Topic 2: The Cell & Tissues
The cell is the basic structural & functional unit of life — every organism in Topic 1's classification is built from it. This single-source file covers cell theory and its discovery, the great prokaryotic vs eukaryotic divide, the plant vs animal cell, and each organelle (membrane, wall, nucleus, mitochondria, chloroplast, ER, Golgi, ribosomes, lysosomes, vacuoles, centrosome, cilia & flagella, capsule, fimbriae), the genetic material & nucleic acids, and how cells group into animal & plant tissues — with labelled diagrams, tables and real dated PYQs with full model answers.
On this page
- 1.The Cell & Cell Theory
- 2.Prokaryotic vs Eukaryotic Cells
- 3.Plant Cell vs Animal Cell
- 4.Cell Boundary — Membrane, Wall & Surface
- 5.Organelles I — Nucleus, ER, Ribosomes, Golgi
- 6.Organelles II — Mitochondria, Plastids, Lysosomes, Vacuoles, Centrosome
- 7.Genetic Material & Nucleic Acids
- 8.Animal Tissues
- 9.Plant Tissues
- 10.Current Affairs Link
- 11.Prelims PYQs
- 12.Mains PYQs + Model Answers
- ★15-Minute Revision Box
Conceptual Clarity — How UPSC Tests the Cell
The cell is a dense, high-return Prelims topic: a single diagram hides dozens of factual hooks. Sort your prep into three question-types:
- Definitional / static — "which organelle does X?" (powerhouse = mitochondria; suicide bag = lysosome; protein factory = ribosome). Pure recall; the commonest style.
- Statement-elimination — two/three statements on, e.g., "prokaryotes lack a nuclear membrane and membrane-bound organelles" where one word decides the answer. Needs the prokaryote/eukaryote and plant/animal contrasts exact.
- Applied / current — a news hook (mitochondrial DNA, stem cells, CAR-T, lab-grown tissue) traced back to a cell fundamental. Also the GS-III bridge to biotech and health.
Highest-frequency themes: organelle functions & nicknames · prokaryote vs eukaryote · plant vs animal cell differences · mitochondria & chloroplast (double membrane, own DNA) · the four animal tissues · meristematic vs permanent plant tissue · xylem & phloem.
1. The Cell & Cell Theory
The cell (Latin cella = small room) is the smallest unit that can carry out all life processes — the basic structural and functional unit of all living organisms. Organisms may be unicellular (one cell does everything, e.g. amoeba, bacteria) or multicellular (division of labour among many cells, e.g. humans).
1.1 Discovery — who found what
| Scientist | Contribution |
|---|---|
| Robert Hooke (1665) | First observed & named "cells" — in a thin slice of cork under his microscope (actually dead cell walls). |
| Anton van Leeuwenhoek | First saw living cells (bacteria, protozoa — "animalcules") — "father of microbiology". |
| Robert Brown (1831) | Discovered the nucleus. |
| Schleiden (1838) & Schwann (1839) | Proposed the cell theory (Schleiden for plants, Schwann for animals). |
| Rudolf Virchow (1855) | Added: "Omnis cellula e cellula" — every cell arises from a pre-existing cell. |
1.2 The cell theory — three tenets
- All living organisms are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells arise from pre-existing cells (Virchow).
2. Prokaryotic vs Eukaryotic Cells
The single most important division in all biology is between prokaryotic ("before nucleus") and eukaryotic ("true nucleus") cells — the basis of the whole five-kingdom system in Topic 1.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus — DNA lies free as a nucleoid; no nuclear membrane. | True, membrane-bound nucleus. |
| Membrane-bound organelles | Absent (no mitochondria, ER, Golgi, plastids). | Present. |
| Ribosomes | Present but smaller (70S). | Larger (80S); 70S inside mitochondria/chloroplasts. |
| Cell wall | Present (peptidoglycan/murein in bacteria). | Present in plants (cellulose)/fungi (chitin); absent in animals. |
| Size & DNA | Small (~1–10 µm); single circular DNA, often plasmids. | Larger (~10–100 µm); DNA on multiple linear chromosomes with histones. |
| Examples | Bacteria, cyanobacteria, archaea, mycoplasma. | Protists, fungi, plants, animals. |
3. Plant Cell vs Animal Cell
Both are eukaryotic, but differ in a few decisive structures that come up almost every year.
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) — rigid. | Absent — only a membrane. |
| Plastids (incl. chloroplast) | Present — enable photosynthesis. | Absent. |
| Vacuole | One large central vacuole. | Small or absent. |
| Centrosome / centrioles | Usually absent. | Present (help cell division). |
| Shape | Fixed, usually rectangular. | Irregular, rounded. |
| Storage food | Starch. | Glycogen. |
4. Cell Boundary — Membrane, Wall & Surface Structures
4.1 Cell (plasma) membrane
- The living outer boundary of every cell; a fluid mosaic of a phospholipid bilayer with embedded proteins (Singer & Nicolson model).
