Metabolism
‫السالم عليكم ورحمة هللا وبركاته‬
Objective
Explain what is metabolism, catabolism and
anabolism. Be able to apply the latter two terms to
various metabolic reactions.
Describe in outline the metabolism of carbohydrates,
lipids and proteins at the level of tissues and organs.
What is metabolism?
Metabolism is defined as the sum total of all
chemical reactions that occur in the body.
Metabolism is usually divided into two categories:
Catabolism - larger molecules are broken down into
smaller ones in a process that usually releases
energy.
Anabolism - larger molecules are made from small
ones in a process the usually requires energy.
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Two faces of metabolism
• Catabolism - degradation
• Anabolism - biosynthesis
Catabolism
Catabolism is that part of metabolism that
involves the break down of large, complex
molecules into smaller, more simplified
products. This occurs during digestion, removal
of hydrogen (dehydrogenation), carboxyl groups
(decarboxylation)
and
amino
groups
(deamination), oxidation, etc.
Anabolism
Anabolism is that part of metabolism that
involves the synthesis of larger, more complex
molecules from small, simple reactants.
Examples of anabolism would include the
synthesis of glycogen from glucose, protein from
amino acids, fat from glycerol and fatty acids
and construction of new antibodies and new
enzymes.
Anabolism
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Key Biochemicals
Most of the structures that make up animals, plants and
microbes are made from three basic classes of molecule:
amino acids, carbohydrates and lipids (often called fats).
As these molecules are vital for life, metabolic reactions
either focus on making these molecules during the
construction of cells and tissues, or by breaking them
down and using them as a source of energy, by their
digestion. These biochemicals can be joined together to
make polymers such as DNA and proteins, essential
macromolecules of life.
.
Overview of Dietary Catabolism
Large molecules in food are broken
down into smaller units. No useful
energy is captured in this phase.
These numerous small molecules are
degraded to a few simple units that
play a central role in metabolism.
Some ATP is generated in this
stage.
Cell
ATP is produced from the complete
oxidation of the acetyl unit of acetyl
CoA, the final common pathways in
the oxidation of fuel molecules.
Three stages in the generation of energy from the oxidation of foodstuffs
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Carbohydrate Metabolism:
Fate of Glucose
1. Glycolysis (enzymatic breakdown and interconversions of
carbon atoms derived from glucose) ultimately yields
ATP and CO2.
2. Converted to glycogen, a storage polymer of potential
metabolic energy.
3. Generates reducing equivalents (NADPH) for fatty acid
synthesis and ribose for nucleotide synthesis (via the
pentose phosphate pathway)
4. Triose phosphate can be metabolized to the glycerol
backbone of triacylglycerols (fat).
5. Pyruvate (and other metabolites) can provide the carbon
skeletons for synthesis of nonessential amino acids;
acetyl CoA necessary for long chain fatty acid and
cholesterol synthesis.
Carbohydrate Metabolism
Lipid metabolism
1. Esterified and stored as triglycerides.
2. b-Oxidation of fatty acids generates energy
and leads to acetyl-CoA (which enters the
citric acid cycle).
3. It is the precursor for synthesis of cholesterol
and other steroids.
4. Can form ketone bodies in the liver (energy
sources during starvation conditions).
Lipid metabolism
Amino Acid Metabolism:
Essential and Non-Essential
1. The amino acids are required for protein
synthesis. Some must be supplied in the diet
since they cannot be synthesized in the body
(essential amino acids).
2. The remainder are non-essential amino acids
which are supplied in the diet, but can also
formed from metabolic intermediates by
transamination and deamination reactions.
Amino Acid Metabolism