Metabolism
Overview.
Catabolism.
Anabolism.
Carbohydrates.
Proteins.
Fats.
Nucleotides metabolism.
Energy storage.
Evolution of metabolism.
Overview.
A large number of chemical reactions,
are taking place within a living organism, and within a cell.
The sum total of all the chemical reactions, is called as metabolism.
We will discuss metabolism in the specific context of human beings.
Catabolism.
The breaking down of macro molecules, into micro molecules,
through chemical reactions, is called catabolism.
In this process the bonds, in the larger molecule are broken.
The energy stored in these bonds is harvested.
This energy can be used for normal living processes.
The energy can also be used, for building other macro molecules,
from the micro molecules.
Food is broken down, by the process of digestion.
It is broken down into a limited number of micro molecules,
that the body can absorb.
For example, carbohydrates can be broken down to glucose.
Glucose can be readily absorbed by the body.
It can be transported in the blood stream.
Cells can absorb glucose.
Further catabolic processes can break down glucose to provide energy.
All these are catabolic processes.
Anabolism.
The process of building macro molecules from micro molecules,
is called Anabolism.
Anabolism involves creation of new chemical bonds.
This requires input of energy.
This energy typically comes from other catabolic processes.
For example, a human body uses a large number of proteins,
to perform distinct functions.
Each protein has a unique composition and structure.
We eat proteins for food.
These are not the same proteins, that the body uses.
The proteins that we eat, are broken down into amino acids,
through catabolic processes.
Amino acids are the building blocks of proteins.
A protein is a chain of amino acids.
Proteins are the molecular machinery of the human body.
New proteins required by the body are synthesised,
from the amino acids, by anabolic processes.
These proteins like muscle protein, enzymes, hormones,
and large variety of other proteins, are used by the body,
for varied internal functions.
Metabolism is a balance of anabolic and catabolic processes,
that are constantly taking place in the body.
Carbohydrates.
All carbohydrates like starch and sugars,
are eventually broken down into glucose.
Glucose is the primary source of energy, for the body.
The body tries to maintain a certain level of glucose,
in the blood stream.
Glucose is readily absorbed by all cells.
Chemical reactions in the cell convert glucose to pyruvate,
in a process called glycolysis.
Pyruvate is converted to acetyl CoA.
Some energy is harvested as ATP.
ATP is the universal currency for energy,
in living organisms.
Acetyl CoA is a two carbon molecule.
This enters the Krebs cycle.
The Krebs cycle, works in the presence of oxygen.
The Krebs cycle works only when energy is required.
Oxygen is brought to the cell, from the lungs, via the blood stream.
The carbon bonds are broken down.
Energy is harvested in the form of ATP molecule.
ATP is a universal currency for energy.
Chemical reactions of breaking down glucose,
to derive ATP is a catabolic process.
Carbon dioxide and water are the by products,
of the Krebs cycle.
Carbon dioxide diffuses out of the cell into the blood stream.
We breathe out carbon dioxide from the lungs.
The body needs a constant supply of glucose,
even when we are resting.
For this it needs to store glucose in a macro molecule.
The body stores glucose as glycogen.
Glycogen is a polysaccharide of glucose.
Polysaccharide are sugars.
Soon after eating, typically a lot of glucose will be generated.
The surplus glucose is synthesised into glycogen,
in the liver and muscle tissue.
This is an anabolic process.
Glycogen acts as a energy reserve.
When glucose level goes down, between meals,
the liver breaks down glucose and supplies it,
to the cells via the blood stream.
This ensures that all the cells have a constant supply of glucose.
Muscle tissue can also break down glycogen,
when energy is needed.
The synthesis and breaking down of glycogen,
is controlled by hormones.
Insulin and glucagon are the hormones,
which manage blood glucose levels.
The pancreas secretes these hormones.
When glucose concentration becomes high,
the pancreas releases insulin into the blood stream.
When glucose concentration becomes low,
the pancreas releases glucagon.
Insulin is the hormone which facilitates the cells to absorb glucose.
Glucagon is the hormone, which stimulates the liver,
to breakdown glycogen and release glucose.
Insulin is an anabolic hormone,
which promotes the synthesis of glycogen and fats, from glucose.
Glucagon is a catabolic hormone, which breaks down glycogen,
into glucose.
These hormones help to maintain a metabolic homeostasis.
A typical 70Kg man stores about 480 grams as glycogen.
Glycogen can yield 4 Kilo calories of energy per gram.
Typically this man would require 2000 kilo calories, per day.
This means that only about one day's reserve of energy,
can be stored as glycogen.
If more energy needs to be stored, the preferred storage medium,
of the body, is in the form of fat.
Proteins.
Proteins are large molecules, comprising of a long chain of amino acids.
Proteins are hydrolysed, and broken down into the basic amino acids.
Chemical reactions convert the amino acids to pyruvate or acetyl CoA.
The amino group in the amino acid becomes Urea, which is released as a byproduct.
This is excreted from the bladder.
Acetyl CoA enters the Krebs cycle.
Energy in the form of ATP is harvested.
This is a catabolic process.
The amino acid can be converted to pyruvate,
which in turn can be converted to glycogen to store energy.
The proteins that we consume, are very different from the proteins,
that the body needs.
Many proteins are synthesised within the cell.
Each protein is a customised bio molecule.