- Selectively permeable — controls what enters/leaves (by diffusion, osmosis, active transport). This regulation is its defining role.
4.2 Cell wall
- A non-living, rigid layer outside the membrane; gives shape and protection. Composition varies: cellulose (plants), chitin (fungi), peptidoglycan/murein (bacteria). Absent in animals.
4.3 Bacterial surface extras (from the syllabus)
| Structure | Role |
|---|---|
| Capsule | Extra slimy protective layer outside the wall in some bacteria; aids in resisting drying & the host immune system (virulence). |
| Fimbriae (pili) | Tiny hair-like projections for attachment to surfaces/host cells; sex pili aid gene transfer (conjugation). |
| Flagella | Long whip-like structure for movement. |
| Cilia | Short, numerous hair-like projections for movement or moving fluids (in some eukaryotes, e.g. Paramecium, human respiratory tract). |
5. Organelles I — Nucleus, ER, Ribosomes, Golgi
5.1 Nucleus — the control centre
- Bounded by a double nuclear membrane with pores; contains chromatin (DNA + protein), which condenses into chromosomes during division, and a dense nucleolus (makes ribosomes).
- Directs all cell activity and carries hereditary information — the "brain" of the cell.
5.2 Endoplasmic reticulum (ER)
- A network of membrane tubes/sheets. Rough ER (with ribosomes) — makes/transports proteins; Smooth ER (no ribosomes) — makes lipids & detoxifies.
5.3 Ribosomes — the protein factory
- Tiny granules of RNA + protein; the site of protein synthesis. Found free in cytoplasm or on rough ER. Present in both prokaryotes (70S) and eukaryotes (80S).
5.4 Golgi apparatus — the packaging & despatch unit
- Stacked flattened sacs that modify, package and secrete proteins/lipids; also forms lysosomes and cell-wall material in plants. Discovered by Camillo Golgi.
6. Organelles II — Mitochondria, Plastids, Lysosomes, Vacuoles, Centrosome
6.1 Mitochondria — the powerhouse
- Double-membraned; inner membrane folded into cristae. Site of aerobic respiration — makes ATP (energy). Hence "powerhouse of the cell".
- Has its own DNA & 70S ribosomes (semi-autonomous) — evidence for the endosymbiotic theory (mitochondria & chloroplasts were once free-living bacteria). Mitochondrial DNA is maternally inherited.
6.2 Plastids (plant cells only)
| Plastid | Role |
|---|---|
| Chloroplast | Green (chlorophyll); site of photosynthesis. Double-membraned; own DNA & ribosomes. |
| Chromoplast | Coloured (red/yellow/orange) — flowers, fruits, carrots. |
| Leucoplast | Colourless — stores starch/oil/protein. |
6.3 Lysosomes, vacuoles & centrosome
- Lysosome — "suicide bag": contains digestive enzymes; breaks down waste & worn-out parts; self-destructs the cell when needed.
- Vacuole — fluid-filled sac; large & central in plant cells (stores cell sap, gives turgidity); small in animals.
- Centrosome/centrioles — in animal cells; organise the spindle fibres during cell division.
- Peroxisome — breaks down fatty acids & detoxifies (e.g. hydrogen peroxide).
7. Genetic Material & Nucleic Acids
The nucleus stores the cell's instructions as nucleic acids. (Heredity & genetics are covered in depth in Topic 4; here we fix the structural basics that Prelims tests.)
| Feature | DNA | RNA |
|---|---|---|
| Full form | Deoxyribonucleic acid | Ribonucleic acid |
| Strands | Double helix (two strands) | Usually single strand |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U (uracil), G, C |
| Role | Stores hereditary information | Carries out protein synthesis (mRNA, tRNA, rRNA) |
- A gene is a segment of DNA that codes for a protein/trait. DNA is packaged with proteins into chromosomes; humans have 46 (23 pairs).
- The double helix was described by Watson & Crick (1953) (using Rosalind Franklin's X-ray data). Base pairing: A–T, G–C.
- The flow of information: DNA → RNA → Protein (the "central dogma").
