cells and molecules - membrane biology, biosynthesis, tonicity

lipid biosynthesis - phosphatidylcholine

• CoA helps assemble the fatty acid and glycerol phosphate into phosphatidic acid

• a phosphatase is used to create diacylglycerol

• choline phosphotransferase adds a choline group

• flippases move the lipid from one leaflet to the other

why membranes are asymmetric

• enzymes involved usually located on one side of the membrane

• 2 sides of membrane have slightly different compositions with different functions - composition dictated by the function of the membrane

• asymmetry in protein orientation gives 2 membrane faces their different properties

why aren’t membranes more homogenous

• lipids can move laterally easily but flip-flopping is slow as the membrane is hydrophobic so it is unfavourable to move the polar head

• membranes have rotational movement

• flipping requires specialised enzymes - flippases (top to bottom) or floppases (bottom to top)

effect of lipids on structure and function of the membrane

• different lipids are present in different leaflets - impacts structure and determines function of membrane

• some lipids longer than others

• cholesterol - stabilises tails and makes membrane longer

• different shaped lipids can create curvature

sphingolipid biosynthesis

• different hydrolysis pathways which have different functional consequences

• sphingomyelin - protects cell membrane

• ceramide - causes cell proliferation, differentiation or cell death

• sphingosine-1-P - stimulates cell growth

• sphingosine - inhibits cell proliferation

• sphingolipid, ceramide and its metabolic products are important signalling molecules

lipid bilayer

• selectively permeable

• allows through: 

small hydrophobic molecules

some small polar molecules

neutral gases

• doesn’t allow through (without help):

large molecules

ions

concentration gradient

• differences in solute concentration which can be equalized by diffusion

isotonic solution

water moves equally between the cell and its environment, does not mean no water movement

hypotonic solution

water moves into cell, water potential in cell lower than water potential of surroundings

hypertonic solution

water moves out of cell, water potential of cell higher than water potential of surroundings

in aggregate

water moves in both directions but primarily one direction