Cell: The Unit of Life

NCERT Line-by-Line Breakdown for NEET 2026

Unit 3: Cell Structure & Function

The cell is the fundamental structural and functional unit of all living organisms. Whether it’s a tiny bacteria or a giant blue whale, everything starts with a cell. In this chapter, we explore Prokaryotic vs. Eukaryotic cells, the Cell Theory, and the intricate machinery of Cell Organelles.

1. The Cell Theory

📜 The Timeline

1838 (Schleiden): German Botanist. Observed that all plants are composed of different kinds of cells.
1839 (Schwann): British Zoologist. Reported that cells had a thin outer layer (plasma membrane) and plant cells have a unique cell wall.
1855 (Rudolf Virchow): “Omnis cellula-e cellula” (New cells arise from pre-existing cells).

Modern Cell Theory: (1) All living organisms are composed of cells and products of cells. (2) All cells arise from pre-existing cells.

2. Prokaryotic Cells (Bacteria)

They lack a well-defined nucleus and membrane-bound organelles. Genetic material is naked (Nucleoid).

[Image of prokaryotic cell structure]

A. Cell Envelope & Modifications

Layer	Composition & Function
Glycocalyx (Outermost)	If loose sheath = Slime Layer. If thick/tough = Capsule.
Cell Wall	Determines shape, prevents bursting.
Plasma Membrane	Selectively permeable. Forms Mesosomes (extensions).

🧪 NEET Gem: Mesosomes
Extensions of plasma membrane into the cell (vesicles, tubules, lamellae).
Functions: Cell wall formation, DNA replication, Respiration, Secretion, Increasing surface area.

Ribosomes: 70S type (50S + 30S subunits). Site of protein synthesis.
Polysome: Several ribosomes attached to a single mRNA.

3. Eukaryotic Cells

Possess an organized nucleus with a nuclear envelope and compartmentalization via membrane-bound organelles.

[Image of plant cell vs animal cell diagram]

A. Cell Membrane

Fluid Mosaic Model (1972) by Singer and Nicolson.

Lipids: Phospholipid bilayer (Polar heads out, Hydrophobic tails in).
Proteins: Integral (buried) and Peripheral (surface).
Fluidity: Quasi-fluid nature of lipids enables lateral movement of proteins (essential for growth, endocytosis).

[Image of fluid mosaic model of cell membrane]

B. Cell Wall

Non-living rigid structure.
Algae: Cellulose, Galactans, Mannans, Calcium carbonate.
Plants: Cellulose, Hemicellulose, Pectins, Proteins.
Middle Lamella: Calcium pectate (glues cells together).

4. The Endomembrane System

Organelles whose functions are coordinated. (ER → Golgi → Lysosome → Vacuole).

1. Endoplasmic Reticulum (ER)
• RER: Has ribosomes. Protein synthesis.
• SER: No ribosomes. Lipid & Steroid hormone synthesis.

2. Golgi Apparatus
• Camillo Golgi (1898).
• Packaging of materials.
• Site of formation of Glycoproteins & Glycolipids.

3. Lysosomes
• “Suicidal Bags”.
• Rich in Hydrolytic enzymes (Lipases, Proteases) active at acidic pH.

4. Vacuoles
• Tonoplast (membrane).
• Contractile vacuole (Amoeba) = Excretion.
• Food vacuole (Protists).

5. Semi-Autonomous Organelles

These organelles have their own Circular DNA and 70S Ribosomes.

[Image of mitochondria and chloroplast structure]

Mitochondria	• Double membrane. • Inner membrane forms Cristae (increase surface area). • Site of Aerobic Respiration (ATP production).
Plastids	• Chloroplasts: Chlorophyll (Photosynthesis). Thylakoids stacked into Grana. • Chromoplasts: Carotenoids (Red/Orange color). • Leucoplasts: Colorless storage (Amyloplast=Carbs, Elaioplast=Oil, Aleuroplast=Protein).

6. Cytoskeleton & Nucleus

A. Cilia and Flagella

Hair-like outgrowths of cell membrane.

Core (Axoneme): Possesses 9 + 2 array of microtubules.
Basal Body: Structure similar to centriole (9 + 0) from which they emerge.

B. The Nucleus

The director of the cell. Contains Chromatin (DNA + Histones).

Classification of Chromosomes (Based on Centromere):
[Image of chromosome types metacentric submetacentric acrocentric telocentric]

Metacentric: Middle centromere (V-shape).
Sub-metacentric: Slightly away from middle (L-shape).
Acrocentric: Close to end (J-shape).
Telocentric: Terminal centromere (i-shape).

📝 Rapid Fire MCQs

Q1. Which of the following is correct for the origin of Lysosome?

A) ER → Golgi bodies → Lysosome
B) Golgi bodies → ER → Lysosome
C) Nucleus → Golgi bodies → Lysosome
D) Mitochondria → ER → Lysosome

Click to check Answer

Answer: A) ER → Golgi bodies → Lysosome (GERL system).

Q2. The arrangement of microtubules in a centriole is:

A) 9 + 2
B) 9 + 0
C) 9 + 1
D) 9 + 3