Cell and cell structure
Most human and animal cells have the following parts:
- a nucleus, which controls the activities of the cell
- cytoplasm, in which most of the chemical reactions take place
- a cell membrane, which controls the passage of substances into and out of the cell
- mitochondria, which is where most energy is released in respiration
- ribosomes, which is where protein synthesis occurs.
Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell. Plant cells often have:
- chloroplasts, which absorb light energy to make food
- a permanent vacuole filled with cell sap.
A bacterial cell consists of cytoplasm and a membrane surrounded by a cell wall; the genes are not in a distinct nucleus.
Yeast is a single-celled organism. Yeast cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.
Cells may be specialised to carry out a particular function.
Dissolved substances can move into and out of cells by diffusion.
Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion.
Oxygen required for respiration passes through cell membranes by diffusion.