Page 976 - Fundamentals of anatomy physiology

Chapter 25 Metabolism and Energetics 963

Figure 25–1 An Introduction to Cellular Metabolism. Cells obtain organic molecules from the interstitial fluid and break

them down to produce ATP. Only about 40 percent of the energy released by catabolism is captured in the form of ATP; the rest is
lost as heat. The ATP generated by catabolism provides energy for all vital cellular activities, including anabolism.

INTERSTITIAL Plasma membrane
FLUID

Results of Anabolism

• Maintenance and
repairs

• Growth
• Secretion
• Nutrient reserves

CATABOLISM ANABOLISM

Organic Molecules NUTRIENT POOL
• Amino acids
• Lipids Anaerobic catabolism in the ATP
• Simple sugars cytosol releases small amounts

HEAT of ATP that are significant only

under unusual Other ATP Expenses

conditions. Aerobic Metabolism • Movement
• Contraction
(in mitochondria) • Intracellular transport
• Cytokinesis
60% 40% ATP • Endocytosis
CYTOSOL • Exocytosis

lipids, and simple sugars cross the plasma membrane and join together, they capture about 40 percent of the energy released
nutrients already in the cytosol. All the cell’s organic building and use it to convert ADP to ATP. The other 60 percent escapes
blocks collectively form a nutrient pool that the cell draws on as heat that warms the cell and the surrounding tissues.
to provide energy and to create new intracellular components.
The ATP produced by mitochondria provides energy to
The breakdown of organic molecules is called catabolism.
This process releases energy that can be used to synthesize ATP 25support both anabolism, which is the synthesis of new or-
or other high-energy compounds. Catabolism proceeds in a se-
ries of steps. In general, the first steps take place in the cytosol. ganic molecules, and other cell functions. Those functions vary
Enzymes there break down large organic molecules previously among cell types, and they include ciliary or cell movement,
assembled by the cell (such as glycogen, triglycerides, or pro- contraction, active transport, and cell division. For example,
teins) into smaller fragments that join the nutrient pool. For muscle fibers need ATP to provide energy for contraction, but
example, carbohydrates are broken down into simple sugars, gland cells need ATP to synthesize and transport secretions. We
triglycerides are split into fatty acids and glycerol, and proteins have considered such specialized functions in other chapters,
are broken down to individual amino acids. so here we focus on anabolic processes.

These preparatory steps produce relatively little ATP. How- In terms of energy, anabolism is an “uphill” process that
ever, further catabolic activity yields smaller organic molecules involves the formation of new chemical bonds. Cells synthesize
that mitochondria can absorb and process. Mitochondrial ac- new organic components for four basic reasons:
tivity releases significant amounts of energy. As mitochondrial
enzymes break the covalent bonds that hold these molecules 1. To Carry Out Structural Maintenance or Repairs. All cells
must expend energy for ongoing maintenance and repairs,
because most structures in the cell are temporary rather