Specialized trans-membrane proteins recognize the substance and allows it access (or, in the case of secondary transport, expend energy on forcing it) to cross the membrane when it otherwise would not, either because it is one to which the phospholipid bilayer of the membrane is impermeable or because it is moved in the direction of the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, normally uses the chemical energy of ATP. The other cases, which usually derive their energy through exploitation of an electrochemical gradient, are known as secondary active transport and involve pore-forming proteins that form channels through the cell membrane.
Sometimes the system transports one substance in one direction at the same time as cotransporting another substance in the other direction. This is called antiport. Symport is the name if two substrates are being transported in the same direction across the membrane. Antiport and symport are associated with secondary active transport, meaning that one of the two substances is transported in the direction of its concentration gradient utilizing the energy derived from the transport of second substance (mostly Na+, K+ or H+) down its concentration gradient.
Particles moving from areas of low concentration to areas of high concentration (i.e., in the opposite direction as the concentration gradient) require specific trans-membrane carrier proteins. These proteins have receptors that bind to specific molecules (e.g., glucose) and thus transport them into the cell. Because energy is required for this process, it is known as 'active' transport. Examples of active transport include the transportation of sodium out of the cell and potassium into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine.
Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient.
Read more about this topic: Secondary Active Transport
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