Active Transport

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•  Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport

Active transport uses energy to move molecules against a concentration gradient

This energy may either be generated by: 

  • The direct hydrolysis of ATP (primary active transport)
  • Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

  • A specific solute will bind to the protein pump on one side of the membrane
  • The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
  • The solute molecule is consequently translocated across the membrane (against the gradient) and released

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•  Structure and function of sodium-potassium pumps for active transport and potassium channels for 

   facilitated diffusion in axons

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

  • At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
  • When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Sodium-Potassium Pump

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

The process of ion exchange against the gradient is energy-dependent and involves a number of key steps:

  1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
  2. A phosphate group is transferred to the pump via the hydrolysis of ATP
  3. The pump undergoes a conformational change, translocating sodium across the membrane
  4. The conformational change exposes two potassium binding sites on the extracellular surface of the pump
  5. The phosphate group is released which causes the pump to return to its original conformation
  6. This translocates the potassium across the membrane, completing the ion exchange

Steps Involved in Ion Exchange via a Sodium-Potassium Pump

sodium-potassium pump