Active Transport

VIII Active transport is the pumping of particles against their gradients

Active transport = Energy-requiring process during which a transport protein pumps a molecule across a membrane, against its concentration gradient.
  • Is energetically uphill and requires the cell to use energy.
  • Helps cells maintain steep ionic gradients across the cell membrane (e.g. Na+, K+, Mg+, Ca++, and Cl-)
  • Transport proteins involved in active transport harness energy from ATP to pump molecules against their concentration gradients.

An example of Active transport system that moves ions against steep concentration gradients is the sodium-potassium pump. Major features of the pump are:
  1. The transport protein alternates between two configurations.
    1. High affinity for Na+ (sodium) with binding sites toward cytoplasm
    2. High affinity for K+ (potassium) with binding sites toward cell's exterior.

  1. ATP adds a phosphate to the protein to power the conformational change.
  2. As the transport protein changes conformation, it moves bound particles across the membrane.
  3. Na+ K+ pump move 3 Na+ molecules out of the cell for every 2 K+ ions pumped into the cell.

IX. Some ion pumps generate voltage across membranes
Because positively and negatively charged ions are unequally distributed across the plasma membrane, all cells have voltages across their plasma membranes.

This difference in charge across a membrane allows multiple cellular process such as action potentials along nerve cells, and chemiosmosis which is used to produce ATP.

Particles are moved across a membrane to set up a charge difference by an electrogenic pump = a transport protein the generates voltage across a membrane.
  • Na+/K+ ATPase is the major electrogenic pump in animal cells.
  • a Proton pump is the major electrogenic pump in plants, bacteria, and fungi. Also, mitochondria and chloroplast use a proton pump to drive ATP synthesis.
  • Voltage created by electrogenic pumps are sources of potential energy available to do cellular work.2010_11_09_09_35_19.pdf000.jpg