Cell Membrane Transport

CELL MEMBRANE Transport:

                                                           The cell membrane controls the interaction of the cell with the extracellular environment. All the materials that enter or leave the cell must pass through the cell membrane. Different substances are transported across the cell membrane by three major mechanisms:

1.  Passive transport.

2.  Active transport.

3. Vesicular transport.

1. Passive Transport:

                                     A type of transport, in which substances cross the cell membrane down their concentration gradient, from higher concentration region to low concentration region in which energy is not required in this process.

Passive transport involves two types of process:

v Passive diffusion.

v Facilitated diffusion.

A - Passive Diffusion: Some substances can pass through the lipid bilayer of cell membrane by simple diffusion down their concentration gradient, called passive diffusion or simple diffusion. Fats and fat-soluble molecules, small uncharged (hydrophobic) molecules, and dissolved gases (O2 and CO2).

B - Facilitated Diffusion: Water and water - soluble molecules like glucose and amino acids, and various sons (Na, K Ca and Cl) also cross the cell membrane down their concentration gradient but they require a special variety of transmembrane proteins that transport them, called channel proteins. These proteins form a hydrophilic-channels that regulate the movement of ions and specific molecules across the cell membrane. Some channels are gated.

Passive transport

2.  Active Transport:

                                     Cells can transport ions and small molecules against their concentration gradients by spending energy. Such energy requiring active-transport is mediated through a special variety of transmembrane proteins called carrier proteins. The best example of active transport is the continuous extrusion of Na⁺ out of the cell by the sodium pumps.

Active transport

3. Vesicular Transport:

                                         The mechanism by which large molecules and small particles enter, leave or move within the cell is called vascular transport. The vesicles are membrane - bound spherical structures which are formed either by originating from the cell membrane or from the membranous organelles of the cell. These vesicles move within the cytoplasm of the cell to reach target sites where these vesicle fuses with the target membrane and releases its contents.

The vesicular transport involving the two mechanisms:

A - Endocytosis: The term used for those processes of vesicular transport by which substances enter the cells. Generally, three mechanisms of endocytosis are recognized:

1)   Pinocytosis. (Also called fluid - phase endocytosis or potocytosis, is the process by which the fluids and small molecules enter the cell.)

2)   Receptor - mediated endocytosis. (The mechanism by which specific macromolecules are allowed enter the cell. This process depends on the presence of receptor proteins, called cargo receptors in the membrane.)

3)   Phagocytosis. (The process by which la particles, e.g, bacteria, cell debris and other unwanted extracellular materials, are engulfed by the phagocytic cells (which belong to the mononuclear phagocyte body)).

Endocytosis

B - Exocytosis: In secretory cells of the exocrine glands, the secretory products are released from the cell surface by exocytosis. The membrane - bound vesicles containing the secretory product originate in the cytoplasm of the cell and move toward the cell membrane to fuse with it. The cell membrane then opens at the site of fusion and the contents of the vesicle are discharged into the extracellular space.

Exocytosis

 

 

Read more… Cell Membrane Structure

                       Cell characteristics.