Nutrients
absorbed from the GI tract are treated in 3 ways:
1.
Broken down immediately to provide energy
2.
Used as building blocks for synthesis of body molecules
3.
Stored for future use
METABOLISM---the
sum of all the chemical reactions in the body. The 2 phases are:
1. Anabolism---synthesis or
building-----simpler molecules are linked to from more complex
molecules---amino acids are linked by peptide bonds to form proteins---these
reactions require energy, so they can also be described as endergonic.
2. Catabolism---degradation or
breakdown---complex compounds are broken down to simpler ones—digestion is an
example. These reactions release energy, so they can also be described as
exergonic.
The
body’s metabolism is a constant balancing act between the 2 types of reactions.
Energy released in catabolism is stored in the form of ATP. Adenosine
diphosphate is phosphorylated (a phosphorus is added) to form ATP. The
high-energy bond in ATP is broken to release the energy when needed for
anabolic reactions.
During
digestion, the polysaccharides and disaccharides that form carbohydrates are
broken down to the monosaccharides glucose, fructose, and galactose. These are
absorbed in the small intestine and carried to the liver. Most of the fructose
and galactose are converted to glucose, the body’s preferred source of energy.
Since all carbohydrates end up as glucose,
their further metabolism is studied as glucose metabolism.
Glucose
will be treated in the following ways, depending on conditions at the time:
1. Transported directly into cells and used
to form ATP
2. Amino acid synthesis---some amino acids
can be synthesized by the body using glucose as the starting point
3. Converted to glycogen and stored in the
liver and skeletal muscle cells (glycogenesis). This can quickly be converted
back to glucose when needed
(glycogenolysis).
4. Lipogenesis (triglyceride synthesis)---excess glucose can be converted to fat
and stored in adipose tissue.
5. Excretion in urine when blood glucose is
very high.
Glucose
enters body cells from the blood by facilitated diffusion. Insulin greatly
increases the number of glucose transporters, so it is required for normal use
of glucose by the body.
The
oxidation of glucose is known as cellular respiration and occurs in every cell
of
the body except red blood cells. It is the
body’s chief source of energy, and complete oxidation produces carbon dioxide,
water and energy. The energy is stored as ATP.
There
are 4 steps in glucose catabolism:
1.
GLYCOLYSIS---a six-carbon molecule of glucose is broken down to 2
three-carbon molecules of pyruvic acid, producing a small amount of ATP. This
step occurs in the cytosol and is anaerobic (no oxygen required). It is called
anaerobic cellular respiration.
2.
FORMATION OF ACETYL COENZYME A--in the presence of oxygen, glucose
breakdown continues to produce much more ATP. Pyruvic acid enters the
mitochondria and is converted to acetyl Coenzyme A, which enters the Kreb’s
cycle.
The
following 2 steps require large amounts of oxygen, and together are known as
aerobic cellular respiration.
3. KREB’S CYCLE---this is a series of oxidation-reduction
reactions controlled by enzymes. It occurs inside the mitochondria. Carbon
dioxide is released.
4. ELECTRON TRANSPORT CHAIN---carrier molecules pass electrons
along the chain, releasing energy and storing it as ATP. The ATP is produced by
a process called chemiosmosis. The final electron acceptor is oxygen. This also
occurs in the mitochondria.
If
no oxygen is present, or if the amount of oxygen is insufficient, the pyruvic
acid from glycolysis can be changed to lactic acid, producing a little more
ATP, but not nearly as much as in the Kreb’s cycle and electron transport
chain.
Lipids
are second choice as a source of energy (after carbohydrates). Following
absorption, they are transported to the liver.
Lipids
may be oxidized to produce ATP. If not immediately needed, they may be
stored as adipose tissue. Lipids are also
important in synthesizing such substances as cell membrane phospholipids,
myelin sheaths, and steroid hormones.
During
digestion, proteins are broken down into amino acids. These are absorbed by the
blood capillaries in the villi and transported to the liver. Amino acids enter
body cells by active transport and are synthesized into proteins. Essential
proteins of the body function as enzymes, clotting chemicals, hormones, muscle
fibers, and structural elements.
