The building blocks of life
A building is made of bricks; a living body is made of cells. The cell is the basic structural and functional unit of life.
In 1665 Robert Hooke looked at a thin slice of cork under a simple microscope and saw tiny box-like compartments, which reminded him of the rooms in a monastery. He called them cells. (What he saw were actually the walls of dead cells.)
Number of cells
Unicellular organisms are made of a single cell that does everything: eating, breathing, excreting and reproducing. Examples: Amoeba, Paramecium, bacteria, yeast.
Multicellular organisms are made of many cells. A human body has trillions of them.
An organism made of many cells begins life as a single cell, the fertilised egg, which divides again and again.
Shape and size
Cells come in many shapes, each suited to its job.
Cell | Shape | Why |
|---|---|---|
Amoeba | no fixed shape; pushes out pseudopodia | to move and catch food |
Red blood cell | round, disc-like | to flow easily through blood vessels |
Nerve cell | long, branched | to carry messages over long distances |
Muscle cell | spindle-shaped | to contract and relax |
White blood cell | can change shape | to reach and engulf germs |
Most cells are microscopic, measured in micrometres (1 μm = one-millionth of a metre). Bacteria are only a few micrometres long. The largest single cell is the egg of an ostrich. A nerve cell in the leg can be about a metre long. The size of a cell has no relation to the size of the animal: an elephant's cells are not larger than a mouse's.
Cells, tissues and organs
A group of similar cells doing the same job is a tissue. Several tissues working together form an organ, such as the stomach, heart or leaf. Organs work together in an organ system.
Parts of a cell
Part | Description | Function |
|---|---|---|
Cell membrane (plasma membrane) | thin, flexible outer boundary | separates cells from each other and from the surroundings; controls what enters and leaves |
Cell wall (plants only) | thick, rigid layer outside the membrane, made of cellulose | gives shape and protection against temperature changes, wind and moisture |
Cytoplasm | jelly-like substance between the membrane and the nucleus | contains the other cell parts (organelles) |
Nucleus | usually round, near the centre, enclosed by a nuclear membrane | controls the activities of the cell; contains chromosomes that carry genes, passing features from parents to offspring |
Nucleolus | small dense body inside the nucleus | |
Vacuole | blank-looking space | stores water and wastes; large in plant cells, small or absent in animal cells |
Plastids (plants only) | coloured bodies in the cytoplasm | chloroplasts contain chlorophyll and carry out photosynthesis |
The whole living substance of the cell, including cytoplasm and nucleus, is called protoplasm, the "living substance of the cell".
Bacterial cells have no well-defined nucleus; their genetic material is not enclosed in a nuclear membrane. Such cells are prokaryotic. Cells with a proper nucleus are eukaryotic.
Plant cells and animal cells
Feature | Plant cell | Animal cell |
|---|---|---|
Cell wall | present | absent |
Plastids (chloroplasts) | present | absent |
Vacuole | one large central vacuole | small or absent |
Shape | usually regular, box-like | usually irregular or rounded |
Looking at cells yourself
Peel the thin skin from the inside of an onion layer, place it on a slide in a drop of water, add a drop of safranin stain and a cover slip, and look under a microscope. You will see box-like cells with clear walls and a dark-stained nucleus in each.
Cheek cells, scraped gently from the inside of your cheek and stained with methylene blue, show irregular cells with no cell wall.
Summary
The cell is the basic unit of life; Robert Hooke named it in 1665.
Cells vary in shape and size according to their job.
Main parts: cell membrane, cytoplasm and nucleus; plants also have a cell wall, plastids and a large vacuole.
Cells form tissues, tissues form organs, and organs form organ systems.