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Metabolism-brief look
When nutritional
materials are oxidised in the cells of the body,
energy is released, some in the form of heat.
Energy may be used immediately to do the work,
e.g., to synthesis new muscle cells from amino
acids, or it may be stored in chemical form as
adenosine triphosphate(ATP). Heat is used to
maintain the body temperature at the optimum
level for the chemical activity (36.8C or 98.4F)
. Excess heat is disposed of though the skin and
the body excreta .The energy produced in the
body may be measured and expressed in units of
work (joules) or units of heat (Calories). A
Calorie (capital C) is the amount of heat
required to raise the temperature of 1 liter of
water through 1 degree Celsius.
1 Calorie = 4184 joules (J) = 4.184
kilojoules (kJ)
The
nutritional value of carbohydrates, protein and
fats eaten in the diet may be expressed in
kilojoules per gram or Calories per gram.
1 gram of carbohydrate
provides 17 kilojoules (4 Calories)
1 gram of protein provides
17 kilojules (4 Calories)
1 gram of fat provides 38 kilojoules (9
Calopries)
The metabolic rate is
the rate at which energy is released from the
nutrients inside cells. As most of the processes
involved require oxygen and produce carbon
dioxide as waste, the metabolic rate can be
estimated by measuring oxygen uptake or carbon
dioxide excretion. The basal metabolic rate
(BMR) is the rate of metabolism when the
individual is at rest and is in the
post-absorptive state, i.e., has not had a meal
for at least 12 hours. In this state the release
of the energy is sufficient to meet only the
essential needs of vital organs, such as the
heart, lungs, nervous system, kidneys. The
post-absorptive state is important because the
intake of food, especially protein, stimulates
an increase in metabolic rate, possibly due to
increased energy utilisation by the liver. This
is called the specific dynamic action (SDA) of
food. In measuring the BMR, the surface area of
the body is taken into account because energy in
the form of heat is lost through the skin.
Surface areas in square meters is calculated
from the height and weight of the individual.
Some of the wide variety of factors that affect
the metabolic rate is shown below:
Factor
Effect
Age
Gradually reduced with age
Sex
Higher in men than women
Height,
Weight
Relatively higher in small people
Pregnancy,menstruation,laction
Higher
Ingestion of
food
Higher
Muscular
activity
Higher
Elevated body
temperature
Higher
Excess thyroid
hormones
Higher
Starvation
Lower
Most foods
contain a mixture of different amounts of
carbohydrate, protein, fat, minerals, vitamins,
roughage and water. Carbohydrates, proteins and
fats are the sources of energy and they are
obtained from the variety of food, usually in
the following proportions:
Protein -
15-20%
** Fat
-
20-25%
**
Carbohydrates - 50-60%
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Metabolism of
Carbohydrate
When digested,
carbohydrate, mainly glucose, is absorbed into
the blood capillaries of the villi of the small
intestine. It is transported by the portal
circulation to the liver, where it is dealt with
in several ways. 1. Glucose may be used to
provide the energy necessary for the
considerable metabolic activity which takes
place in the liver. 2. Some of the glucose may
remain in the circulating blood to maintain the
normal blood glucose of about 2.5 to 5.3
millimoles per liter(mmol/l) (45 to 95 mg/100
ml). 3. Some of the glucose may be converted to
the insoluble polysaccharide, glycogen, in the
liver and in the muscles. Insulin is the hormone
necessary for this change to take place. The
formation of glycogen inside cells is a means of
storing carbohydrate without upsetting the
osmotic equilibrium. Before it can be used it
must be broken down again into its constituent
monosacharides. Liver glycogen constitutes a
store of glucose used for liver activity and to
maintain the blood glucose level. Muscle
glycogen provides the glucose requirement of
muscle activity. Adrenaline, thyroxine and
glucogen are the main hormones associated with
the conversion of glycogen to glucose.
4.Carbohydrate in excess of that required to
maintain the blood glucose level and glycogen
level in the tissues is converted to fat and
stored in the fat depots.
All the cells of the
body require energy to carry out their metabolic
process including: multiplication of cells for
replacement of worn out cells; contraction of
muscle fibres, synthesis of secretions produced
by the cells of glands. The oxidation of
carbohydrate and fat provides most of the energy
required by the body.
Oxidation of
Carbohydrate
Complete oxidation of glucose requires an
adequate supply of oxygen. This is the process
by which energy is released during prolonged
physical activity, e.g., the man who runs 1500
meters in 4 minutes depends upon aerobic
oxidation. The energy release takes place slowly
and is balanced by oxygen intake. Complete
oxidation of carbohydrate in the body results in
the production of energy, carbon dioxide and
water. Some energy can be provided by glucose in
the absence of oxygen. This anaerobic process
does not release all the energy from the glucose
molecule and, using disposes, effort can be
maintained for only a limited period of time.
This is the energy used in a sudden spurt of
activity over a very short period of time, e.g.,
the men who runs 100 meters in 10 seconds could
not take in enough oxygen in that time to
provide energy by complete oxidation of glucose,
so he has to be depend upon the aerobic process.
One of the end products of this process is
lactic acid, and if it accumulate in excess in
the muscle it causes the pain associated with
unaccustomed exercise.
Fate of the end products of
carbohydrate metabolism
1.Lactic acid. Some of the
lactic acid produced by anaerobic catabolism of
glucose may be oxidised in the cells to carbon
dioxide and water but first it must be changed
to pyruvic acid. It complete oxidation does not
take place lactic acid passes to the liver in
the circulatory blood where it is converted to
the glucose and may then take any of the
pathways open to the glucose.
