Cell Structure and Function

Definition

A cell is the structural and functional unit of all living organisms. Every living process, from breathing to movement, occurs at the cellular level.

Core Principles (Cell Theory)

• Proposed by Schleiden and Schwann, later expanded by Virchow.
• All living organisms are composed of cells.
• The cell is the basic unit of life.
• All cells arise from pre-existing cells (Omnis cellula-e cellula).

Examples

• Unicellular: Amoeba (single cell performs all life functions).
• Multicellular: Humans (trillions of cells working in coordination).

Comparison

Think of a Prokaryotic cell as a studio apartment (one open room) and a Eukaryotic cell as a luxury mansion (separate rooms/organelles).

A. Cell Envelope & Cell Wall

A rigid outer layer providing shape and structural support, found in plants, fungi, and bacteria. (e.g., Cellulose in mango trees).

B. Cell Membrane (Plasma Membrane)

A thin, flexible, semi-permeable layer. The Fluid Mosaic Model (Singer and Nicolson) describes it as a "sea" of lipids with "iceberg" proteins floating within it.

• Mitochondria: The "Powerhouse"; produces ATP (energy).
• Ribosomes: The "Protein Factories"; build proteins.
• Endoplasmic Reticulum (ER): The "Highway"; Rough ER (proteins), Smooth ER (lipids).
• Golgi Apparatus: The "Post Office"; modifies and packages proteins.
• Lysosomes: The "Suicide Bags"; contain digestive enzymes for waste.
• Vacuoles: The "Storage Tanks"; store water and nutrients.

Components

• Nuclear Envelope: Double membrane protection.
• Nucleolus: Site of ribosome production.
• Chromatin: Loose DNA; condenses into Chromosomes during division.

Enzymes are proteins that speed up reactions using the Lock and Key Model. The enzyme (lock) has an Active Site that only fits a specific substrate (key).

Enzyme Action Equation

E + S → [ES] → E + P
(Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product)

A. The Cell Cycle

Consists of Interphase (Growth/DNA replication) and M-Phase (Division).

B. Mitosis vs. Meiosis

• Mitosis (Equational): Two identical daughter cells; used for growth/repair. (Phases: PMAT).
• Meiosis (Reduction): Four non-identical daughter cells; produces gametes (sperm/egg) with half chromosomes.

Numerical Illustration: DNA Content

Problem: A cell in the G₁ phase has 2C amount of DNA. How much DNA will it have after the S-phase and during the G₂ phase?

Step 1: Initial State (G₁) = 2C
Step 2: S-Phase (Replication) = DNA doubles
Step 3: Calculation: 2C × 2 = 4C
Answer: In G₂ phase, DNA = 4C
        

• Electron Microscopy: High-resolution imaging for nanotechnology.
• CRISPR-Cas9: Precise gene-editing tool.
• Flow Cytometry: Used in cancer diagnostics.

1. Which organelle is known as the "suicide bag"?
   (a) Golgi apparatus (b) Lysosome (c) Ribosome (d) Vacuole
2. The 'Fluid Mosaic Model' describes which structure?
   (a) Cell Wall (b) Nucleus (c) Cell Membrane (d) Cytoplasm
3. Which of these is a prokaryotic organism?
   (a) Amoeba (b) Mushroom (c) Bacteria (d) Human
4. The primary monomer of proteins is:
   (a) Glucose (b) Fatty acids (c) Nucleotides (d) Amino acids
5. In which phase of mitosis do chromosomes line up in the middle?
   (a) Prophase (b) Metaphase (c) Anaphase (d) Telophase
6. Which organelle produces ATP?
   (a) Ribosome (b) Mitochondria (c) ER (d) Nucleus
7. Enzymes are chemically made of:
   (a) Lipids (b) Carbohydrates (c) Proteins (d) Nucleic acids
8. Meiosis results in how many daughter cells?
   (a) 2 (b) 4 (c) 8 (d) 1
9. What is the main function of the Golgi apparatus?
   (a) Energy production (b) Protein synthesis (c) Packaging/Shipping (d) DNA storage
10. The cell wall of plants is primarily composed of:
    (a) Chitin (b) Peptidoglycan (c) Cellulose (d) Lipids

1. Differentiate between Mitosis and Meiosis.
   Mitosis produces two identical diploid cells for growth/repair. Meiosis produces four non-identical haploid cells for sexual reproduction.
2. Why is the cell membrane called "selectively permeable"?
   Because it allows only specific substances to pass through while blocking others, maintaining internal balance.
3. Explain the role of Ribosomes in a cell.
   Ribosomes are the sites of protein synthesis. They read mRNA instructions to assemble amino acids into proteins.
4. What would happen to a cell if its lysosomes burst?
   Digestive enzymes would leak into the cytoplasm and begin digesting the cell's own organelles (autolysis).
5. Describe the structure of a Nucleotide.
   A nucleotide consists of a Nitrogenous base, a Pentose sugar, and a Phosphate group.
6. How does temperature affect enzyme activity?
   Low temperatures slow activity; very high temperatures cause enzymes to denature (lose shape) and become non-functional.
7. State the importance of the S-phase in the cell cycle.
   The S-phase is when DNA replication occurs, ensuring daughter cells receive complete genetic instructions.
8. What is the difference between Rough ER and Smooth ER?
   Rough ER has ribosomes for protein synthesis; Smooth ER lacks ribosomes and handles lipid synthesis/detoxification.
9. Define Cell Theory.
   All living things are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells.
10. Why do plants need a cell wall while animals do not?
    Plants need rigid walls for structural support and protection against osmotic pressure; animals require flexibility for movement.

• Cell Theory: Schleiden, Schwann, Virchow.
• Prokaryote: No nucleus, circular DNA.
• Eukaryote: Nucleus, linear DNA, organelles.
• Mitochondria: ATP production.
• Lysosome: Waste disposal.

Mnemonics:

• Mitosis Phases: P-M-A-T (Please Make Another Two).
• Organelle Tricks:
  • Mighty Mitochondria Makes ATP.
  • Golgi = Golden Gift Wrap (packaging).
  • Lysosome = Lysol (cleaning/waste).