Last reviewed 30 Sept 2026 · 9 min read
The cell
The cell is the basic structural and functional unit of life. All living things are made of one or more cells.
- Robert Hooke (1665) first observed cells in a thin slice of cork using a simple microscope and coined the word "cell".
- Anton van Leeuwenhoek (1674) first saw free-living cells (bacteria, protozoa).
- Robert Brown (1831) discovered the nucleus.
- Schleiden and Schwann (1838–39) proposed the cell theory: all plants and animals are made of cells. Rudolf Virchow (1855) added that all cells arise from pre-existing cells (omnis cellula e cellula).
- Purkinje coined the word protoplasm (the living substance); the cell's contents = protoplasm.
- Electron microscope (Knoll and Ruska) allowed us to see the fine structure.
Unicellular organisms (amoeba, paramecium, bacteria, yeast); multicellular (humans, plants). The smallest cell is that of Mycoplasma (PPLO); the largest single cell is the ostrich egg; the longest cell is the nerve cell (neuron); the human egg (ovum) is the largest human cell and the sperm the smallest; human red blood cells are about 7 µm in diameter.
Prokaryotes and eukaryotes
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | no true nucleus (no nuclear membrane); a nucleoid | true nucleus with a nuclear membrane |
| Organelles | no membrane-bound organelles | present |
| Size | small (1–10 µm) | larger (10–100 µm) |
| Ribosomes | 70S | 80S (70S in mitochondria and chloroplasts) |
| Examples | bacteria, blue-green algae (cyanobacteria) | plants, animals, fungi, protists |
Plant cell and animal cell
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | present (cellulose) | absent |
| Plastids (chloroplasts) | present | absent |
| Vacuole | one large central vacuole | small or absent |
| Centrioles | absent (in higher plants) | present |
| Shape | fixed, usually rectangular | irregular, round |
| Lysosomes | rare | present |
Cell organelles and their functions
| Organelle | Structure | Function |
|---|---|---|
| Cell membrane (plasma membrane) | thin, flexible, selectively permeable layer of lipids and proteins | controls entry and exit of substances |
| Cell wall | rigid, of cellulose in plants (chitin in fungi, peptidoglycan in bacteria) | protection, support, shape |
| Nucleus | double-membraned; contains chromatin (chromosomes/DNA) and a nucleolus | control centre; heredity; nucleolus makes ribosomes |
| Cytoplasm | jelly-like fluid between the membrane and nucleus | site of many chemical reactions |
| Mitochondria | double membrane; own DNA; inner folds are cristae | "powerhouse of the cell" — produce ATP by aerobic respiration |
| Chloroplast | double membrane, contains chlorophyll, grana and stroma | photosynthesis (in plants) |
| Other plastids | chromoplasts (colour), leucoplasts (storage: starch, oil, protein) | colour, storage |
| Endoplasmic reticulum (ER) | network of membranes; rough ER (ribosomes attached) and smooth ER | rough — protein synthesis; smooth — lipid synthesis, detoxification |
| Golgi apparatus (Golgi complex) | stacks of flattened sacs (discovered by Camillo Golgi) | packaging, modifying and secretion of materials |
| Lysosomes | small sacs with digestive enzymes | "suicide bags"; digest worn-out organelles and foreign material |
| Ribosomes | tiny granules (RNA + protein) | protein synthesis ("protein factories") |
| Vacuoles | fluid-filled sacs | storage; in plants, maintain turgidity |
| Centrosome / centrioles | in animal cells | help in cell division (form the spindle) |
| Peroxisomes | vesicles | break down hydrogen peroxide, fatty acids |
Key points: Mitochondria and chloroplasts have their own DNA and ribosomes and can multiply on their own (the "semi-autonomous" organelles). Ribosomes are found in both prokaryotic and eukaryotic cells. Tonoplast is the membrane of the vacuole. A plasmid is a small circular DNA in bacteria, used in genetic engineering.