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Membrane structure

Topic 1
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Davson and J
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J
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L
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They found that:
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Not all membranes are identical and symmetrical

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Membranes with different functions have different compositions and
structure
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This can be seen by electron micrograph

Proteins are non-polar and wouldn’t interface with water (polar)
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It’s unlikely to appear as a layer on the polar “heads” of the
phospholipids

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Proteins stay between the non-polar “tails”

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Very few proteins are polar

➢ More evolved electron micrograph and their studies on cells’ action in different
environments and solutions gave birth to a new membrane model
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The fluid mosaic model or the Singer-Nicolson model
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Proteins are inserted in a fluid layer of phospholipids

The phospholipid bilayer
➢ The plasma membrane is made up of phospholipids
➢ Each phospholipid is made of three-carbon compound named glycerol
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The molecules have hydrophilic a head
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One carbon is attached to a highly polar organic alcohol that includes
a bond to a phosphate group

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It is polar, due to the organic alcohol => soluble in water

And two hydrophobic tails

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Membrane structure

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Sites for hormone-binding
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Have receptors with specific shape exposed on the exterior that fit
the shape of a specific hormone

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When the hormone attaches to the protein, the shape of the protein
changes and this causes reactions in the cell

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Cell adhesion

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Membrane structure
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Cell-to-cell communication
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These proteins have carbohydrate molecules attached, which provide
the cell with an identification label

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Pumps for active transport
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Proteins transfer substances from one side of the membrane to the
other by changing shape

Carbohydrates
➢ Carbohydrates are used for cell recognition
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The immune system uses them to identify foreign cells
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Like disease causing bacteria

Organ transplants and blood transfer sometimes fail because the body of
the person doesn’t recognise them
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This means that the organ and blood of the donor have different
carbohydrates than the receiver

➢ Carbohydrates attached to a protein form a glycoprotein
➢ Carbohydrates with a hydrophobic group (phospholipid) form a glycolipid
➢ When somebody is allergic to something, their immune system is alarmed, because
it doesn’t recognise the carbohydrates of the thing they are allergic to
Cholesterol
➢ Molecules of cholesterol can be found in the hydrophobic region of the membrane
➢ Cholesterol determines the fluidity and permeability of the membrane, which
changes with temperature
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More cholesterol means fewer molecules that can pass through the
membrane

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Helps the body produce some hormones
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Like testosterone and oestrogen

Allows membranes to function effectively in a wider range of temperatures
than when it’s not present

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Membrane structure
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Topic 1

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