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Jan 18, 2018


#"Active transport"# #to# #"uses carrier proteins only"#
#"Faciliated diffusion"# #to# #"uses both channel and carrier proteins."#


Hydrophobic(water-fearing) and non-polar molecules conveniently cross the plasma membrane by sliding from lipid bilayer. The small polar molecules e.g #C_2H_5OH# also get their way through plasma membrane. But entry of large polar molecules & ions into plasma membrane usually get restricted because of internal non-polar hydrophobic lipid bilayer

Then membrane proteins i.e carrier and channel proteins, embedded between these lipid bilayers are used.

Carrier proteins and Active transport:
Sometimes molecules & ions need to move against their concentration gradient which means from an area of lower concentration to an area of higher concentration by consuming energy. In this case, carrier proteins help them to accomplish their task and the process is called active transport.

  • In primary active transport ; ions & molecules move against their concentration by using the energy provided by hydrolysis of #"ATP"#.
    First of all the ion or molecule binds to a carrier protein at binding site. #"ATP"# released by mitochondria is hydrolyzed forming #"ADP"# and #"Pi"#. This phosphate binds to the carrier protein which then changes its conformation(structure) a bit. The molecule or ion is thus transferred from its crowded area to the one with low concentration. And the bound phosphate then is released from carrier protein causing it to flip back to its previous shape.
    Sodium-potassium #(Na^+#/#K^+)# pump.

  • In secondary active transport ; the co-transporter carrier proteins i.e sympoters and antipoters are used. In this case, #"ATP"# is not used as an energy source.
    In a nutshell, one solute molecule is transported against its concentration gradient by utilizing the energy generated by gradient of another solute which is transported along it's concentration gradient.
    Glucose and amino acids are reabsorbed from kidney tubules via secondary active transport.

Facilitated Diffusion and Channel proteins:
As it is a type of diffusion so it would definitely be along the concentration or electrochemical gradient i.e from an area of higher concentration to the one with lower concentration. The only difference is that it uses membrane proteins while diffusion doesn't.

  • Channel proteins essentially allow movement of certain ions and water molecules down their concentration gradient. Each type of channel protein is very selective for passage of a specific substance.
  • Through Simple channel proteins ions & molecules simply slide. But some channel proteins may be gated. They need electrical or chemical stimulus to open.

Facilitated Diffusion and Carrier Proteins:
Facilitated diffusion uses uniporters as carrier proteins. Uniporters normally transport organic molecules, such as sugars and amino acids.

  • The specific molecule attach to the uniporter(carrier protein) at its binding site. The carrier protein then flip over and releases the molecule down to the area with lower concentration.

Hope it helps...

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