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Passive Assisted Diffusion (Facilitated Diffusion)

Literature Review

Diffusion of molecules through plasma membrane of cells would occur naturally unassisted

for molecules which has any of two characteristics, the molecules is a lipid-soluble molecules,thus it will be able to directly go through bilayer structure of plasma membrane or the

molecules must be small enough to go through protein channel, for example certain ion and

water. Molecules that aren’t small enough or water -soluble, such as protein, must pass the

membrane with different mechanism. Using unique protein called carrier-protein, this

mechanism called assisted membrane transport.

Carrier proteins are special protein planted in plasma membrane that stretch along the

thickness of membrane. These proteins have binding site that normally exposed to

extracellularfluid (ECF). Once specific mollecules are bound to its binding site, the protein

structure will transform resulting in it having its binding site exposed to intracellularfluid

(ICF). The molecule will then be released into ICF and the protein transform back into its

 previous form. This process can be reversed, from ICF to ECF.

This process may be passive by going down the gradient thus require no energy (facilitated

diffusion) or going against gradient thus require energy (active transport). Since molecules

are more likely to bind with carrier-protein binding site in a high-concentration than in low-

concentration, facilitated difussion requires no energy or ATP because the molecules binding

and releasing happen by themselves. For example in glucose transport. Cells regularly use

glucose for producing ATP while blood continuously supply fresh supplies of glucose.

Therefore the glucose concentration in ICF always lower than concentration in ECF. Glucose

will automatically bound into carrier- protein and automatically released once it’s exposed to

ICF since the concentration there is low. The carrier protein will then transform back into its

 previous form, which expose its binding site to ECF. As a result, facilitated diffusion requires

no energy.

While diffusion rate at unassisted diffusion has no limit for its only related to its

concentration, assisted diffussion has. Since carrier protein has switch between its two forms

to transport molecules, one carrier protein can only transport molecules at limited rate. As a

result, facilitated diffusion transport rate not only related to the molecules concentration, butalso limited by how many carrier protein the membrane has.

Another factor that may reduce the molecule transport rate is competition with another

similar molecule. Some type of carrier-proteins may bind with more than one type of

molecules. It may bind with more than one type of molecules with similar structure. If these

molecules present at same place, these molecules will compete at binding with protein’s

 binding-site and resulting in both molecules transport rates.

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Material and Method

The experiment is done using PhysioEx software version 8.0 which may be freely accessed at

http://media.pearsoncmg.com/bc/bc_physioex_8/experiments/index.htm. We ran this

experiment using “Facilitated Diffusion” section of the software designed specially to

simulate facilitated diffusion. The steps used are taken from “Penuntun Praktikum Fisiologi”

for “Cell and Genetics Module” by Faculty of Medicine Universitas Indonesia. 

Membrane used is created with 500 glucose carriers on its surface. The membrane then

 placed between two tanks and locked. We then put a solution consist of 2 mM of glucose into

the left tank to simulate ECF and deionized water in the right tank to simulate ICF. The

membrane cover then released so that the fluid will have contact with the membrane and

 placed again after 60 minutes. Experiment then repeated with using 300, 700, and 900

glucose carrier proteins with same glucose concentration. The experiment was then repeated

again using different glucose concentration, 8 mM. In addition, we also try placing same

amount of glucose concentration in both tanks.

Results

The result shows that the glucose-transport rate goes up as more carrier-proteins are available.

0.0005

0.0008

0.001

0.0012

0.00220.0023

0.0031

0.0038

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

300 CP 500 CP 700 CP 900 CP

Result Graphic

2 mM

8 mM