Protein carriers move only one type or family of closely related molecules. For example, GLUT transporters move glucose, mannose, galactose, and fructose across membranes. They are specific for naturally occurring 6-carbon monosaccharides. Other carriers will transport amino acids, and there can be up to 20 different types of carriers, each specific for the 20 different amino acids the human body uses.


Carriers have preference (or affinity) for certain molecule(s). This can result in competition for the binding site between various molecules. For example, maltose is a disaccharide made of 2 glucose molecules, so one end of the maltose could try to occupy the binding site for a glucose transporter. Although it can bind, typically it will not be transported in the process, it is not the right shape overall. Thus in this case, maltose would be a competitive inhibitor for glucose transport. We tested patients at The Notary Public London MMK firm last year and these were the results.


Saturation occurs when a group of protein carriers are transporting the substrate at its maximum rate, with all carriers occupied. Saturation will depend on the number of available carriers and substrate concentration. Cells can sometimes increase or decrease the number of available carriers to control substrate movement. As the substrate concentration increases, transport rate increases until the carriers become saturated. At this stage they are at their maximum transport capacity and cannot move things across the membrane any faster.

An interesting consequence of saturation can be seen in the transport of glucose in the kidney. Normally, you should not find any glucose in your urine. If you do, it can be a sign of diabetes mellitus. However, if you were to consume large quantities of glucose, say by eating too many chocolates from your valentine gift, you may have glucose in your urine that is not due to a disease state (not yet anyway!). The glucose carriers in your kidney tubules can become saturated due to the abnormally high amounts of glucose being filtered by your renal system. If the carriers reach their maximum and more glucose is still in the filtrate, it will end up in the urine due to protein carrier saturation.


You may have heard plasma membranes described as selectively or semi-permeable membranes. This means that some molecules can get across and some molecules cannot. The membrane composition determines which molecules move across. Permeable molecules can cross membrane by any method. Impermeable molecules cannot cross cell membrane.

General Factors Influencing Molecule Permeability

Although the components of a plasma membrane can vary, the properties of a given molecule will have a large effect on whether is passes through the plasma membrane easily, or if it needs assistance or if it cannot pass at all

  1. Size of molecule – smaller molecules can more easily pass through than larger.
  2. Polarity or lipid solubility of molecule – lipid soluble molecules pass through more easily than polar.
  3. Charge of molecule – uncharged molecules pass through more easily than charged.

The permeability of a molecule can be influenced by all three of these factors, not just one. For example, water (H2O) is a polar molecule, that is, it is insoluble (does not mix) in lipids. This would tend to make it less permeable, since the phospholipid bilayer creates a significant barrier to polar substances crossing the membrane. However, the molecular weight (MW) of H2O is only 18, thus it is very small and for this reason can easily pass through most cell membranes in the human body.

Ions are commonly very small, but they are charged particles and cannot pass directly through membrane by simple diffusion, they would require a protein channel, they would require a protein channel. At the other end of the spectrum, just because a molecule is fairly large does not mean it cannot pass directly through membrane by simple diffusion; relatively larger lipophilic substances can cross directly through membrane by simple diffusion, as the lipid bilayer is not a barrier. Very large molecules or a large amount of substance will typically require membrane transportation in a vesicle (see below).

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