Just as a building is made of bricks, every living organism is made of cells. A cell is the structural and functional unit of life – the smallest unit that can carry out all the activities of a living being. Some organisms like bacteria and Amoeba are just a single cell, while the human body has trillions of them. Cell biology is one of the most important areas of Biology in all competitive exams.
Discovery of the Cell
| Year | Scientist | Contribution |
|---|---|---|
| 1665 | Robert Hooke | Observed thin slices of cork under a self-made microscope and saw small compartments, which he named "cells". Described in his book Micrographia. He actually saw dead cell walls. |
| 1674 | Antonie van Leeuwenhoek | First to observe living cells (bacteria, protozoa, sperm) using an improved microscope. |
| 1831 | Robert Brown | Discovered the nucleus in plant cells. |
| 1839 | J.E. Purkinje | Coined the term "protoplasm" for the living fluid substance of the cell. |
| 1838–1839 | M.J. Schleiden (botanist) and Theodor Schwann (zoologist) | Proposed the cell theory. |
| 1855 | Rudolf Virchow | Stated that new cells arise only from pre-existing cells – "Omnis cellula e cellula". |
The invention of the electron microscope (by Ernst Ruska and Max Knoll, 1931) made it possible to see the fine structure of cell organelles.
Cell Theory
- All living organisms are made up of one or more cells.
- The cell is the basic unit of structure and function of life.
- All new cells come from pre-existing cells (added by Virchow).
Exception: Viruses do not have a cellular structure. They show life-like activity only inside a host cell, so they are considered a link between living and non-living things.
Size and Number of Cells – Exam Facts
| Smallest known cell | Mycoplasma (PPLO – Pleuro-Pneumonia-Like Organism), about 0.1–0.3 µm |
|---|---|
| Largest single cell | Egg of the ostrich |
| Longest cell in the human body | Nerve cell (neuron) |
| Largest cell in the human body | Ovum (female egg) |
| Smallest cell in the human body | Sperm (male gamete) |
| Human cells without a nucleus | Mature red blood cells (RBCs) of mammals |
| Unit used to measure cells | Micrometre (µm) = 10-6 m |
- Unicellular organisms: made of one cell – Amoeba, Paramecium, Chlamydomonas, bacteria, yeast.
- Multicellular organisms: made of many cells – plants, animals, humans, most fungi.
Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No true nucleus; DNA lies in a region called the nucleoid | True nucleus enclosed by a nuclear membrane |
| Membrane-bound organelles | Absent (no mitochondria, ER, Golgi) | Present |
| Ribosomes | 70S type | 80S type in the cytoplasm |
| Size | Generally small (1–10 µm) | Generally larger (10–100 µm) |
| Examples | Bacteria, cyanobacteria (blue-green algae), Mycoplasma | Plants, animals, fungi, protozoa |
Plant Cell vs Animal Cell
| Plant cell | Animal cell |
|---|---|
| Has a rigid cell wall made of cellulose | No cell wall |
| Has plastids (chloroplasts) | No plastids |
| One large central vacuole | Small and temporary vacuoles, or none |
| Centrioles absent in higher plants | Centrioles present |
| Lysosomes are rare | Lysosomes present |
| Usually fixed, rectangular shape | Usually irregular or round shape |
| Stores food as starch | Stores food as glycogen |
Cell Organelles and Their Functions
1. Cell Membrane (Plasma Membrane)
The outermost covering of an animal cell, made of lipids and proteins. It is selectively permeable – it allows only some substances to enter or leave. Its structure is explained by the Fluid Mosaic Model given by Singer and Nicolson (1972).
2. Cell Wall
A rigid, non-living outer layer found in plant cells, fungi and bacteria. In plants it is made of cellulose; in fungi of chitin; and in bacteria of peptidoglycan. It gives shape, strength and protection, and prevents plant cells from bursting in dilute solutions.
3. Nucleus – the Control Centre
- Discovered by Robert Brown (1831); surrounded by a double nuclear membrane with pores.
- Contains chromatin, which condenses into chromosomes during cell division. Chromosomes carry genes made of DNA.
- The nucleolus inside it makes ribosomal RNA and helps form ribosomes.
- It controls all cell activities and carries hereditary information.
4. Cytoplasm
The jelly-like fluid between the cell membrane and the nucleus, in which all organelles are suspended. Many chemical reactions, such as glycolysis, take place here.
5. Mitochondria – the Powerhouse of the Cell
- Site of aerobic respiration; produce energy in the form of ATP (adenosine triphosphate), called the "energy currency" of the cell.
- Double-membraned; the inner membrane is folded into cristae.
- Have their own DNA and ribosomes, so they can make some of their own proteins – they are called semi-autonomous organelles.
- Absent in prokaryotes and in mature mammalian RBCs.
6. Plastids
| Type | Colour / pigment | Function |
|---|---|---|
| Chloroplast | Green – chlorophyll | Photosynthesis – the "kitchen of the cell" |
| Chromoplast | Yellow, orange, red – carotenoids | Gives colour to flowers and fruits (e.g., tomato, carrot) |
| Leucoplast | Colourless | Stores starch, oils and proteins (e.g., potato) |
Like mitochondria, chloroplasts have their own DNA and ribosomes.
7. Endoplasmic Reticulum (ER)
- Rough ER – has ribosomes on its surface; helps in protein synthesis and transport.
- Smooth ER – no ribosomes; makes lipids and helps detoxify poisons and drugs, especially in liver cells.
