Summary

1. Metabolism is the collective term for all of the reactions going on inside a cell. These reactions are divided into catabolic—those that break down chemical compounds to provide energy—and anabolic—those that build up complex molecules from simpler ones. Catabolic reactions are oxidative and anabolic reactions are reductive.

2. The Krebs cycle is at the center of the cell's metabolism. It can act to oxidize two carbon units derived from carbohydrates, fats, or amino acids and acts as a central switching yard for the molecules in metabolism.

3. Glycolysis converts glucose to pyruvate. If the pyruvate is reduced to lactate, glycolysis can continue in the absence of oxygen as this reduction regenerates the NAD+ needed for glycolysis.

4. Glycogen can act as a reserve of glucose for glycolysis.

5. Glucose-6-phosphate can be converted to a pentose sugar for nucleotide manufacture with production of NADPH for biosynthesis.

6. Fats (triacylglycerols) are concentrated fuel stores. Their fatty acid components are oxidized to two-carbon units by j oxidation.

7. Amino groups must be removed before excess dietary amino acids can be used as fuels. This is done by transferring them to make aspartate or glutamate, and thence to urea for excretion.

8. Gluconeogenesis allows the synthesis of glucose from noncarbohydrate precursors (but mammals cannot make glucose from fatty acids).

9. Biosynthetic pathways for molecules follow different routes from the catabolic pathways. Good examples are fatty acid synthesis and breakdown (j oxidation) and glycogen synthesis and breakdown.

10. Plants use light energy to generate ATP and reducing power as NADPH and hence to fix carbon dioxide into sugars.

11. Metabolic reactions are controlled by feedforward and feedback mechanisms, which make use of allosteric control and covalent modification of key enzymes.

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