Hydrophilic
Attracted to water
Hydrophobic
Not attracted to water
Amphipathic substance
A substance where part of the molecule is hydrophilic and part is hydrophobic. Such as a phospholipid.
Hydrophilic part of a phospholipid
The phosphate group (the phosphate head)
Hydrophobic part of a phospholipid
Consists of two hydrocarbon chains (hydrocarbon tails)
Chemical structure of phospholipids
The structure can be represented simply using a circle for the phosphate group and two lines for the hydrocarbon chains.
What happens when phospholipids are mixed with water?
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Phospholipid bilayer
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Significance of Gorter and Grendel.
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Significance of Davson and Danielli.
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Significance of Singer and Nicolson.
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Peripheral protein
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Integral protein
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Significance of freeze-etched electron micrographs.
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Significance of structure of membrane proteins.
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Significance of fluorescent antibody tagging.
-An example of experimental evidence that did not fit with the Davson-Danielli model.
-Red or green fluorescent markers were attached to antibodies that bind to membrane proteins.
-The membrane proteins of some cells were tagged with red markers and other cells with green markers.
-The cells were fused together.
-Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.
-This showed that membrane proteins are free to move within the membrane rather than being fixed in a peripheral layers.
Primary function of cell membranes
To form a barrier through which ions and hydrophilic molecules cannot easily pass. This is carried out by the phospholipid bilayer.
6 functions of membrane proteins
1. Hormone binding sites (receptors), for example the insulin receptor.
2. Immobilized enzymes with the active site on the outside, for example in the small intestine.
3. Cell adhesion to form tight junctions between groups of cells in tissues and organs.
4. Cell to cell communication, for example receptors for neurotransmitters at synapses.
5. Channels for passive transport to allow hydrophilic particles across by facilitated diffusion.
6. Pumps for active transport which use ATP to move particles across the membrane.
Why/how are membrane proteins diverse?
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Structure of membranes
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Protein content of membranes
-The protein content of membranes is very variable, because the function of membranes varies.
-The more active a membrane, the higher is its protein content.
-The protein content of most plasma membranes on the outside of the cell is about 50%.
-The highest protein contents are in the membranes of chloroplasts and mitochondria, which are active in photosynthesis and respiration.
Membrane structure drawing
Diagram should show phospholipids, integral proteins, peripheral proteins and cholesterol.
Two main components of cell membranes
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Cholesterol
-Cholesterol is a type of lipid, but it is not a fat or oil.
-Instead it belongs to a group of substances called steroids.
-Most of ac cholesterol molecule is hydrophobic so it is attracted to the hydrophobic hydrocarbon tails in the centre of the membrane, but one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic.
-This is attracted to the phosphate heads on the periphery of the membrane.
-Cholesterol molecules are therefore positioned between phospholipids in the membrane.
-The amount of cholesterol in animal cell membranes varies.
Which of the three states of matter do cell membranes correspond to?
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Importance of membrane fluidity of animal cells needing to be carefully controlled
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Role of cholesterol in membrane
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Permeability
The state of a membrane that causes it to allow liquids or gases to pass through it. The membrane is said to be semi-permeable