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Crossing Membranes

  • Substances need to move around the body of an organism. To do this they must cross Biological Membranes. There are a number of ways in which this may happen.

Passive Transport

  • When placed in a system, a substance will tend to even out - it will move from a region of high concentration to a region of low concentration. This is called Diffusion. Within a system, each molecule has its own Concentration Gradient, and therefore will Diffuse irrespective of other molecules.
  • Whenever Diffusion occurs, molecules will always be moving in both directions, but it there is a Concentration Gradient, their Net Movement will be towards the region of low concentration.
  • Substances can be transported across Biological Membranes by Diffusion. In cells, a number of processes may ensure that the Concentration Gradient is maintained and the Equilibrium is not met. Useful substances are constantly being used up and waste products are constantly being produced.
  • Lipid based molecules are able to diffuse across Biological Membranes unaided, since they are Hydrophobic, and are not repelled by the Phospholipid 'heads'.
  • Very small molecules are small enough to pass through the Phospholipid Bilayer, even if they are charged. Water can diffuse in this way, as can Carbon Dioxide and Oxygen.
  • Some charged molecules normally are unable to pass through Biological Membranes since they are repelled by the Hydrophobic Phosphate 'heads'. However, special Proteins, called Channel Proteins, provide pores though which these molecules can diffuse. These Proteins are often shaped in such a way as to only allow certain molecules to pass through.
  • Larger molecules, like Glucose, are too big to diffuse through the Phospholipid Bilayer unaided. These require special Proteins in the Membrane called Carrier Proteins. Once a specific molecule collides with the Carrier Protein, it 'flip flops' carrying the molecule to the other side of the membrane.
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Active Processes

  • Sometimes substances are required to be moved against the Concentration Gradient, or faster than they would by Passive Transport. For example, plants need to take up Magnesium Ions in their roots against the Concentration Gradient. In these cases, Active Processes are used, which require energy.

Active Transport

  • Active Transport involves the movement of molecules across a Membrane using Proteins in the Bilayer similar to Carrier Proteins. These use energy in the form of ATP to 'pump' molecules in one direction across a membrane.
  • The Carrier Proteins involved in Active Transport are Complementary to the molecule they transport. Once the molecule collides with the Protein, it is transported across the membrane and released the other side. It cannot then be transported back since the Carrier Protein is no longer Complementary to it.
  • Active Transport means that a substance can move against the Concentration Gradient and at a much faster rate than Diffusion alone.
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Bulk Transport

  • Bulk Transport involves the movement of large amounts of molecules across a membrane. For example, cells that produce Hormones in large quantities transport them out in bulk. This is done via the movement of Vesicles within the cell, which can easily fuse of be 'pinched off' from a membrane.
  • The movement of substances into a cell is called Endocytosis, and the movement of substances out of a cell is called Exocytosis. The prefixes 'phago' and 'pino' can be used to donate the movement of solid and liquid material respectively.
  • Energy, in the form of ATP, is used in the movement of Vesicles around the cell and fusion with, and pinching off from, the membrane.
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