ScienceThe cell: unit of life
What a cell is, its main parts, and how plant cells differ from animal cells.
A building is made of bricks. In the same way, the body of every living thing is made of tiny units called cells. The cell is the basic unit of structure and function in all living things: it is the smallest part that is itself alive.
Discovery
In 1665 the English scientist Robert Hooke looked at a thin slice of cork under a microscope he had built. He saw rows of tiny empty boxes, like the cells of a honeycomb, and named them "cells". Most cells are far too small to be seen without a microscope.
One cell or many
Some living things are made of just one cell. They are called unicellular: Amoeba, Paramecium and bacteria are examples. That single cell does everything: it feeds, breathes, moves and reproduces. Plants, animals and human beings are multicellular. A human body has many trillions of cells.
Cells come in many shapes and sizes, each suited to its work. Red blood cells are round discs that slip through narrow vessels. Nerve cells are long and branched to carry messages. Muscle cells are spindle-shaped so that they can contract. The yolk of a hen's egg is a single cell, large enough to see.
Parts of a cell
- Cell membrane: the thin outer skin of the cell. It holds the cell together and controls what goes in and out.
- Cytoplasm: the jelly-like material that fills the cell. Smaller working parts, called organelles, float in it, and most of the cell's activities take place here.
- Nucleus: a round body, usually near the centre. It is the control room of the cell. It contains thread-like chromosomes, which carry genes, the instructions passed from parents to children.
- Mitochondria: the powerhouses, which release energy from food.
- Vacuoles: sacs that store water, food or waste.
Plant cell and animal cell
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present, outside the cell membrane; it gives shape and strength | Absent |
| Chloroplasts | Present in green parts; they contain chlorophyll for photosynthesis | Absent |
| Vacuole | One large central vacuole | Small vacuoles, or none |
| Shape | Usually fixed and box-like | Usually rounded or irregular |
From cells to a body
In a multicellular body, cells do not work alone. Similar cells doing the same job form a tissue, such as muscle tissue. Different tissues join to form an organ, such as the heart. Organs working together make an organ system, such as the circulatory system. All the systems together make the organism.
Cell → Tissue → Organ → Organ system → Organism
Key terms
- Cell
- the basic unit of structure and function of living things
- Unicellular
- made of a single cell
- Nucleus
- the control centre of the cell
- Cytoplasm
- the jelly-like material inside the cell
- Chloroplast
- the green part of a plant cell where photosynthesis happens
- Tissue
- a group of similar cells doing the same job
Try it yourself
Peel the thin, transparent skin from the inner side of an onion layer and spread it on a glass slide with a drop of water. If your school has a microscope, look at it: you will see neat, brick-like plant cells, each with a wall and a nucleus.
Quick questions
Answer in your head or on paper first, then tap to check.
Who discovered the cell, and when?
Robert Hooke, in 1665.
Which part of the cell is called its control room?
The nucleus.
Name two parts found in plant cells but not in animal cells.
The cell wall and chloroplasts.
Arrange in order: organ, cell, organism, tissue, organ system.
Cell, tissue, organ, organ system, organism.
This is a simple explanation to help you understand. For examinations, read the same chapter in your prescribed textbook. Last checked: 11 November 2026.