AMP and calcium ions, Ca2+.

Which of the following enables a cell to concentrate and take in a specific kind of molecule? a) channel proteins b) receptor-mediated endocytosis c) passive transport d) facilitated diffusion e) osmosis

receptor-mediated endocytosis

In general, the hydrolysis of ATP drives cellular work by __________. a) yielding free phosphate b) releasing heat c) acting as a catalyst d) releasing free energy that can be coupled to other reactions e) lowering the free energy of the reaction

releasing free energy that can be coupled to other reactions In general, the hydrolysis of ATP drives cellular work by releasing free energy that can be coupled to other reactions. Energy coupling is a key feature in the way cells manage their energy resources to do work, which is the use of an exergonic process to drive an endergonic one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy (not a catalyst) that powers cellular work. By-products such as heat are not effective energy sources for such work.

What event would activate a G protein? a) phosphorylation of GDP to GTP b) phosphorylation of GTP to GDP c) replacement of GDP with GTP d) hydrolysis of GTP to GDP e) hydrolysis of GDP to GTP

replacement of GDP with GTP A replacement of GDP with GTP would activate a G protein. A G protein-coupled receptor (GPCR) is a cell-surface transmembrane receptor that works with the help of a G protein, a protein that binds the energy-rich molecule GTP. Many different signaling molecules, including yeast mating factors, epinephrine and many other hormones, and neurotransmitters, use G protein-coupled receptors in the regulation of enzyme activity. When GDP is bound to the G protein, the G protein is inactive. When the appropriate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape. Its cytoplasmic side then binds an inactive G protein, causing a GTP to displace the GDP. The activated G protein leaves the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme's shape and activity.

Most cells cannot harness heat to perform work because __________. a) temperature is usually uniform throughout a cell b) heat is not a form of energy c) cells do not have much heat; they are relatively cool d) heat can never be used to do work e) heat must remain constant during work

temperature is usually uniform throughout a cell Although heat is a form of energy, most cells cannot harness heat to perform work because temperature is usually uniform throughout a cell. A system can put heat to work only when there is a temperature difference that results in the heat flowing from a warmer location to a cooler one. If temperature is uniform, as it is in a living cell, then the only use for heat energy generated during a chemical reaction is to warm a body of matter, such as the organism.

Early work on signal transduction and glycogen metabolism by Sutherland indicated that __________. a) epinephrine is involved in response to stress b) the cell-signaling pathway involves two separate steps: transduction and response c) the signal molecule worked equally well with intact or disrupted cells d) the signal molecule did not interact directly with the cytosolic enzyme but required an intact plasma membrane before the enzyme could be activated e) the signal molecule combined directly with a cytosolic enzyme to form an active quaternary structure

the signal molecule did not interact directly with the cytosolic enzyme but required an intact plasma membrane before the enzyme could be activated

The plasma membrane is referred to as a "fluid mosaic" structure. Which of the following statements about that model is true? a) the fluid aspect of the membrane is due to the lateral and rotational movement of phospholipids, and embedded proteins account for the mosaic aspect b) the fluid aspect of the membrane is due to the behavior of phospholipids, and the mosaic aspect is due to the presence of carbohydrates c) the fluid aspect of the membrane describes its structure at normal temperatures, and the mosaic aspect describes the behavior of the membrane as the temperature is lowered d) the mosaic aspect of the membrane is due to the glycosylation of inner leaflet phospholipids e) only phospholipids are capable of moving in the membrane

the fluid aspect of the membrane is due to the lateral and rotational movement of phospholipids, and embedded proteins account for the mosaic aspect The fluid aspect of the membrane is due to the lateral and rotational movement of phospholipids, and embedded proteins account for the mosaic aspect of the fluid-mosaic model of the cell plasma membrane. A mosaic is a structure, like a tile wall mural, made up of smaller pieces embedded within a larger background.

