Active Transport – Exchange and Transport Ep 4

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 lower concentration to an area of higher 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.

As with diffusion and osmosis, there are a few factors that can affect the rate of active transport.

1. The number of carrier proteins available: more proteins means a faster rate.
2. How fast the proteins work: a faster carrier protein means a faster rate.
3. Availability of ATP: if respiration is inhibited and not enough ATP is being produced, the rate will be slower.

Glucose absorption (co-transport)

A special type of carrier protein that can be used for move substances against a concentration gradient is a co-transporter protein. Using these, substances get transported across a membrane with a buddy. One substance will get transported across down its concentration gradient and will bring another substance with it 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.

Sometimes the concentration of glucose in the epithelial cells lining the ileum is higher than the concentration in the lumen of the ileum, but we still want to absorb that glucose. It gets transported 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 (against the glucose concentration gradient). 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 epithelial 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. 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

• Active transport is the movement of substances against a concentration gradient across a cell membrane.
• Energy from ATP is required.
• It uses carrier proteins.
• Glucose is absorbed by co-transport in the ileum. Co-transport involves one substance moving down a concentration gradient and one moving against a concentration gradient.

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