Contents
Evolutionary Biology Topics allow us to trace the history of life on Earth.
The Cell Membrane: Passive and Active Transport
Mediated Transport
We have seen that the cell/plasma membrane acts as an effective barrier to the free diffusion of most molecules of biological significance. Yet, it is essential that some materials enter and leave the cell. Nutrients such as sugars and materials of growth such as amino acids must enter the cell, and the wastes of metabolism must be thrown out. Such molecules are moved across the membrane by special proteins called transport proteins or permeases. Permeases form a small passageway through the membrane, enabling the solute molecule to cross the phospholipid bilayer. Permeases are usually quite specific, only a limited group of chemical substances or perhaps even a single substance they recognize and transport.
Types of Mediated Transport
It is of the following two types:
- Facilitated transport/diffusion: In this case, the permease assists a molecule to diffuse through the membrane that it cannot otherwise penetrate.
- Active transport: In this case, the energy is supplied to the system (called a pump) to transport molecules in a direction opposite to a concentration gradient.
Facilitated diffusion, therefore, differs from active transport in that it promotes movement in a downhill direction (i.e., in the direction of concentration gradient) only and requires no metabolic energy to drive the transport system. In many animals, facilitated transport (or facilitated diffusion) aids in the transport of glucose (blood sugar) into the body cells that oxidize it to get ATPs. The concentration of glucose is greater in the blood than in the cells that consume it, favouring inward diffusion. Glucose is a water-soluble molecule that by itself is unable to penetrate the membrane rapidly enough to support the metabolism of many cells. The carrier system (i.e., mediated transport) increases the inward flow of glucose.
Differences between Active Transport and Diffusion
| Active Transport | Diffusion |
| 1. It is a rapid process. | 1. It is a slow process. |
| 2. It can move materials through a biological (cellular) membrane against the concentration gradient. | 2. It can move materials across a biomembrane down the concentration gradient. |
| 3. It takes place in one direction only. | 3. It takes place in both directions. |
| 4. It needs carrier (or transport) proteins to occur. | 4. It occurs without the help of carrier proteins. |
| 5. It uses the energy of ATP. | 5. It does not use energy. |
| 6. It brings about selective uptake of materials. | 6. It allows all transmissible molecules to pass through the membrane. |
| 7. It leads to the accumulation of materials in the cell. | 7. It does not accumulate material in the cell. |
In active transport, molecules are moved uphill against the forces of passive diffusion. Active transport always involves the expenditure of energy (from ATP) because materials are pumped against the concentration gradient. The most important active transport system in all animals is those, that maintain sodium and potassium gradients between cells and the surrounding extracellular fluid or external environment.
Most animal cells require a high internal concentration of potassium ions for protein synthesis by the ribosomes and for certain enzymatic functions. The potassium ion concentration maybe 20 to 50 times greater inside the cell than outside. Sodium ions, on the other hand, maybe 10 times more concentrated outside the cell than inside. Both of these ionic gradients are maintained by the active transport of potassium ions into and sodium ions out of the cell.