Much of the suitability of ATP as an energy intermediary is related to the instability of the bonds between the phosphate groups. These bonds are unstable because __________. a) the valence electrons in the phosphorus atom have less energy on average than those of other atoms b) they are hydrogen bonds, which are only about 10% as strong as covalent bonds c) the phosphate groups are nonpolar and are repelled by water in the cell d)the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP e) the bonds between the phosphate groups are unusually strong and breaking them releases free energy

the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP ATP is useful to the cell because the energy it releases on losing a phosphate group is somewhat greater than the energy most other molecules could deliver. This hydrolysis releases so much energy because all three phosphate groups are negatively charged. These like charges are crowded together, and their mutual repulsion contributes to the instability of this region of the ATP molecule. The triphosphate tail of ATP is the chemical equivalent of a compressed spring.

A chemical reaction is designated as exergonic rather than endergonic when __________. a) activation energy is required b) the products are less complex than the reactants c) activation energy exceeds net energy release d) the potential energy of the products is less than the potential energy of the reactants e) it absorbs more energy

the potential energy of the products is less than the potential energy of the reactants otential energy is energy that is not kinetic; it is stored energy that matter possesses because of its location or structure. Free energy is the portion of a system's energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell. Biologists find it most informative to focus on the change in free energy (ΔG) during the chemical reactions of life. ΔG represents the difference between the free energy of the final state and the free energy of the initial state. Exergonic reactions have a negative ΔG and therefore provide energy that can be used to perform work.

Some bacteria are metabolically active in hot springs because __________. a) they use molecules other than proteins or RNAs as their main catalysts b) their enzymes have high optimal temperatures c) they are able to maintain a lower internal temperature d) high temperatures make catalysis unnecessary e) their enzymes are completely insensitive to temperature

their enzymes have high optimal temperatures Temperature and pH are environmental factors important in the activity of an enzyme. Up to a point, the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the molecules move rapidly. Above that temperature, however, the speed of the enzymatic reaction drops sharply. The thermal agitation of the enzyme molecule disrupts the hydrogen bonds, ionic bonds, and other weak interactions that stabilize the active shape of the enzyme, and the protein molecule eventually denatures. Each enzyme has an optimal temperature at which its reaction rate is greatest. Without denaturing the enzyme, this temperature allows the greatest number of molecular collisions and the fastest conversion of the reactants to product molecules. Most human enzymes have optimal temperatures of about 35-40°C (close to human body temperature). The thermophilic bacteria that live in hot springs contain enzymes with optimal temperatures of 70°C or higher.

Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because __________. a) they amplify the original signal manyfold b) they are species specific c) they always lead to the same cellular response d) they counter the harmful effects of phosphatases e) the number of molecules used is small and fixed

they amplify the original signal manyfold Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because they amplify the original signal manyfold. Many of the relay molecules in signal transduction pathways are protein kinases (enzymes that transfer phosphate groups from ATP to a protein), and they often act on other protein kinases in the pathway. This phosphorylation cascade amplifies the original signal many times over because each kinase in the cascade, once activated, can act on many molecules of its substrate.

Which of the following correctly describes a general property of all electrogenic pumps? a) they create a voltage difference across a membrane b) they pump sodium out of the cell and potassium into the cell c) they can pump a large variety of solutes across a membrane against their concentration gradients d) they create a cell interior that is positively charged relative to the outside of the cell e) they create a high concentration of protons within a cell

they create a voltage difference across a membrane Electrogenic pumps create a voltage difference across a membrane. The sodium-potassium pump appears to be the major electrogenic pump of animal cells. This transport system pumps ions against steep concentration gradients: Sodium ion concentration ([Na+]) is high outside the cell and low inside, while potassium ion concentration ([K+]) is low outside the cell and high inside. The pump oscillates between two shapes in a cycle that moves three Na+ out of the cell for every two K+ pumped into the cell. The two shapes have different binding affinities for Na+ and K+. ATP powers the shape change by transferring a phosphate group to the transport protein (phosphorylating the protein). By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work.