Each cell expresses certain genes,
which results in synthesis of a specialised protein.
There are 20 basic amino acids.
The cell can synthesise all the proteins, that it requires,
by assembling long chains of amino acids.
Synthesis of proteins is an anabolic process.
The liver is involved in the metabolism of proteins, for the body.
Amino acids derived from breaking down of proteins,
travel to the liver.
There are multiple metabolic pathways for use of the amino acids,
in the liver.
1. They can be used for protein synthesis.
2. They can be converted to glucose and glycogen.
- These are called glycogenic amino acids.
3. They can be converted into fatty acids, via acetyl CoA.
- This can be sent for storage in adipose tissue.
- These are called Ketogenic amino acid.
Muscles are built of proteins.
Muscles can synthesis proteins from amino acids.
A typical 70Kg man, has about 6Kg of proteins.
4 kilo calories of energy, can be harvested from one gram of protein.
The energy requirements per day is about 2000 kilo calories.
Theoretically twelve days of energy required,
can be provided by the proteins in the body.
Protein is not the preferred source of energy storage.
A malnourished person will tend to lose his muscle mass.
Fats.
Fats are also referred to as lipids.
Fats have a glycerol backbone.
Glycerides are fatty acids esters of glycerol.
Three fatty acids are attached to the glycerol group.
This is called as a triglyceride.
Catabolic processes break down fats into glycerol and fatty acids.
Fatty acids are long chains of carbon and hydrogen.
Fatty acids are metabolised by breaking down two carbon atoms at a time,
and convert it to acetyl CoA.
This process continuous in a cycle, till the whole chain is consumed.
Acetyl CoA enters the Krebs cycle.
Energy in the form of ATP, is harvested.
Glycerol can be converted to pyruvate,
which in turn can be converted to glucose or glycogen.
Glucose to pyruvate conversion can be two way process.
Pyruvate to acetyl CoA conversion is a one way process.
Fatty acids which is converted into acetyl CoA,
cannot convert into pyruvate.
Acetyl CoA can convert into adipose tissue,
where energy is stored as fat.
It is interesting that carbohydrates, proteins, and fats,
can produce acetyl CoA.
Acetyl CoA from any of these sources can turn into adipose tissue.
Adipose tissue is body fat.
When the body needs energy,
adipose tissue releases fatty acid.
This is sent to the liver, where it is oxidised to form glucose.
If we eat more than required, the excess food of any form,
is converted to body fat.
A sedentary life style, and excess food,
causes people to become overweight or obese.
A typical 70Kg man has about 12Kg of adipose tissue or fat.
Fat is an energy rich molecule.
About 9 kilo calories of energy can be harvested,
from 1 gram of fat.
This means that the body can theoretically store 60 days of energy,
at the rate of 2000 kilo calories per day.
Nucleotides metabolism.
Nucleotides are organic molecules, which are the subunits of nucleic acids.
DNA and RNA are examples of nucleic acids.
Nucleotides also function as energy carriers, like in ATP.
Nucleotide are made up of 5 basic bases.
Adenine.
Guanine.
Cytosine.
Thymine.
Uracil.
Adenine and guanine are purine bases.
Cytosine, thymine and uracil are pyrimidine bases.
Glucose can be a starting point for synthesising nucleotides.
When a ribose is added to a base, we get a nucleoside.
For example,
Adenosine.
When we add ribose-5 phosphate, it becomes a nucleotide.
Deoxyribonucleic acid can be derived from this.
Energy storage.
Glycogen is the convenient means of short term energy storage.
Fat is the preferred means of storing energy for the long term.
Glycogen associates itself with water molecules.
This makes it heavier.
It requires more quantity and weight of glycogen to store more energy.
Fat is a more efficient way to store energy.
Fatty acids have a long chain of carbon and hydrogen atoms.
For example,
CH2 single bond CH2 single bond CH double bond CH single bond CH2 single bond CH3.
The single bond are saturated with hydrogen.
The double bond are unsaturated.
The carbon bonds store energy.
It can be oxidised to release energy as ATP.
A long chain of carbon atoms, is broken down into smaller molecules,
to release energy.
Glucose is soluble in water.
Fats are not soluble in water.
This makes fat relatively chemically inert.
This property makes it suitable for energy storage.
Proteins have many functional roles in the body.
Fats have no major functional role.
The main functional role, of fat is to store energy.
Fat is hydrophobic, unlike polar molecules.
Polar molecules attract water, which adds to their weight.
One gram of glycogen or protein is associated with,
3 grams of water.
If we want to store, the energy in 12Kg of fat, as glycogen,
we would need 90Kg of glycogen.
This would not be practical.
The body prefers to store long term energy needs as fat.
Evolution of metabolism.
Metabolism is common to all living organisms,
both plants and animals.
Metabolism co-evolved with the evolution of life.
We can trace the major steps in the evolution of metabolism.
1. Storage of energy in the ATP molecule.
2. Glycolysis, which exists in all forms of life.
- It is relatively inefficient.
3. Anaerobic photosynthesis using hydrogen sulphate.
4. Use of water in photosynthesis, and release of oxygen.
5. Nitrogen fixation.
6. Aerobic respiration.
In the long history of evolution of life, aerobic respiration is quiet recent.
This is the metabolism, we have been discussing in this module.