Page 977 - Fundamentals of anatomy physiology

964 Unit 5 Environmental Exchange

than permanent. Their removal and replacement are part in the body. They perform a variety of vital functions for

of the process of metabolic turnover. p. 85 the cell, and when energy is available, cells synthesize ad-

2. To Support Growth. Cells preparing to divide increase in size ditional proteins. However, when glucose or fatty acids are
and synthesize extra proteins and organelles. unavailable, cells can break down proteins into their com-
ponent amino acids, and use the amino acids as an energy
3. To Produce Secretions. Secretory cells must synthesize their source. The primary function of proteins is not to serve as
products and deliver them to the interstitial fluid. an energy source, but proteins are so abundant and acces-
sible that they represent an important “last-ditch” nutrient
4. To Store Nutrient Reserves. Most cells “prepare for a rainy reserve.
day”—a period of emergency, an interval of extreme
activity, or a time when the supply of nutrients in the The nutrient pool is the source of the substrates for both
bloodstream is inadequate. Cells prepare for such times catabolism and anabolism. As you might expect, cells tend
by building up reserves—nutrients stored in a form that to conserve the materials needed to build new compounds
can be mobilized as needed. The most abundant storage and break down the rest. Cells continuously replace mem-
form of carbohydrate is glycogen, a branched chain of branes, organelles, enzymes, and structural proteins. These
glucose molecules. The most abundant storage lipids are anabolic activities require more amino acids than lipids, and
triglycerides, consisting primarily of fatty acids. Muscle few c­ arbohydrates. In general, when a cell with excess carbo-
cells and liver cells, for example, store glucose in the form hydrates, lipids, and amino acids needs energy, it first breaks
of glycogen, but adipocytes and liver cells store triglycer- down carbohydrates. Lipids are the second choice. Amino
ides. Proteins are the most abundant organic components
acids are seldom broken down if

other energy sources are available.

Figure 25–2 Nutrient Use in Cellular Metabolism. Cells use the contents of the nutrient pool Mitochondria provide the en-
ergy that supports cellular opera-
to build up reserves and to synthesize cellular structures. Catabolism within mitochondria provides

the ATP needed to sustain cell functions. Mitochondria are “fed” small carbon chains produced by the tions. The cell feeds its mitochondria

breakdown of carbohydrates (primarily glucose, stored as glycogen), lipids (especially fatty acids from from its nutrient pool. In return, the
triglycerides), and proteins (amino acids). The mitochondria absorb these breakdown products for cell gets the ATP it needs. However,
further catabolism by means of the citric acid cycle and the electron transport system. mitochondria are picky eaters: They

accept only specific organic mole-

cules for processing and energy pro-

Structural, functional, and storage components duction. For this reason, chemical

Triglycerides Glycogen Proteins reactions in the cytosol take which-
ever organic nutrients are available

Organic compounds Nutrient Fatty acids Glucose Amino acids and break them down into smaller
that can be absorbed pool fragments that the mitochondria
by cells are can process. The mitochondria then
25 distributed to cells break the fragments down further,
throughout generating carbon dioxide, water,
the body by the and ATP (Figure 25–2). This mito-
bloodstream. chondrial activity involves two path-

Three-carbon chains

Two-carbon chains ways: the citric acid cycle and the
electron transport system. We describe

MITOCHONDRIA these important catabolic and ana-
ATP bolic cellular reactions in the next

section.

Citric Coenzymes Electron O2
acid transport
KEY cycle system Checkpoint
= Catabolic pathway
1. Define metabolism.
H2O 2. Define catabolism.
3. Define anabolism.

= Anabolic pathway CO2 See the blue Answers tab at the back of
the book.