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Chapter 3: Problem 9

Which of the following reactions will occur only if coupled to a second, energetically favorable reaction? A. glucose \(+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\) B. \(\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow\) glucose \(+\mathrm{O}_{2}\) C. nucleoside triphosphates \(\rightarrow\) DNA D. nucleotide bases \(\rightarrow\) nucleoside triphosphates E. \(A D P+P_{i} \rightarrow\) ATP

Short Answer

Expert verified
Reactions B, D, and E require coupling; however, the most commonly textbook-coupled is E (ADP + Pi 鈫� ATP).

Step by step solution

01

Understanding Energetic Favorability

For a reaction to occur spontaneously, it must be energetically favorable, meaning it has a negative Gibbs free energy change (9G < 0). Reactions that convert high-energy molecules to lower-energy products are typically favorable, while reactions that build complex molecules from simpler ones often require energy input.
02

Analyzing Reaction A

Reaction A: Glucose + O鈧� 鈫� CO鈧� + H鈧侽 is a combustion reaction. This reaction releases energy and is highly exergonic (energetically favorable, 9G < 0). Thus, it can occur spontaneously without coupling.
03

Analyzing Reaction B

Reaction B: CO鈧� + H鈧侽 鈫� Glucose + O鈧� is photosynthesis. This is an endergonic process (energetically unfavorable, 9G > 0), requiring energy input to proceed. It will only occur when coupled with energy from sunlight or another favorable reaction.
04

Analyzing Reaction C

Reaction C: Nucleoside triphosphates 鈫� DNA, this polymerization reaction requires energy to form the phosphodiester bonds in DNA, thus it is not spontaneous and needs to be coupled with an energy-releasing reaction.
05

Analyzing Reaction D

Reaction D: Nucleotide bases 鈫� Nucleoside triphosphates requires energy to attach phosphate groups to the nucleotide bases, making it an energetically unfavorable process (endergonic), and it must be coupled with an energetically favorable reaction.
06

Analyzing Reaction E

Reaction E: ADP + Pi 鈫� ATP involves forming a high-energy phosphate bond and is an energetically unfavorable process (endergonic), typically coupled with reactions such as cellular respiration or photosynthesis to proceed.
07

Identifying the Correct Option

Reactions B, D, and E require coupling to energetically favorable reactions to occur due to their endergonic nature. As the question is asking for the one out of multiple possibilities, let's identify any characteristic difference.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Gibbs Free Energy Change
The concept of Gibbs free energy is crucial in determining whether a chemical reaction can occur spontaneously. A reaction is considered spontaneous when it proceeds without needing extra energy input from its surroundings. This is determined by the Gibbs free energy change, often denoted as \( \Delta G \). The formula to calculate this change is: \[ \Delta G = \Delta H - T \Delta S \], where \( \Delta H \) is the change in enthalpy, \( T \) is the temperature in Kelvin, and \( \Delta S \) is the change in entropy.
  • \( \Delta G < 0: \) The reaction is exergonic and energetically favorable.
  • \( \Delta G = 0: \) The reaction is at equilibrium and no net change occurs.
  • \( \Delta G > 0: \) The reaction is endergonic and requires energy input to proceed.
Understanding the Gibbs free energy change helps in evaluating whether a reaction needs to be coupled to a more favorable one.
Exergonic Reactions
Exergonic reactions are a core type of energetic reactions driven by the release of energy. These reactions have a negative Gibbs free energy change (\( \Delta G < 0 \)). An example of an exergonic reaction is the combustion of glucose: \[ \text{glucose} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \]In this reaction, the energy stored in the chemical bonds of glucose is released, resulting in low-energy molecules like carbon dioxide and water.
  • Highly energetic molecules break down into products with less stored energy.
  • No additional energy from the environment is required for the reaction to proceed.
  • Exergonic reactions are often associated with processes like respiration where energy is made available for cellular work.
These characteristics make exergonic reactions crucial for many biological processes.
Endergonic Reactions
Endergonic reactions involve the uptake or requirement of energy to proceed. They are characterized by a positive Gibbs free energy change (\( \Delta G > 0 \)). An example of an endergonic process is photosynthesis:\[ \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \text{glucose} + \mathrm{O}_2 \]Here, energy not spontaneously available in the reactants is required to drive the production of high-energy glucose molecules.
  • The reaction converts lower-energy molecules into higher-energy products.
  • Energy is typically supplied from ATP or environmental sources like sunlight.
  • Endergonic reactions are vital for biosynthetic processes, like the creation of complex molecules from simpler ones.
These reactions embody the concept that biological systems need external energy input to build new complex structures.
Reaction Coupling
Reaction coupling is a fundamental process that allows energy requiring (endergonic) reactions to proceed. In biological systems, many critical reactions are not favorable on their own. Hence, they are coupled to exergonic reactions. For instance: The synthesis of ATP from ADP and Pi:\[ \text{ADP} + P_i \rightarrow \text{ATP} \] is endergonic, but can occur when coupled with energy-releasing reactions like cellular respiration. With reaction coupling:
  • The total Gibbs free energy change (G) of the combined reactions must be negative.
  • Coupling helps in harnessing the energy released from exergonic reactions for driving endergonic ones.
  • It is a vital strategy in metabolism to allow for the achievement of complex cellular functions.
This coupling mechanism ensures that energy is efficiently utilized, allowing biological processes to maintain structure and function.

