The cell membrane is a delicate organ of the cell that regulates the movement of substances into and outside the cell. Cell membrane transport occurs in two major ways like
1. Passive transport
- Passive diffusion
- Facilitated diffusion
- Osmosis.
2. Active transport.
- Sodium potassium pump
- Bulk transport (phagocytosis and pinocytosis)
Cell Membrane Transport
The structure of the cell membrane is designed so 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 the cell membrane is classified as
Passive transport.
This is transport that occurs without the use of energy.
Here the substances move from a region of higher concentration to a lower concentration.
This is of three types
a) Passive diffusion:
Here the solute molecules move from a region of higher concentration to a region of lower concentration.
This diffusion occurs until the concentration of substance indie and outside the cell is equal.
Small molecules move down the concentration gradient through the plasma membrane by diffusion. This is possible due to the permeable nature of the membrane. The substance which diffuses include
Gases like oxygen and carbon dioxide.
Lipids like fatty acids and steroids diffuse by dissolving in the lipid part of the membrane.
Ions of sodium, potassium, and calcium diffuse through water-filled channels.
Some substances that cannot diffuse across the membrane pass by facilitated diffusion.
b) Facilitated diffusion (passive-mediated transport)
This route is used by those materials that cannot diffuse across the cell membrane without some aid.
For this, specialized carrier protein molecules help in moving substances 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.
Examples of substances using this route are glucose and amino acids.
The carrier channels are specific for one particular substance and also are limited.
Hence, the rate of transport is dependent on the availability of free carrier proteins.
This limitation of the number of molecules that can be transported in a given time is called transport maximum.
c) Osmosis
This process is similar to diffusion, but here instead of the solute, the solvent moves down the concentration gradient.
Here the water molecules move from a region of lower solute concentration to a region of higher solute concentration.
When the solute molecules are large and unable to diffuse, this osmosis occurs.
Like diffusion, this osmosis occurs until the equilibrium is reached.
Thus, in all the above three methods, like passive diffusion, facilitated diffusion, and osmosis, there is no energy spent on transport.
However, in the next methods, transport across the membrane occurs through the use of energy (ATP).
Active transport
In this process, the substances move from one region with a high concentration to another with a low concentration across the cell membrane.
So, this is against the concentration gradient, and hence, chemical energy in the form of ATP is spent.
This is of the following types like
Sodium potassium pump
Here the movement of solutes occurs by the use of energy in the form of ATP. However, the mechanism involves the movement of one sodium ion into an exchange of potassium ions outside. This method is said to use 30% of cellular ATP requirements.
Na+ is higher in concentration on the outside of the cell, while K+ is in higher concentration on the inside. But due to the 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+.
Bulk transport
The above-described methods are suitable for small-size molecules. But for large particles, the transport occurs by bulk transport. Here the particle is engulfed in the cytoplasm. Solid engulfment is called phagocytosis (cell eating), while for the liquid, it is called 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 from the inside occurs through the reverse process. This is called exocytosis.
Related Questions
1. why is membrane transport vital for normal cell function?
Ans: In a normal cell, membrane transport is vital for the movement of glucose and amino acids into the cells for the production of energy and protein synthesis, respectively.
In the nerve cell, the conduction of nerve impulses occurs by polarization, depolarization, and repolarization processes. The process of repolarization involves the movement of K+ (Potassium ions) into the nerve against the expulsion of Na+ (Sodium ion).
This process is facilitated by the action of a Sodium-potassium pump by the use of almost 30% of cellular energy.
2. What facilitates passive transport across a cell membrane?
Ans: The Carrier proteins facilitate passive movement across the cell membrane. This is possible due to the concentration gradient across the membrane.
3. Which form of passive transport allows small molecules, such as oxygen, to cross the cell membrane?
Ans: As discussed above, small molecules like oxygen, carbon-dioxide move across the membrane by passive diffusion.
4. Why does the cell membrane have transport proteins?
Ans: The cell membrane has transport proteins to facilitate the movement of molecules by passive facilitated diffusion or active transport. Molecules like glucose move by transporting protein by the passive process.
In contrast, potassium and sodium ions move across the nerve membrane against the concentration gradient through the transport of proteins by an active process.
5. How do carrier proteins transport substances across cell membranes?
When the molecules bind to a specific site of the carrier protein, the carrier proteins under a change in their shape, leading to the movement of molecules from one side to another side.
6. Explain why carbon dioxide can cross a cell membrane without the aid of a transport protein.
Carbon dioxide and oxygen get dissolved in the lipid fraction of the cell membrane and get diffused across. Hence, CO2 does not help transport protein.