Hormones
are the primary regulators of metabolism. The main thing that determines what type of metabolic reactions are occurring is how recently
you have eaten.
Nutrients
are being absorbed from the GI tract and glucose is readily available. Insulin
is the major hormone in charge. Processes are:
1. Transport of glucose into body cells
2. Transport of amino acids into cells
3. Glycogen synthesis
4. Protein synthesis
5. Triglyceride synthesis
No
glucose is being absorbed from the GI tract, so the major effort is directed at
maintaining a normal blood glucose level (70-110 mg/100 ml). Hormones are
glucagon, thyroxine, and cortisol. Processes:
1. Glycogen breakdown
2. Triglyceride breakdown
3. Protein breakdown (we try to use glucose
and fat before this)
4. Gluconeogenesis---making glucose from
non-carbohydrate sources
The
basal metabolic rate (BMR) is a measure of the rate at which the quiet,
resting, fasting body breaks down foods and releases heat. Thyroid hormones are
the major regulator of the BMR.
Most
body heat is produced by the oxidation of foods we eat. The rate at which heat
is produced is affected by exercise, strong sympathetic stimulation, hormones
and body temperature. We use some of this heat to maintain our normal 98.6o ,
but often excess heat is produced and this must be removed. Radiation,
conduction, convection
and evaporation are ways of removing body heat.
Body
temperature is regulated by neurons in the hypothalamus (preoptic area)
which act as the thermostat. Input from
temperature receptors in the skin, mucous membranes, and internal structures
cause the preoptic area to become more active if body temp increases and less
active if it decreases.
If
the need is to warm up, the heat-promoting center causes:
1. Vasoconstriction of skin vessels
2. Sympathetic impulses to increase cellular
metabolism
3. Increased tone of skeletal muscle or
shivering
4. Increased thyroid secretion
If
we need to cool off, the heat-losing center causes:
1. Vasodilation of skin vessels
2. Decreased rate of cellular metabolism
3. Decreased skeletal muscle tone
4. Sweating
Fever
is an abnormally high body temp. The most common cause is infection by viruses
or bacteria. White blood cells involved in phagocytosis release interleukin-1
which “resets” the body thermostat in the preoptic area to a higher
temperature. The mechanisms listed above go into action to raise body temp.
Beneficial
effects of fever:
1. Intensifies the effects of interferons
2. May inhibit growth of pathogens
3. Speeds antibody production and T cell
proliferation
4. May aid in repair of cells by speeding up
chemical reactions
Harmful
effects of fever:
1. Dehydration
2. Acidosis
3. Brain damage if severe enough
In
the hypothalamus are clusters of neurons that receive signals that indicate
hunger or satiety (a feeling of fullness). Adipocytes secrete a hormone called
leptin, which acts on the hypothalamus to inhibit circuits that encourage
eating. With low levels of leptin, a neurotransmitter called neuropeptide Y is
released and stimulates food intake. Higher levels of leptin cause release of a
different neurotransmitter, melanocortin, which acts to inhibit food intake.
Distention of the stomach and duodenum also contributes to satiety.
Other
substances required for metabolism are vitamins and minerals.
MINERALS---inorganic
substances needed by the body for essential functions, including calcium,
phosphorus, iron, iodine, sodium, potassium, and many others. Some are needed
in large amounts and some only in tiny traces.
VITAMINS---organic
nutrients required in tiny amounts to maintain growth and metabolism. The 2
main groups of vitamins are:
1. Fat-soluble vitamins---A,D,E,K---these are absorbed from the small intestine
dissolved in dietary fat and can be stored in cells.
2. Water-soluble vitamins---B,C---these are absorbed in water and any excess of these is
excreted in the urine
Most
vitamins cannot be synthesized by the body and must be ingested in food or
pills. Vitamin K is synthesized by bacteria in the digestive tract. Vitamin A
can be synthesized if the proper raw materials are present.