AQA 4.1.3.2 Osmosis

Welcome to the show everyone! I'm Lottie, here with Mr H. And Mr H, I need to start today with a minor kitchen tragedy. I made a beautiful salad at 1 PM, poured the dressing over it, and by 3 PM... my crisp lettuce was COMPLETELY limp and sad. You dehydrated your lettuce, Lottie. You bathed it in a salty vinaigrette, created a concentration gradient, and the water SIMPLY left the cells. Welcome to AQA specification 4.1.3.2: Osmosis. Right. So the vinaigrette LITERALLY pulled the water out of the leaves? In a manner of speaking, yes. But let's be PRECISE for the examiners. Osmosis is a very specific type of diffusion. It is the diffusion of water from a dilute solution to a concentrated solution, through a... partially permeable membrane. From a dilute solution to a concentrated solution. So, dilute just means there's a HIGH concentration of water, and concentrated means there's a LOW concentration of water because it's full of other stuff... like the salt in my dressing. Exactly. The water moves from where there is a LOT of it, to where there is less of it. But here is the Mr. H Mark Scheme Warning: if you write that on an exam and forget the phrase "partially permeable membrane," that is a one-way ticket to ZERO marks. You are no longer describing osmosis; you are just describing a puddle spreading. A puddle spreading! Okay, so the partially permeable membrane is the NON-NEGOTIABLE part. It's like a microscopic sieve that only lets the tiny water molecules through, but blocks the bigger sugar or salt molecules. Precisely. Now, let's face the music with the Required Practical. You will ALMOST CERTAINLY see this on your Paper 1. We test this using potato cylinders. I actually remember doing this in school! We cut up a potato into little identical cylinders, weighed them, and then dropped them into beakers with different concentrations of sugar solution. [challenging tone] Let's test you. If a potato cylinder is left in a beaker for an hour, and it gains mass... WHY did that happen? If it gains mass, water must have moved INTO the potato cells by osmosis. Which means the sugar solution in the beaker was MORE dilute than the inside of the potato. [short pause] Spot on. And if it LOSES mass? [responds quickly] Then the water moved out. The solution in the beaker was more concentrated than the potato's cytoplasm. Exactly like my sad salad lettuce! Correct. But here is where students THROW marks away. The Maths Trap. When you record your results, you do NOT just write down the raw change in grams. You MUST calculate the percentage change in mass. [short pause] Why? Oh, I know this. Because no matter how careful you are with a scalpel, the potato cylinders might not have all weighed exactly the same at the start. If one started at 5 grams and another at 6 grams, just looking at a 1-gram change isn't a FAIR comparison. Percentage change levels the playing field. Excellent. It is about PROPORTIONALITY. Change in mass, divided by the initial mass, multiplied by 100. And what about the... sweet spot? The cylinder that doesn't change mass at ALL? The sweet spot means there's no NET movement of water, right? The concentration of the sugar solution outside is EXACTLY the same as the concentration inside the potato cells. Good. Now, a final crucial biological distinction. What happens if TOO MUCH water moves into an animal cell? Say, a red blood cell placed in pure, distilled water? Well, animal cells don't have that rigid cell wall. So... if water just keeps rushing in from the dilute outside to the concentrated inside, it's going to swell up and pop—like a water balloon. [clears throat] It BURSTS. 'Pop' is for balloons, Lottie. But yes, it bursts. However, a plant cell—like your lettuce—has a strong cell wall made of cellulose. Right, the cellulose wall! So it acts like a structural cage? It stops the cell membrane from expanding too far? Exactly. The cell becomes TURGID. The internal pressure increases, but the cellulose wall holds it rigid. It is LITERALLY what keeps the plant upright. Without enough water, the cells become flaccid... and the plant wilts. [summing up] Flaccid cells equal wilted lettuce. Got it. So the checklist is: water moves from dilute to concentrated, partially permeable membrane is NON-NEGOTIABLE, always calculate percentage change for the potatoes, and cellulose saves the plant from bursting. A very solid performance. No colloquialisms, just pure, marks-focused science. Next time, we finish the transport trilogy with Active Transport. See you then.