Passive Transport

Simple transport is the movement of a substance across a membrane, due to a difference in concentration, without any help from other molecules. The difference in the concentrations of the molecules in the two different areas is known as the concentration gradient. The substance moves from the side of the membrane where it is in more concentration to the side where it is less concentrated. This transport will continue until this gradient has been eliminated. Transport moves materials from an area of higher concentration to an area of lower concentration. Hence, it is referred to as moving solutes "down the concentration gradient."

The end result of transport is an equal concentration, or equilibrium, of molecules on both the sides of the membrane. Those substances that can squeeze between the lipid molecules in the plasma membrane by simple transport are usually very small, hydrophobic molecules. Examples of such molecules include the molecules of oxygen and carbon dioxide. Figure 1 describes simple passive transport of dye into the water.

The cytoplasm of a cell contains ions and molecules such as sugars and amino acids dissolved in water. The mixture of these substances and water is called an aqueous solution. Water, the most common of the molecules in the mixture, is the solvent, and the substances dissolved in the water are solutes. The ability of water and solutes to diffuse across membranes has important consequences.

Both water and solutes spread from high concentration regions to low concentration regions, that is, they spread their concentration gradients downwards. When a membrane separates two regions, the result depends on whether or not the solutes can cross that membrane freely. Most solutes, including ions and sugars, are not lipid-soluble and, therefore, are unable to cross the lipid bilayer of the membrane. 

Even water molecules which are very polar are not able to cross a lipid bilayer. Water flows through aquaporins which are specialized channels for water. A simple experiment demonstrates this. If we place an amphibian egg in hypotonic spring water, it does not swell. If we then inject aquaporin mRNA into the egg, the channel proteins are expressed and the egg swells. Dissolved solutes interact with water molecules, which form hydration shells about the charged solute. When there is a concentration gradient of solutes, the solutes will move from a high concentration to a low concentration, dragging with them their hydration shells of water molecules. When a membrane separates two solutions, hydration shell water molecules move with the diffusing ions, creating a net movement of water towards the low solute. This net water movement across a membrane by diffusion is called osmosis (figure 2).

The concentration of all solutes in a solution determines the osmotic concentration of the solution. If two solutions have unequal osmotic concentrations, the solution with the higher concentration is hyperosmotic, and the solution with the lower concentration is hypoosmotic. If the osmotic concentrations of two solutions are equal, the solutions are isosmotic.
In cells, a plasma membrane separates two aqueous solutions, one inside the cell and one outside. The direction of the net diffusion of water across this membrane is determined by the osmotic concentrations of the solutions on either sides. For example, if the cytoplasm of a cell were hypoosmotic to the extracellular fluid, water would diffuse out of the cell toward the solution with the higher concentration of solutes. This loss of water from the cytoplasm would cause the cell to shrink until the osmotic concentrations of the cytoplasm and the extracellular fluid becomes equal.

Carriers refer to the class of membrane proteins, transport ions, as well as other solutes throughout the membrane, such as sugars and amino acids. Carriers are like channels, are specific to a particular type of solution and can transport substances across the membrane in either direction. If the cytoplasm concentration is higher, the solute on the cytoplasmic side of the membrane is more likely to bind to the carrier and release on the extracellular side. If the extracellular fluid has a higher concentration, the net movement will be from outside to inside. Thus, net movement always occurs from high to low concentration areas, just as it takes place in simple transport, but the process is facilitated by carriers. Hence, this transport mechanism is referred to as facilitated transport.

Various examples of facilitated transport by carrier proteins can be found in the membranes of vertebrate red blood cells (RBCs). One RBC carrier protein, for example, transports a different molecule in each direction: Cl^– in one direction and bicarbonate ion (HCO3^–) in the opposite direction. This carrier is important in transporting carbon dioxide in the blood.
Another important facilitated transport carrier in red blood cells is the glucose transporter. RBC keep their internal concentration of glucose low through a chemical trick: they immediately add a phosphate group to any entering glucose molecule converting it to highly charged glucose phosphate that cannot pass back across the membrane. This maintains a steep concentration gradient for glucose, favouring its entry into the cell. Instead, the transmembrane protein appears to bind the glucose and then flip its shape, dragging the glucose through the bilayer and releasing it on the inside of the plasma membrane. Once it releases the glucose, the glucose transporter reverts to its original shape. Figure 3 describes the difference between simple transport and facilitated transport.

The characteristic of selective channel transport is that its rate is saturable. In other words, if a substance's concentration gradient is gradually increased, it will also increase its transport rate to a certain point and then level off. Further gradient increases will not result in any additional increase in the rate. The scientific explanation for this kind of observation is that the membrane contains a limited number of carriers.

Facilitated transport has three main features: