Cell — The Unit of Life

Why This Chapter Matters

Cell biology is the most important chapter for NEET Biology — 8-12 marks every year. Cell organelles, cell wall, plasma membrane, nucleus, and differences between prokaryotic and eukaryotic cells are frequently tested.

Prerequisites

•Basic cell structure from Class 9 (NCERT)

•Microscopy (light vs electron microscope)

Core Concepts

1. Cell Theory

Schleiden (1838) — all plants are made of cells

Schwann (1839) — all animals are made of cells

Virchow (1855) — "Omnis cellula e cellula" — cells arise from pre-existing cells

Modern Cell Theory: Cell is the structural and functional unit of life.

2. Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic

|---|---|---|

Nucleus	No true nucleus (nucleoid region)	True membrane-bound nucleus
Size	1-10 μm	10-100 μm
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi)
Ribosome	70S (50S+30S)	80S (60S+40S)
DNA	Circular, no histone	Linear, with histones
Cell wall	Present (peptidoglycan in bacteria)	Present in plants (cellulose), fungi (chitin), absent in animals
Examples	Bacteria, Archaea, Mycoplasma	All other organisms

3. Plasma Membrane (Cell Membrane)

Fluid Mosaic Model — Singer and Nicolson (1972):

•Phospholipid bilayer (fluid — can move laterally)

•Proteins embedded (mosaic — scattered proteins)

•Cholesterol — maintains fluidity at different temperatures

Phospholipid: Hydrophilic head (faces water) + Hydrophobic tail (faces inward)

Membrane transport:

•Passive: diffusion, osmosis, facilitated diffusion (no energy — along concentration gradient)

•Active: pumps (Na⁺/K⁺ ATPase) — against gradient, requires ATP

•Endocytosis: phagocytosis (solid), pinocytosis (liquid)

•Exocytosis: release of materials from cell

4. Cell Wall

Plants: Cellulose (primary wall) + Lignin/Suberin/Pectin (secondary wall)

Fungi: Chitin

Bacteria: Peptidoglycan (murein)

Plants have plasmodesmata — cytoplasmic connections through cell walls

5. Nucleus

•Nuclear envelope = outer + inner nuclear membrane with nuclear pores

•Nucleoplasm = fluid inside nucleus

•Chromatin = DNA + proteins (histone) — forms chromosomes during cell division

•Nucleolus = site of rRNA synthesis and ribosome assembly — disappears during cell division

•Nuclear matrix = fibrous network providing structural support

Eukaryotic DNA is linear, associated with histone proteins (H1, H2A, H2B, H3, H4).

Nucleosome = 8 histone proteins (octamer) + DNA wrapped around them.

6. Mitochondria

•Double membrane (outer smooth, inner folded = cristae)

•Matrix contains: enzymes, circular DNA, 70S ribosomes

•Site of aerobic respiration (Krebs cycle in matrix, ETC on inner membrane)

•"Powerhouse of the cell"

•Semiautonomous — own DNA and ribosomes

•Evolutionary origin: endosymbiosis theory (engulfed bacteria)

7. Chloroplast

•Double membrane (outer, inner + thylakoid membrane)

•Grana (stack of thylakoids) = light reaction

•Stroma = dark reaction (Calvin cycle)

•Contains: DNA (circular), 70S ribosomes, enzymes

•Found in leaf mesophyll cells, guard cells

•Semiautonomous — own DNA and ribosomes

8. Endoplasmic Reticulum (ER)

Rough ER: Ribosomes attached — protein synthesis + modification

Smooth ER: No ribosomes — lipid synthesis, detoxification, calcium storage

9. Golgi Body (Golgi Complex)

•Stack of membrane-bound sacs (cisternae)

•cis face (receives from ER) → trans face (sends to destination)

•Functions: protein modification, packaging, secretion, lysosome formation

•"Trafficking policeman of the cell"

10. Lysosomes

•Contain hydrolytic enzymes (acid hydrolases)

•pH 4.5-5.0 (acidic)

•"Suicide bags" — can digest cell contents (autolysis)

•Formed by Golgi body

•Digest worn-out organelles (autophagy) and foreign particles

11. Vacuoles

•Plants: large central vacuole (tonoplast membrane), provides turgor pressure

•Animals: small, temporary food vacuoles or contractile vacuoles

12. Cytoskeleton

Microfilaments: Actin — cell movement, cytokinesis

Microtubules: Tubulin — cilia/flagella formation, spindle fibres, cell shape

Intermediate filaments: Give mechanical strength

13. Cilia and Flagella

Structure: 9+2 arrangement of microtubules

9 doublet microtubules surrounding 2 central singlet microtubules

Used for movement

Cilia (shorter, many) | Flagella (longer, few or one)

