Every living organism — whether a tiny bacterium, a mighty elephant, or a huge banyan tree — is made up of cells. The cell is the structural and functional unit of life. Some organisms, like Amoeba and Paramecium, consist of just a single cell and are called unicellular organisms, while others, like humans and trees, are made of millions of cells and are called multicellular organisms. The cell was first discovered by Robert Hooke in 1665, when he observed thin slices of cork under his self-made microscope and saw box-like compartments, which he called "cells".
Organisms differ greatly in their number of cells. A human body has about 10 trillion (10^13) cells, while a single drop of pond water can contain hundreds of tiny single-celled organisms. All the functions of life — nutrition, respiration, growth, movement, reproduction and responding to the environment — take place inside cells. This chapter explores the structure of the cell, its various components called cell organelles, and the functions each organelle performs.
2. Discovery of the Cell
The cell was discovered by Robert Hooke in 1665. While examining a thin slice of cork under a microscope, he observed many small, box-like compartments which reminded him of the small rooms (cells) where monks lived, and so he named them cells. However, what Hooke saw were actually the dead cell walls of plant cells.
Later, the study of cells advanced with better microscopes. In 1674, Anton van Leeuwenhoek was the first to observe living cells, such as bacteria and red blood cells. Today, with the help of powerful microscopes, we know that cells are the basic units of all living organisms and that the structure of a cell is complex and highly organised.
3. Discovery of the Cell
The branch of science that deals with the study of cells is called cell biology or cytology. A cell shows enormous diversity in size, shape and number:
Variation in number: Unicellular organisms have a single cell; multicellular organisms have millions or billions of cells.
Variation in shape: Cells may be spherical, oval, elongated, spindle-shaped or irregular. Amoeba has no fixed shape and constantly changes it, while human red blood cells are disc-shaped and nerve cells are long and branched.
Variation in size: Some cells are microscopic, while others are large enough to be seen with the naked eye. The smallest cell is that of the bacterium Mycoplasma (0.0001 mm), and the largest cell is the egg of an ostrich (about 170 mm in diameter, roughly 17 cm).
Variation in function: Different cells perform different functions — muscle cells contract, nerve cells carry messages, and white blood cells fight infection.
4. The Structure of a Cell
Although cells vary in shape and size, most cells have three basic parts:
Plasma membrane (cell membrane): The outer boundary of the cell. It separates the cell from its surroundings and controls the movement of substances into and out of the cell. Because it allows only some materials to pass through, it is called a selectively permeable membrane.
Cytoplasm: A jelly-like fluid that fills the space between the cell membrane and the nucleus. It contains many cell organelles and is the site of many cellular activities.
Nucleus: The control centre of the cell. It contains chromosomes, which carry genes — the units of inheritance. The nucleus is usually spherical and lies in the centre of the cell. It is surrounded by a double membrane called the nuclear membrane, which has pores. The nucleus contains a fluid called nucleoplasm, and inside the nucleoplasm is a dense spherical body called the nucleolus.
The nucleus controls all activities of the cell and is responsible for cell division and the transmission of hereditary characters from parents to offspring.
5. Cytoplasm and Cell Organelles
The cytoplasm contains several tiny, specialised structures called cell organelles, each with a specific function:
Mitochondria: Known as the powerhouse of the cell, they release energy in the form of ATP during respiration. Cells that need more energy have more mitochondria.
Golgi apparatus: It stores, modifies and packages substances for secretion, and helps in the formation of lysosomes.
Endoplasmic reticulum (ER): A network of tubes that helps in the synthesis of proteins and fats, and transports materials inside the cell. It is of two types: rough ER (with ribosomes, helps in protein synthesis) and smooth ER (helps in fat synthesis).
Ribosomes: Tiny particles on the ER that are the sites of protein synthesis.
Lysosomes: Known as the "suicide bags" of the cell, they contain digestive enzymes that break down foreign materials and worn-out cell parts.
Vacuoles: Fluid-filled sacs that store food, water and waste. In plant cells a large central vacuole is present which provides rigidity.
6. Plant Cell vs Animal Cell
Plant cells and animal cells differ in several ways:
Cell wall: Plant cells have a rigid outer cell wall made of cellulose, which gives them shape and support. Animal cells have no cell wall, only a cell membrane.
Chloroplasts: Plant cells contain chloroplasts which contain the green pigment chlorophyll and perform photosynthesis. Animal cells do not have chloroplasts.
Vacuoles: Plant cells have a large central vacuole; animal cells have many small vacuoles.
Centrosome: Animal cells contain a centrosome which helps in cell division; plant cells do not have a centrosome.
Both plant and animal cells contain a nucleus, cytoplasm, mitochondria, ribosomes, ER, Golgi apparatus and lysosomes.
