Porth's Essentials of Pathophysiology, 4e

10

Cell and Tissue Function

U N I T 1

Cell metabolism is the process that converts dietary fuels from carbohydrates, proteins, and fats into adenosine triphosphate (ATP), which provides energy for the cell. Adenosine triphosphate is formed through three major pathways: (1) the glycolytic pathway, (2) the citric acid cycle, and (3) the electron transport chain. In fuel metabolism, which is an oxidation–reduction reaction, the fuel donates electrons and is oxidized, and the coenzymes nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) accept electrons and are reduced. U N D E R S T A N D I N G Cell Metabolism

C

1

O

C

Glycolytic Pathway. Glycolysis, which occurs in the cytoplasm of the cell, involves the splitting of the six- carbon glucose molecule into two three-carbon molecules of pyruvic acid. Because the reaction that splits glucose requires two molecules of ATP, there is a net gain of only two molecules of ATP from each mol- ecule of glucose that is metabolized. The process is anaerobic and does not require oxygen (O 2 ) or produce carbon dioxide (CO 2 ). When O 2 is present, pyruvic acid moves into the mitochondria, where it enters the aerobic citric acid cycle. Under anaerobic conditions, pyruvate is converted to lactic acid, allowing glycolysis to continue as a means of supplying cells with ATP when O 2 is lacking. Citric Acid Cycle. Under aerobic conditions, both of the pyruvic acid molecules formed by the glycolytic pathway enter the mitochondria, where each combines with acetyl- coenzyme to form acetyl-coenzyme A (acetyl-CoA). The formation of acetyl-CoA begins the reactions that occur in the citric acid cycle. Some reactions release CO 2 and some transfer electrons from the hydrogen atom to NADH or FADH. In addi- tion to pyruvic acid from the glycol- ysis of glucose, fatty acid and amino acid breakdown products can also enter the citric acid cycle. Fatty acids, which are the major source of fuel in the body, are oxidized by a process called β - oxidation to acetyl-CoA for entry into the citric acid cycle.

C

C

C C

1 Glucose

2

ATP

2 ADP P +

2

C C C

C C C P

P

2 NAD +

P 4 ADP + 4

NADH + 2H +

4

ATP

2

Lactic acid (anaerobic)

Pyruvic acid (aerobic)

Pyruvic acid

2

Acetyl-coenzyme A

To electron transport chain

NADH + H +

CoA

FADH 2

Citric acid cycle

NADH + H +

CO 2

CO 2

ATP

NADH + H +

To electron transport chain