Metabolism is the foundation of all biological processes. It represents the network of thousands of chemical reactions that occur inside living cells.

These reactions are tightly controlled and interconnected through enzymes, energy transfer systems, and regulatory pathways.

Every living organism — from a single-celled bacterium to a complex human being — depends on metabolism to:

· Convert nutrients into energy.

· Synthesize necessary biomolecules.

· Maintain cellular structures.

· Adapt and respond to environmental changes.

Without metabolism, life cannot exist, because all physiological and cellular activities depend upon it.

2. Definition of Metabolism

Metabolism can be defined as:

“The sum total of all chemical reactions that occur within a living cell or organism to sustain life, growth, and reproduction.”

It includes both energy-producing and energy-consuming reactions.

The word metabolism originates from the Greek term “metabole”, meaning “change” — referring to the continuous chemical transformations taking place in living systems.

3. Basic Components of Metabolism

Metabolic reactions are grouped into two interdependent categories:

A. Catabolism (Degradative Phase)

· Involves the breakdown of large, complex molecules (such as carbohydrates, proteins, and lipids) into smaller units.

· These reactions release energy, usually stored temporarily in the form of ATP (Adenosine Triphosphate) or NADH.

· Catabolic processes are exergonic (energy-releasing).

Examples:

· Glycolysis (breakdown of glucose → pyruvate)

· Citric Acid Cycle (oxidation of acetyl-CoA)

· β-oxidation of fatty acids

B. Anabolism (Biosynthetic Phase)

· Involves the synthesis of complex biomolecules (proteins, nucleic acids, lipids, and polysaccharides) from smaller precursors.

· These reactions consume energy (ATP, NADPH).

· Anabolic processes are endergonic (energy-consuming).

Examples:

· Protein synthesis (from amino acids)

· Photosynthesis (CO₂ → glucose)

· Lipid synthesis (fatty acids → triglycerides)

4. Energy Coupling in Metabolism

Cells couple catabolic and anabolic reactions through energy transfer molecules like ATP.

· During catabolism, energy is released and stored as ATP.

· During anabolism, ATP is hydrolyzed to provide energy for biosynthesis.

Thus, ATP acts as the energy currency of the cell.

5. Enzymatic Regulation of Metabolism

Metabolic reactions are catalyzed by enzymes, which:

· Increase the speed of reactions.

· Ensure reactions occur under mild temperature and pH conditions.

· Provide regulation (activation/inhibition).

Enzymes allow complex reaction sequences to be controlled, preventing energy loss and maintaining balance.

6. Overview of Primary Metabolism

Definition:

Primary metabolism includes all the biochemical pathways and reactions essential for the normal growth, development, and reproduction of an organism.

These reactions are vital to life — their interruption results in the death or malfunctioning of the organism.

Characteristics of Primary Metabolism

· Occurs universally in all living cells.

· Produces primary metabolites, which are directly involved in physiological functions.

· Operates throughout the growth phase.

· Highly conserved among species (similar pathways across plants, animals, and microbes).

Major Primary Metabolic Pathways

Carbohydrate Metabolism

· Includes glycolysis, gluconeogenesis, and Krebs cycle.

· Responsible for energy production (ATP).

Protein Metabolism

· Includes synthesis and degradation of amino acids.

· Provides building blocks for enzymes and structural proteins.

Lipid Metabolism

· Fatty acid synthesis and degradation.

· Provides energy storage and membrane components.

Nucleic Acid Metabolism

· DNA and RNA synthesis for genetic continuity.

Photosynthesis (in plants)

· Converts light energy to chemical energy (glucose).

Examples of Primary Metabolites

Category	Examples	Functions
Carbohydrates	Glucose, sucrose, starch	Energy source
Amino acids	Glycine, alanine, valine	Protein building blocks
Nucleotides	ATP, GTP, DNA, RNA bases	Genetic information, energy
Lipids	Phospholipids, triglycerides	Membrane structure, energy storage

7. Overview of Secondary Metabolism

Definition:

Secondary metabolism involves biochemical reactions that produce secondary metabolites — compounds not essential for growth or reproduction, but vital for ecological survival, defense, and adaptation.

These metabolites are often species-specific and are formed during the stationary or late growth phase.

Characteristics of Secondary Metabolism

· Not present in all organisms.

· Usually occurs in plants, fungi, and some bacteria.

· Involves specialized biosynthetic pathways.

· Secondary metabolites are derived from intermediates of primary metabolism.

Functions of Secondary Metabolites

1. Defense mechanisms – Protect plants from herbivores, pathogens, or UV light.

2. Signaling – Act as attractants (colors, fragrances) for pollination and seed dispersal.

3. Competition – Inhibit the growth of neighboring organisms (allelopathy).

4. Symbiosis and communication – Microbes use them for intercellular signaling.

Examples of Secondary Metabolites

Group	Examples	Sources/Functions
Alkaloids	Morphine, quinine, nicotine, caffeine	Defense, medicinal uses
Terpenoids	Menthol, carotenoids, camphor	Aroma, pigments, protection
Phenolics	Tannins, flavonoids, lignin	Antioxidants, structural support
Glycosides	Digitalis, saponins	Medicinal and defense roles
Antibiotics	Penicillin, streptomycin	Produced by bacteria/fungi for protection