CHAPTER
Metabolic Homeostasis
22.1
Metabolic Homeostasis
The functions of cells and tissues in all organisms require
energy that normally is obtained from ingested food con-
taining proteins, lipids, and carbohydrates. Human beings
follow an intermittent food/fast schedule; however, en-
ergy must be supplied to cells and tissues continuously.
Thus, excess chemical substances capable of supplying
energy are stored and released as needed to maintain
homeostasis,
the tendency for biological systems to main-
tain relatively constant chemical conditions in their in-
ternal environments. Some organs, tissues, and cells are
metabolic providers; others are users. Each cell membrane
acts as a barrier to hold its cellular components and, by ex-
pending energy, is able to regulate the gradient of ions and
metabolites moving into and out of the cell. This chapter
discusses how energy is stored and released by tissues and
how homeostasis is maintained.
Organs serve as reservoirs or sites for synthesis of the
metabolites required for homeostasis; at all levels home-
ostasis is regulated by the endocrine system. Fluxes of
metabolites through biochemical pathways are regulated
by stoichiometric need, allosteric control of enzyme ac-
tivity, modification of regulatory enzymes, and changes in
their rate of enzyme synthesis or degradation. Hormones
often influence these processes and, in turn, metabo-
lites modulate the secretion of hormones. Thus, many-
interconnected feedback loops provide the positive and
negative regulation that maintains homeostasis.
Human beings possess considerable metabolic versatil-
ity for utilization of major fuel classes such as carbohy-
drate, lipid, and protein to maintain energy requirements.
Metabolism of these substances is organized to:
1. Maintain the blood glucose level within narrow
limits;
2. Maintain an optimal supply of glycogen;
3. Maintain an optimal supply of protein.
If carbohydrates or proteins are ingested in excess of
the amounts necessary to maintain optimal supplies of
glycogen in tissues or protein, the excess is converted to
fat. Conversion of excess carbohydrate and/or protein into
fat is biochemically an irreversible process. As a result,
the body conserves the compounds that it can interconvert,
uses them to supply energy when needed, or converts them
to fatty acids when it is more efficient to store the carbon
in the form of fat. The body tends to conserve its protein
reserves and to draw on fat reserves preferentially in time
of energy demand.
The tricarboxylic acid (TCA) cycle and
ft
-oxidation
(Chapter 14) are tightly coupled to electron transport
via the nicotinamide and flavin nucleotides and by ADP
and ATP. The TCA cycle in cells functions only when
ADP is present (i.e., when ATP is being utilized). Thus,
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