Membranes

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Basic Functions of Cell Membrane

  • 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)

                          (http://images.tutorvista.com/content/cellular-micromolecules/lipid-bilayer-structure.jpeg)

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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.
  • Both are examples of exocytosis
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Pinocytosis and Phagocytosis (cont)

(http://upload.wikimedia.org/wikipedia/commons/thumb/b/b7/Pinocytosis.svg/250px-Pinocytosis.svg.png)(http://img.tfd.com/dorland/thumbs/phagocytosis.jpg)

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