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Biology · CDS

BC01 — Cell Biology (Cytology)

BC01 · CDS General Knowledge — Biology ★ High Priority

The cell is the fundamental unit of all life. Every biological process — from muscle contraction to memory — happens inside cells. CDS consistently asks about organelle functions, prokaryote vs eukaryote, and cell division. This chapter builds the vocabulary you need for every other biology chapter.

CDS Focus: Mitochondria = powerhouse (ATP); ribosome = protein synthesis; lysosome = suicidal bag; Golgi = packaging; nucleus controls heredity. Mitosis = equational (growth); meiosis = reductional (gametes). Prokaryotes have no membrane-bound nucleus. These exact facts repeat across CDS papers.

Part 1 — Cell Theory & Types

Cell theory, proposed by Schleiden (1838) and Schwann (1839) and later completed by Virchow (1855), states three things: all living organisms are made of cells; the cell is the basic structural and functional unit of life; and all cells arise from pre-existing cells. The one major exception is viruses — they are non-cellular and show life only inside a host.

1. Prokaryotic vs Eukaryotic Cells

Nucleus (control centre)

Prokaryote

NO membrane-bound nucleus; naked DNA

Eukaryote

TRUE nucleus with nuclear membrane

Organelles

Prokaryote

ABSENT (no membrane-bound)

Eukaryote

PRESENT (mitochondria, Golgi etc.)

Cell wall

Prokaryote

PRESENT (peptidoglycan in bacteria)

Eukaryote

Plant: cellulose wall; Animal: no cell wall

Size & Examples

Prokaryote

Smaller (1–10 μm) — Bacteria, Blue-green algae

Eukaryote

Larger (10–100 μm) — Plant, Animal, Fungal cells

Ribosomes are present in BOTH types (70S in prokaryotes; 80S in eukaryotes).

Part 2 — Cell Organelles

2. Cell Organelles — Structure and Function

Each organelle is like a specialist department in a factory. The key for CDS is knowing the specific job of each organelle and the memorable nickname associated with it.

Nucleus

Membrane

Double membrane

Main Function

Contains DNA (hereditary material). Controls all cell activities.

Key Fact

"Control Centre" — Nucleolus makes rRNA

Mitochondria

Membrane

Double membrane

Main Function

Cellular respiration; produces ATP (energy currency).

Key Fact

"Powerhouse of cell" — Has own DNA (semi-autonomous)

Ribosome

Membrane

No membrane (naked)

Main Function

Site of protein synthesis (translation of mRNA).

Key Fact

"Protein factory" — 70S (prokaryote); 80S (eukaryote)

Golgi Body

Membrane

Single membrane

Main Function

Packaging and secretion of proteins and lipids.

Key Fact

"Post office of cell" — Forms lysosomes

Lysosome

Membrane

Single membrane

Main Function

Digests worn-out organelles and foreign material (autolysis).

Key Fact

"Suicidal bag" — Contains digestive enzymes

Chloroplast

Membrane

Double membrane

Main Function

Photosynthesis — converts light energy to chemical energy.

Key Fact

Plants only! — Contains chlorophyll pigment

Vacuole

Membrane

Single membrane

Main Function

Storage of water, nutrients, and waste products.

Key Fact

Plant: one large central — Animal: many small vacuoles

ER (Rough): has ribosomes; makes proteins | ER (Smooth): no ribosomes; makes lipids & detoxification

Part 3 — Cell Division

3. Cell Division — Mitosis and Meiosis

Cells divide for two purposes: to grow and repair (mitosis) and to form sex cells for reproduction (meiosis). Understanding which is which — and why — is critical for CDS.

Mitosis — Equational Division

Purpose

Growth, repair, and replacement of somatic (body) cells

Result

2 daughter cells; same chromosome number as parent (2n → 2n); genetically identical

Stages

Prophase → Metaphase → Anaphase → Telophase

Meiosis — Reductional Division

Purpose

Formation of gametes (sperm and egg cells)

Result

4 daughter cells; HALF the chromosome number of parent (2n → n); genetically varied

Stages

Meiosis I (reductional) + Meiosis II (equational)

Human = 46 Chromosomes (23 pairs): After meiosis, gametes (sperm and egg) each carry 23 chromosomes (haploid, n). When sperm and egg fuse at fertilisation, the resulting zygote restores the full complement of 46 chromosomes (diploid, 2n). This halving and doubling keeps the chromosome number stable across generations. It is one of the most directly tested CDS biology facts.

