Life activity of cells
In a living cell the cytoplasm moves constantly; this can be seen under a microscope in the leaf cells of elodea (Elodea), because the leaf is thin and transparent. The cell wall has tiny pores through which water and nutrients pass between cells. The wall and cytoplasm do not let all substances through equally: some enter, others do not. The cell processes the substances that come in and turns them into what it needs; it also breathes and feeds. A young cell is small and grows larger and its wall thickens; in an old cell the vacuole takes up more room than the cytoplasm. Only cells of the formative tissue divide: first the nucleus divides in two, then a partition forms in the cytoplasm, and the mother cell gives two young cells. This is how a plant grows — by more cells being made and the cells growing.
Elodea leaf under the microscope: take one leaf from an elodea shoot from an aquarium and make a slide in a drop of water. Watch the slow circling movement of the chloroplasts. If you can, compare the speed in cool and in warm water (never above 40 °C). Safety: handle glass carefully and wash hands. Iodine test: iodine solution stains clothes; do not drink or taste it and wear gloves.