How cells were discovered, what cell theory says, how microscopes work, and how prokaryotic, plant and animal cells differ.
Every living thing you have ever seen, from a mushroom to a mosquito to the person next to you, is made of cells, and some living things are only one cell. This article tells the story of how cells were discovered, what cell theory says, how microscopes show us cells, and how the main types of cell (bacterial, plant and animal) differ. For a close-up of the parts inside a human cell, see the companion article on the human cell.
Written for students up to O-Level (grades 8 to 10), and useful for parents and teachers.
A cell is the smallest unit that can carry out all the processes of life: it takes in materials, releases energy, builds new molecules, responds to its surroundings, and can reproduce. Some organisms are unicellular (one cell), such as an amoeba, a yeast cell or a bacterium. Others are multicellular, such as a tree or a human, made of millions to trillions of cells that work together.
Cells were not seen until microscopes were invented, and the idea that all life is cellular took about two hundred years to develop.
Hooke saw only the empty walls of dead cork cells, which looked like small rooms, which is why he used the word 'cell'. Leeuwenhoek, using tiny lenses he made himself, was the first to observe living microorganisms. Later scientists, with better microscopes, saw that plant and animal tissues are made of cells, and that cells do not appear from nothing.
Cell theory is one of the great unifying ideas of biology. In its usual form it says:
Modern biology adds that cells contain hereditary information (DNA) that is passed to daughter cells. There are some interesting edge cases: viruses are not cells (they cannot carry out life processes on their own), and some tissues, such as skeletal muscle fibres, contain many nuclei in a single long cell. Scientists treat these as refinements, not as reasons to discard the theory.
Two ideas matter. Magnification is how many times larger an image is than the real object. Resolution is the ability to show two close points as separate: better resolution means more detail. Increasing magnification beyond the resolution limit only makes a bigger blur.
| Feature | Light microscope | Electron microscope |
|---|---|---|
| Uses | Light and glass lenses | A beam of electrons and electromagnets |
| Useful magnification | Up to about ×1500 | Hundreds of thousands of times |
| Resolution | About 0.2 µm, enough to see cells and large organelles | Roughly a thousand times finer; shows fine detail such as membranes and ribosomes |
| Specimens | Can show living cells | Specimens are in a vacuum, so they are dead and specially prepared |
| Size and cost | Small, cheap, found in schools | Large, expensive, used in research and hospitals |
Units. 1 millimetre (mm) = 1000 micrometres (µm), and 1 µm = 1000 nanometres (nm).
magnification = image size ÷ actual size, using the same units.
On a photograph of a cell, a scale bar 20 mm long is labelled '10 µm'. A nucleus on the photograph measures 30 mm. Find the magnification and the real size of the nucleus.
Check: a nucleus of about 15 µm is reasonable inside a cell of 50 µm or more.
Most cells are too small to see without a microscope. They stay small because of the surface-area-to-volume ratio: as a cell grows, its volume increases faster than its surface, so a large cell cannot exchange materials fast enough. A few cells are large, such as egg cells and some nerve cells, but they have special features to cope.
There are two basic kinds of cell. Prokaryotic cells (bacteria and archaea) have no nucleus. Eukaryotic cells (animals, plants, fungi and protists) have a nucleus and other membrane-bound organelles.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No; DNA lies in the cytoplasm (nucleoid) | Yes; DNA is inside a nuclear envelope |
| DNA | Usually one circular chromosome, often with extra small rings (plasmids) | Several linear chromosomes |
| Membrane-bound organelles | None | Many (mitochondria, ER, Golgi and others) |
| Ribosomes | Slightly smaller | Slightly larger |
| Typical size | About 1 to 10 µm | About 10 to 100 µm |
| Cell wall | Present in most (made of peptidoglycan in bacteria) | Plants: cellulose; fungi: chitin; animals: none |
| Examples | Bacteria, such as E. coli | Humans, oak trees, yeast |
Plant and animal cells are both eukaryotic. Both have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells have extra features that suit a plant's way of life.
| Structure | Animal cell | Plant cell | Function |
|---|---|---|---|
| Cell wall | No | Yes (cellulose) | Supports and protects the cell; stops it bursting |
| Chloroplasts | No | Yes, in green parts | Photosynthesis |
| Large permanent vacuole | No (small, temporary ones only) | Yes | Stores cell sap; helps keep the cell firm |
| Shape | Often irregular or rounded | Often regular, fixed by the wall | |
| Stored carbohydrate | Glycogen | Starch | Energy store |
A plant cell in pure water takes in water by osmosis, and the vacuole swells. The cell wall resists, so the cell becomes firm but does not burst; it is turgid. This pressure holds up soft stems and leaves. In a strongly concentrated solution, water leaves, the cell becomes flaccid and, in extreme cases, the membrane pulls away from the wall in a process called plasmolysis. That is why a plant wilts if it is short of water.
Potato cylinders are weighed (2.0 g each) and placed in pure water and in a strong sugar solution for one hour. Predict the masses.
The masses are examples only; your results will differ. To compare fairly, calculate percentage change = (change ÷ starting mass) × 100: for 2.3 g it is (0.3 ÷ 2.0) × 100 = +15%.
Unicellular organisms do everything in one cell: an amoeba moves, feeds and reproduces, and yeast cells divide to make more yeast. Multicellular organisms have advantages: they can grow larger, and their cells can specialise, with some cells doing one job very well (division of labour). The price is that the cells must coordinate and be supplied by systems such as blood vessels in animals and xylem in plants.
Cells specialise by differentiation: switching on particular genes while all cells keep the same DNA. Stem cells are unspecialised cells that can divide and become specialised cells. Stem cells in bone marrow replace blood cells throughout life, and bone marrow transplants rely on this.
| Organelle | Main job | Found in |
|---|---|---|
| Nucleus | Stores DNA, controls cell activities | Eukaryotes |
| Mitochondrion | Releases energy from glucose (aerobic respiration) | Most eukaryotes |
| Ribosome | Builds proteins | All cells |
| Endoplasmic reticulum and Golgi | Fold, process and package proteins | Eukaryotes |
| Chloroplast | Photosynthesis | Plants and algae |
| Cell wall | Support and protection | Plants, fungi, most bacteria |
The article on the human cell explains how the main organelles cooperate in making and exporting a protein.
| Term | Meaning |
|---|---|
| Cell theory | All organisms are made of cells; the cell is the basic unit; cells come from cells |
| Magnification | How much larger an image is than the real object |
| Resolution | The ability to see two close points as separate |
| Prokaryote | A cell with no nucleus |
| Eukaryote | A cell with a nucleus and membrane-bound organelles |
| Turgid | Firm because the vacuole is full of water |
| Differentiation | Development of a specialised cell from an unspecialised one |
| Stem cell | An unspecialised cell that can divide and differentiate |
Continue with these related Biology articles on EDUSAMBAM:
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