Quick Summary
The cell is the fundamental unit of life. This chapter covers the distinction between prokaryotic and eukaryotic cells, the structure and function of all major organelles, and the plasma membrane model. Cell organelles and their functions are among the most heavily tested topics in NEET biology.
Cell Theory
- Schleiden (1838): All plants are made of cells
- Schwann (1839): All animals are made of cells
- Virchow (1855): "Omnis cellula e cellula" — all cells arise from pre-existing cells
Exceptions to cell theory: Viruses (not made of cells), RBCs (no nucleus in mature form), sieve tubes (no nucleus).
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus (nucleoid) | True membrane-bound nucleus |
| Organelles | No membrane-bound organelles | Mitochondria, ER, Golgi, etc. |
| Ribosome | 70S (50S + 30S) | 80S (60S + 40S) |
| Size | 1–10 μm | 10–100 μm |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants), absent (animals) |
| DNA | Circular, no histones | Linear, with histones |
| Examples | Bacteria, Cyanobacteria | All plants, animals, fungi |
Cell Wall (Plant Cells)
- Primary wall: Thin, cellulose, pectin; present in young cells
- Secondary wall: Thick, lignin; in mature cells
- Middle lamella: Pectin cement between adjacent cells
- Plasmodesmata: Cytoplasmic connections through cell walls
Plasma Membrane
Fluid Mosaic Model (Singer & Nicolson, 1972):
- Phospholipid bilayer (hydrophilic heads outward, hydrophobic tails inward)
- Proteins: integral (transmembrane) and peripheral (surface)
- Cholesterol (in animal cells) maintains fluidity
- Functions: Selective permeability, cell signalling, transport
Cell Organelles
Endoplasmic Reticulum (ER)
| Type | Features | Function |
|---|---|---|
| Rough ER (RER) | Ribosomes on surface | Protein synthesis + transport |
| Smooth ER (SER) | No ribosomes | Lipid synthesis, detoxification |
Golgi Apparatus
- Stack of flattened membranous sacs (cisternae)
- Cis face: Receives vesicles from ER
- Trans face: Ships processed products
- Functions: protein modification (glycosylation), packaging, secretion
- "Post office of the cell"
Mitochondria ("Powerhouse")
- Double membrane; inner membrane folded into cristae
- Matrix contains enzymes for Krebs cycle
- Own DNA and 70S ribosomes → semi-autonomous organelle
- Site of aerobic respiration → ATP production
Chloroplast ("Kitchen of the cell")
- Double membrane; inner membrane forms thylakoids stacked into grana
- Stroma: Site of dark reactions (Calvin cycle)
- Thylakoid membrane: Site of light reactions
- Contains own DNA and 70S ribosomes → semi-autonomous
- Present only in plant cells and algae
Lysosomes ("Suicidal bags")
- Membrane-bound vesicles with hydrolytic enzymes
- Digest worn-out organelles (autophagy), foreign particles
- Enzymes work at pH 5 (acidic)
Vacuoles
- Plant cells: Large central vacuole (up to 90% of cell volume); maintains turgor pressure
- Animal cells: Small, temporary
- Tonoplast: membrane surrounding vacuole
Ribosomes
- Site of protein synthesis
- 70S in prokaryotes and organelles (mitochondria, chloroplast)
- 80S in eukaryotic cytoplasm
Nucleus
- Nuclear envelope: Double membrane with nuclear pores
- Nucleolus: Site of rRNA synthesis and ribosome assembly
- Chromatin: DNA + histone proteins (condenses into chromosomes during division)
Centrosome / Centriole
- Present in animal cells; absent in most plant cells
- Forms spindle fibres during cell division
Important Questions
One Mark
- Who proposed the fluid mosaic model of the plasma membrane?
- What is the ribosome size in prokaryotes?
- Name the organelle called the "suicidal bag" of the cell.
- Where does the Krebs cycle occur in the cell?
Short Answer (3 marks)
- Compare the structure and function of RER and SER.
- Describe the structure of mitochondria. Why is it called semi-autonomous?
- Explain the fluid mosaic model of the plasma membrane.
Long Answer (5 marks)
- Describe the structure and function of chloroplast. How is it similar to mitochondria?
- Compare prokaryotic and eukaryotic cells on the basis of five structural features.
Common Mistakes
- Mitochondria ribosome: it has 70S ribosomes (like prokaryotes), not 80S — this confuses students.
- Lysosome enzymes: they are acid hydrolases (work at pH 5), not neutral — the acidity is essential.
- Vacuoles: large in plant cells, small in animal cells — not the other way round.
- Cell wall: only in plants (cellulose), fungi (chitin), bacteria (peptidoglycan) — NOT in animal cells.
Board Exam Tips
- Prokaryotic vs eukaryotic comparison table (5 characters) is a standard 5-mark question.
- Organelle diagrams (mitochondria, chloroplast, Golgi) with labels appear regularly.
- The fluid mosaic model — description with integral vs peripheral proteins — is a 3-mark question.
- NEET: Cell organelles and their functions contribute 3–4 questions per paper — this is a high-yield topic.
Frequently Asked Questions
1. All living organisms are composed of cells. 2. The cell is the basic structural and functional unit of life. 3. All cells arise from pre-existing cells (Virchow). Proposed by Schleiden (plants) and Schwann (animals) in 1838–39.