Inside the cell: organelles and the watery cytoplasm
Inside the cell membrane is the cytoplasm, a gel that is mostly water. The nucleus controls the cell and holds the genetic material; it is in plant and animal cells, usually near the centre of an animal cell. Chloroplasts, with chlorophyll, are only in plant cells and carry out photosynthesis. Vacuoles store food, water and extra salts: a plant cell has one or a few large ones, an animal cell many small ones. Mitochondria release energy from food for the cell (they do not “make” energy). The endoplasmic reticulum is a system of membranes that moves materials around inside the cell. Water matters in four ways. It is the universal solvent: it dissolves salts and nutrients and carries gases and wastes. It forms the watery cytoplasm, which keeps the cell’s shape and its internal pressure. Its high specific heat steadies the body temperature. And it takes part in metabolic reactions.
| Part | Job |
|---|---|
| nucleus | controls the cell, holds genetic material |
| chloroplast (plants only) | photosynthesis |
| mitochondria | release energy from food |
| vacuole | stores food, water, extra salts |
| endoplasmic reticulum | moves materials inside the cell |
Cell city, on paper. Draw a town and give every part of the cell a job in it: the nucleus as the office that gives orders, mitochondria as power stations, vacuoles as storerooms, the cell membrane as the gate, the cytoplasm as the water in which everything works. Write one sentence for each. Then mark the one part that would be missing in the town of an animal cell and say why.