AQA 4.1.3.1 Diffusion

Welcome to GCSE Science Unlocked everyone! I'm Lottie, here with Mr. H. And Mr. H, I need to start with something I was staring at in my kitchen this morning. I dropped a tea bag into a mug of hot water, didn't stir it at all, and just watched the brown colour slowly spread out until the whole drink was perfectly brewed. I used to think that was magic. Then I decided it was physics. [clears throat] Welcome to the show. It is BIOLOGY, Lottie. Though the principles of Physics certainly underpin it. Energy cannot be created or destroyed, Physics 101, but we are looking at biological cellular processes today. Specifically, AQA specification point 4.1.3.1. Diffusion. Specification 4.1.3.1. Right, let me get the official definition out of the way before you start sighing at me. Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a NET movement from an area of higher concentration to an area of lower concentration. Almost perfect. But here is the Mr. H MARK SCHEME WARNING. You MUST state that this happens down a concentration gradient. And crucially, it is a PASSIVE process. If the word isn't precise, the mark isn't yours. Passive. So that means it requires absolutely NO energy from the cell itself? The molecules are just doing their own thing, bumping around on their own kinetic energy until they’re evenly spread out. Precisely. In a biological context, this passive movement is how gas exchange operates. Oxygen and carbon dioxide moving into and out of cells. It is also how urea, a cellular waste product, moves from cells into the blood plasma so it can be excreted by the kidney. Urea moving into the blood plasma. That sounds slightly less appetizing than my morning tea. It is considerably more essential for your survival than your morning tea. Focus, Lottie. In the exam, they often ask what makes this diffusion happen faster. Let's test you. Okay, I’ve got three factors written down. Number one: the concentration gradient. The bigger the difference in concentration between two areas, the faster the diffusion. Correct. Think of the concentration gradient like a slide -- the steeper the slide, the faster you travel down it. And the second? The temperature. Higher temperatures mean particles have more kinetic energy, so they move faster. Which entirely explains why my tea brews instantly in boiling water, but takes ages if I try to make iced tea with cold water. I will accept that analogy, though please do NOT write about PG Tips in your biology paper. [stifled laugh] The examiner will have a fit. What is the third factor? The surface area of the membrane. A larger surface area means more physical space for the particles to pass through all at once. Exactly. But don't let the simplicity of those three factors fool you. The surface area component is where students consistently drop marks. Because of the surface area to volume ratio, right? This is the part that always gives me a headache. Why do single-celled organisms have it so easy compared to us? It is simple mathematics. A single-celled organism has a relatively large surface area in comparison to its volume. This immense surface area allows sufficient transport of molecules into and out of the cell to meet all of its metabolic needs. Simple diffusion is enough to keep it alive. A relatively large surface area compared to its volume. So because we are massive, multicellular humans, our outside "surface" -- our skin -- isn't nearly big enough to supply our massive inside "volume"? Spot on. We have a SMALL surface area to volume ratio. Therefore, we cannot rely on simple diffusion across our skin. We would suffocate. We need specialised organ systems for exchange. So to compensate for our small ratio, we’ve developed internal structures. Things like lungs and small intestines. Yes. And the AQA spec explicitly wants you to know WHY these exchange surfaces are so remarkably effective. Take the lungs -- specifically the alveoli -- or the small intestine with its villi. They all share four key features. You already gave me the first one. A large surface area. Correct. Second? A membrane that is very thin, to provide a short diffusion path. "Short diffusion path" is the EXACT phrasing you need for the mark scheme. Excellent. Now, what about the blood? In animals, they have an efficient blood supply. That constantly moves the absorbed substances away, which maintains that steep concentration gradient you mentioned earlier. And for gaseous exchange in the lungs, they're ventilated -- breathing in and out -- to keep the air gradient steep too. So basically... the villi in my small intestine and the alveoli in my lungs are just HIGH-SPEED diffusion stations? That is a very accurate, if highly informal, way of putting it. [trying not to laugh] I will tolerate "high-speed diffusion stations" in this room, Lottie, but that is a one-way ticket to ZERO marks on paper. Stick to "large surface area, thin membrane, efficient blood supply." Message received! So to quickly recap 4.1.3.1: Diffusion is high to low concentration, it's totally passive. The speed depends on the gradient, temperature, and surface area. And multicellular organisms need specialized exchange surfaces because our surface area to volume ratio is simply too small. If you can articulate those points exactly as we just discussed, you have mastered diffusion. Brilliant. Next time, we’re looking at the one that specifically involves water, right? Osmosis. I’ll bring the potatoes. See you then.