Metabolism
Every living organism—whether a tiny microbe or a human being—contains thousands of biomolecules working in harmony. But these molecules are not static; they are constantly being broken down and rebuilt through a series of chemical reactions. This dynamic process is called metabolism.
Metabolism is not just a biochemical concept—it is the very essence of life. It enables organisms to grow, reproduce, maintain their structure, and respond to environmental changes. Without metabolism, life simply cannot exist.
Metabolic Pathways
Metabolism operates through metabolic pathways, which are sequences of enzyme-catalysed reactions where one molecule is transformed step by step into another.
These pathways are of three types:
1. Catabolic Pathways (Breaking Down)
These pathways decompose complex molecules into simpler ones, releasing energy—hence they are exothermic. This released energy is often stored in the form of ATP (Adenosine Triphosphate).
Examples:
- Glycolysis: Glucose → Pyruvate
- Cellular Respiration: Glucose → CO₂ + H₂O + ATP
- Protein Catabolism: Proteins → Amino acids
Think of catabolism as burning fuel to generate energy.
2. Anabolic Pathways (Building Up)
These pathways construct complex molecules from simpler ones, and they require energy input—hence they are endothermic.
Examples:
- Photosynthesis: Light energy → Chemical energy (glucose)
- Gluconeogenesis: Non-carbohydrates → Glucose
- Protein Synthesis: Amino acids → Proteins
Anabolism is like using energy to build a house brick by brick.
3. Amphibolic Pathways (Dual Role)
Some pathways act as a bridge between catabolism and anabolism—these are called amphibolic pathways.
The best example is the Krebs Cycle (Citric Acid Cycle):
- Catabolic role: Breaks down carbohydrates, fats, and proteins to release energy
- Anabolic role: Provides intermediates for synthesising amino acids, fatty acids, and nucleotides
So, it behaves like a central junction in metabolism, serving both breakdown and synthesis.
Enzymes
Now, imagine if all these reactions happened at normal speed—it would take years for a single reaction! This is where enzymes come in.
Enzymes are biological catalysts (mostly proteins, sometimes RNA called ribozymes) that accelerate chemical reactions without being consumed.
They ensure that metabolism is fast, efficient, and regulated.
Six Major Classes of Enzymes
Each enzyme is specialised:
| Enzyme Class | Reaction Catalysed | Example |
| Hydrolase | Hydrolysis (catabolic) | Lipase, protease |
| Isomerase | Rearrangement of atoms within a molecule | Phosphohexoisomerase |
| Lyase | Splitting chemicals into smaller parts without using water (catabolic) | Decarboxylases, aldolases |
| Oxidoreductase | Transfers electrons or hydrogen atoms from one molecule to another | Dehydrogenases, oxidases |
| Synthetases | Joining of two molecules by the formation of new bonds (anabolic) | DNA ligase, DNA polymerase |
| Transferase | Moving a functional group from one molecule to another | Kinases, transaminase |
Factors Affecting Enzyme Activity
Enzyme efficiency is not constant—it depends on several factors:
- Temperature:
Activity increases up to an optimum (~37°C in humans), but high heat denatures enzymes. - pH Level:
Each enzyme has an optimal pH; extremes can denature it. - Substrate Concentration:
Increases reaction rate until all active sites are saturated. - Enzyme Concentration:
More enzymes → faster reaction (if substrate is sufficient). - Inhibitors:
- Competitive inhibitors: Compete with substrate for active site
- Non-competitive inhibitors: Bind elsewhere and change enzyme shape
Co-factors: Helpers of Enzymes
Some enzymes cannot function alone—they need co-factors, which are non-protein components that activate the enzyme (apoenzyme).
Types:
- Prosthetic Groups: Permanently attached
- Co-enzymes: Temporarily bind (often derived from vitamins)
- Metal Ions: Assist in catalysis (e.g., Zn²⁺, Mg²⁺)
Without co-factors, many enzymes remain inactive.
Metabolites
During metabolism, various organic compounds are produced—these are called metabolites.
They are of two types:
1. Primary Metabolites
- Essential for survival (growth, development, reproduction)
- Found in all organisms
- Examples: Amino acids, carbohydrates, lipids, proteins, nucleic acids
Functions → Energy production, Structural roles, Building macromolecules
2. Secondary Metabolites
- Not essential for survival but provide ecological advantages
- Mostly found in plants, fungi, and microbes
Functions:
- Defence (e.g., alkaloids)
- Attraction (pigments, scents)
- Human use (antibiotics, drugs, rubber)
Examples → Alkaloids, flavonoids, antibiotics, essential oils