CH 03 - Energy, catalysis, and biosynthesis

EXAM 1: homework problems

Reactions that build larger molecules in the cell are called ___________; reactions that break down molecules into smaller ones are called ___________.

ANABOLIC, CATABOLIC

Metabolism is the sum total reactions in a cell, which are of two types. Catabolic reactions are the reactions where cells break down the large molecules in foodstuffs into their constituent building blocks. Anabolic reactions are biosynthetic—they build larger molecules from smaller building blocks.

Living systems can generate and maintain order without violating the second law of thermodynamics because they generate

HEAT

Even though living systems produce ordered structures from smaller building blocks, this does not violate the second law of thermodynamics because they still create disorder by releasing energy in the form of heat. Heat is energy in its most disordered form.

Why is sunlight the ultimate source of energy for nearly all living things on Earth?

PHOTOSYNTHETIC ORGANISM PRODUCE FOOD MOLECULE USING LIGHT ENERGY

Organic molecules for energy are obtained by animals through their diet, by consuming either plants or animals that eat plants. Plants produce organic molecules using energy from the sun, thus the source of energy from food molecules in plants is ultimately from the sun.

Cellular respiration ___________ energy and produces ___________, whereas photosynthesis ___________ energy and produces ___________.

PRODUCES: WATER + CARBON DIOXIDE

CONSUMES: OXYGEN + SUGARS

Cellular respiration is the process by which cells break down food molecules for energy, producing water and carbon dioxide as waste products. Photosynthesis uses energy from the sun to make food molecules (sugars) and oxygen.

In an enzymatic reaction, a molecule gains an electron. This is known as a(n) ___________ reaction.

REDUCTION

Gain of electrons by a molecule is called reduction; loss of electrons is called oxidation.

Why is CO₂ an end product of cellular respiration?

IT IS THE MOST STABLE FORM OF CARBON IN OUR ATMOSPHERE

Cellular respiration is the breakdown of carbon-containing molecules through oxidation to release energy. Because the most energetically stable form of carbon in the presence of oxygen is carbon dioxide, this is the final end product of respiration.

If a reaction is energetically favorable (exergonic), then it must produce a(n)

INCREASE IN ENTROPY (DISORDER)

Reactions that are energetically favorable increase the entropy (or disorder) of the universe. This can be through either the release of free energy in different forms, or the release of energy as heat (energy in its most disordered form).

Enzymes increase the speed of a chemical reaction because they

LOWER ACTIVATION ENERGY TO START THE REACTION

Enzymes work by lowering the threshold energy input, called the activation energy, needed to start a reaction.

If the products of a reaction have more free energy than the reactants, then that reaction is

NOT ENERGETICALLY FAVORABLE

Reactions that have products with LESS free energy than the reactants will release free energy and create more disorder in the system. These will proceed spontaneously and are energetically favorable. However, when products of a reaction have MORE free energy than the reactants, there is no release of energy, and the reaction is not energetically favorable.

What is the value of ΔG at equilibrium?

ZERO

When equilibrium is reached, the forward and reverse reactions are equal, so there is no net change in the number of reactants and products. The reaction does not proceed forward or backward at this point and ΔG is equal to zero.

What is the relationship between ΔG and ΔG⁰?

ΔG⁰ IS THE FREE ENERGY CHAGNE AT STANDAR TEMP AND CONCENTRATIONS

The value of the free-energy change of a reaction, ΔG, is dependent upon the concentration of reactants and products, as well as the temperature of the system. ΔG⁰ is the standard free-energy change at standard temperature and concentrations to allow comparison of different reactions.

For a reaction Y à X with a very high equilibrium constant, K, which of the following is true? You can refer to the following two equations in formulating your answer.

ΔG = ΔG° + RT ln [X]/[Y]

THE AMT OF PROD WILL BE HIGHER THAN THE AMT OF SUBSTRATE AT EQUILLIBRIUM

If the equilibrium constant, K, is very high, it means that there is far more of the product X than substrate Y at equilibrium (i.e., when forward and reverse reaction rates are balanced). This corresponds to a free-energy change that is large and negative.

For the following uncatalyzed reaction at equilibrium, how would the reaction change if it was now an enzyme-catalyzed reaction at equilibrium?

FORWARD REACTION A WOULD INCREASE

Enzymes lower the activation energy for reactions and thus increase the probability that the energy barrier can be surpassed for the reaction to proceed. They do not change the equilibrium point (the relative levels of reactants and product at equilibrium).

What is the role of activated carriers in cells?

CAPTURE ENERGY FROM ENERGY RELEASING REACTIONS + TRANSFER IT TO OTHER REACTIONS

Activated carriers collect electrons from oxidation of molecules in catabolic reactions and transfer them to anabolic reactions that require electrons for the reduction of molecules.

Which of the following is a reason why ATP hydrolysis has a negative ΔG⁰?

THE REMOVAL OF THE PHOSPHATE IS ENERGETICALLY FAVORABLE

The products of the ATP hydrolysis reaction—ADP and free phosphate—are more stable and have a lower free energy. Release of free phosphate is energetically favorable because it relieves the repulsion of the negative charges of the neighboring phosphate groups and the aqueous environment to make hydrogen bonds with the phosphate.

In the following condensation reaction, how does ATP power the formation of product A–B?

A PHOSPHATE IS TRANSFERRED TO A-OH TO FORM A HIGH ENERGY INTERMEDIATE

ATP powers the formation of energetically unfavorable bonds between two molecules by first breaking its own high-energy phosphoanhydride bond and transferring the phosphate to another molecule. This new phosphate linkage can then be broken to power the bond formation between molecules.

What purpose does NADPH serve in biosynthetic reactions like the one pictured below?

DONATING ELECTRONS FOR A REDUCTION REACTION

NADPH is a carrier of electrons that are used in reduction reactions often used in biosynthesis of molecules. In the process, NADPH donates electrons, and itself becomes oxidized to NADP+.

Why is the oxidation of NADPH energetically favorable?

THE OXIDIZED FORM OF NADPH IS MORE STABLE THAN THE REDUCED

NADPH carries two high-energy electrons. Losing these electrons in an oxidation reaction is energetically favorable because the oxidized form, NADP+, is more stable than the reduced form (NADPH).

Which of the following has a higher concentration in the cell to allow it to be available to accept electrons from oxidation of food molecules?

• NAD+

• NADH

• NADPH

• NADP+

NAD+

Cells maintain high concentrations of NAD+ so that it is readily available to accept electrons from the oxidation of food molecules in catabolic reactions.

Condensation reactions are energetically ___________ and hydrolysis reactions are energetically ___________.

UNFAVORABLE, FAVORABLE

Condensation reactions reduce the disorder in the universe and are energetically unfavorable. Hydrolysis reactions increase the disorder in the universe and are energetically favorable.