The plasma (or cell surface) membrane surrounds cells and internal membranes form organelles by dividing a cell into compartments (compartmentalisation). Cell membranes form partially permeable barriers that separate the cell contents from its external environment and organelle contents from the cytoplasm. Membranes control which substances enter or leave, so different conditions can be maintained inside the cell or organelle compared to outside, allowing division of labour.

Membranes are partially permeable as they allow some substances to pass freely but prevent the passage of others (which require a transport protein to cross).

Membranes are selectively permeable as they have specific transport proteins which determine which substances can cross.

The partially and selectively permeable properties of membranes means that only small, non-polar molecules are able to freely diffuse across the membrane e.g. oxygen, while other large, polar molecules require a transport protein to cross e.g. glucose and ions. So the presence of specific transport proteins determine which molecules can cross giving membranes the ability to control which substances can enter or leave.

Role of the plasma membrane:

• Separate cell contents from its external environment

• Control transport of substances into or out of cell

• Bulk movement of substances into or out of cell by endocytosis or exocytosis (e.g. secretion of proteins)

• Location of antigens for cell recognition as ‘self’ or ‘non-self’

• Release of chemical signals (e.g., hormones) to allow cell signaling

• Location of receptors for chemical signals (e.g. hormones) to allow cell signaling

• Cell to cell attachment to form tissues

• Site of attachment and organisation of enzymes

Role of internal membranes:

• Separate organelle contents from cytoplasm

• Control transport of substances into or out of the organelle

• Formation of vesicles for intracellular transport (e.g. between ER and Golgi)

• Contain receptors for intracellular recognition and signaling

• Site of attachment for ribosomes on RER

• Site of attachment and organisation of enzymes (e.g. ATP synthase in mitochondria)

• Site of chemical reactions (ATP production in mitochondria)

• Location of photosynthetic pigments for photosynthesis (thylakoid membrane in chloroplast)

• Protection of rest of cell from contents (hydrolytic enzymes in lysosomes)

• Protection of DNA in nucleus