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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
Note 
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