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 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.
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.
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.