1. Metabolic Pathways of Carbohydrates
Digestion of Carbohydrates
Glycolysis: Oxidation of Glucose
Pathways for Pyruvate

2. Digestion of Carbohydrates
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.

3. Digestion of Carbohydrates

4. Glycolysis
In Stage 2, the metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C).

5. 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.

6. Glycolysis: Energy-Investment1 2 3 4 5

7. Glycolysis: Energy-Production
In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules.

8. Glycolysis: Reactions 6-107 8 9 10

10. Glycolysis: Overall Reaction
Glycolysis generates 2 ATP and 2 NADH.
Two ATP are used in energy-investment to add phosphate groups to glucose and fructose-6-phosphate.
4 ATP are formed in energy-generation by direct transfers of phosphate groups to 4 ADP.
Glucose + 2ATP + 4ADP + 4Pi + 2NAD+
2Pyruvate + 4ATP + 2ADP + 2NADH + 2H+
Glucose + 2ADP + 2Pi + 2NAD+
2Pyruvate + 2ATP + 2NADH + 2H+
Reoxidation of 2 NADH through Respiratory Chain will produce 6 ATP
Total ATP gain = 4ATP -2ATP + 6ATP = 8 ATP

11. Glycolysis
In this pathway glucose is converted to pyruvate or lactate, with production of energy (ATP).
Glycolysis is the major oxidative pathway for the utilization of glucose and is found in all cells.
It is unique pathway in that:
It can utilize O2 (aerobic), in the cells with mitochondria to yield pyruvate.
It can function in the absence of O2 (anaerobic) as in exercising muscle, or in cells that lack mitochondria (RBCs), to yield lactate.
All enzymes of glycolysis are found in the cytosol.

12. 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.

13. Pathways for Pyruvate
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
CH3—C—S—CoA + CO2 + NADH + H+
acetyl CoA
Pyruvate Kinase

14. Lactate dehydrogenase
Lactate Formation
When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis. O ||
CH3—C—COO- + NADH + H+
pyruvate OH |
CH3—CH—COO- + NAD+
lactate
Lactate dehydrogenase

15. Lactate in Muscles 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.

16. Fermentation Occurs in anaerobic microorganisms such as yeast.
Decarboxylates pyruvate to acetaldehyde, which is reduced to ethanol.
Regenerates NAD+ to continue glycolysis. O ||
CH3—C—COOH + NADH + H+
Pyruvic acid OH |
CH3—CH2 + NAD+ + CO2
Ethanol

17. Pathways for Pyruvate