I. The flow of energy in the cell is called bioenergetics.
B. According to the second law of thermodynamics, all energy transformation reactions result in an increase in entropy (disorder).
1. As a result of the increase in entropy, there is a decrease in free (usable) energy.
2. Atoms that are organized into large organic molecules thus contain more free energy than more-disorganized, smaller molecules.
C. In order to produce glucose from carbon dioxide and water, energy must be added.
1. Plants use energy from the sun for this conversion, in a process called photosynthesis.
2. Reactions that require the input of energy to produce molecules with higher free energy than the reactants are called endergonic reactions.
D. The combustion of glucose to carbon dioxide and water releases energy in the form of heat.
1. A reaction that releases energy, thus forming products that contain less free energy than the reactants, is called an exergonic reaction.
2. The same total amount of energy is released when glucose is converted into carbon dioxide and water within cells, even though this process occurs in many small steps.
1. Some of the chemical-bond energy in glucose is therefore transferred to the "high energy" bonds of ATP.
2. The breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate results in the liberation of energy.
3. The energy liberated by the breakdown of ATP is used to power all of the energy-requiring processes of the cell. ATP is thus the "universal energy carrier" of the cell.
II. Oxidation-reduction reactions are coupled and usually involve the transfer of hydrogen atoms.
A. A molecule is said to be oxidized when it loses electrons; it is said to be reduced when it gains electrons.
B. A reducing agent is thus an electron donor; an oxidizing agent is an electron acceptor.
C. Although oxygen is the final electron acceptor in the cell, other molecules can act as oxidizing agents.
D. A single molecule can be an electron acceptor in one reaction and an electron donor in another.
1. NAD and FAD can become reduced by accepting electrons from hydrogen atoms removed from other molecules.
2. NADH + H+, and FADH2, in turn, donate these electrons to other molecules in other locations within the cells.
3. Oxygen is the final electron acceptor (oxidizing agent) in a chain of oxidation-reduction reactions that provide energy for ATP production.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.