know the structure and properties of cell membranes
understand how models such as the fluid mosaic model of cell membranes are interpretations of data used to develop scientific explanations of the structure and properties of cell membranes
the cell membrane
● partially permeable
○ small, non-polar molecules can pass through gaps
○ large, polar molecules pass through channel and carrier proteins
cell surface membranes
● under a light microscope the cell membrane looks like a single line, but when viewed
under an electron microscope we can see it is in fact a bilayer (double layer)
● the basic structure of this bilayer is two layers of lipid molecules (essential biological
molecules) called phospholipids
phospholipids
● the phospholipid head is polar
○ making it hydrophilic (interacts / associates with water)
● the fatty acid tails are non-polar
○ making them hydrophobic (does not interact / associate with water)
○ glycerol, phosphate group > phosphate head, two fatty acid tails > lipid tail
● the most stable arrangement is the bilayer
○ this is because the cell is filled with cytoplasm which is aqueous and the cell is
surrounded with tissue fluid which is also aqueous
fluidity (molecules are free to move)
● if the fatty acid tails are unsaturated, this means they have a C=C double bond
○ these C=C cause 'kinks' in the fatty acid tails
● more unsaturated fatty acids in phospholipids = more fluid membrane
● unsaturated fatty acids have kinks in their hydrocarbon tails caused by C=C which
prevents them packing closely together and creates space in which they can move into
● cholesterol sits in between fatty acid tails of the phospholipids and prevents movement
of phospholipids, so reduces fluidity
the fluid (molecules are free to move) mosaic (made up of different molecules, e.g., proteins,
phospholipids) model of cell membranes
fluid mosaic model - proposed by Singer and Nicholson in 1972
● phospholipid bilayer
○ fluid
● integral and peripheral proteins
○ mosaic
■ enzymes
■ carrier proteins
■ channel proteins
● glycoproteins (proteins and polysaccharides) and glycolipids (proteins and lipids)
○ cell-to-cell recognition, receptors
● A large surface area to volume ratio is required for maximum diffusion.
● Diffusion surfaces need to be very thin – one cell thick, so there is a short diffusion
distance.
● Lots of capillaries maintains a concentration gradient.
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