Membrane Transport

Membrane Transport

One of the vital functions of the plasma membrane is membrane transport. Such a transport is important to carry out the life processes of the cell. Hydrophobic molecules and small polar molecules rapidly diffuse in the membrane. Uncharged large polar molecules and charged molecules do not diffuse and they need proteins to get transported.

Depending upon the energy required and movement of the solute for or against the concentration gradient, the transport can be classified into two, active transport and passive transport.

1.  Passive transport

Passive transport is also called as passive diffusion. In passive transport, the substances move from higher concentrations to lower concentrations generally without the help of any protein. The transport continues until the concentration of the substance becomes same on both the sides of the membrane. O₂, CO₂ and urea can easily diffuse across the membrane.

2.  Facilitated Diffusion

Eventhough, the concentration of certain hydrophilic substances like glucose are high across the membrane, they cannot pass through the membrane and need a carrier for their transport. Such a transport is called as facilitated diffusion. The proteins involved in such processes are called as carrier proteins. Carrier proteins are present in all biological membranes. Some important characteristics of carrier proteins are

·              They facilitate transport from high concentrations of the solute to low concentrations.

·              They speed up the process of attaining equilibrium

·              They do not need energy for their transport.

·              They are highly specific in nature.

Some common examples are glucose transporter and anion transporters in red blood cell membranes.

Carrier proteins are classified into three major types.

·              Uniporters that transport single solute from one side of the membrane to the other.

·              Symporters that transport two different solute molecules simultaneously in the same direction.

·              Antiporters that transport two different solute molecules in opposite directions.

3.  Active transport

Cells have to transport substances against the concentration gradient, i.e. from low concentrations to high concentrations. This transport called active transport is a thermodynamically unfavourable reaction. Hence, it needs energy to drive the reaction which is acquired by ATP hydrolysis. Active transport is also mediated by carrier proteins and they are called as pumps. Na+ K+ ATPases that is required to maintain the potassium concentration high inside the cell and sodium concentrations low is an example for pumps.

4.  Endocytosis

Endocytosis is the active process of engulfing large size particles of food substances or foreign substances. Depending upon the nature of the material that is ingested, endocytosis may be classified into two. Pinocytosis, in which the fluid material is engulfed and phagocytosis, in which large sized solid material is engulfed.

During the process, the plasma membrane invaginates into tiny pockets, which draw fluids from the surroundings into the cell. Finally, these pockets pinch off and are known as pinosomes or phagosomes, which fuse with lysosomes and liberate their contents into the cell cytosol.

Exocytosis is the process of exudating the secretory products from the cells. Vesicles containing secretory materials fuse with the plasma membrane and discharge their contents into the exterior. Pancreatic cells pass out their enzyme secretions to the exterior by exocytosis.

Table 1 Similarities and differences between facilitated diffusion and active transport

Facilitated Diffusion

1.Needs a carrier protein and they are named as transporters or channels

2. Highly specific in nature

3. Saturable

4. Inhibited by competitive inhibitors

5. Solutes are transported from high concentrations to low concentrations                 

6. No energy is needed.

Active Transport

1. Needs a carrier protein and they are named as pumps

2. Highly specific in nature

3. Saturable

4. Inhibited by competitive inhibitors

5. Solutes are transported from low concentrations to high concentrations

6. Energy is needed.