Maintenance of the Body
To be oxidized for energy, amino acids are converted to keto
acids that can enter the Krebs cycle. ±is involves transamination,
oxidative deamination, and keto acid modiﬁcation.
Amine groups removed during deamination (as ammonia) are
combined with carbon dioxide by the liver to form urea. Urea is
excreted in urine.
Deaminated amino acids may also be converted to fatty acids and
Amino acids are the body’s most important building
blocks. Nonessential amino acids are made in the liver by
In adults, most protein synthesis serves to replace tissue proteins
and to maintain nitrogen balance.
Protein synthesis requires the presence of all 10 essential amino
acids. If any are lacking, amino acids are used as energy fuels.
Metabolic States of the Body
Catabolic-Anabolic Steady State of the Body
±e amino acid pool provides amino acids for synthesis of
proteins and amino acid derivatives, ATP synthesis, and energy
storage. To be stored, amino acids are ﬁrst deaminated and then
converted to fats or glycogen.
±e carbohydrate-fat pool primarily provides fuels for ATP
synthesis and other molecules that can be stored as energy
±e nutrient pools are connected by the bloodstream; fats,
carbohydrates, and proteins may be interconverted via common
During the absorptive state (during and shortly aFer a meal),
glucose is the major energy source; needed structural and
functional molecules are made; excess carbohydrates, fats, and
amino acids are stored as glycogen and fat.
Events of the absorptive state are controlled by insulin, which
enhances the entry of glucose (and amino acids) into cells and
accelerates its use for ATP synthesis or storage as glycogen or fat.
±e postabsorptive state is the period when bloodborne fuels
are provided by breakdown of energy reserves. Glucose is
made available to the blood by glycogenolysis, lipolysis, and
gluconeogenesis. Glucose sparing begins. During prolonged
fasting, the brain also begins to metabolize ketone bodies.
Events of the postabsorptive state are controlled largely by
glucagon and the sympathetic nervous system, which mobilize
glycogen and fat reserves and trigger gluconeogenesis.
The Metabolic Role of the Liver
±e liver is the body’s main metabolic organ and it plays a crucial
role in processing (or storing) virtually every nutrient group. It
helps maintain blood energy sources, metabolizes hormones, and
detoxiﬁes drugs and other substances.
Cholesterol Metabolism and Regulation of Blood Cholesterol
±e liver synthesizes cholesterol, catabolizes cholesterol and
secretes it in the form of bile salts, and makes lipoproteins. ±e
liver makes a basal amount of cholesterol (85%) even when
dietary cholesterol intake is excessive.
Metabolism of Major Nutrients
Carbohydrate metabolism is essentially glucose metabolism.
Phosphorylation of glucose on entry into cells eﬀectively traps it
in most tissue cells.
Glucose is oxidized to carbon dioxide and water via three
successive pathways: glycolysis, Krebs cycle, and electron
transport chain. Some ATP is harvested in each pathway, but the
bulk is captured in the electron transport chain.
Glycolysis is a reversible pathway in which glucose is converted
to two pyruvic acid molecules; two molecules of reduced NAD
are formed, and there is a net gain of 2 ATP. Under aerobic
conditions, pyruvic acid enters the Krebs cycle; under anaerobic
conditions, it is reduced to lactic acid.
±e Krebs cycle is fueled by pyruvic acid (and fatty acids). To
enter the cycle, pyruvic acid is converted to acetyl CoA. ±e
acetyl CoA is then oxidized and decarboxylated. Complete
oxidation of two pyruvic acid molecules yields 6 CO
, 8 NADH
, 2 ²ADH
, and a net gain of 2 ATP. Much of the energy
originally present in the bonds of pyruvic acid is now present in
the reduced coenzymes.
In the electron transport chain, (a) reduced coenzymes are
oxidized by delivering hydrogen to a series of oxidation-
reduction acceptors; (b) hydrogen is split into hydrogen ions and
electrons (as electrons run downhill from acceptor to acceptor,
the energy released is used to pump H
into the mitochondrial
intermembrane space, which creates an electrochemical proton
gradient); (c) the energy stored in the electrochemical proton
gradient drives H
back through ATP synthase, which uses the
energy to form ATP; (d) H
and electrons are combined with
oxygen to form water.
²or each glucose molecule oxidized to carbon dioxide and
water, there is a net gain of 32 ATP: 4 ATP from substrate-level
phosphorylation and 28 ATP from oxidative phosphorylation.
±e shuttle for reduced NAD
produced in the cytosol may use 2
ATP of that amount.
When cellular ATP reserves are high, glucose catabolism is
inhibited and glucose is converted to glycogen (glycogenesis) or
to fat (lipogenesis). Much more fat than glycogen is stored.
When blood glucose levels begin to fall, glycogenolysis occurs,
in which glycogen stores are converted to glucose. ±e liver can
also perform gluconeogenesis, the formation of glucose from
noncarbohydrate (fat or protein) molecules.
Muscular System; Topic: Muscle Metabolism, pp. 10–22.
End products of lipid digestion (and cholesterol) are transported
in blood in the form of chylomicrons.
Glycerol is converted to glyceraldehyde 3-phosphate and enters
the Krebs cycle or is converted to glucose.
²atty acids are oxidized by beta oxidation into acetic acid
fragments. ±ese are bound to coenzyme A and enter the Krebs
cycle as acetyl CoA. Dietary fats not needed for energy or
structural materials are stored in adipose tissue.
±ere is a continual turnover of fats in fat depots. Breakdown of
fats to fatty acids and glycerol is called lipolysis.
When excessive amounts of fats are used, the liver converts acetyl
CoA to ketone bodies and releases them to the blood. Excessive
levels of ketone bodies (ketosis) lead to metabolic acidosis.
All cells use phospholipids and cholesterol to build their plasma
membranes. ±e liver forms many functional molecules from lipids.