- Works as a transport network inside the cell.
8. Golgi Apparatus
Discovered by Camillo Golgi (1898). It modifies, packages and dispatches proteins and lipids made by the ER – often called the "post office" or "packaging centre" of the cell. It also helps form lysosomes.
9. Lysosomes – the Suicide Bags
Discovered by Christian de Duve (1955). They are membrane sacs full of digestive enzymes. They digest food particles, worn-out organelles and invading germs. When the cell is damaged, lysosomes may burst and digest the cell itself – hence the name "suicide bags".
10. Ribosomes
Tiny non-membranous granules made of RNA and protein, discovered by George Palade. They are the site of protein synthesis – the "protein factories" of the cell. Prokaryotes have 70S ribosomes and eukaryotes have 80S ribosomes.
11. Vacuoles
Fluid-filled storage sacs. In plant cells, the large central vacuole stores water, salts and waste and keeps the cell firm (turgid). Amoeba has a food vacuole, and freshwater protozoa have a contractile vacuole to remove excess water.
12. Centrosome and Centrioles
Found in animal cells near the nucleus. The centrioles help form the spindle fibres during cell division. They also form the base of cilia and flagella.
13. Peroxisomes
Small sacs containing enzymes that break down harmful hydrogen peroxide and fatty acids.
Transport Across the Cell Membrane
- Diffusion: movement of molecules from a region of higher concentration to lower concentration (e.g., exchange of O2 and CO2).
- Osmosis: movement of water through a selectively permeable membrane from a dilute solution to a concentrated solution.
- Endosmosis: water enters the cell (raisins swell in water). Exosmosis: water leaves the cell.
- Plasmolysis: shrinking of the cytoplasm of a plant cell away from the cell wall when kept in a concentrated (hypertonic) solution. That is why salting pickles preserves them and too much fertiliser can "burn" plants.
- Endocytosis: the cell membrane engulfs food particles – as in Amoeba.
Cell Division
| Feature | Mitosis | Meiosis |
|---|---|---|
| Occurs in | Body (somatic) cells | Reproductive (germ) cells |
| Purpose | Growth, repair and replacement of cells | Formation of gametes (sperm and egg) |
| Number of divisions | One | Two |
| Daughter cells | Two, identical to the parent | Four, genetically different |
| Chromosome number | Remains the same (diploid, 2n) | Becomes half (haploid, n) – "reduction division" |
| Crossing over | Does not occur | Occurs – creates variation |
- Stages of mitosis: Prophase → Metaphase → Anaphase → Telophase, followed by division of the cytoplasm (cytokinesis).
- A human body cell has 46 chromosomes (23 pairs); a human sperm or egg has 23 chromosomes.
- Uncontrolled cell division leads to cancer.
Stem Cells
Stem cells are unspecialised cells that can divide and develop into different types of specialised cells. They are found in embryos, bone marrow and umbilical cord blood, and are used in treating blood disorders and in research on repairing damaged tissues.
Quick Revision Points
- Cell discovered by Robert Hooke (1665) in cork; living cells first seen by Leeuwenhoek.
- Cell theory: Schleiden and Schwann; "Omnis cellula e cellula": Virchow.
- Powerhouse: mitochondria; kitchen: chloroplast; suicide bags: lysosomes; protein factory: ribosome; control centre: nucleus.
- Mitochondria and chloroplasts have their own DNA.
- Plant cell wall is made of cellulose; fungal cell wall of chitin.
- Longest human cell: neuron; largest: ovum; mature RBC has no nucleus.
- Mitosis keeps chromosome number the same; meiosis halves it.
Read also: Tissues – Plant and Animal Tissues
Practice MCQs
1. Who discovered the cell?
(a) Robert Brown
(b) Robert Hooke
(c) Leeuwenhoek
(d) Schleiden
2. Which organelle is called the "powerhouse of the cell"?
(a) Ribosome
(b) Golgi apparatus
(c) Mitochondria
(d) Lysosome
3. Which organelle is known as the "suicide bag" of the cell?
(a) Lysosome
(b) Vacuole
(c) Plastid
(d) Centrosome
4. The plant cell wall is mainly made of:
(a) Chitin
(b) Starch
(c) Cellulose
(d) Protein
5. Which of the following has its own DNA?
(a) Golgi apparatus
(b) Mitochondria
(c) Lysosome
(d) Endoplasmic reticulum
6. The longest cell in the human body is:
(a) Muscle cell
(b) Ovum
(c) Nerve cell
(d) Bone cell
7. "Omnis cellula e cellula" was stated by:
(a) Rudolf Virchow
(b) Theodor Schwann
(c) Robert Hooke
(d) Purkinje
8. Protein synthesis takes place in:
(a) Lysosomes
(b) Ribosomes
(c) Vacuoles
(d) Centrioles
9. Which division reduces the chromosome number to half?
(a) Mitosis
(b) Meiosis
(c) Binary fission
(d) Budding
10. Shrinking of the cytoplasm of a plant cell in a concentrated salt solution is called:
(a) Osmosis
(b) Endosmosis
(c) Plasmolysis
(d) Diffusion
Answers: 1 – (b) | 2 – (c) | 3 – (a) | 4 – (c) | 5 – (b) | 6 – (c) | 7 – (a) | 8 – (b) | 9 – (b) | 10 – (c)
Reference: NCERT Science Class IX (The Fundamental Unit of Life) and NCERT Biology Class XI (Cell: The Unit of Life; Cell Cycle and Cell Division).