0610 · 3.3Active transport

How active transport works

0:00Here is a puzzle. A plant needs mineral ions, such as nitrate, from the soil. But the cells in its roots already contain far more of these ions than the water in the soil around them. If ions only moved by diffusion, they would move out of the root and into the soil, from high concentration to low. Yet plants keep on taking ions in. How? The answer is active transport.

0:22By the end of this video you will be able to describe active transport, explain how it differs from diffusion, say what the protein carriers in the membrane do, and explain why it matters, including how root hair cells take up ions. You need two ideas from earlier videos. First, diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

0:46It needs no extra energy. Second, respiration is the set of chemical reactions in cells that release energy from glucose, and much of it happens in the mitochondria. Think of a concentration gradient as a hill. In diffusion, particles roll down the hill, from high concentration to low, all on their own. Active transport moves particles the other way, up the hill. And just as pushing a ball uphill takes effort, moving particles against a concentration gradient takes energy.

1:13The syllabus describes active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, in other words against a concentration gradient, using energy from respiration. Notice the three key parts of that sentence. It happens through a cell membrane. It goes from lower concentration to higher concentration, against the gradient. And it uses energy, which comes from respiration.

1:38Those three parts are exactly what make it different from diffusion. For the extended syllabus, you also need to know how it happens. The syllabus wants you to state that protein carriers move molecules or ions across a membrane during active transport. A protein carrier is a protein molecule built into the cell membrane. Each type of carrier only fits certain molecules or ions. Here is a simple way to picture it.

2:02An ion on the outside of the membrane fits into a carrier. The carrier uses energy from respiration to change its shape. As it changes shape, it carries the ion across to the other side of the membrane and releases it there, inside the cell. Then the carrier goes back to its original shape, ready to collect another ion. It works like a revolving door that only turns when energy is supplied.

Why active transport matters

2:24Now, why does active transport matter? The syllabus wants you to explain its importance for moving molecules or ions across membranes, including ion uptake by root hairs. Let us go back to the puzzle we started with. Root hair cells need mineral ions such as nitrate, which the plant uses to make proteins, and magnesium, which it uses to make chlorophyll. The concentration of these ions in the soil water is usually much lower than inside the root hair cells.

2:49So diffusion cannot bring them in. Instead, protein carriers in the root hair cell membrane use active transport to pump the ions in, against the concentration gradient. This needs a lot of energy, so root hair cells contain many mitochondria, where aerobic respiration releases energy. And it explains something farmers know well. If a field is waterlogged, the soil fills with water and there is very little oxygen around the roots.

3:13Without oxygen, the root cells cannot carry out aerobic respiration, so they cannot release enough energy for active transport, and the plant cannot take up the mineral ions it needs. Active transport matters in animals too. In the small intestine, digested food molecules such as glucose are absorbed into the blood. At first, glucose can diffuse in, while it is more concentrated in the gut than in the blood. But active transport lets the cells of the gut lining keep absorbing glucose even when its concentration in the gut has fallen lower than in the blood.

3:45So almost none of the glucose from your food is wasted. In general, active transport lets a cell build up a substance to a higher concentration than its surroundings, or remove a substance even when there is very little of it left. Diffusion alone could never do that.

Answering active transport questions

4:00Let us try an exam-style question. A bar chart shows that the concentration of nitrate ions is much higher in a root hair cell than in the soil water around it. The question asks you to explain how the root hair cell takes up nitrate ions. Start with the evidence. The concentration inside the cell is higher than outside, so the ions are moving against a concentration gradient. That rules out diffusion.

4:24So the answer is active transport. Then add the details. The nitrate ions are moved through the cell membrane by protein carriers, using energy released by respiration in the mitochondria of the root hair cell. A follow-up question might ask what would happen if the roots were given a substance that stops respiration. The answer is that the uptake of nitrate ions would stop, or slow down a lot, because there would be no energy for active transport.

4:50Let us collect the common mistakes. The first is saying that active transport moves particles down a concentration gradient. It moves them against it, from lower to higher concentration. The second is forgetting the energy, or saying where it comes from wrongly. The energy comes from respiration, not from the sun and not from the particles themselves. The third is saying that respiration makes energy. Respiration releases energy. And the fourth is mixing up the three processes.

5:17Diffusion and osmosis need no energy from respiration, and they go down a gradient. Active transport needs energy from respiration, and it goes against a gradient. Let us recap. Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration. Protein carriers in the membrane move the molecules or ions across.

5:44Active transport lets root hair cells take up mineral ions from the soil, even though the ions are more concentrated inside the cells, and it lets the small intestine absorb glucose even when little is left in the gut. Diffusion and osmosis happen down a gradient and need no energy from respiration. Active transport goes against a gradient, and it does need energy from respiration. Together, these three processes control what enters and leaves every cell.