Definition
A cell is the smallest structural and functional unit of all living organisms. Every living thing — from the simplest bacterium to the most complex plant or animal — is made of one or more cells. Cells carry out all the basic processes of life: obtaining energy, growing, reproducing, and responding to the environment.
Key Characteristics
- Cells are enclosed by a cell membrane that controls what enters and leaves
- Most cells contain DNA, the genetic material that directs cell activity
- Cells carry out metabolism — chemical reactions that sustain life
- Cells can reproduce by dividing to produce new cells
- Prokaryotic cells (bacteria) have no membrane-bound nucleus; eukaryotic cells (plants, animals, fungi) do
- Sizes range from about 1 µm (bacteria) to over 100 µm (human egg cell)
Real-World Analogy
Think of a cell as a tiny factory. The cell membrane is the factory walls controlling entry and exit of materials. The nucleus is the manager's office holding the blueprints (DNA). The mitochondria are the power generators, and ribosomes are the assembly lines building proteins.
A cell is the basic unit of life. All organisms are made of cells, all cells come from pre-existing cells, and all life functions — energy production, growth, reproduction — are carried out by cells. This is summarised in the Cell Theory, one of the foundational principles of biology.
Worked Examples
Animal cell vs. plant cell
An animal cell has a cell membrane, nucleus, mitochondria, and ribosomes but no cell wall. A plant cell has all of those plus a rigid cell wall made of cellulose, a large central vacuole for water storage, and chloroplasts for photosynthesis.
Unicellular vs. multicellular organisms
An amoeba is a single-celled (unicellular) organism — one cell performs every life function. A human body contains about 37 trillion cells, each specialised for a role: muscle cells contract, red blood cells carry oxygen, nerve cells transmit signals.
Practice Problems
What is the smallest unit of life called, and what theory states that all living things are made of it?
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What is the difference between a prokaryotic cell and a eukaryotic cell?
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Name three organelles found in both animal and plant cells and state the function of each.
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A student says that a brick wall is made of bricks and a living organism is made of cells. In what ways is this analogy useful and in what way does it break down?