Chapter 1 — Cell: The Fundamental Unit of Life

Section 1.1 – 1.2

Discovery of the Cell & Cell Theory

Every living thing — from a tiny bacterium to a giant banyan tree — is built from cells, the smallest structural and functional units of life. The branch of science that studies them is cytology (cell biology). The cell could only be discovered after the microscope was invented.

🔬 Robert Hooke (1665)

Looking at a thin slice of cork under his own microscope, Hooke saw a honeycomb of tiny box-like compartments and named them cells (from Latin cella, "a little room"). He published this in his book Micrographia.

🦠 Antonie van Leeuwenhoek

Using a powerful simple microscope, he was the first to observe living cells — tiny moving organisms in water he called "animalcules" (bacteria and protozoa).

✓

Hooke actually saw only the empty cell walls of dead cork cells — the living contents were long gone. So he observed the boxes, not living material.

Year	Scientist	Discovery
1665	Robert Hooke	Observed and named "cells" in cork
1831	Robert Brown	Discovered the nucleus
1839	Purkinje	Coined the term "protoplasm"
1838–39	Schleiden & Schwann	Proposed the cell theory
1855	Rudolf Virchow	"All cells arise from pre-existing cells"
1931	Knoll & Ruska	Designed the electron microscope

❖ The Cell Theory

Proposed by Schleiden and Schwann (1839) and extended by Virchow (1855). Modern form: (1) all living organisms are made of one or more cells; (2) the cell is the basic structural and functional unit of life; (3) all cells arise from pre-existing cells ("Omnis cellula e cellula") — no cell can arise on its own.

💡 Did You Know?

The virus is an exception to the cell theory — it has no cellular structure of its own and can only multiply inside a living host cell.

🔭 Microscopes

Light microscopes use light and glass lenses (simple or compound) for magnification. The electron microscope (Knoll & Ruska, 1931) uses electron beams and magnetic lenses for very high magnification (100,000× to over 1,000,000×), revealing the fine structure of organelles.

Section 1.3

Number, Shape & Size of Cells

Cells vary enormously depending on the job they do.

Unicellular

The whole organism is a single cell — Amoeba, Paramecium, yeast, bacteria, Chlamydomonas.

Multicellular

Made of many cells with a division of labour — plants, fungi, animals, humans. All begin life from a single cell, the zygote.

💡 Did You Know?

A human body has roughly 37 trillion cells. In multicellular bodies there is a division of labour — muscle cells move, nerve cells signal, blood cells transport — each specialised to do its job better.

Cell	Shape	Why
Nerve cell	Long, branched	To carry messages over distances
Red blood cell	Biconcave disc	To squeeze through vessels & exchange gases
Muscle cell	Elongated, contractile	To move bones
Xylem / phloem	Elongated, tubular	To conduct water & food in plants
Guard cells	Kidney-shaped	To open and close stomata

Unit of size

micrometre

1 µm = 10⁻⁶ m. Prokaryotes ~1–10 µm; eukaryotes ~10–100 µm.

Smallest cell

Mycoplasma

About 0.3 µm across — Mycoplasma gallisepticum.

Largest cell

Ostrich egg

About 15 cm long — visible with the naked eye. Longest cells: human nerve cells (~1 m).

Section 1.4

Cell Boundaries: Membrane & Wall

Most cells have three main parts: the outer covering(s), the cytoplasm and the nucleus. The boundary protects the cell, gives it shape and controls what moves in and out.

The plasma membrane is a phospholipid bilayer with embedded proteins — living, flexible and selectively (semi-) permeable

❖ Plasma membrane

The living, thin, elastic outer covering made mainly of phospholipids with some proteins. Being selectively permeable, it lets only certain materials in or out. It enables diffusion, osmosis, transport, endocytosis and exocytosis, and gives the cell shape and flexibility.

Diffusion

Spontaneous movement of a substance from higher to lower concentration until even — e.g. CO₂ and O₂ crossing the membrane. No energy or special membrane needed; fastest in gases.

Osmosis

Movement of water from higher to lower water concentration through a semi-permeable membrane. No energy used. Root hairs absorb water this way.

A cell in hypertonic solution shrinks (exosmosis), in isotonic stays the same, in hypotonic swells/bursts (endosmosis)

Plasmolysis — a plant cell in a hypertonic solution loses water and its protoplasm shrinks away from the cell wall.
Deplasmolysis — placing it back in a hypotonic solution lets water re-enter and the cell regains shape.
Active transport uses energy (ATP) to move ions (e.g. the sodium–potassium balance).
Facilitated transport is passive movement with the help of proteins (e.g. glucose into cells).
Endocytosis takes material in (Amoeba feeding); exocytosis expels waste/secretions.

