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23.4 Digestion of Carbohydrates

23.5 Glycolysis: Oxidation of Glucose

23.6 Pathways for Pyruvate

Chapter 23 Metabolic Pathways for Carbohydrates

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In the mouth, salivary amylase hydrolyzes -glycosidic bonds in polysaccharides to give

smaller polysaccharides (dextrins), maltose, and some glucose.

In the small intestine, pancreatic amylase hydrolyzes dextrins to maltose and glucose.

The disaccharides maltose, lactose, and sucrose are hydrolyzed to monosaccharides.

The monosaccharides enter the bloodstream for transport to the cells.

Stage 1: Digestion of Carbohydrates

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Digestion of Carbohydrates

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Glycolysis In Stage 2, the

metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C).

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Glycolysis: Energy-Investment

In reactions 1-5 of glycolysis: Energy is used to add phosphate groups to

glucose and fructose. Glucose is converted to two three-carbon

molecules.

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Glycolysis: Energy Investment

1

2

3

4

5 5

7

Glycolysis: Energy-Production

In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules.

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Glycolysis: Reactions 6-10

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7

8

9

10

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Glycolysis generates 2 ATP and 2 NADH. Two ATP are used in energy-investment to add

phosphate groups to glucose and fructose-6-phosphate.

Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP.

Glucose + 2ADP + 2Pi + 2NAD+ 2Pyruvate + 2ATP + 2NADH + 2H+

Glycolysis: Overall Reaction

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Regulation of Glycolysis Reaction 1 Hexokinase is inhibited by high

levels of glucose-6-phosphate, which prevents the phosphorylation of glucose.

Reaction 3 Phosphofructokinase, an allosteric enzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP.

Reaction 10 Pyruvate kinase, another allosteric enzyme is inhibited by high levels of ATP or acetyl CoA.

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When oxygen is present in the cell, (aerobic conditions), pyruvate from glycolysis is decarboxylated to produce acetyl CoA and CO2. O

|| CH3—C—COOH + HS—CoA + NAD+

pyruvic acid O

|| CH3—C—S—CoA + CO2 + NADH + H+

acetyl CoA

Pathways for Pyruvate

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When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis.

O lactate || dehydrogenase

CH3—C—COO- + NADH + H+

pyruvate OH |

CH3—CH—COO- + NAD+

lactate

Lactate Formation

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Under anaerobic conditions (strenuous exercise): Oxygen in the muscles is depleted. Lactate accumulates in the muscles. Muscles tire and become painful. Rest is needed to repay the oxygen debt and to

reform pyruvate in the liver.

Lactate in Muscles

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Fermentation

Fermentation: Occurs in anaerobic microorganisms such as yeast. Decarboxylates pyruvate to acetaldehyde, which is

reduced to ethanol. Regenerates NAD+ to continue glycolysis. O OH

|| | CH3—C—COOH + NADH + H+ CH3—CH2 + NAD+ + CO2

Pyruvic acid Ethanol

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Pathways for Pyruvate