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The individual steps in the catabolism of carbohydrates are known in great detail.
For the most part, cells break down carbohydrates by similar metabolic pathways whether they are plant cells, animal cells, or bacterial cells.
The central pathway, glycolysis, found in most cells is outlined in Figure 10-2; however, there are alternative pathways for oxidizing carbohydrates. 
Sugars are cleaved from polysaccharides such as starch and glycogen by the action of phosphorylase enzymes.
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The phosphorolysis introduces inorganic phosphate and produces glucose-1-phosphate as shown in the upper half of Figure 10-3.
Disaccharides, such as sucrose and maltose, are hydrolyzed by saccharases without the addition of phosphate, thereby releasing their constituent monosaccharides. These sugars are phosphorylated by enzymatic reaction with ATP.
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The function of this phosphorylation is twofold:
(1) It increases the energy content of the molecule and
(2) It introduces a charged moiety into the molecule, making it relatively impermeable to membranes and therefore unlikely to diffuse out of the cell or into an organelle.
The phosphorylated sugars produced from either of these sources enter the enzymatic reactions outlined in Figure 10-2, producing the intermediate pyruvate. This central pathway of carbohydrate metabolism is called glycolysis.
Glycolysis:
The major features of glycolysis are as follows:
1. The sugars are first doubly phosphorylated. In the case of mono-saccharides such as glucose, fructose, mannose, and the like, 2 moles of ATP per mole of monosaccharide are utilized. Sugars that are derived from glycogen and starch require only 1 mole of ATP per mole of glucose-equivalent, because the first inorganic phosphate is acquired during the phosphorolysis of the polysaccharide.
2. The six-carbon sugar diphosphate is split by aldolase (10-8), producing two three-carbon units, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate; the latter subsequently forms a second mole of glyceraldehyde-3-phosphate (10-9).
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3. A major oxidation and phosphorylation of the substrate is catalyzed by glyceraldehydeS-phosphate dehydrogenase. Two moles of hydrogen are removed per mole of substrate and reduce 2 moles of the coenzyme NAD +. In the same reaction, inorganic phosphate is incorporated into the acid (10-10).
4. In the final steps of glycolysis, the intermediates are dephosphorylated by reaction with ADP For each mole of monosaccharide oxidized to pyruvate, 2 miles of ATP are consumed and 4 moles of ATP are produced, resulting in a net production of 2 moles of ATP. Note that when the original sugar molecule is derived from glycogen or starch by phosphorolysis, there is a net production of 3 moles of ATP per mole of glucose.
Pyruvate does not accumulate in very large amounts in cells. Instead, it is converted into other products (Fig. 10-4). The enzymes that act on pyruvate vary among different kinds of organisms and with the nature of the environment.
The more common fates of pyruvate are:
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(1) Its fermentation to ethanol and carbon dioxide in cells such as yeast,
(2) Its anaerobic conversion into lactate in cells such as muscle cells, and
(3) Its conversion into acetate in the mitochondria of cells and organisms living under aerobic conditions.