8. Animal Tissues
A tissue is a group of similar cells performing a common function. In animals there are four basic types.
| Tissue | Function | Examples |
|---|---|---|
| Epithelial | Covering & lining of body surfaces, cavities, organs; protection, absorption, secretion. | Skin, lining of mouth, gut & blood vessels; glands. |
| Connective | Binds, supports & connects; has a matrix between cells. | Blood (fluid matrix), bone, cartilage, tendon, ligament, adipose (fat). |
| Muscular | Contraction → movement. | Skeletal (voluntary, striated), cardiac (heart, involuntary, striated), smooth (involuntary, unstriated). |
| Nervous | Receives & transmits impulses; control & coordination. | Neurons + supporting glial cells. |
9. Plant Tissues
Plant tissues are broadly meristematic (dividing) or permanent (differentiated).
| Type | Sub-type / feature | Role |
|---|---|---|
| Meristematic (actively dividing) | Apical (tips of root/shoot) | Increase in length (primary growth). |
| Lateral (cambium) / Intercalary | Increase in girth; intercalary (at nodes) regrows grass. | |
| Permanent — Simple (one cell type) | Parenchyma | Packing, storage, photosynthesis (chlorenchyma), buoyancy (aerenchyma). |
| Collenchyma | Flexible mechanical support (young stems, leaf stalks). | |
| Sclerenchyma | Hard, dead cells — rigidity (husk, nut shell, fibres). | |
| Permanent — Complex (vascular) | Xylem | Conducts water & minerals upward (roots → leaves); also support. Mostly dead cells. |
| Phloem | Conducts food (sugars) both ways (translocation). Living cells. |
10. Current Affairs Link (2024–2026)
Cell biology is static, but modern medicine and biotech keep giving it news hooks. Verify the latest before the exam. check for latest update or data
| Recent theme | Fundamental it tests | Why it matters for UPSC |
|---|---|---|
| Stem-cell therapy | Cell differentiation; tissues | GS-III health & ethics; regulation. |
| CAR-T cell therapy (India's first, indigenous) | Cells & immunity | Links to Topics 5 & 10; a strong example. |
| Cultured / lab-grown meat | Animal tissue & cell culture | GS-III food-tech & sustainability. |
| Mitochondrial-replacement | Mitochondria & mtDNA | Prelims fact + GS-IV ethics angle. |
- Recurring exam hooks: powerhouse of the cell · suicide bag · prokaryote vs eukaryote · plant vs animal cell · is blood a tissue? · xylem vs phloem · organelle with its own DNA.
11. Prelims PYQs
Objective questions anchored to genuinely tested UPSC themes on the cell & tissues. Each carries a worked rationale.
Q: The "powerhouse of the cell", the site of aerobic respiration and ATP production, is the —
Answer: (b) Mitochondrion — double-membraned, has its own DNA, and produces ATP by aerobic respiration.
Q: Which of the following is characteristic of a prokaryotic cell?
Answer: (c) In prokaryotes DNA is free (nucleoid) with no nuclear membrane; they lack mitochondria and have 70S ribosomes.
Q: Which of the following is a connective tissue?
Answer: (b) Blood is a connective tissue with a fluid matrix (plasma). Bone and cartilage are also connective tissues.
Q: Which structures are present in a plant cell but absent in a typical animal cell?
Answer: (b) Cell wall (cellulose) and chloroplast/plastids are plant-specific; the others are common to both.
Q: The dictum "Omnis cellula e cellula" (every cell arises from a pre-existing cell) was given by —
Answer: (c) Virchow (1855). Hooke named the cell; Schleiden & Schwann framed the cell theory; Virchow completed it.
Q: In plants, the upward conduction of water and minerals from roots to leaves is carried out by —
Answer: (a) Xylem (mostly dead cells) conducts water upward; phloem translocates food (sugars) in both directions.
Q: Which two organelles possess their own DNA and ribosomes?
Answer: (c) Mitochondria and chloroplasts are semi-autonomous (own DNA + 70S ribosomes) — the basis of the endosymbiotic theory.
Q: Which nitrogenous base is present in RNA but not in DNA?
Answer: (c) RNA has uracil (U) in place of thymine (T); RNA sugar is ribose, DNA sugar is deoxyribose.
Likely: organelle nicknames (powerhouse / suicide bag / protein factory) · prokaryote vs eukaryote statements · plant vs animal cell · mitochondria & chloroplast own DNA · blood as connective tissue · xylem vs phloem · DNA vs RNA (uracil, ribose, strands) · capsule/fimbriae/flagella/cilia roles · cell-theory attributions. check for latest update or data
12. Mains PYQs + Model Answers
Cell fundamentals anchor applied GS-III questions on health, biotech and food-tech. The frameworks below show how to deploy them analytically.
Q: Stem cells hold great promise for regenerative medicine but raise scientific and ethical concerns. Discuss.
Model Answer
- Introduction — define: Stem cells are unspecialised cells able to divide and differentiate into specialised cell types — the biological basis of tissue repair.
- Types & promise:
- Embryonic (pluripotent) · adult/tissue (multipotent) · induced pluripotent (iPSCs) — reprogrammed adult cells.
