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).

There are 2 ways a molecule can transported across a cell membrane: Passive & Active

  1. Passive Transport: does not require energy (ATP). Movement down a gradient.

1) Diffusion

                        2) Facilitated diffusion

                        3) Filtration

1) Diffusion

Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration. In other words, the molecule is moving down its concentration gradient. This is a passive transport mechanism. Getting in a kayak and going down stream with the river is an example of passive transport. No energy expenditure is required; you can just sit there and be moved down stream. In the body, the net movement of molecules continues down its gradient until equilibrium is reached. Diffusion can occur in open regions or across a partition such as a membrane.

Factors that Effect the Rate of Diffusion

Diffusion is a very common and important mode of transport in the human body. The oxygen (O2) that enters our blood stream from our lungs does so by simple diffusion. A very important issue in human physiology is what factors affect the rate of diffusion of a molecule from one side of a plasma membrane to the other. Listed below are some of the important factors that affect the rate (how quickly) diffusion takes place.

Some factors that effect the rate of diffusion:

  1. Size of the molecule (as indicated by its MW) – smaller = faster
  2. Distance – shorter = faster
  3. Temperature – warmer = faster
  4. Surface area of membrane – greater = faster
  5. Thickness of barrier – thinner = faster
  6. Steepness of concentration gradient – greater = faster

As we continue in physiology, all of these factors will be revisited in the various organ systems.

2) Facilitated Diffusion

Some molecules that are polar or too big to use simple diffusion to get across a membrane can use protein carriers to move down their concentration gradient. This requires no energy. The molecules must bind to the membrane carrier (as discussed above), so in this way in needs the ‘help’ of a carrier to move down its gradient across the membrane. As long as it is going down its concentration gradient, it is still diffusion. The term ‘facilitated’ indicates that the molecule is getting some assistance. As we have seen, this process is also prone to specificity, competition and saturation.

3) Filtration

Filtration is the net movement of water and solutes across a membrane due to the force of hydrostatic pressure. Hydrostatic pressure can be defined as the force of a fluid on the walls of its container. It can also be described as the force of gravity on a fluid. For example, if you place ground coffee on a paper filter and pour water over the top of it, the filter allows water and small solutes to pass through, but not the bigger coffee grinds. What you get on the other side is a filtrate of what was above, that is, anything small enough to pass through the holes of the filter, thus the term ‘filtration’. In the human body, the hydrostatic pressure of blood in a blood vessel pushes water and solutes across the blood vessel wall and into the interstitium. This is a normal function of certain blood vessels, as we shall see later!

  1. Active Transport: requires energy input (ATP)

1) Primary (direct) active

2) Secondary (indirect) active

                        3) Vesicular transport

Active transport requires the input of energy from ATP. This ATP id required because molecules are being moved up or against their concentration gradients. It is proteins that move these molecules against their concentration gradients. It is important to realize that creating and maintaining disequilibrium across a membrane in the body is very important and useful. Again, this requires the input of energy from ATP. The movement can be of one or more substances across a membrane:

  1. Uniport: When a protein moves only one (uni) kind of molecule.
  2. Co-transport: When a protein moves more than one molecule at a time.
  3. Symport: Moves molecules in the same direction.
  4. Antiport: Moves molecules in the opposite directions.

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