❖ Cell wall

A rigid, non-living, freely permeable extra covering outside the membrane in plant, fungal and bacterial cells. In plants it is made of cellulose (chitin in fungi; peptidoglycan in bacteria). It gives strength and shape and stops the cell bursting when it takes in excess water.

	Cell wall	Cell membrane
Living?	Non-living	Living
Found in	Plant/fungal/bacterial cells	All cells
Made of	Cellulose	Lipids & proteins
Permeability	Freely permeable	Selectively permeable

Section 1.4 (contd.)

Cytoplasm

The semi-fluid, jelly-like material between the membrane and the nucleus, where most of the cell's activities happen.

Remember

Protoplasm = Cytoplasm + Nucleus

The cytoplasm is the cytosol (fluid, mostly water with dissolved salts, sugars, amino acids and proteins) plus the suspended organelles.

Site of many metabolic reactions (e.g. steps of respiration).
Holds and supports the organelles.
Allows transport of materials within the cell.
Helps movement & feeding in Amoeba (forms pseudopodia).
Temporarily stores food, water, ions and wastes.

Section 1.4 (contd.)

The Nucleus

The large, usually central, control centre of the cell.

The nucleus: a double-membrane envelope with pores, fluid nucleoplasm, the RNA-rich nucleolus and chromatin (DNA + protein)

Nuclear envelope — double membrane with pores for transfer between nucleus and cytoplasm; continuous with the ER.
Nucleolus — rich in RNA & protein; the "factory of ribosomes".
Chromatin — thread-like DNA + protein that condenses into chromosomes during division; functional DNA segments are genes.

💡 Did You Know?

Humans have 46 chromosomes (23 pairs — the diploid condition). Mature mammalian red blood cells lose their nucleus so they can carry more oxygen.

Functions of the nucleus

The chief controlling centre of the cell.
Holds chromosomes/genes that pass on hereditary traits.
Controls metabolic activities and regulates enzyme activity.

Section 1.4 (contd.)

Prokaryotic vs Eukaryotic, Plant vs Animal

Cells are grouped by whether they have a true (membrane-bound) nucleus.

Feature	Prokaryotic cell	Eukaryotic cell
Size	1–10 µm	5–100 µm
True nucleus	Absent (DNA in a nucleoid)	Present (membrane-bound)
Nuclear membrane	Absent	Present
Membrane-bound organelles	Absent	Present
Examples	Bacteria, cyanobacteria	Protozoa, fungi, plants, animals

Plant cells are box-like with a cell wall, plastids and a large central vacuole; animal cells are rounder with no wall or plastids and small vacuoles

Feature	Plant cell	Animal cell
Cell wall	Present (cellulose)	Absent
Plastids	Present	Absent
Vacuole	One large central vacuole	Small or absent
Centrosome/centriole	Absent	Present
Shape	Usually fixed, rectangular	Usually rounded, flexible

Section 1.5

Cell Organelles

Membrane-bound structures, each with a definite shape and job — a key feature of eukaryotic cells.

Mitochondrion

⚡ Mitochondria — powerhouse

Rod-shaped, double membrane; the inner one folds into cristae (large surface for reactions), the space inside is the matrix. Site of cellular respiration, releasing energy from glucose as ATP. Has its own DNA & ribosomes (semi-autonomous).

Chloroplast

🍃 Plastids (plant cells only)

Double-membrane, semi-autonomous (own DNA & ribosomes). Three types: chloroplasts (green, photosynthesis — the "kitchen of the cell", with grana/thylakoids in a stroma), leucoplasts (colourless, store starch/oil/protein) and chromoplasts (colour of flowers & fruits).

🌐 Endoplasmic Reticulum (ER)

A network of tubes and sheets continuous with the nuclear membrane. Rough ER (with ribosomes) makes proteins; Smooth ER (no ribosomes) makes lipids and detoxifies poisons/drugs in liver cells. It transports materials and helps build new membranes (membrane biogenesis).

📦 Golgi apparatus

Stacks of flattened sacs (cisternae); discovered by Camillo Golgi. It modifies, packages and dispatches the products made in the ER, forms lysosomes, and secretes enzymes/hormones. Called dictyosomes in plants.

🪥 Lysosomes

Single-membrane sacs of hydrolytic (digestive) enzymes made by the Golgi. They digest food, worn-out organelles and invading germs. Called "suicidal bags" — if a cell is damaged they burst and self-digest it (autolysis).

⚙️ Ribosomes

Tiny granules of RNA & protein, without a membrane, on the rough ER or free in the cytoplasm. They are the protein factories of the cell.

💧 Vacuoles

Fluid-filled (cell sap) cavities bounded by a tonoplast. Large in plants (50–90% of the cell), giving turgidity and storage; small/absent in animals. A contractile vacuole expels excess water in Amoeba.