2. Carbon dioxide is excreted from the
body as a gas by the lungs.
3. Water. The water of metabolism is added
to the considerable amount of water already
present in the body.
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Metabolism of Protein
Protein foods taken as
part of the diet consist of a number of amino
acids. About 20 amino acids have been named and
about 8 of those are described as essential
because they cannot be synthesised in the body.
The remainders are described as non-essential
amino acids because they can be synthesised by
many tissues. The enzymes in this process are
called transaminases. Digestion breaks down the
protein of the diet to its constituent amino
acids in preparation transfer into the blood
capillaries of the villi in the wall of the
small intestine. In the portal circulation amino
acids are transported to the liver then into the
general circulation thus making them available
to all the cells and tissues of the body.
Different cell choose form those available the
particular amino acids required for building or
repairing their specific type of tissue and
synthesising their secretions, e.g., antibodies,
enzymes, hormones.
Amino
acids not required for building and repairing
body tissues are broken down in the liver. 1.
The nitrogenous part is converted to urea by the
process of deamination and excreted in the
urine. 2. The remaining part is used to provide
energy, or stored as fat, in this excess of
immediate requirements.
Amino Acid
Pool
A pool
of amino acids is maintained within the body.
This is the source from which the different
cells of the body draw the amino acids they need
to synthesis their own materials, e.g., new
cells, secretion such as enzymes and hormones,
blood proteins.
Source of amino acids
1 .Exogenous : These are
derived from the protein eaten in the diet. 2.
Endogenous : These are obtained from the
breakdown of body protein. In an adult About 80
to 100g of protein arebroken down and replaced
each day. Intestinal mucosa has the most rapid
turnover of cells.
Loss of amino acids
1. Deamination : Amino
acids not needed by the
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deamainated, mainly in the liver. The
nitrogenous part is excreted as urea by the
kidneys, and the remainder is used to provide
energy and heat. 2. Excretion : The faeces
contain a considerable amount of protein
consisting of desquamated cells from the lining
of the alimentary tract. Endogenous and
exogenous amino acids are mixed in the pool and
the body is said to be in nitrogen balanced when
the rate of removal from the pool is equal to
the additions to it. Unlike carbohydrates, the
body has no capacity for the storage of amino
acids except for this relatively small pool.
Depicts what happens to amino acids in the
body.
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Metabolism of Fat
Fats that have been
digested and absorbed into the lacteals are
transported via the receptaculum chyli and the
thoracic duct to the blood stream and so, by a
circuitous route, to the liver. Fatty acids and
glycerol circulating in the blood are used by
organs and glands to provide energy and in the
synthesis of some of their secretions. In the
liver some fatty acids and glycerol are used to
provide energy and heat, and some are
reorganized and recombined to form a verity
compounds. Excess fatty acids are taken up by
fat cells which change when to the neutral fat,
the form in which they are stored. When it is
required it is converted back to the fatty acids
and circulated to be taken up and used by body
cells to provide energy. The ends of the
products of the fat metabolism are energy, heat,
carbon dioxide, and water.
Ketone bodies are the keto acids produced
during the process of oxidation of the fats and
are always present in the blood in very small
amounts. They are excreted in the urine and in
the expired air as acetone. Fat is used in
increased amounts when there is not enough
carbohydrate available for metabolism. When the
final stage of metabolism are blocked, due to
the deficiency in the supply of the products of
carbohydrate metabolism, excess keto acids are
formed and a state of ketosis develops. These
may occur in starvation of diabetes mellitus
when there is deficiency of insulin, which
facilitates the amount of carbohydrate into
cells. Excess keto acids have a toxic effect on
brain cells.Fat is synthesised from
carbohydrates and proteins which are taken into
the body in excess of its needs and stored in
the fat deposits, e.g., under the skin in the
omentum around the kidneys.
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Other Details about
Metabolism
Relationships between
carbohydrates,fatty acids,glycerols,and
deaminated amino acids as energy-releasing
substances :
The
degradation of carbohydrates, fatty acids,
glycerol and the residue after amino acids are
deaminated occurs inside the cells, releasing
energy and forming the waste products carbon
dioxide and water. The catabolism of these
molecules occurs in a series of steps, a little
energy being related at each stage. Upto a
certain point each nutrient passes through a
series of separate and distinct stages but there
after, they all follow a common pathway of
degradation. This final common pathway is called
the citric acid cycle or Kreb's cycle.
Carbohydrates go through a
series of stage to pyruvate and acetyl co-enzyme
A. It is in this form that it joins oxaloacetic
acid in the cytric acid cycle. Fatty acids
passes through a series of oxidative stages to
acetyl co-enzyme A and, under normal
circumstances, progress to oxaloacetic acid and
the citric acid cycle. If, however, an excessive
amount of acetyl co-enzyme A is produced some of
it develops into keto acids. Deaminated amino
acid are two types: those which go through a
series of stages to oxailoacetic acids and so to
citric acid cycle and those which follow
different series of changes to become acetyl
co-enzyme A and thereafter take a pathway either
to oxaloacetic acid or to keto acids.
The formation of abnormal
amounts of keto acids occurs in starvation and
in diabetes mallitus when excessive amounts of
fat and amino acids are used to provide energy,
that is, when excess keto acids are produced. In
both these examples there is an insufficiency of
carbohydrates inside the cells. In diabetes this
is due to storage in the supply of the hormone
insulin which facilitates the transportation of
carbohydrate from the extra cellular fluid
across the cell membrane and its subsequent
metabolism. Excess keto acids are excreted in
the urine and in expired air as acetone.
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