- Describe the structure of animal and plant cells
- State the function of each major organelle
- Compare plant and animal cells
- Explain how specialised cells match their function
The building blocks of life
Every organism — you, a mushroom, a whale — is made of cells: microscopic compartments where the chemistry of life happens. Some organisms are a single cell; you are about 30 trillion of them, working together.
A cell is the smallest unit of life. It is surrounded by a membrane, contains genetic material, and carries out the chemical processes that keep an organism alive.
Cells were first seen in the 1660s when Robert Hooke looked at cork through a microscope and named the little boxes he saw "cells". Modern microscopes reveal what's inside those boxes: specialised structures called organelles — literally "little organs".
The organelles
Here's the cast of characters you need to know:
- Nucleus — contains the genetic material (DNA) and controls the cell's activities: which proteins get made, when the cell divides.
- Cell membrane — the boundary. It holds the cell together and controls what enters and leaves.
- Cytoplasm — the jelly-like substance filling the cell, where most chemical reactions happen.
- Mitochondria — the site of aerobic respiration: they release energy from glucose to power the cell. Active cells (muscle!) have many.
- Ribosomes — tiny structures where proteins are made (protein synthesis).
- Cell wall (plants, algae, fungi only) — a tough outer layer of cellulose that supports and strengthens the cell.
Cells that do a lot of work need a lot of energy, so they contain more mitochondria. A muscle cell may have thousands; a skin cell far fewer. Structure matches demand.
Plant cells: three extras
Plant cells contain everything above plus three structures animal cells lack:
- Chloroplasts — contain chlorophyll, the green pigment that absorbs light for photosynthesis.
- Permanent vacuole — a large sac of cell sap that keeps the cell rigid and stores substances.
- Cell wall — made of cellulose, giving extra strength and support.
carbon dioxide + water → glucose + oxygen
glucose + oxygen → carbon dioxide + water (+ energy released)
Notice the two reactions are near-mirror images — plants and animals are chemically intertwined.
Specialised cells
Cells differentiate to become specialised: their structure changes to suit one job.
- Root hair cells grow long hairs to absorb water from soil — huge surface area, no chloroplasts (it's dark underground).
- Palisade leaf cells are packed with chloroplasts and tall and thin to catch maximum light.
- Sperm cells have a tail for swimming, masses of mitochondria for energy, and enzymes to digest into the egg.
- Red blood cells lose their nucleus entirely to make room for haemoglobin, and are biconcave discs for maximum surface area.
Whenever an exam asks about a specialised cell, answer in two moves: what is the job? then which feature helps it do that job? Structure always serves function.
Organisation: cells to systems
Cells don't work alone. The hierarchy:
organelles → cells → tissues → organs → organ systems
A tissue is a group of similar cells doing one job (muscle tissue). An organ combines tissues (the stomach). An organ system combines organs (the digestive system).
Practice
Cell wall, chloroplasts and the permanent vacuole.
Mitochondria release energy in aerobic respiration. Chloroplasts capture light for photosynthesis.
They live underground in darkness, absorbing water — there is no light for photosynthesis, so chloroplasts would be useless. They are specialised for absorption instead, with long hairs for surface area.
Organelle → cell → tissue → organ → organ system.
Quick check
Which organelle contains genetic material and controls the cell?
Where does protein synthesis happen?
Which feature gives plant cells extra strength?
Why do muscle cells contain many mitochondria?
- Cells are the basic units of life; organelles are their specialised parts.
- Nucleus (control + DNA), mitochondria (respiration), ribosomes (protein synthesis), membrane (boundary).
- Plant cells add: cell wall, chloroplasts (photosynthesis) and a permanent vacuole.
- Specialised cells adapt their structure to their function — always link the two.
- Organisation: organelles → cells → tissues → organs → organ systems.