Edexcel AS Biology Unit 1 Topic 2.1

For Edexcel AS Biology Unit 1, Topic 2.1: The cell membrane- structure and function

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  • Created on: 19-05-13 20:01

Plasma Membrane

  • Fluid Mosaic Model
  • Plasma membrane is made up of:
    • Cholesterol
    • Proteins 
      • Peripheral: attached to surface of lipid bilayer
      • Integral: embedded in lipid bilayer
    • Phospholipids
    • Polysaccharides
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  • Made up of:
    • Two non-polar, hydrocarbon fatty acid tails
    • Glycerol group
    • Phosphate group
  • The head: 
    • Made from a glycerol group, which is attached to an ionised phosphate group
  • The tails:
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Cholesterol, glycoproteins and glycolipids

  • Cholesterol:
    • Disturbs the close-packing of the phospholipids, increasing the flexibility of the membrane
  • On outer surface of cell, antenna-like carbohydrate molecules form complexes with some of the membrane proteins (forming glycoproteins) and lipids (glycolipids). The functions of these may be:
    • Cell-cell recognition
    • Receptor sites for chemical signals
    • involved in the binding of cells into tissues
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The free passage of molecules from a region of their high concentration to a region of low concentration.

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Facilitated Diffusion

  • A substance that is otherwise unable to diffuse across the plasma membrane does so due to its effect on particular molecules present in the membrane
  • These molecules form into pores big enough for diffusion
  • They close up when the substance is no longer present
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Active Transport

  • Occurs against a concentration gradient
  • Requires energy from ATP
  • Involves 'pumps' (carrier proteins)
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Bulk Transport

  • Occurs through the movement of vesicles of matter across the membrane by cytosis
  • Uptake = endocytosis and export= exocytosis
  • Uses energy from metabolism (ATP)
  • Transports large particles and macromolecules
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Factors that speed up diffusion

  • The surface area available for gas exchange: the greater the repiratory surface's surface area, the greater the rate of diffusion
  • The difference in concentration: the greater the gradient in concentrationacross the respiratory surface, the greater the rate of diffusion
  • The length of the diffusion path: the shorter the diffusion path, the greater the rate of diffusion
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Specialized Respiratory Systems

  • Have:
    •  a very large area in a small space
    • a ventilation mehcnaism to maintain the supply of air or oxygenated water
  • May have:
    • an internal transport system to delvier oxygen to tissues
    • a respiratory pigment to improve the efficiency of transport
  • E.g
    • Gills (compact but with large surface area)
    • Lungs (large surface area)
    • Tracheae (internal tubes that divide and reach the cells of the body) (present in many insects)
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Respiratory Systems in Large Animals

  • Larger animals generally have specialised organs for gas exchange
  • They provide a large, thin surface are for gas exchange
  • Conditions for diffusion are also often improved by 3 refinements:
    • a ventilation mechanism: pumping mechanism that moves water or air over the gills or into and out of the lungs or tubes, which maintains the concentration gradient for diffusion
    • a blood circulatory system: accelerates the removal of dissolved oxygen from the respiratory surface as soon as it has diffused in, maintaining the concentration gradient
    • a respiratory pigment: increases the gas-carrying ability of the blood- e.g. haemoglobin
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Process of inspiration (inhalation) in mammals

  • Diaphragm: contracts and flattens, pushing down on abdomen
  • External Intercostal Muscles: contract, moving rib cage in and out
  • Internal Intercostal Muscles: relax
  • Thorax: volume increases, air pressure falls below atmospheric pressure
  • Air flow: in
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Process of expiration (exhalation) in mammals

  • Diaphragm: relax, pressure from abdominal contents pushes diaphragm back up into dome shape
  • External Intercostal Muscles: relax
  • Internal Intercostal Muscles: contract, moving rib cage down and in
  • Thorax: volume decreases, the air pressure rises above atmospheric pressure
  • Air flow: out
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