AP Biology Chapter 6 Study Module

Which of the following has the most free energy per molecule?

A starch molecule Among the possible choices, the molecule with the most free energy per molecule is starch. An animal obtains starch, proteins, and other complex molecules from the food it eats. As catabolic pathways break these molecules down, the animal releases carbon dioxide and water—small molecules that possess less chemical energy than the food did. Biochemical pathways, carried out in the context of cellular structures, enable cells to release chemical energy from food molecules and use the energy to power life processes, such as assembling sugars, amino acids, fatty acids, and cholesterol molecules into more complex molecules to support life.

Which of the following states the relevance of the first law of thermodynamics to biology?

Energy can be freely transformed among different forms as long as the total energy is conserved. The first law of thermodynamics is relevant to biology because acquiring and using energy are necessary tasks for survival. The first law of thermodynamics, the energy of the universe is constant: Energy can be transferred and transformed, but it cannot be created or destroyed. The first law is also known as the principle of conservation of energy. For example, the electric company does not make energy, it merely converts one form of energy to another that is more convenient for us to use. By converting sunlight to chemical energy, a plant acts as an energy transformer, not an energy producer.

If the entropy of a living organism is decreasing, which of the following is most likely to be occurring simultaneously?

Energy input into the organism must be occurring to drive the decrease in entropy. 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. The second law of thermodynamics states that every energy transfer or transformation increases the entropy of the universe. Although order can increase locally, there is an unstoppable trend toward randomization of the universe as a whole. Living systems increase the entropy of their surroundings, as predicted by thermodynamic law. It is true that 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. At the organismal level as well, complex and beautifully ordered structures result from biological processes that use simpler starting materials. However, an organism also takes in organized forms of matter and energy from the surroundings and replaces them with less ordered forms. For example, an animal obtains starch, proteins, and other complex molecules from the food it eats. As catabolic pathways break these molecules down, the animal releases carbon dioxide and water—small molecules that possess less chemical energy than the food did. The depletion of chemical energy is accounted for by heat generated during metabolism. On a larger scale, energy flows into most ecosystems in the form of light and exits in the form of heat.

Which of the following reactions would be endergonic?

Glucose + fructose → sucrose 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).

When 1 mole of ATP is hydrolyzed in a test tube without an enzyme, about twice as much heat is given off as when 1 mole of ATP is hydrolyzed in a cell. Which of the following best explains these observations?

In the cell, the hydrolysis of ATP is coupled to other endergonic reactions. When the terminal phosphate bond in ATP is broken by addition of a water molecule, a molecule of inorganic phosphate leaves the ATP, which becomes adenosine diphosphate, or ADP. The reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed. This is the free-energy change measured under standard conditions. In the cell, conditions do not conform to standard conditions, primarily because reactant and product concentrations differ from 1 M. For example, when ATP hydrolysis occurs under cellular conditions, the actual ΔG is about -13 kcal/mol, 78% greater than the energy released by ATP hydrolysis under standard conditions. 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 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.

What do the sign and magnitude of the ΔG of a reaction tell us about the speed of the reaction?

Neither the sign nor the magnitude of ΔG have anything to do with the speed of a reaction. The sign and magnitude of the ΔG of a reaction tell us nothing about the speed of the reaction. The speed of the reaction is determined by the activation energy (EA) barrier of the reaction and the temperature. However, enzymes can be used to speed a reaction, without affecting the free-energy change (ΔG) for a reaction, by reducing its activation energy.

Organisms are described as thermodynamically open systems. Which of the following statements is consistent with this description?

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 an example of the cellular work accomplished with the free energy derived from the hydrolysis of ATP, involved in the production of electrochemical gradients?

Proton movement against a gradient of protons The bonds between the phosphate groups of ATP can be broken by hydrolysis. When the terminal phosphate bond is broken by addition of a water molecule, a molecule of inorganic phosphate leaves the ATP, which becomes adenosine diphosphate, or ADP. The reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed. The cell uses this energy to perform transport work, such as the pumping of substances across membranes against the direction of spontaneous movement. A key process in metabolism is the transport of hydrogen ions (H+) across a membrane to create an electrochemical gradient.

Which of the following is changed by the presence of an enzyme in a reaction?

The activation energy 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 change the ΔG for a reaction; it cannot make an endergonic reaction exergonic. 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.

Metabolic pathways in cells are typically far from equilibrium. Which of the following processes tend(s) to keep these pathways away from equilibrium?

The continuous removal of the products of a pathway to be used in other reactions AND an 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.

Which of the following situations does not represent an energy transformation?

The coupling of ATP hydrolysis to the production of a proton gradient across a membrane by a proton pump The energy stored in ATP is used to create an electrochemical gradient of protons that contains energy. 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. Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, but the term is misleading. The phosphate bonds of ATP are not unusually strong bonds, as "high energy" may imply; rather, the reactants (ATP and water) themselves have high energy relative to the energy of the products (ADP and phosphate). The release of energy during the hydrolysis of ATP comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves. When ATP is hydrolyzed in a test tube, the release of free energy merely heats the surrounding water. In most cases in the cell, however, the generation of heat alone would be an inefficient (and potentially dangerous) use of a valuable energy resource. Instead, the cell's proteins harness the energy released during ATP hydrolysis in several ways to perform the three types of cellular work—chemical, transport, and mechanical.

Which of the following determines the sign of ΔG for a reaction?

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.

According to the second law of thermodynamics, which of the following is true?

The decrease in entropy associated with life must be compensated for by increased entropy in the environment in which life exists. According to the second law of thermodynamics, spontaneous processes, those requiring no outside input of energy, increase the disorder of the universe. That is, every energy transfer or transformation increases the disorder of the universe. Although order can increase locally, there is an unstoppable trend toward randomization of the universe as a whole. 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.

Molecules A and B contain 110 kcal/mol of free energy and molecules B and C contain 150 kcal/mol of energy. A and B are converted to C and D. What can be concluded?

The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants. Molecules A and B contain 110 kcal/mol of free energy and molecules B and C contain 150 kcal/mol of energy. If A and B are converted to C and D then it can be concluded that the reaction is endergonic because 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 is the quantity of energy required to drive the reaction.

Which of the following statements correctly describes some aspect of ATP hydrolysis being used to drive the active transport of an ion into the cell against the ion's concentration gradient?

This is an example of energy coupling. Energy coupling is key feature in the way cells manage their energy resources to do work, which is the use of an exergonic (spontaneous) process to drive an endergonic (nonspontaneous) 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 or transport protein) that powers cellular work.

An exergonic (spontaneous) reaction is a chemical reaction that __________.

releases energy when proceeding in the forward direction Based on their free-energy changes, chemical reactions can be classified as either exergonic ("energy outward") or endergonic ("energy inward"). An exergonic reaction proceeds with a net release of free energy (ΔG < 0). From the equation ΔG = ΔH - TΔS, for ΔG to be negative then either ΔH must be negative (the system gives up enthalpy and H decreases) or TΔS must be positive (the system gives up order and S increases), or both. More than a century of experiments has shown that only processes with a negative ΔG are spontaneous. When ΔH and TΔS are tallied, ΔG has a negative value (ΔG < 0) for all spontaneous processes. In other words, every spontaneous process decreases the system's free energy, and processes that have a positive or zero ΔG are never spontaneous.

In general, the hydrolysis of ATP drives cellular work by __________.

releasing free energy that can be coupled to other reactions Energy coupling is 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. Byproducts such as heat are not effective energy sources for such work.

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 __________.

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.