Porth's Essentials of Pathophysiology, 4e

11

Cell Structure and Function

C h a p t e r 1

INTACT MITOCHONDRIUM

3

Intermembrane space

Electron Transport Chain. At the completion of the citric acid cycle, each glucose molecule has yielded only four new molecules of ATP (two from glycolysis and two from the citric acid cycle). In fact, the principal function of these ear- lier stages is to make the electrons (e – ) from glucose and other food substrates available for oxidation. Oxidation of the electrons carried by NADH and FADH 2 is accom- plished through a series of enzy- matically catalyzed reactions in the mitochondrial electron transport chain. During these reactions, pro- tons (H + ) combine with O 2 to form water (H 2 O), and large amounts of energy are released and used to add a high-energy phosphate bond to adenosine diphosphate (ADP), con- verting it to ATP. There is a net yield of 36 molecules of ATP from 1 mol- ecule of glucose (2 from glycolysis, 2 from the citric acid cycle, and 32 from the electron transport chain). In general, the net amount of ATP formed from each gram of protein that is metabolized is less than for glucose, whereas that obtained from fat is greater (e.g., each 16-carbon fatty acid molecule yields about 129 molecules of ATP).

Inner membrane

Outer membrane

Matrix

Outer membrane

ATP synthesis

H + H + H +

Inner membrane

Matrix

e -

O 2

NADH H +

H +

H + H +

ADP ATP

+ Pi

2 H 2

O

H +

Electron-transport chain

the cytoplasm and the aerobic pathways in the mitochondria. The most efficient of these pathways is the aerobic citric acid cycle and electron transport chain in the mitochondria. This pathway, which requires oxygen, produces carbon dioxide and water as end products and results in the release of large amounts of energy that is used to convert adenosine diphosphate (ADP) to ATP. The glycolytic pathway in the cytoplasm involves the breakdown of glucose to form ATP. This pathway can function without oxygen by producing lactic acid.

SUMMARY CONCEPTS

■■ Metabolism is the process whereby the carbohydrates, fats, and proteins from the foods we eat are broken down and subsequently converted into the energy needed for cell function. Energy is stored in the high-energy phosphate bonds of adenosine triphosphate (ATP), which serves as the energy currency for the cell. ■■ Two sites of energy conversion are present in cells: the anaerobic glycolytic pathway in