- Applications: repair of damaged tissue (spinal cord, cardiac), bone-marrow transplants, drug testing, and organ regeneration.
- Concerns: ethical use of embryos; risk of tumour formation (teratoma); immune rejection; unregulated/unproven "stem-cell clinics" exploiting patients.
- India's framework: ICMR-DBT National Guidelines for Stem Cell Research; only bone-marrow transplant is an approved therapy — the rest is research.
- Way forward: favour ethically-sound iPSCs, strict clinical-trial oversight, public awareness against quackery, and indigenous R&D.
- Conclusion: harnessing stem cells needs the cell fundamentals plus firm ethical & regulatory guardrails so promise is not lost to malpractice.
Q: "The endosymbiotic origin of mitochondria and chloroplasts is one of the most compelling ideas in cell biology." Explain, and note its relevance.
Model Answer
- Introduction: The endosymbiotic theory holds that mitochondria & chloroplasts were once free-living prokaryotes engulfed by an ancestral eukaryotic cell, becoming permanent organelles.
- Evidence:
- Both have a double membrane, their own circular DNA, and 70S (bacterial-type) ribosomes.
- They divide independently (binary fission) and their DNA resembles bacterial DNA.
- Significance: explains eukaryotic complexity & energy capacity; underpins maternal inheritance of mtDNA (ancestry, forensics), mitochondrial-disease research, and mitochondrial-replacement therapy.
- Conclusion: a fundamental that links evolution (Topic 1) to modern medicine and biotechnology — a good cross-cutting example.
Q: Cultured (lab-grown) meat is proposed as a sustainable protein source. Explain its cell-biology basis and its significance for India.
Model Answer
- Introduction: Cultured meat is real animal muscle tissue grown from a small sample of animal cells in a bioreactor, without rearing/slaughtering animals.
- Cell-biology basis: animal (muscle) stem cells are multiplied in a nutrient medium and differentiated into muscular tissue on a scaffold — a direct application of cell culture & tissue science.
- Significance for India: lower land, water & GHG footprint; food security & protein for a large population; animal-welfare gains; new agri-biotech industry.
- Challenges: high cost, scale-up, regulation (FSSAI), consumer acceptance.
- Conclusion: a promising but nascent technology resting squarely on cell & tissue fundamentals.
Q: Explain how the structure of the cell membrane enables it to regulate the internal environment of the cell.
Model Answer
- Introduction: The plasma membrane is the living boundary that separates the cell's interior from its surroundings and controls exchange — central to homeostasis.
- Structure (fluid mosaic model):
- A phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) with embedded & mobile proteins, cholesterol and glycoproteins.
- Fluidity lets components move; proteins act as channels, carriers, pumps & receptors.
- How it regulates: selective permeability — small/non-polar molecules diffuse freely; water by osmosis; ions & large molecules via specific channels/carriers; active transport moves substances against gradients using ATP; receptors handle signalling.
- Significance: maintains ion balance, nutrient uptake, waste removal & internal stability; its selectivity is also a target for drugs & toxins.
- Conclusion: structure fits function — the fluid-mosaic design makes precise, selective regulation possible.
Likely: stem cells & regenerative medicine ethics · endosymbiotic theory & mitochondrial medicine · cultured meat & tissue engineering · cell membrane & drug targeting · cell as the base of biotechnology. check for latest update or data
15-Minute Revision Box
Must-Remember Facts — Cell & Tissues
- Cell = basic structural & functional unit of life
- Hooke named cell (cork); Leeuwenhoek saw living cells; Brown found nucleus
- Cell theory = Schleiden & Schwann; Virchow: cells from pre-existing cells
- Largest cell = ostrich egg; longest = neuron; smallest = mycoplasma
- Prokaryote: no nuclear membrane, no membrane-bound organelles, 70S ribosomes, DNA = nucleoid
- Eukaryote: true nucleus + organelles, 80S ribosomes
- Plant only: cell wall (cellulose), plastids, large central vacuole; store starch
- Animal only: centrosome; store glycogen
- Mitochondria = powerhouse (double membrane, own DNA, ATP)
- Chloroplast = photosynthesis (own DNA); Ribosome = protein factory
- Golgi = packaging/post office; Lysosome = suicide bag
- Rough ER = protein; Smooth ER = lipids & detox; Nucleus = control centre
- Surface: capsule = protection, fimbriae = attachment, flagella = movement, cilia = short & many
- DNA: double, deoxyribose, A-T-G-C; RNA: single, ribose, uracil; Watson & Crick 1953
- Animal tissues: epithelial, connective (blood!), muscular, nervous
- Muscle: skeletal (voluntary striated), cardiac (involuntary striated), smooth (involuntary unstriated)
- Plant: meristematic (growth); xylem = water up (dead), phloem = food both ways (living)