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4.1 Structure of the cell surface membrane (Phospholipid bilayer…
4.1 Structure of the cell surface membrane
Phospholipid bilayer
Hydrophilic heads
of both phospholipid layers point outwards - attracted by water on both sides
Hydrophobic tails
of both phospholipid layers point inwards - repelled by the water on both sides
Lipid soluble material can move through the phospholipid bilayer. Water soluble material cannot.
Makes the membrane flexible and self sealing
Proteins
Some are embedded in the surface of the bilayer and give mechanical support, or sometimes act as cell receptors for molecules such as hormones (in conjunction with glycolipids)
Some go through the whole phospholipid bilayer. These are
protein channels
(water filled tubes which allow water-soluble ions to diffuse across) and
carrier proteins
(bind to ions or molecules like glucose and amino acids and change shape to move the molecules across the membrane)
provide structural support
act as channels transporting water-soluble substances across the membrane
allow active transport across the membrane through carrier proteins
form cell-surface receptors for identifying cells
help cells adhere together
act as receptors for example for hormones etc
Cholesterol
Occurs within the phospholipid bilayer
Adds strength to membranes
very hydrophobic, so prevent water loss and loss of dissolved ions from cells
pull together the fatty acid tails of the phospholipid molecules to limit their movement without making the membrane too rigid.
make the membrane less fluid at high temps
Fluid-mosaic model
The structure in which all the various molecules are combined in the cell surface membrane.
fluid
because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape
mosaic
because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic
Glycolipids
Carbohydrate covalently bonded with a lipid.
The carb portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell surface receptor for specific chemicals.
act as recognition sites
help maintain the stability of the membrane
help cells to attach to one another and so form tissues
Permeability of the cell-surface membrane
Most molecules do not freely diffuse across because many are:
not soluble in lipids and therefore cannot pass through the phospholipid bilayer
too large to pass through the channels in the membrane
of the same charge as the charge on the proteins channels and so, even if they are small enough to pass through they are repelled
electrically charged (polar) and therefore have difficultly passing through the non-polar hydrophobic tails in the phospholipid bilayer
Glycoprotiens
Carbohydrate chains attached to many extrinsic proteins on the outer surface of the cell membrane. Glycoproteins act as cell-surface receptors, more specifically for hormones and neurotransmitters
act as recognition sites
help cells to attach to one another and so form tissues
allow cells to recognise one another, for example lymphocytes can recognise and organism's own cells