Diffusion and active transport across the cell membrane

When nobody pushes it, where does a substance go on its own? Drop a tea bag into a mug of hot water, leave it alone, and watch. Nobody stirs it. Ten minutes later the whole mug is brown, because the colour has spread itself out, from where there was lots of it to where there was none. Now picture a plant's root in damp soil. The soil water around it holds hardly any nitrate, far less than is already packed inside the root's cells. Going by the tea, that nitrate should leak out of the root and into the soil. Instead, the root keeps pulling more of it in. First, what the tea is doing, and then back to that root.

The tea is showing you diffusion. Diffusion is the spreading out of the particles of a gas, or of a substance dissolved in a liquid. Those particles are always moving, bumping about at random in every direction. Nothing aims them anywhere, and no cell spends any energy on it. Concentration means how much of a substance is packed into a space. Right next to the tea bag the brown particles are crowded, and across the mug they are thin. Particles drift both ways at random. But more of them leave the crowded part than come back into it, because there are more of them there to leave. That overall flow is called the net movement, and it always runs from a higher concentration to a lower concentration. Now picture the same mug a few hours later. The brown is even all the way through, so the concentration is the same everywhere. Have the particles stopped moving, or are they still moving about? Still moving. The particles never stop, but now as many drift one way as the other, so only the net movement stops. The difference in concentration between two places is called the concentration gradient. Moving from the higher concentration to the lower one is moving down the concentration gradient. So a diffusion sentence names the particle and both ends. Oxygen moves from a higher concentration to a lower concentration, down the concentration gradient. Cells follow the same rule. The cell membrane, the thin layer around every cell, lets small particles such as oxygen diffuse straight across it. Oxygen diffuses into a cell from the blood, because the cell keeps using oxygen up. Carbon dioxide diffuses out, because the cell keeps making it. Urea, a waste substance, diffuses out of cells into the blood plasma, the pale liquid that carries the blood cells. Each one runs from a higher concentration to a lower one.

Back to the root, which goes against the rule the tea just showed you. The soil water holds very little nitrate, and the root's cells hold much more of it. Left to diffusion alone, would nitrate move into the root, or out of it? Out of it. Diffusion only ever runs down the gradient, and here the higher end is inside the root. So the root is moving nitrate the other way, from the dilute soil water into its more concentrated cells. That is against the concentration gradient. Moving a substance from a more dilute solution to a more concentrated solution, against the concentration gradient, is called active transport. Now, why should moving nitrate in cost the root cell energy? Random movement alone would carry the nitrate out. Moving it in means the cell has to do work, and doing work takes energy. That energy is released by respiration, inside the root's own cells. Respiration releases energy from food; it never creates it. Plants need mineral ions such as nitrate for healthy growth. That is why root hair cells spend energy taking them in from very dilute soil water.

Every case is decided by one comparison: where the substance is more concentrated, and where it is heading. Heading from higher to lower, it can be diffusion. Heading from lower to higher, it must be active transport, and that needs energy released by respiration. Try the first case. Oxygen moves from the blood into a muscle cell that is using oxygen up quickly. Oxygen into that busy muscle cell: diffusion, or active transport? It's diffusion. The blood has the higher oxygen concentration and the busy muscle cell has the lower one, so the oxygen runs down the gradient with no energy spent. Now a seaweed cell. It already holds iodide ions at many times the concentration of the sea water around it, and it takes in still more iodide. Iodide going into that seaweed cell: diffusion, or active transport? Active transport. The iodide is heading from lower to higher, against the gradient, so the seaweed cell has to spend energy released by respiration. A different one now, to explain. A yeast cell is respiring, and carbon dioxide is leaving it. Why is carbon dioxide leaving the yeast cell diffusion, not active transport? The yeast cell makes carbon dioxide as it respires, so its inside is the higher concentration. Leaving the cell runs from higher to lower, down the gradient. One more, and this time the comparison is already done for you. Glucose is moving from the last part of the gut, where little is left, into blood that already holds more. So which process is moving the glucose, and what does it have to spend? Active transport, paid for with energy released by respiration. That is the gut's own example: sugar moved into the blood, because your cells use sugar for respiration. Notice what decided all four cases: the direction, compared with the two concentrations. The energy follows from the direction, never the other way round. One examiner's report on a Foundation paper records this. Almost half of the students knew that nitrate ions are absorbed from the soil by active transport, with the remaining students being equally split between diffusion and osmosis. The fix is to compare the two concentrations before naming anything. Diffusion would carry nitrate out, and osmosis moves only water, never an ion, so neither one brings nitrate in.

Active transport needs energy, and a flooded field shows what happens when that energy runs short. A field is flooded for a week, and the waterlogged soil holds almost no oxygen. The crop's roots take in far less nitrate than usual. The floodwater carried plenty of fertiliser. Explain the drop in nitrate, one step at a time. Pause it there and work it out for yourself. The chain starts with oxygen. Respiring needs oxygen, so with almost none, far less energy is released. Nitrate gets into root hair cells by active transport, because the soil water is more dilute than the cells. Active transport needs energy released by respiration. With less energy released, less nitrate is moved in. Now the trap hidden in the question: all that extra fertiliser around the roots. Why does the extra fertiliser not help the roots take in nitrate? The soil water is still more dilute than the root cells, so nitrate must still come in by active transport. Without oxygen, the root cannot release the energy to pay for it.

Here is a sentence a student wrote about oxygen and a cell, and it is yours to mark. Oxygen moves across the concentration gradient into the cell by diffusion, and the cells move towards the blood. The process named is right. Two of the other words in it would not be credited. What are the two slips in that student's sentence? First, across the gradient gives no direction at all. Second, cells don't move about; the oxygen particles do. Those are the two slips. One examiner's report on a Higher paper says this. The use of the term concentration gradient was confused by many students throughout this question. Movement along or across a concentration gradient, the report says, gives no clear indication of direction and requires further qualification. Some students described cells moving, instead of ions or particles. The fix is to name the particle and both ends. Oxygen diffuses from the blood, where its concentration is higher, into the cell, where its concentration is lower. That leaves the two processes side by side. Diffusion runs down the concentration gradient and costs the cell no energy. Active transport runs against it, using energy released by respiration.

Four quick ones to finish, and every one is decided by the comparison you have been making. What does active transport need, and where does it come from? Active transport needs energy, released by respiration in the cell doing the moving. Now a poison stops respiration in a root. Which process stops working? Active transport stops, because its energy supply has gone. Diffusion carries on, because it never needed any. Try a new one: glucose enters a cell using it fast, from blood holding more. Which process? Diffusion. The blood is the higher concentration and the cell the lower, so the glucose runs down the gradient. One more, back to the damp soil: why can't diffusion explain the root? The root's cells already hold more nitrate than the soil water. Pulling more in runs from lower to higher, so it has to be active transport. The tea spreads out for free. The root pays for every bit of nitrate it gathers, with energy released by respiration.

If telling diffusion from active transport now feels solid, give this video a thumbs up. You've got this one down, so it never needs watching again. If it is still wobbly, save it and come back to it. Three questions in, it starts to feel normal.

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