NEET Important Points

Smallest cell: Mycoplasma (PPLO — Pleuropneumonia-like organisms)

Largest cell: Ostrich egg

Longest cell: Nerve cells (neurons)

Cell without nucleus: RBC (mature), platelets, sieve tube cells (phloem)

Cell with multiple nuclei: Muscle fibres (syncytium), osteoclasts, liver cells

Robert Hooke (1665) — coined the term "cell" while observing cork cells

Leeuwenhoek — first observed living cells (bacteria)

PYQs (NEET)

NEET 2023: Which of the following is not a feature of prokaryotic cells?

(A) 70S ribosomes (B) Circular DNA (C) Membrane-bound nucleus (D) Cell wall

Answer: C — Prokaryotes do NOT have a membrane-bound nucleus.

NEET 2022: The fluid mosaic model of plasma membrane was proposed by:

(A) Singer and Nicolson (B) Davson and Danielli (C) Robertson (D) Gorter and Grendel

Answer: A

NEET 2021: Mitochondrial matrix contains all EXCEPT:

(A) Circular DNA (B) 70S ribosomes (C) Enzymes for Krebs cycle (D) Grana

Answer: D — Grana are found in chloroplasts, NOT in mitochondria.

NEET 2020: Which cell organelle is involved in protein sorting and packaging?

(A) ER (B) Golgi apparatus (C) Lysosome (D) Vacuole

Answer: B

MCQ Practice

Q1. Ribosome of chloroplast is:

(A) 80S (B) 70S ✓ (C) 60S (D) 50S

Q2. The unit of chromatin organisation is:

(A) Chromosome (B) Chromatin fibre (C) Nucleosome ✓ (D) Nucleolus

Q3. Plasmodesmata connect:

(A) Animal cells (B) Plant cells through cell walls ✓ (C) Prokaryotic cells (D) Fungal cells

Q4 (Hard). The Na⁺/K⁺ pump moves:

(A) 3 Na⁺ out and 2 K⁺ in ✓ (B) 2 Na⁺ out and 3 K⁺ in (C) 3 Na⁺ in and 2 K⁺ out (D) Equal exchange

Q5 (Hard). Which has 9+0 arrangement (no central pair)?

(A) Flagella (B) Cilia (C) Centrioles ✓ (D) Both A and B

Revision Notes

CELL THEORY: Schleiden+Schwann (all cells) + Virchow (cells from cells)

PROKARYOTE vs EUKARYOTE:
Prokaryote: No nucleus, 70S ribosome, circular DNA, 1-10μm
Eukaryote: True nucleus, 80S ribosome, linear DNA+histones, 10-100μm

PLASMA MEMBRANE: Fluid Mosaic Model (Singer & Nicolson 1972)
Phospholipid bilayer + embedded proteins + cholesterol

ORGANELLES:
Mitochondria: Double membrane, 70S ribosome, Krebs cycle(matrix), ETC(inner membrane)
Chloroplast: Double membrane, 70S, grana(light reaction), stroma(dark reaction)
Rough ER: ribosomes, protein synthesis
Smooth ER: lipid synthesis, detoxification
Golgi: cis→trans, protein modification/packaging/secretion
Lysosome: hydrolytic enzymes, pH 4.5, "suicide bags"
Nucleus: nuclear envelope, nucleoplasm, chromatin, nucleolus

NUCLEOSOME: 8 histones (octamer H2A×2, H2B×2, H3×2, H4×2) + H1 + DNA wrapped

CILIA/FLAGELLA: 9+2 microtubule arrangement
CENTRIOLES: 9+0 (no central pair)

SMALLEST CELL: Mycoplasma | LARGEST CELL: Ostrich egg
NO NUCLEUS: RBC, platelets, sieve tube cells

Cell — The Unit of Life

Cell — The Unit of Life

Why This Chapter Matters

Prerequisites

Core Concepts

1. Cell Theory

2. Prokaryotic vs Eukaryotic Cells

3. Plasma Membrane (Cell Membrane)

4. Cell Wall

5. Nucleus

6. Mitochondria

7. Chloroplast

8. Endoplasmic Reticulum (ER)

9. Golgi Body (Golgi Complex)

10. Lysosomes

11. Vacuoles

12. Cytoskeleton

13. Cilia and Flagella

NEET Important Points

PYQs (NEET)

MCQ Practice

Revision Notes

Related Topics