So far we have looked at the passive movement of substances across cell membranes by diffusion and osmosis. Sometimes substances need to move against a concentration gradient, which is when active transport comes into action.
Active transport
Active transport uses energy from ATP to transport substances from an area of low concentration to an area of high concentration across a cell membrane. These substances can be molecules (e.g. glucose) or ions (e.g. Ca2+). The process uses carrier proteins like we saw in facilitated diffusion. The substance binds to the carrier protein, the protein then changes shape and releases the substance on the other side of the cell membrane. Take a look at the diffusion article for a diagram.
A special type of carrier protein that can be used for active transport is a co-transporter protein. Using these, substances get transported across with a buddy. One substance will get transported across down its concentration gradient and will bring a buddy substance across against a concentration gradient. One example of this is how glucose can be absorbed in the ileum (small intestine), which we will look at below.
As with diffusion and osmosis, there are a few factors that can affect the rate of active transport.
- The number of carrier proteins available: more proteins means a faster rate.
- How fast the proteins work: a faster protein means a faster rate.
- Availability of ATP: if respiration is inhibited and not enough ATP is being produced, the rate will be slower.
Glucose absorption (co-transport)
Sometimes the concentration of glucose in the epithelial cells lining the ileum is higher than what is in the lumen of the ileum, but we still want to absorb that glucose. It gets brought across with sodium (Na+) ions using a sodium-glucose co-transporter protein. Na+ ions diffuse into the cell down a concentration gradient and bring glucose molecules with them. Glucose is then transported into the blood through a channel protein by facilitated diffusion.
To maintain the concentration gradient of Na+ ions, they are actively transported out of the cell into the blood by a sodium-potassium pump, which also pumps potassium (K+) ions into the cell. It’s a fairly complicated process, so perhaps try drawing it out from memory a few times. Know that it is the diffusion of Na+ ions that is driving the active transport of glucose. Check out this article for more on absorption in the ileum.
Another example of co-transport in the phloem vessels of plants. The companion cells next to the phloem vessels use energy to transport hydrogen ions into the surrounding tissue. When they re-enter the cell down the concentration gradient that has been created, they bring sucrose with them through a co-transporter protein. We will look at this in more detail when we cover transport in plants.
Summary
- The movement of substances against a concentration gradient across a cell membrane.
- Requires energy from ATP.
- Uses carrier proteins and co-transporter proteins.
- Glucose is absorbed by active transport in the ileum, which is driven by the co-transport of sodium ions.
