Singapore-Cambridge A-Level Biology (H2)

## The cell theory The **cell theory** has three parts. Living organisms are composed of cells, the cell is the smallest unit of life, and all cells come from pre-existing cells. The second part means that nothing smaller than a cell carries out all the processes of life (metabolism, growth, response to the environment and reproduction) by itself. The third part rules out cells arising from non-living matter under present conditions, and is the reason every cell in a body can be traced back to a single fertilised egg through repeated division. ## Eukaryotic cells and their compartments A eukaryotic cell has a nucleus and a set of membrane-bound compartments. Membranes divide the cell into regions, so enzymes and substrates for one process can be kept together at high concentration and incompatible reactions (for example the breakdown of proteins by lysosomal enzymes) are kept away from the rest of the cytoplasm. Membrane systems also give a large surface area on which enzymes and carriers can be arranged in order. The **cytoplasm** is everything inside the cell surface membrane except the nucleus. Its fluid part, the **cytosol**, is a solution of ions, sugars, amino acids and proteins in which many metabolic reactions, including glycolysis, take place. ### Structures and their functions | Structure | Structure seen in drawings and micrographs | Function | |---|---|---| | Cell surface membrane | Thin phospholipid bilayer around the cell, about 7 nm thick | Controls what…