The cell membrane is an organ of the cell that regulates the movement of substances into and outside the cell. The transport across it occurs in two significant ways, like
1. Passive transport
- Passive diffusion
- Pore transport
- Ion-pair transport
- Facilitated diffusion
- Osmosis.
2. Active transport.
- Sodium potassium pump
- Bulk transport (phagocytosis and pinocytosis)
Transport across the Cell Membrane
- The structure of the cell membrane is such that it does not allow free movement of substances.
- However, it is permeable to a few substances to enable them to 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 a transport process that occurs without the usage of energy.
- Here, the substances move from a region of higher concentration to a lower concentration.
This is of the following types
A) Passive diffusion
- Here, the solute molecules move from a region of higher concentration to an area of lower concentration.
- This diffusion occurs until the concentration of substance inside 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 that 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.
Further, substances that cannot diffuse across the membrane pass by facilitated diffusion.
B) Pore transport
- Here, the substances move through protein channels or pores meant for them.
- The pores permit the movement of specific molecules through diffusion.
c) Ion-pair transport
- Here, an ion moves concurrently through the pore with an opposite-charged ion.
As such, the ion finds it hard to move alone due to charge and hence gets through along with the oppositely charged ion.
D) 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 transport rate depends 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 only the water molecules move down the concentration gradient instead of the solute.
- The water molecules move from a region having a lower solute concentration region to a higher solute concentration.
- This osmosis occurs when the solute molecules are large and unable to diffuse.
- Like diffusion, this osmosis occurs until the equilibrium is reached.
- Thus, no energy is spent on transport using all three methods: passive diffusion, facilitated diffusion, and osmosis.
However, in the following methods, transport across the membrane occurs without 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; 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 using ATP as energy.
- 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 outside the cell, while K+ is higher on the inside.
- However, the ions tend to move to the other side due to the concentration gradient.
- 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.
- However, 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.
Frequently Asked Questions and Answers.
Why is membrane transport vital for normal cell function?
In a normal cell, membrane transport is vital for moving glucose and amino acids into the cells to produce energy and protein synthesis.
In the nerve cell, the conduction of nerve impulses occurs by polarization, depolarization, and repolarization processes. 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 using almost 30% of cellular energy.
What facilitates passive transport across a cell membrane?
The Carrier proteins facilitate passive movement across the cell membrane. This is possible due to the concentration gradient across the membrane.
Which form of passive transport allows small molecules, such as oxygen, to cross the cell membrane?
As discussed above, small molecules like oxygen and carbon dioxide move across the membrane by passive diffusion.
Why does the cell membrane have transport proteins?
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.How do carrier proteins transport substances across cell membranes?
When the molecules bind to a specific site of the carrier protein, the carrier proteins change their shape, leading to the movement of molecules from one side to another.
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 require transport protein to cross the cell membrane.
Cell Membrane Transport Quiz
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