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Most popular questions from this chapter

Which of the following statements are correct? Explain your answers. A. Some enzyme-catalyzed reactions cease completely if their enzyme is absent. B. High-energy electrons (such as those found in the activated carriers NADH and NADPH) move faster around the atomic nucleus. C. Hydrolysis of ATP to AMP can provide about twice as much energy as hydrolysis of ATP to ADP. D. A partially oxidized carbon atom has a somewhat smaller diameter than a more reduced one. E. Some activated carrier molecules can transfer both energy and a chemical group to a second molecule. F. The rule that oxidations release energy, whereas reductions require energy input, applies to all chemical reactions, not just those that occur in living cells. G. Cold-blooded animals have an energetic disadvantage because they release less heat to the environment than warm-blooded animals do. This slows their ability to make ordered macromolecules. H. Linking the reaction \(X \rightarrow Y\) to a second, energetically favorable reaction \(Y \rightarrow Z\) will shift the equilibrium constant of the first reaction.

A reaction in a single-step biosynthetic pathway that converts a metabolite into a particularly vicious poison (metabolite \(\rightleftharpoons\) poison) in a mushroom is energetically highly unfavorable. The reaction is normally driven by ATP hydrolysis. Assume that a mutation in the enzyme that catalyzes the reaction prevents it from utilizing ATP, but still allows it to catalyze the reaction. A. Do you suppose it might be safe for you to eat a mushroom that bears this mutation? Base your answer on an estimation of how much less poison the mutant mushroom would produce, assuming the reaction is in equilibrium and most of the energy stored in ATP is used to drive the unfavorable reaction in nonmutant mushrooms. B. Would your answer be different for another mutant mushroom whose enzyme couples the reaction to ATP hydrolysis but works 100 times more slowly?

The rate of a simple enzyme reaction is given by the standard Michaelis-Menten equation: \\[ \text { rate }=V_{\max }[\mathrm{S}] /\left([\mathrm{S}]+\mathrm{K}_{\mathrm{M}}\right) \\] If the \(V_{\max }\) of an enzyme is 100 \mumole/sec and the \(K_{M}\) is \(1 \mathrm{mM},\) at what substrate concentration is the rate 50 \mumole/sec? Plot a graph of rate versus substrate (S) concentration for \([\mathrm{S}]=0\) to \(10 \mathrm{mM}\). Convert this to a plot of 1/rate versus \(1 /[\mathrm{S}] .\) Why is the latter plot a straight line?

Consider the equation \\[ \begin{array}{c} \text { light energy }+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \\\ \text { sugars }+\mathrm{O}_{2}+\text { heat energy } \end{array} \\] Would you expect this reaction to occur in a single step? Why must heat be generated in the reaction? Explain your answers.

The phosphoanhydride bond that links two phosphate groups in ATP in a high- energy linkage has a \(\Delta G^{\circ}\) of -7.3 kcal/mole. Hydrolysis of this bond in a cell liberates from 11 to 13 kcal/mole of usable energy. How can this be? Why do you think a range of energies is given, rather than a precise number as for \(\Delta G^{\circ} ?\)
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