Role of membranes

What is the overall role of membranes

To control which substances enter or leave, so different conditions can be maintained inside the cell or organelle compared to outside, allowing division of labour.

How are membranes partially permeable

They allow some substances to pass freely but prevent the passage of others

How are membranes selectively permeable

They have specific transport proteins which determine which substances can cross.

What substances can diffuse freely across the membrane

Small, non-polar molecules e.g. oxygen

What substances requires specific transport proteins to cross membranes

large, polar substances e.g. glucose, and ions

What gives membranes the ability to control which substances leave or enter

The presence of specific transport proteins

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