2.7: Facilitated Diffusion

In Topic 2.6 we started to discuss the different forms of membrane transport that allow substances to enter and exit the cell. Topic 2.7 dives deeper into the various types of membrane transport.

Vocab List

Written Explanation

Types of Membrane Proteins:

In order to transport large polar molecules and ions across the cell membrane, membrane proteins are needed, as the molecules are too large to pass through the gaps in the plasma membrane, and their charge makes them unable to mix with portions of the membrane. This type of assisted diffusion, known as facilitated diffusion, uses proteins that span the entire length of the membrane (such proteins are called integral or transmembrane proteins).

Diagram of transmembrane proteins embedded in the plasma membrane

There are two types of transmembrane proteins used in facilitated diffusion. These include channel proteins, used mostly by ions, which allow the passage of any material which is of the right size and charge to pass through, and carrier proteins, which bind to specific molecules in specific quantities and transport them across the membrane. This is still all done passively, meaning that an additional input of energy is unneeded.

Diagram of channel (left) and carrier (right) proteins in action

Lastly, there are also aquaporins, which are channel proteins that allow more water to enter and exit the cell, participating in the process of osmosis. This will be further talked about in topic 2.8.

Membrane Proteins in Active Transport:

Membrane proteins are also used for active transport. When a solute is not being transported in large quantities across the membrane through endo and exocytosis (see 2.6), proteins are used to move it across its concentration gradient into or out of the cell.

The most common example of energy being used with membrane proteins to transport ions across the cell membrane is the sodium-potassium pump. This pump uses a carrier protein in order to transport sodium ions out of the cell and potassium ions into it. First, three sodium ions in the cell enter the carrier protein, and a phosphate group from ATP (an energy storing molecule) gets broken off and added to the protein, providing energy for the action. Using the energy, the carrier protein changes shape and is now open facing the environment, while the portion inside the cell is closed. This allows the sodium ions to exit the cell and for two potassium ions to enter the carrier protein. The phosphate group then detaches, causing the protein to return to its original position, depositing the potassium into the cell.

Diagram of sodium-potassium pump and the steps in its functioning

Membrane Potential:

Finally, through both active and passive transport an ionic concentration gradient could be established on both sides of the membrane, which generates an electrical charge from the charges of the ions. Membrane potential is what the electric potential as a result of this concentration gradient formed by ions is called. Together with the concentration gradient this creates what is called an electrochemical gradient which can affect the ions' movement across the membrane, due not only to the difference in solute concentration but also the difference in electrical charge on either side of the membrane.

Diagram of electrochemical gradient. The triangles show how the quantities of their associated atom differ on both sides of the membrane.

The sodium-potassium pump is so essential to cells because it assists in maintaining the membrane potential of the cell. By removing 3 positive ions from the cell (Na+) and only bringing in 2 positive ions (K+), the pump has a net effect of moving positive ions out of the cell, making the cell have a more negative charge