Cell Membrane Notes

Cell Membrane

• The membrane is composed of 2 layers: extracellular monolayer, and cytosol monolayer
• “mono” = one, single

Functions:

• Barrier to water-soluble (polar) molecules, charged molecules and large molecules
• Regulates the transport in and out of organelle or cell
• Separate inside from outside (different environments need to be maintained);
• Help create little compartments where specific environment can be maintained
• For example lysosomes; appropriate pH level for hydrolytic acid in order for it to perform what it was supposed to
• Maintain ion gradients (energy conservation and signalling)
• Involved in signalling
• The membrane is selectively permeable
• Small, nonpolar molecules (O2, CO2, N2) readily diffuse through bilayer with the concentration gradient
• Small, uncharged polar molecules (H2O, glycerol) can move through but very slowly
• Large, uncharged polar molecules (glucose, sucrose) very rarely move through membrane
• Small ions, do not move through the membrane
• The plasma membrane is an Amphipathic (composed of a hydrophobic, and hydrophilic compartments)
• Phospholipid structure
• Polar head region (phosphate group and a glycerol)
• 2 fatty acid tails
• Glycerol is linked to fatty acid tails through ester bonds
• Polar head groups are different depending on monolayer
• if temp is decreasing and high proportion of saturated fatty acids it is going to reach a non-motion or solidified state faster vs when there is un saturated fatty acids
• membranes need an optimal fluidity in order to allow the cell to work properly
• Non homogeneous (not all the same)
• There is an asymmetry in monolayers because of the functions
• Different Phospholipids require for different jobs (cellular functions)
• Membranes are not static, they can move depending on the function of the particular part of the membrane
• VERY dynamic

Membrane Fluidity 

• Lateral diffusion
• Phospholipids are changing positions (flexion; flex their tails wider, rotation; spin tails around, flip flop (rarely occurs); phospholipids flip sides)
• Temperature, cholesterol, chain length, and ratio of saturated vs unsaturated fatty acids affect membrane fluidity
• Unsaturated fatty acids push phospholipid tails away a little ways so there is more room to move (less packed = more fluid), kinks in tails
• Saturated fatty acids are more closely packed and cannot move as well, they also have longer tails

Desaturases 

• family of enzymes that create unsaturated fatty acids
• activity of desaturase increase when there is a decrease of temp
• at low temperatures, there is an increase of unsaturated fatty acids to allow for permeability (fluidity)
• at high temperatures, there will be less because we don’t want to have a membrane that is too fluid

Regulation of Fluidity

• organisms adjust fatty acid composition at different temperatures in order to maintain optimum fluidity for the function of that membrane
• the length of fatty acid chains affect fluidity (the longer (more carbon atoms) = less permeable)

Cholesterol 

• an important component of the plasma membrane
• associated with animal cells only
• inserted between phospholipids
• needed in order for membranes to function normally
• varies on temperature
• polar head groups incorporated with nonpolar tails
• pushes phospholipids aside
• important for function of body
• buffering effect by;
• reducing packing opportunities in lower temperatures maintains fluidity
• by reducing phospholipid movement in higher temperatures it maintains fluidity