Membrane
Outer layer of a cell or of an organelle made of a lipid bilayer
Selectively permeable
[!abstract]+ Lipid bilayer
Double layer of mainly phospholipid, also cholesterol, glycolipid molecules (incl. fat) and proteins
Hydrophilic heads form the surfaces, hydrophobic tails form the coreUnsaturated hydrocarbon tails increase fluidity by preventing packing.
Saturated hydrocarbon tails decrease fluidity by favouring packing.
At moderate temperature, cholesterol decreases fluidity by reducing lipid movement.
At low temperature, cholesterol increases fluidity by preventing solidification.
Associated proteins
Integral protein
Protein that penetrates the membrane
Transmembrane protein
Integral protein that extends through the membrane (instead of partway)
Types:
- Channel: has a hydrophilic channel to transport hydrophiles
- Carrier: like channel, but also has binding sites for active transport
Peripheral protein
Protein that loosely associates to the surface of the membrane
Transport across membrane
Depends on type and size of molecule and fluidity of the membrane
Passive transport
Moves solute down its concentration gradient (from higher concentration to lower concentration)
Doesn't require energy
Diffusion
For hydrophobes
Facilitated diffusion
Variant of diffusion
For large polar molecules (e.g. glucose) or ions
Requires channel or carrier protein
Osmosis
Special case of facilitated diffusion
For water, which moves against the concentration gradient of solute (a.k.a. down its concentration gradient)
Requires aquaporin protein
Continues until the two sides are isotonic
[!abstract] Aquaporin
Channel protein for osmosis
Active transport
Moves solute against its concentration gradient (from lower concentration to higher concentration) in exchange for another solute
Requires carrier protein
Requires energy provided by the hydrolysis of ATP
Steps:
- Solute 1 in solution 1 binds to the carrier
- The carrier switches sides with energy provided by the hydrolysis of ATP
- Solute 1 enters solution 2
- Solute 2 in solution 2 (most commonly
) binds to the carrier - The carrier switches sides
- Solute 2 enters solution 1
Ajitani Hifumi