Cell membrane is a delicate organ of the cell which regulates movement of substances into and outside the cell. The cell membrane transport occurs in two major ways like
1. Passive transport
- Passive diffusion
- Facilitated diffusion
2. Active transport.
- Sodium potassium pump
- Bulk transport (phagocytosis and pinocytosis)
Cell Membrane Transport
The structure of cell membrane is designed in such a way that it does not allow free movement of substances. However, it is semipermeable due to which certain substances can still move in and out of the cell. Based on the mechanism of movement, the transport across cell membrane is classified as
This is transport where the substances move from a region of higher concentration to lower concentration without the use of energy.
This is of three types like
a) Passive diffusion:
Here the solute molecules move from a region of higher concentration to the region of lower concentration. Small molecules move down the concentration gradient like
Oxygen, carbon dioxide, fatty acids diffuse by dissolving in the lipid part of the membrane.
b) Facilitated diffusion (passive-mediated transport)
This route is used by those materials which cannot diffuse across the cell membrane without some aid.
Specialized carrier protein molecules help in moving substance from one side of the membrane to the other. When the substance molecules bind, the carrier protein changes its shape so that the molecules move to the other end of the channel in the protein.
The specialized carrier proteins are limited. Hence, the rate of transport is dependent on the availability of free carrier proteins.
This is a process of movement of water molecules from a region of lower concentration to a region of higher concentration.
This is a method of transport where the substances move from one region to other region across cell membrane by use of chemical energy in the form of an ATP. This is of following types like
Sodium potassium pump
Here the movement of solutes occurs by use of energy in the form of ATP. However, the mechanism involves movement of one sodium ion into a side for an exchange of potassium ion to outside. This method is said to use 30% of cellular ATP requirement.
Na+ is higher in concentration on the outside of the cell while K+ is in higher concentration on the inside. But due to concentration gradient, the ions tend to move to the other side. For this excess Na+ is kept on higher levels by being constantly pumped out in exchange for K+.
The above described methods are suitable for small size molecules. But for large particles, the transport occurs by bulk transport. Here the particle are engulfed into the cytoplasm. Solid engulfment is called phagocytosis (cell eating) while for the liquid, it is called a pinocytosis (Cell drinking).
In this process the solid material is entrapped in a membranous vacuole. The lysosomes bind to these vacuoles and release the lysosomal enzymes which digest the material inside the vacuole.
The removal of waste matter form inside occurs by reverse process. This is called as exocytosis.