7. Shape, Size and Specialised Cells
The shape of a cell is related to its function. White blood cells (WBCs) in human blood change their shape to engulf and destroy foreign particles. Red blood cells (RBCs) are disc-shaped and transport oxygen. Nerve cells (neurons) are long with branching ends to transmit nerve impulses. Muscle cells are spindle-shaped to allow contraction. This specialisation, called differentiation, allows cells to perform specific jobs in a multicellular organism.
8. Cell Division
Cell division is the process by which a cell divides to produce new cells. It is important for growth, repair and reproduction. During cell division, the nucleus first divides and then the cytoplasm divides. Cell division enables organisms to grow, replace worn-out cells, heal wounds, and (in unicellular organisms) reproduce. The ability of cells to divide and form new cells is what keeps living things alive and growing.
Quick Revision Tables
Table 1: Cell Organelles and Their Functions
Organelle
Function
Nickname
Nucleus
Controls cell activities, carries hereditary information
Control centre
Mitochondria
Releases energy by respiration
Powerhouse of the cell
Ribosomes
Protein synthesis
Protein factory
Endoplasmic reticulum
Synthesis and transport of proteins and fats
Intracellular highway
Golgi apparatus
Modifies, stores and packages substances
Packaging centre
Lysosomes
Digest foreign materials and worn-out cells
Suicide bags
Chloroplasts
Photosynthesis (plant cells)
Kitchen of the cell
Table 2: Plant Cell vs Animal Cell
Feature
Plant Cell
Animal Cell
Cell wall
Present (cellulose)
Absent
Chloroplasts
Present
Absent
Vacuoles
One large central vacuole
Many small vacuoles
Centrosome
Absent
Present
Shape
Fixed, regular
Irregular, variable
Table 3: Discovery Timeline
Scientist
Year
Discovery
Robert Hooke
1665
Discovered cells in cork
Anton van Leeuwenhoek
1674
First observed living cells
Mind Map
graph TD
A["Cell - Structure and Functions"] --> B["Discovery"]
B --> B1["Robert Hooke 1665: cork cells"]
B --> B2["Leeuwenhoek 1674: living cells"]
A --> C["Cell Parts"]
C --> C1["Plasma membrane: selectively permeable"]
C --> C2["Cytoplasm: jelly-like, holds organelles"]
C --> C3["Nucleus: control centre, genes"]
A --> D["Organelles"]
D --> D1["Mitochondria: powerhouse"]
D --> D2["Ribosomes: protein synthesis"]
D --> D3["Lysosomes: suicide bags"]
D --> D4["Golgi apparatus: packaging"]
A --> E["Plant vs Animal Cell"]
E --> E1["Plant: cell wall, chloroplast, large vacuole"]
E --> E2["Animal: centrosome, small vacuoles"]
Important Diagrams (SVG)
Diagram 1: The Animal Cell
Diagram 2: Plant Cell with Cell Wall and Chloroplasts
Common Mistakes
Saying Robert Hooke discovered living cells: Hooke observed dead cork cell walls in 1665; Leeuwenhoek first observed living cells in 1674.
Thinking the cell wall is present in animals: Only plant cells have a cellulose cell wall; animal cells have only a cell membrane.
Calling the nucleus the powerhouse: Mitochondria release energy and are the powerhouse; the nucleus is the control centre carrying genetic information.
Believing all cells are the same size and shape: Cells vary hugely — Mycoplasma is the smallest, an ostrich egg is the largest, and nerve cells are long and branched.
Saying lysosomes build proteins: Lysosomes digest and destroy waste and foreign material ("suicide bags"); ribosomes build proteins.
Forgetting plant cells also have mitochondria and a nucleus: Both plant and animal cells share nucleus, cytoplasm and most organelles.
Writing that the membrane lets everything pass through: The cell membrane is selectively permeable — it lets only some substances pass.
Exam Tips
Memorise the function nickname pairs: mitochondria-powerhouse, nucleus-control centre, lysosome-suicide bag, ribosome-protein factory, chloroplast-kitchen.
Draw and label plant vs animal cell diagrams — a very common long question; keep plant cell rectangular with cell wall, animal cell rounded without.
Quote Hooke 1665 and Leeuwenhoek 1674 for any discovery question.
For vacuoles, state plant cells have one large central vacuole while animal cells have many small ones.
Link structure to function: WBCs change shape to engulf germs; nerve cells are long to transmit impulses.
Conclusion
The cell is the basic unit of all living things, discovered by Robert Hooke in 1665 and further explored by Leeuwenhoek. Despite the enormous variety in the size, shape and number of cells, all cells share a plasma membrane, cytoplasm and a nucleus that controls their activities. Within the cytoplasm, organelles such as mitochondria, ribosomes, lysosomes, the Golgi apparatus and the endoplasmic reticulum perform specialised jobs, while plant cells add a cell wall, chloroplasts and a large vacuole to their animal-cell counterparts. Understanding how cells are organised helps us appreciate how life works at its most fundamental level — every breath, every heartbeat and every thought depends on the ceaseless activity of trillions of tiny cells.