AQA 4.1.3.3 Active Transport

Welcome to GCSE Science Unlocked, everyone! I'm Lottie, here with Mr. H. And Mr. H, we've just spent the last two episodes coasting downhill. Diffusion and Osmosis both felt like rolling a ball down a hill—they just happen on their own. But today, we are staring at a massive, steep incline. An apt description, Lottie. We have arrived at specification point 4.1.3.3... ACTIVE TRANSPORT. And unlike our previous two cellular mechanisms, this process most certainly does NOT happen for free. It is the literal uphill struggle of the cellular world. Right, because in active transport, we aren't following the crowd. We're moving substances from a more dilute solution to a more concentrated solution. So... against the concentration gradient. Exactly. But don't let the simplicity of the phrase "against the gradient" fool you. Let's have the formal definition for the mark scheme, please. If the word isn't PRECISE, the mark isn't yours. Okay, getting my exam hat on. [clears throat] Active transport moves substances from a more dilute solution to a more concentrated solution, AGAINST a concentration gradient. And because it's going the wrong way, it makes energy from respiration to do the pumping. And there it is. The classic trap. Here is the Mr. H Mark Scheme Warning, Lottie. You just said it "makes energy." Energy cannot be created or destroyed. Physics 101. Active transport REQUIRES energy transferred by respiration. Say "makes energy" in an AQA exam, and that is a one-way ticket to zero marks. Transferred by respiration. Got it. So the cell has to ACTIVELY spend its energy currency just to force these molecules to go where they naturally wouldn't. Diffusion and Osmosis are completely passive, but Active Transport is... well, ACTIVE. Precisely. If a question mentions mitochondria or respiration in relation to the movement of substances, your mind should go straight to Active Transport. It requires EFFORT from the cell. So where is this actually happening? Because I can't imagine a cell would want to spend its hard-earned energy—transferred by respiration—unless it ABSOLUTELY had to. Necessity is the mother of invention. Let's look at plants first. Think back to our root hair cells. They need mineral ions from the soil—specifically things like NITRATES, which the plant uses for making proteins. But surely there are way more nitrates already inside the plant than in the vast amount of dirt outside? [questioning tone] The soil water is usually really dilute, isn't it? Spot on. The concentration of minerals in the soil is often much lower than inside the root hair cell. If the plant relied on diffusion, what would happen? The minerals would just diffuse out of the plant and back into the soil! The plant would essentially BLEED OUT all its nutrients. A rather dramatic way of putting it, but yes. Active transport allows the plant to absorb these mineral ions against the gradient. This is exactly why root hair cells are PACKED with mitochondria. They need a constant supply of energy transferred by respiration to keep those transport proteins pumping. Okay, so plants use it for mineral ions like nitrates in the roots. What about us? I'm guessing our digestive system has something to do with this, right? Indeed it does. In the human small intestine, active transport allows sugar molecules—GLUCOSE—to be absorbed from lower concentrations in the gut into the blood, where the blood sugar concentration is already HIGHER. Ah, so even if we've already absorbed most of our breakfast through passive diffusion, active transport kicks in to make sure we don't waste a single remaining molecule of glucose? It FORCES those last few sugars into the blood. Exactly. It ensures every possible glucose molecule is absorbed so it can be transported to cells where it is needed for respiration. Now, let's face the music. The exam board loves a classic six-marker asking you to compare all three transport methods. Let's test you. Bring it on. Okay... Diffusion: moves ANY substance, goes from high to low concentration, and it's passive. No energy needed. Correct. [short pause] Next? Osmosis. This one is WATER ONLY. It moves from a dilute solution to a concentrated solution—so high water concentration to low water concentration—through a partially permeable membrane. Also passive. Excellent inclusion of the partially permeable membrane. And finally, today's topic? Active Transport. Moves any substance, but it goes from LOW to HIGH concentration—against the gradient. And the CRUCIAL difference: it requires energy transferred by respiration. A perfect conclusion to our study of cell transport. Just remember your specific examples—mineral ions into root hair cells, and glucose from the gut into the blood. I think you're ready for the big leagues, Lottie. Next time, we step up from individual cells to look at how they form tissues and organs. I'll bring my organ donor card just in case! Just bring your notebook, Lottie.