Unit 9
Carbohydrate Metabolism

Overview

Carbohydrate metabolism (the breakdown of sugars) is considered the central metabolic pathway in vivo. The overall reaction for glucose can be written

C6 H12 O6 + 6 O2 → 6 CO2 + 6 H2O

Considerable energy is released during carbohydrate metabolism. Part of the released energy escapes as heat (which is why we are at 37°C while the temperature of the room around us is 19–20°C). However, ~67% of the released energy is used to synthesise “high-energy” phosphate compounds, such as ATP and creatine phosphate. ATP and creatine phosphate, in turn, are hydrolyzed to provide the energy for all biological needs, such as motion, ion transport, and biosynthesis.

Carbohydrate metabolism can be divided into two stages:

  1. glycolysis (glucose → pyruvate + ATP)
  2. tricarboxylic acid cycle (pyruvate → NADH/FADH2 + CO2 )

A third stage uses the energy stored in the above two processes and converts it into ATP:

electron transport chain (O2 + NADH/FADH2 → ATP + H2O )

Note that each stage produces high-energy compounds (ATP, NADH, FADH2 or some combination), but only one stage (the electron transport chain) uses oxygen to do so.

In addition to the stages of carbohydrate metabolism shown above, this unit discusses glycogen synthesis and degradation. Glycogen is the polymeric storage form of glucose in vivo. The balance between its synthesis and degradation ensures a steady supply of glucose for carbohydrate metabolism.

Unit 9 is divided into six lessons:

  1. Energy I—Introduction to Metabolism
  2. Glycolysis( Glucose → Pyruvate)
  3. Glycogen
  4. Citric Acid Cycle (Pyruvate → NADH/FADH2)
  5. Electron Transport Chain (NADH/FADH2 → ATP)
  6. Energy II—ATP

Objectives

After completing this unit, you should be able to

  1. explain the difference between standard free energies ( ΔG°′) and physiological free energies (ΔG).
  2. describe the three stages of glucose metabolism.
  3. explain how the energy released when glucose is oxidized through the TCA cycle is chemically “stored.”
  4. explain the major mechanisms by which the three stages of glucose metabolism are regulated.
  5. describe the synthesis of ATP by oxidative phosphorylation.
  6. explain what glycogen is, and describe how it is synthesized and degraded.
  7. describe a general cyclic cascade, and define the term “covalent modification.”
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Glossary

acetyl-CoA coenzyme A attached to an acetate group (CH3COO−) via a high-energy bond; acetyl-CoA is the central molecule of metabolism
aerobe oxygen-requiring organism; uses glycolysis, the tricarboxylic acid cycle and the electron transport chain to produce ATP
anaerobe non-oxygen-requiring organism; uses only glycolysis to produce ATP
electrostatic repulsion the force repelling two ionic groups each holding the same charge
entropy thermodynamic measure of disorder
NADH2 flavin adenine dinucleotide (reduced form); a molecule, central to metabolism, which carries a pair of electrons only slightly less energetic than those of NADH
metabolism a highly integrated set of reactions by which energy and reducing power are extracted from the environment and biological macromolecules are synthesized
NADH nicotinamide adenine dinucleotide (reduced form); a molecule, central to metabolism, which carries a pair of high-energy electrons
nonequilibrium thermodynamics the steady-state conditions that exist
in vivo
physiological free energy the amount of energy available to a organism based on both the standard free energy of a reaction and also the actual concentrations of the reactants and products of this reaction in vivo
resonance presence of (2n + 4) double bond electrons, in alternating bonds, in a planar organic compound
standard free energy thermodynamic value; amount of energy released when a mole of reactant is allowed to come to equilibrium (with product) under fixed conditions of temperature, pressure, and hydronium ion concentration
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