✶ Centrosome & centrioles

Found near the nucleus in animal cells only. Help form the spindle fibres during cell division, and form cilia and flagella.

⚫ Peroxisomes

Small single-membrane sacs of oxidative enzymes in plant and animal cells; they detoxify harmful substances (and help photo-respiration in plants).

Section 1.7

Biomolecules

Carbon-based chemicals that build the cell and run its life processes. Inorganic: water, minerals, acids, bases. Organic: carbohydrates, proteins, lipids, nucleic acids, vitamins.

Biomolecule	Main role
Carbohydrates	Quick energy & energy store; structural in some plant cells
Proteins	Enzymes, transport, growth, repair; part of membranes
Lipids (fats)	Make the membrane bilayer; long-term energy; some hormones
Nucleic acids	Genetic material (DNA/RNA); control heredity & protein-making
Vitamins	Co-factors for enzymes; regulate metabolism
Water	Major part of cytoplasm; solvent; supports reactions
Minerals	Water balance, nerve & enzyme function

Section 1.8

Mitosis, Meiosis & Cancer

Cell division makes new cells for growth, repair and reproduction. The DNA is copied first so each new cell gets a full set of genes.

The stages of mitosis — one division producing two identical daughter cells

Basis	Mitosis	Meiosis
Where	Body (somatic) cells	Reproductive organs
Divisions	One	Two
Daughter cells	Two	Four
Chromosomes	Same (2n → 2n)	Halved (2n → n)
Genetic nature	Identical to parent	Genetically different
Role	Growth & repair (similarity)	Gametes (variation)

Mitosis → similarity

One parent cell → two identical daughter cells with the same chromosome number. Drives growth, repair and replacement of cells.

Meiosis → variation

Forms gametes with half the chromosomes (haploid). Crossing over and independent assortment create variation, so offspring differ from parents.

⚠️

Cancer: when DNA mutations break the control of the cell cycle, cells divide uncontrollably and form a tumour. A benign tumour stays put; a malignant one invades and spreads (metastasis).

Glossary

Key Terms

Quick definitions for fast revision.

Cell: The basic structural and functional unit of all living organisms.
Cytology: The branch of biology that studies cells.
Cell theory: All organisms are made of cells; the cell is the basic unit of life; all cells come from pre-existing cells.
Plasma membrane: The living, selectively permeable outer boundary of the cell (phospholipids + proteins).
Cell wall: The rigid, non-living, freely permeable cellulose covering of plant cells.
Diffusion / Osmosis: Movement of a substance high→low concentration / movement of water through a semi-permeable membrane.
Plasmolysis: Shrinking of a plant cell's protoplasm away from the wall when it loses water.
Cytoplasm / Protoplasm: Jelly-like material outside the nucleus / cytoplasm together with the nucleus.
Nucleus: The membrane-bound control centre containing the genetic material.
Mitochondria: Double-membrane "powerhouse" that releases energy as ATP in respiration.
Plastids / Chloroplast: Plant organelles; chloroplast carries out photosynthesis.
Endoplasmic reticulum: Membrane network; rough ER makes protein, smooth ER makes lipid.
Golgi apparatus: Modifies, packages and dispatches cell products; forms lysosomes.
Lysosome: Digestive "suicidal bag" full of hydrolytic enzymes.
Ribosome: Membrane-less granule that is the site of protein synthesis.
Prokaryotic / Eukaryotic: Cell without a true nucleus / cell with a true nucleus.
Mitosis / Meiosis: Division giving two identical cells / division giving four genetically varied gametes.

Exam Strategy

Top Exam Tips

High-yield reminders that catch most students out.

Hooke saw dead cork

Hooke observed empty cell walls of dead cork; Leeuwenhoek first saw living cells.

Virus = exception

Viruses break the cell theory — no cellular structure, multiply only inside a host.

Diffusion vs osmosis

Osmosis is only water through a semi-permeable membrane; diffusion is any substance, any medium.

Hyper/hypo/iso

Hyper→cell shrinks, hypo→cell swells, iso→no change. "Hypo" = lots of water outside.

Semi-autonomous pair

Only mitochondria and plastids have their own DNA & ribosomes.

Wall = non-living

Cell wall is non-living & freely permeable; membrane is living & selectively permeable.

Plant vs animal

Wall, plastids & big vacuole = plant; centrosome/centriole = animal.

Mitosis vs meiosis

Mitosis → 2 identical (growth/repair); meiosis → 4 varied gametes (2n→n).

Self-Evaluation

Practice Question Bank — 150 Questions

CBSE-pattern questions across all five sections with toggle answers. Section A: 50 MCQ & Assertion-Reason (1 mark) · Section B: 30 Very Short Answer (2 marks) · Section C: 30 Short Answer (3 marks) · Section D: 20 Long Answer (5 marks) · Section E: 20 Case-Based (4 marks).