Active transport and co-transport

In a nutshell

Active transport is a method of transporting molecules. It is an active process which means it requires energy to occur. Larger molecules can also be transported through vesicles.

Active transport

Definition

Active transport is the movement of molecules or ions into or out of a cell against a concentration gradient using carrier proteins. This means molecules are transported from a region of lower concentration to a region of higher concentration. This is an active process meaning it requires ATP.

Biology; Exchange and transport; KS5 Year 12; Active transport and co-transport

1.	Carrier proteins are located in the plasma membrane.
2.	Molecules bind to receptor sites on the carrier proteins.
3.	On the inside of the cell, ATP binds to the carrier protein.
4.	The ATP is hydrolysed into ADP and a phosphate molecule.
5.	This causes a conformational change in the shape of the protein and opens it to the other side of the membrane.
6.	The molecule travels through the carrier protein through to the other side of the membrane.
7.	The molecule of phosphate is released and the carrier protein returns to its original shape.
8.	The phosphate recombines with ADP to form ATP during respiration.

Note: Sometimes a molecule can move into a cell while another molecule is being transported out of that same cell. This is the case for the sodium-potassium pump. The sodium ions are being transported out of the cell and potassium ions are being transported into the cell. The sodium-potassium pump is crucial for nerve impulses in animal cells.

Co-transport

Definition

Co-transport occurs when the transport of one molecule is coupled with the transport of another molecule in the same direction and through the same carrier protein.

This is possible as the concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.

Example

Biology; Exchange and transport; KS5 Year 12; Active transport and co-transport

1.	Sodium ions are actively transported out of the epithelial cells and into the blood through the sodium-potassium pump.
2.	Therefore, there is a higher concentration of sodium ions in the lumen of the intestine than in the epithelial cells.
3.	Sodium ions diffuse into the epithelial cells from the ileum. As they pass through the protein carrier (co-transporter), glucose molecules are also carried into the epithelial cells.
4.	The glucose can then be transported into the blood by facilitated diffusion.

Bulk transport

Some larger molecules are transported in vesicles. There are two types of vesicular transport you need to know about: exocytosis and endocytosis.

Endocytosis

Endocytosis is a form of active transport of molecules into the cell. Part of the cell membrane will surround the molecules to be transported. A vesicle will form around these molecules allowing them to travel within the cell. Large molecules like cholesterol are transported via endocytosis.

Exocytosis

Exocytosis is a form of active transport of molecules out of the cell. Small membrane-bound sacs called vesicles fuse with the cell membrane and release their contents to the outside of the cell. ATP is needed to fuse the membranes together. Proteins and polysaccharides are transported via exocytosis.

Note: Phagocytosis (the process of phagocytes engulfing pathogens) is an example of endocytosis.

Adaptations for rapid transport

Cells have adaptations to increase the rate of active transport. Some of these adaptations are explained below.

Adaptation	Explanation
Microvilli	Some epithelial cells have microvilli on their surface. These are similar (but much smaller) than villi and they are also finger-like projections. The microvilli increase the surface area for diffusion, facilitated diffusion and active transport as there is more membrane for the carrier and channel proteins to be located.
Increased number of channel and carrier proteins	Increasing the density of channel and carrier proteins will increase the number of molecules that can be transported at any given time. This will increase the rate of active transport.
Increased respiration rate	An increased rate of respiration will produce more ATP which can be used to power active transport.