Cells primarily need membranes to separate their internal contents from their external environment.
The membrane also has special metabolic pathways to control movement of substances in and out of cell.
There are also receptor sites on the membrane to recognise external stimuli (e.g hormones).
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Membrane Composition
Membranes are arranged in a phospholipid bilayer.
The head group of each phospholipid is polar and hydrophilic.
The tail is non-polar and hydrophobic.
The tails pack together tightly because of Van der Waals forces between them.
The lipid bilayer is energetically favourable to form, so it forms spontaneously.
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Membrane Composition (cont)
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Membrane Composition (cont)
The fatty acid tails may have double bonds (unsaturated). The double bonds cause kinks, as it is harder to make a regular packed structure.
This increases membrane permeability, and fluidity.
Cholestrol fits between kinks in unsaturated hydrocarbon chains, decreasing permeability.
The bilayer is a dynamic structure, in which the phospholipids are constantly moving, and proteins can move about freely.
This is known as the fluid mosaic model.
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Membrane Composition (cont)
Glycolipids are lipids with a carbohydrate on the extracellular side. They help protect epithilial cells, are are involved in cell-recognition processes.
Integral proteins span through the entire membrane, peripheral proteins bond little with membrane.
They can act as channel proteins, transport proteins, receptors or recognition sites.
Glycoproteins are proteins with a carbohydrate on the extracellular side. They are important in cell interactions, cell recognition, and as receptors. They stabalise the membrane structure (H bonds).
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Transport Across Membranes
Simple diffusion is the net passive movement of particles from a region of high concentration to a region of low concentration, until equilibrium is reached.
Facilitated diffusion is the net passive movement of particles down the concentration gradient. Still no enery required. A channel protein is required, which contains a hydrophilic pathway. It is specific to certain molecules.
A transfer of kinetic energy by particles allows the channel to make comformation change (open/close)
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Transport Across Membranes (cont)
Active transport is the active movement of particles from a low to a high region of concentration.
Energy is required, which comes from ATP (adenosine tri-phosphate).
The process across membranes requires protein transporters.
A key example is the sodium-potassium pump, which pumpes Na+ in and K+ out, both against the concentration gradient.
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Transport Across Membranes (cont)
Osmosis is the net passive momement of water molecules from a region of high water potential to a region of low water potential across a semi-permeable membrane.
When in high water potential (hypotonic solution), water moves into a cell. An animal cell becomes lysed (bursts), while a plant cell becomes turgid.
When in low water potential (hypertonic solution), water moves out of cell. An animal cell becomes crenated (shrivelled), while a plant cell becomes flaccid, then plasmolised (loses cell wall).
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Pinocytosis and Phagocytosis
Pinocytosis is cell drinking- liquids are taken in via vesicles. It is an active process.
Phagocytosis is cell eating- the membrane engulfs solid food into a giant vesicle. It is an active process.
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