B2.1.1 Cells and cell structure
Most human and animals cells have the following parts:
- a nucleus - controls the activities of the cell
- cytoplasm - where most of the chemical reactions take place
- a cell membrane - controls the passage of substances into and out of the cell
- mitochondria - where most energy is released in respiration
- ribosomes - where protein synthesis occurs.
Plant and algal cells also have:
- cell wall made of cellulose - strengthens the cell.
- chloroplasts - absorb light energy to make food
- 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.
B2.1.2 Dissolved substances
Dissolved substances can move in and out of cells by diffusion.
Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in movement from an area of a higher concentration to 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.