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Chapter 17: Problem 33

What is a coupled reaction? What is its importance in biological reactions?

Short Answer

Expert verified
A coupled reaction is a chemical reaction where an energy-releasing (exergonic) reaction happens alongside an energy-absorbing (endergonic) reaction. The energy released by the exergonic reaction drives the endergonic reaction. In biological systems, coupled reactions are crucial, allowing energetically unfavorable reactions, necessary for life processes, to take place by utilizing the energy produced from energetically favorable reactions.

Step by step solution

01

Understand and Define Coupled Reaction

A coupled reaction is a type of chemical reaction where an energetically unfavorable reaction occurs simultaneously with an energetically favorable one. The favorable reaction provides the energy necessary for the unfavorable one to occur, and hence the two reactions are 'coupled'.
02

Importance in Biological Reactions

The importance of coupled reactions in biological context lies in the energy transfer. In a living organism, many processes require energy, that is often derived from unfavorable reactions. These reactions can't proceed without an external energy source. This is where coupled reactions come into play. The energy released from favorable reactions, structurely or spatially related, can be used to drive the unfavorable reactions, enabling biological processes to take place.

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Key Concepts

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

Chemical Reaction
In the realm of chemistry, a chemical reaction is a process in which substances, known as reactants, interact to form new substances called products. During a chemical reaction, bonds between atoms in the reactants are broken, rearranged, and reformed into new configurations. This process can release or absorb energy.
  • Reactants: Starting substances that undergo change.
  • Products: New substances formed after the reaction.
  • Bond Energy: Energy needed to break and form chemical bonds.
Understanding chemical reactions is fundamental since they are involved in everything from making medicine to producing energy, and are essential to countless daily activities.
Energy Transfer
Energy transfer is a core concept in science that describes the movement of energy from one place, system, or form to another. In the context of chemical reactions, it refers to how energy is released or absorbed when reactants are converted into products.

Energy can exist in different forms, such as:
  • Kinetic energy: Energy of motion.
  • Potential energy: Stored energy due to position or structure.
  • Thermal energy: Heat energy generated during reactions.
In chemical reactions, particularly in biological contexts, energy transfer is pivotal as it powers cellular processes necessary for life.
Biological Processes
Biological processes encompass the complex sequences of chemical reactions and physical changes within living organisms that are essential for life's maintenance and growth. These processes include respiration, digestion, and cellular metabolism, all of which rely heavily on chemical reactions.

Here are a few key points about biological processes:
  • Metabolism: Network of chemical reactions in cells.
  • Photosynthesis: Conversion of light energy into chemical energy by plants.
  • Respiration: Release of energy from glucose in cells for survival.
Coupled reactions are critical in these processes as they manage the energy requirements for various cellular activities.
Unfavorable Reaction
An unfavorable reaction is a type of chemical reaction that, on its own, would not naturally proceed due to insufficient energy. In other words, it absorbs more energy than it releases, making it non-spontaneous.

Characteristics of unfavorable reactions:
  • Require input energy to proceed.
  • Often linked with surmounting an energy barrier.
  • Drive various necessary but energy-intensive biological activities.
These reactions become feasible in biological systems when coupled with favorable reactions that provide the needed energy, illustrating their practicality in controlled ecological settings.
Favorable Reaction
A favorable reaction is a chemical reaction that occurs spontaneously, releasing more energy than it consumes. It is considered energetically advantageous and is often used to drive other processes that require energy input.

Key characteristics of favorable reactions include:
  • Provide surplus energy beneficial for other reactions.
  • Usually have a negative change in Gibbs free energy (\( riangle G < 0 \)).
  • Occur naturally and help maintain equilibrium in nature.
In biological systems, these reactions often couple with unfavorable ones, allowing necessary biological processes to proceed efficiently.

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Most popular questions from this chapter

The reaction \(\mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\) proceeds spontaneously at \(25^{\circ} \mathrm{C}\) even though there is a decrease in the number of microstates of the system (gases are converted to a solid). Explain.

Consider two carboxylic acids (acids that contain the \(-\mathrm{COOH}\) group \(): \mathrm{CH}_{3} \mathrm{COOH}\) (acetic acid, \(\left.K_{\mathrm{a}}=1.8 \times 10^{-5}\right)\) and \(\mathrm{CH}_{2} \mathrm{ClCOOH}\) (chloroacetic acid, \(K_{\mathrm{a}}=1.4 \times 10^{-3}\) ). (a) Calculate \(\Delta G^{\circ}\) for the ionization of these acids at \(25^{\circ} \mathrm{C}\). (b) From the equation \(\Delta G^{\circ}=\Delta H^{\circ}-T \Delta S^{\circ},\) we see that the contributions to the \(\Delta G^{\circ}\) term are an enthalpy term \(\left(\Delta H^{\circ}\right)\) and a temperature times entropy term \(\left(T \Delta S^{\circ}\right) .\) These contributions are listed below for the two acids: $$ \begin{array}{lcc} \hline & \Delta H^{\circ}(\mathrm{k} \mathrm{J} / \mathrm{mol}) & T \Delta S^{\circ}(\mathrm{k} \mathrm{J} / \mathrm{mol}) \\ \hline \mathrm{CH}_{3} \mathrm{COOH} & -0.57 & -27.6 \\ \mathrm{CH}_{2} \mathrm{ClCOOH} & -4.7 & -21.1 \\ \hline \end{array} $$ Which is the dominant term in determining the value of \(\Delta G^{\circ}\) (and hence \(K_{\mathrm{a}}\) of the acid)? (c) What processes contribute to \(\Delta H^{\circ} ?\) (Consider the ionization of the acids as a Br酶nsted acid-base reaction.) (d) Explain why the \(T \Delta S^{\circ}\) term is more negative for \(\mathrm{CH}_{3} \mathrm{COOH}\)

Consider the following reaction at \(25^{\circ} \mathrm{C}\) : $$ \mathrm{Fe}(\mathrm{OH})_{2}(s) \rightleftharpoons \mathrm{Fe}^{2+}(a q)+2 \mathrm{OH}^{-}(a q) $$ Calculate \(\Delta G^{\circ}\) for the reaction. \(K_{\mathrm{sp}}\) for \(\mathrm{Fe}(\mathrm{OH})_{2}\) is \(1.6 \times 10^{-14}\)

As an approximation, we can assume that proteins exist either in the native (or physiologically functioning) state and the denatured state: $$ \text { native } \rightleftharpoons \text { denatured } $$ The standard molar enthalpy and entropy of the denaturation of a certain protein are \(512 \mathrm{~kJ} / \mathrm{mol}\) and \(1.60 \mathrm{~kJ} / \mathrm{K} \cdot \mathrm{mol},\) respectively. Comment on the signs and magnitudes of these quantities, and calculate the temperature at which the process favors the denatured state.

The molar heat of vaporization of ethanol is 39.3 kJ/mol and the boiling point of ethanol is \(78.3^{\circ} \mathrm{C}\). Calculate \(\Delta S\) for the vaporization of 0.50 mol ethanol.
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