Part 4 — Tissues

4. Animal Tissues

Epithelial

Structure & Location

Cells tightly packed; line body surfaces — skin, gut lining, lung surface

Function

Protection, absorption, secretion, filtration

Connective

Structure & Location

Cells scattered in matrix — bone, cartilage, blood, adipose, tendons

Function

Support, binding, transport (blood), insulation (fat)

Muscular

Structure & Location

Skeletal (voluntary, striated); Smooth (involuntary, unstriated); Cardiac (heart muscle, involuntary striated)

Function

Movement and locomotion

Nervous

Structure & Location

Neurons (nerve cells) with axon and dendrites; found in brain, spinal cord, nerves

Function

Transmission of electrical signals; coordination

CDS PYQs — Cell Biology

Q1. Which organelle is called the "powerhouse of the cell"?CDS PYQ

(a) Nucleus

(b) Ribosome

(d) Golgi apparatus

✓ Answer: (c) Mitochondria

The mitochondria produces ATP (adenosine triphosphate) through cellular respiration — the energy currency that powers every cellular activity. It has its own DNA and ribosomes, making it semi-autonomous. This is why it is called the powerhouse. The number of mitochondria is highest in metabolically active cells like liver cells, muscle cells, and sperm (flagellum movement). This is the single most repeated CDS cell biology question.

Q2. Lysosomes are known as "suicidal bags" because:CDS PYQ

(a) They produce energy

(b) They contain digestive enzymes that can destroy the cell itself

(d) They synthesise proteins

✓ Answer: (b) Contain digestive enzymes

Lysosomes contain around 40 different hydrolytic (digestive) enzymes. When a cell is damaged or during programmed cell death (apoptosis), the lysosome membrane ruptures and releases these enzymes into the cell cytoplasm, digesting the entire cell from within — hence "suicidal bags." They also digest foreign particles and worn-out organelles (autophagy). This CDS question appears in multiple years.

Q3. Cell division that produces gametes (sperm/egg) is called:CDS PYQ

(a) Mitosis

(b) Meiosis

(d) Budding

✓ Answer: (b) Meiosis

Meiosis is called reductional division because it halves the chromosome number from 2n (diploid) to n (haploid). It produces 4 genetically varied gametes. Mitosis produces 2 genetically identical cells with the same chromosome number (equational division) — used for growth and repair. Germ cells (in testes and ovaries) undergo meiosis; all other body cells undergo mitosis.

Q4. Which of the following is NOT found in a plant cell?⚡ Tricky

(a) Cell wall

(b) Chloroplast

(d) Vacuole

✓ Answer: (c) Centriole

Centrioles are absent in plant cells. They are present in animal cells and play a key role in forming the spindle fibres during cell division. Plants manage cell division without centrioles. Cell wall (cellulose), chloroplasts, and large central vacuole are all found in plant cells but not in animal cells — the reverse of this question also appears in CDS. This four-way plant-animal comparison is extremely important.

Quick Memory Chart — BC01

Organelle Nicknames

Mitochondria: Powerhouse
Ribosome: Protein factory
Lysosome: Suicidal bag
Golgi: Post office / packaging
Nucleus: Control centre

Division Key Facts

Mitosis: 2 cells, same 2n; growth & repair
Meiosis: 4 cells, half n; gametes
Human: 46 chromosomes (2n)
Gametes: 23 chromosomes (n)
Stages: P-M-A-T (Pro-Meta-Ana-Telo)

Plant vs Animal Cell

Plant only: cell wall, chloroplast, large vacuole
Animal only: centriole, cholesterol membrane
Prokaryote: no membrane-bound nucleus
Ribosome: both pro & eukaryote
Viruses: non-cellular (exception to cell theory)

Practice Exercise

E1. Which organelle is the site of protein synthesis?

(a) Golgi body

(b) Lysosome

(d) Mitochondria

E2. Ribosomes of prokaryotic cells are:

(a) 80S

(b) 70S

(d) 90S

E3. The process of a cell digesting its own components using lysosomal enzymes is called:

(a) Phagocytosis

(b) Autophagy

(d) Pinocytosis

E4. Mitosis results in:

(a) 4 haploid cells

(b) 2 diploid cells genetically identical to parent

(d) 4 diploid cells

Answers:
E1 → (c) Ribosome [both free and ER-attached ribosomes translate mRNA into proteins] | E2 → (b) 70S [Eukaryotic ribosomes = 80S; the 'S' stands for Svedberg sedimentation units; antibiotics target 70S] | E3 → (b) Autophagy [auto = self; phagocytosis = engulfing external particles] | E4 → (b) 2 diploid cells genetically identical [mitosis preserves both chromosome number and genetic content]

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