🔗 Nature of Bonds Linking Monomers

Living systems are built from polymers, which are formed by joining small units called monomers. The type of chemical bond linking monomers determines the nature and function of the polymer.

🧬 1️⃣ Peptide Bond – Proteins

Proteins are polymers of amino acids.

How is a peptide bond formed?

  • The carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH₂) of the next amino acid.
  • A molecule of water (H₂O) is released (dehydration reaction).
  • The resulting covalent bond is called a peptide bond (-CO–NH-).

📌 Key Points to Remember:

  • Links amino acids
  • Present in proteins
  • Formed by dehydration

🧠 Memory Line:

Protein = Amino acids + Peptide bonds

🍞 2️⃣ Glycosidic Bond – Polysaccharides

Polysaccharides are polymers of monosaccharides (simple sugars).

How is a glycosidic bond formed?

  • Formed between two carbon atoms of adjacent monosaccharides
  • Involves elimination of a water molecule
  • The bond formed is called a glycosidic bond

📌 Examples:

  • Glucose + Glucose → Starch / Cellulose / Glycogen

🧠 Memory Line:

Sugar + Sugar = Glycosidic bond

🧬 3️⃣ Phosphodiester Bond – Nucleic Acids

Nucleic acids (DNA & RNA) are polymers of nucleotides.

How is a phosphodiester bond formed?

  • The phosphate group links:
  • the 3′-carbon of sugar of one nucleotide
  • to the 5′-carbon of sugar of the next nucleotide
  • Since phosphate forms two ester bonds, it is called a phosphodiester bond

📌 Key Role: Forms the sugar–phosphate backbone of DNA and RNA

🧠 Memory Line:

DNA backbone = Sugar + Phosphate + Phosphodiester bond

🔄 Concept of Metabolism – Chemistry of Life

Living organisms are dynamic, not static. Thousands of chemical reactions occur every second to maintain life.

🔁 1️⃣ Metabolism

Metabolism is defined as the sum total of all chemical reactions occurring in a living cell or organism.

  • Includes both breaking down and building up reactions
  • Ensures continuous turnover of biomolecules

🧠 Key Idea:

Life = Continuous chemical activity

🛣️ 2️⃣ Metabolic Pathways

Metabolic reactions occur in organized sequences, not randomly. These sequences are called metabolic pathways.

  • Linear pathways: One step follows another (e.g., Glycolysis)
  • Circular pathways: End product regenerates starting compound (e.g., Krebs cycle)

📌 Analogy: Like traffic moving through fixed city routes

⚖️ Metabolic Basis of Living

Based on energy flow, metabolism is of two types:

🏗️ A. Anabolism (Biosynthesis)

  • Direction: Simple → Complex
  • Energy: Requires energy (endergonic)
  • Purpose: Growth, repair, storage
  • Examples:
  • Amino acids → Proteins
  • Photosynthesis
  • Acetic acid → Cholesterol

🧠 Memory Line:

Anabolism = Building + Energy needed

🔨 B. Catabolism (Degradation)

  • Direction: Complex → Simple
  • Energy: Releases energy (exergonic)
  • Purpose: Energy supply
  • Examples:
  • Glucose → Lactic acid (Glycolysis)
  • Cellular respiration

🧠 Memory Line:

Catabolism = Breaking + Energy released

🔋 Energy Currency of Cell

  • The universal energy currency of the cell is ATP (Adenosine Triphosphate)
  • Energy released during catabolism is stored in ATP
  • ATP is used to drive anabolic reactions, muscle contraction, transport, etc.

🧠 Key Line:

ATP = Energy money of the cell

⚖️ The Living State – Steady but Not Still

  • Living systems maintain a steady-state
  • Concentration of biomolecules remains constant
  • Example: Blood glucose = 4.2–6.1 mmol/L

❌ Why Not Equilibrium?

  • At equilibrium → No work can be done
  • Living organisms must continuously perform work

✅ Therefore:

Living state = Non-equilibrium steady-state

🧠 Ultimate Concept:

Life is a constant fight against equilibrium

🧠 Memory Capsules

  • Protein → Peptide bond
  • Polysaccharide → Glycosidic bond
  • Nucleic acid → Phosphodiester bond
  • All polymer bonds form by removal of water
  • Anabolism: Build up | Energy needed
  • Catabolism: Break down | Energy released
  • Catabolism → ATP formation
  • ATP → Anabolism + Work
  • Steady-state + Non-equilibrium = Life

💊 One-Look Table

Keyword Recall
Peptide Protein
Glycosidic Sugar
Phosphodiester DNA/RNA
ATP Energy
Equilibrium Death

🧠 Final Exam Hack:

If reactions stop → equilibrium → death

💡 Questions & Answers

Q1. What is metabolism?

A1.

Metabolism is the sum of all chemical reactions occurring in a living organism that help in growth, maintenance, and energy supply.

🔑 Key Points: Total reactions; life processes.

Q2. Differentiate between anabolism and catabolism.

A2.

Anabolism builds complex molecules using energy, while catabolism breaks complex molecules and releases energy.

🔑 Key Points: Build vs break; energy use vs release.

Q3. Which bond links amino acids in proteins?

A3.

Peptide bond.

Q4. Why do living systems never reach equilibrium?

A4.

Because at equilibrium no work can be done, and living organisms must continuously perform work to survive.

🔑 Key Points: Work requires non-equilibrium.

Q5. What is the energy currency of the cell?

A5.

ATP (Adenosine Triphosphate) is the energy currency of the cell.

Q6. Which bond links nucleotides in DNA?

A6.

Phosphodiester bond.