Water inside living things: kinds of cells and the kingdoms
Water is the most abundant substance in living things. Water makes up about 70% of the body mass of a mammal: about 47% inside the cells and about 23% outside them (in blood plasma and other fluids). The cell is the unit of structure and function of all living things. Cells are of two kinds. In a prokaryote, such as a bacterium, the DNA lies free in the cell with no membrane around it, and the cell has a wall that contains peptidoglycan. In a eukaryote the genetic material is inside a nucleus with a membrane, and the cell has organelles such as mitochondria. So it is wrong to say that every living cell has a nucleus: bacteria have none. Eukaryotes form four kingdoms. Protists are mostly single-celled or simple, often aquatic; some make their own food and some do not. Fungi have cell walls of chitin and feed on others; most are multicellular, while yeast is single-celled. Plants have cellulose walls and make food by photosynthesis. Animals have no cell wall, can usually move and feed on others; they include invertebrates and vertebrates. The plant cell wall lets water and salts through; the cell membrane is semi-permeable.
| Kingdom | Cell wall | Food |
|---|---|---|
| protists | some | make or take in |
| fungi | chitin | feed on others |
| plants | cellulose | photosynthesis |
| animals | none | feed on others |
Kingdom card game, with a friend. Write the names of ten living things on cards (for example a rose, a mushroom, a goat, an amoeba, a bacterium, a fern, a mould, a fish, yeast, a moss). Shuffle and sort the cards into prokaryote and the four kingdoms of eukaryotes. Check each one with a book or an adult and say one clue for every choice, such as “has a chitin wall” or “makes food by photosynthesis”.