Singapore-Cambridge O-Level Biology

## Cells as the basic unit of life Living things differ from non-living things in that they grow, reproduce, move and respond to changes around them. All of them are built from cells, and the activities of an organism can be explained by what its cells do. Cells are organised into tissues (groups of similar cells doing one job), tissues into organs, organs into organ systems, and the systems together make up the organism. Most cells are far too small to see without a microscope. A **light microscope** shows the nucleus, cytoplasm, cell membrane and, in plants, the cell wall, vacuole and chloroplasts. Finer detail, such as the endoplasmic reticulum, Golgi body, mitochondria and ribosomes, is only visible in an **electron micrograph**, which uses a beam of electrons and has a much higher resolution. ## Cell structures and their functions ### Structures seen with a light microscope The **cell membrane** surrounds the cytoplasm of every cell. It is partially permeable, so it controls which substances enter and leave the cell. The **cytoplasm** is the jelly-like material inside the membrane where most chemical reactions of the cell take place, and in which the organelles are suspended. The **nucleus** contains the cell's DNA in the form of chromosomes. It controls the activities of the cell, since the genes it holds carry the instructions for making the proteins (including enzymes) the cell uses. The **cell wall** is found in plant cells, outside the cell membrane. It is made of…