go
back to Diffusion 2Diffusion III
The cell membrane acts as a diffusion barrier to the movement of substances into and out of the cell. Because of the hydrophobic core of the lipid bilayer that forms the base of the membrane, only substances that are hydrophobic will be able to pass through the membrane by the process of simple diffusion. In general, all hydrophilic substances will not be able to pass through this hydrophobic core (the one exception is water which is permeable through most membranes).
How do hydrophilic substances get across the membrane? There are two major mechanisms by which this is accomplished.
1. Pores or Channels: For small ions such as sodium potassium, chloride, calcium, etc., membranes may have pores or channels which serve as holes or tunnels through the hydrophobic core of the membrane. These channels are integral membrane proteins that span the membrane and are generally specific for a given ion (thus there can be a sodium channel and a potassium channel). In addition, there can be several types of a channel for a given ion, for example, there are over five different types of potassium channels found in some cells. Movement through the channels is through diffusion (simple diffusion) and thus the direction of movement (into or out of the cell) is determined solely by the concentration gradient. It is important to remember that pores or channels only exist for small inorganic ions and not for larger biochemicals such as glucose or amino acids.
2. Carrier Proteins: Carrier proteins are integral membrane proteins that bind to specific molecules or ions and transports them through the lipid bilayer of the membrane. Unlike pores or channels that can only transport small ions, carrier proteins can also transport small organic molecules such as glucose and amino acids (although a given carrier protein can are specific for a single substance). Transport across the membrane utilizing carrier proteins is usually bidirectional (can move the substance into or out of the cell) which is dependent upon the concentration gradient - thus it is diffusion! (There is another process that utilizes carrier proteins that is able to move substances against the concentration gradient but more of that later). This type of diffusion is termed carrier mediated diffusion. Carrier meidated diffusion differs from simple diffusion directly through the membrane in two primary ways:
a. Specificity: Carrier proteins are very specific for the molecule that is transported. For example, glucose carriers only are able to transport glucose.
b. Saturation: There are only a certain number of carrier proteins for a given solute and under the conditions of high solute concentration, all of the carriers may be being used, thus there is a maximum rate of transport which is determined by the number of available carrier proteins. Niether simple diffusion nor diffusion through pores can be saturated.
If you imagine that the membrane is a barrier to transport similar to a river, you can imagine that the pores or channels are similar to bridges or tunnels; carrier proteins are like ferry boats. Also, you should understand that both pores (and channels) and carrier proteins can be regulated. That means the cell may have mechanisms that turn these proteins on or off. For example, the glucose carriers found in kidney cells require the presence of the hormone insulin in order to transport glucose into the kidney cells. When there is no insulin, the glucose carriers will not transport glucose (they are still found in the cells, they just no longer will transport glucose across the membrane). Also, remember that the presence of these transport mechanisms does not change the fact that diffusion is the driving force - thus, as always, High to Low.
go
back to Diffusion 2
Take a diffusion quiz
