B2.1 Cells and simple cell transport

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.

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.