The central theme of metabolism is to extract energy from nutrients like carbohydrates, lipids, and proteins, store excess energy for future use, and maintain tissue proteins. The body primarily uses glucose and fatty acids as fuels, breaking them down through metabolic pathways into smaller units like glucose and fatty acids to obtain energy. Metabolism involves both the breakdown of macromolecules through catabolic pathways to generate energy and the synthesis of complex molecules from simple precursors through anabolic pathways.

1. Central Theme of Metabolism
• Metabolic fuels (carbohydrates, lipids and proteins) are broken down to as well
as build up from smaller units such as glucose, fatty acids and amino acids by
several metabolic pathways.
• The central theme of metabolism is to extract energy from food, store the
excess for future needs and to maintain tissue proteins for performing various
vital functions.
• The body uses glucose and fatty acids as the primary fuels to obtain energy.
Only under dire circumstances, amino acids are used for this purpose.
• Glucose can be obtained from other carbohydrates, glucogenic amino acids
and the ‘glycerol’ portion of fat.
• To synthesize tissue proteins, amino acids are required.
• Although glucose can be channelized to make some non-essential amino acids,
fats cannot serve this purpose.
• When energy consumed is in surplus, the body converts all the three major
nutrients into stored fat.

2. -Metabolism-
• The entire spectrum of chemical reactions, occuring in the living
system, are collectively referred to as metabolism.
-Metabolic pathway-
• A metabolic pathway (or metabolic map) constitutes a series of
enzymatic reactions to produce specific products.
-Metabolite-
• The term metabolite is applied to a substrate or an
intermediate or a product in the metabolic reactions.

3. -Division of metabolism-
• Divided into two categories
1. Catabolism
• The degradative processes concerned with the breakdown
of complex molecules to simpler ones, with a concomitant
release of energy.
2. Anabolism
• The biosynthetic reactions involving the formation of
complex molecules from simple precursors.
-Amphibolism-
• The term amphibolism is used for reactions which are both
catabolic and anabolic in nature.

5. - Catabolism –
-Purpose of catabolism-
• To trap the energy of the biomolecules in the form of
ATP.
• To generate the substances (precursors) required for
the synthesis of complex molecules.
-Stages of catabolism –
• Three stages:
1. Conversion of complex molecules into their building
blocks.
2. Formation of simple intermediates
3. Final oxidation of acetyl CoA

7. -Anabolism-
• For the synthesis of complex molecules, the starting
materials are relatively few. E.g. pyruvate, acetyl CoA and
the intermediates of citric acid cycle.
• The anabolic reactions are dependent on the supply of
energy (as ATP or GTP) and reducing equivalents (as NADPH
+ H+).
• The anabolic and catabolic pathways are not reversible and
operate independently.
• The metabolic pathways occur in specific cellular locations
(mitochondria, microsomes etc.) and are controlled by
different regulatory signals.
-Intermediary metabolism-
• Intermediary metabolism refers to the entire range of
catabolic and anabolic reactions, not involving nucleic acids.

9. -Types of metabolic reactions-
• The biochemical reactions are mainly of four types
1. Oxidation-reduction
2. Group transfer
3. Rearrangement and isomerisation
4. Make and break of carbon-carbon bonds