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.