Introduction
Okay, so every cell you own is wrapped in a thin skin called the cell membrane, and it's basically a very picky bouncer, some stuff gets in, some stuff gets kicked out, and some stuff is on the permanent no-entry list. This whole chapter is about how things actually get across that bouncer.
Good news: there are only three ways in or out, and one simple question tells them apart every single time, is the substance going the "easy way" (spreading out, downhill, for free) or the "hard way" (uphill, which costs energy)? Nail that one question and diffusion, osmosis, and active transport basically sort themselves out. Let's get into it 🚦
Diffusion: Stuff Spreads Out On Its OwnE3.1
Ever notice how if someone opens a bag of chips across the room, you smell it within seconds? Nobody fanned it towards you. The smell particles just spread out on their own, from where they were crowded (the bag) to where there were none (your nose). That's diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, because particles are always moving randomly.<br><br>The word "net" is doing heavy lifting: particles zoom around in all directions, but more of them drift from the crowded side to the empty side, so the overall flow is downhill. And because it runs on the particles' own random motion, diffusion is completely free, the cell doesn't spend any energy on it. That's how oxygen gets into your cells and carbon dioxide gets out.
> 🔑 Key idea: diffusion = net movement of particles from high to low concentration (down the gradient), driven by random motion. No energy needed.
Worked Example: Naming the Free RideE3.1
Problem A drop of food colouring is added to still water and slowly spreads until the colour is even throughout. Which process is this, and does it need energy?
Step 1 Check the direction: the dye moves from where it's concentrated (the drop) to where it's dilute (the rest of the water). That's downhill, high to low concentration.
Step 2 Downhill movement caused by random motion, with no membrane needed and no energy spent, is the definition of diffusion. $$\text{high concentration} \longrightarrow \text{low concentration (down the gradient)}$$
Step 3 So the answer is diffusion, and no, it needs no energy from the cell, the particles' own random movement does all the work.
What Makes Diffusion Faster (and Why Size Matters)E3.1
Diffusion isn't always the same speed, and examiners love asking what changes it. Five dials: bigger surface area = faster, steeper concentration gradient (bigger difference) = faster, shorter distance (thinner barrier) = faster, higher temperature = faster (particles get more energy and move quicker), and smaller/uncharged molecules = faster. That's why lungs, guts, and leaves are all built big, thin, and with a good blood supply, they're maxing out every dial.<br><br>Here's the deeper flex: surface area to volume ratio. As something gets bigger, its volume grows faster than its surface, so its SA:V drops. A tiny cell has loads of surface for its size, so diffusion supplies it fine. A big animal has too little surface and the inside is too far away, which is exactly why you have lungs and blood instead of just absorbing oxygen through your skin.
> 🔑 Key idea: faster diffusion = bigger surface area + steeper gradient + shorter distance + higher temp + smaller molecules. Small things have a high SA:V (diffusion works); big active things need exchange surfaces + transport systems.
Worked Example: Big Cube vs Little CubeE3.1
Problem An agar cube with 1 mm sides and a cube with 2 mm sides are cut from the same jelly. Which one has the bigger surface-area-to-volume ratio?
Step 1 Do the 1 mm cube: surface area = 6 × (1 × 1) = 6, volume = 1 × 1 × 1 = 1. $$\text{SA:V} = 6 : 1$$
Step 2 Now the 2 mm cube: surface area = 6 × (2 × 2) = 24, volume = 2 × 2 × 2 = 8. $$\text{SA:V} = 24 : 8 = 3 : 1$$
Step 3 The smaller (1 mm) cube wins with 6:1 versus 3:1, so it would soak up acid or dye by diffusion all the way through faster. Smaller = higher SA:V = better for diffusion.
Osmosis: Diffusion's Water-Only CousinE3.2
Osmosis is just diffusion, but the VIP guest is water and there's a membrane involved. Definition to lock in: osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.<br><br>"Water potential" sounds scary but it's simple: pure water has the highest water potential (we call it zero), and adding solute lowers it, making it negative. Water always slides from high water potential to low. The classic trap is mixing up water and solute: a concentrated solution has lots of solute but little water, so it has a low water potential, and water moves towards it. And yes, water potentials are negative numbers, so "higher" means closer to zero (−50 is higher than −200). Osmosis, like all diffusion, is free.
> 🔑 Key idea: osmosis = net movement of water, from high to low water potential, through a partially permeable membrane. Pure water = highest (zero) water potential; solute lowers it.
Worked Example: Which Way Does Water Go?E3.2
Problem A cell with a water potential of −200 kPa sits next to a solution with a water potential of −50 kPa. Which way does water move?
Step 1 The one rule: water moves from higher water potential to lower water potential.
Step 2 Compare the numbers on a line. −50 is closer to zero than −200, so −50 is the higher water potential. $$-50 \text{ kPa (higher)} \longrightarrow -200 \text{ kPa (lower)}$$
Step 3 So water moves from the solution into the cell. Don't get tricked by "200 is a bigger number", with negatives, closer to zero wins.
Osmosis in Animal Cells: No Wall, Big ProblemE3.2
Animal cells (like red blood cells) have no cell wall, just the membrane, so osmosis hits them hard. Drop one in pure water (higher water potential outside) and water floods in until the cell swells and bursts, that bursting is called lysis. Drop one in a concentrated solution (lower water potential outside) and water leaves until the cell shrivels up. Only in a solution of equal water potential does it keep its normal shape (water in = water out).<br><br>This is exactly why your body works so hard to keep the concentration of your blood steady, put your cells in the wrong surroundings and they either pop or shrink. You'll see this again as osmoregulation when you study the kidney.
> 🔑 Key idea: animal cells have no wall, so they burst (lysis) in dilute surroundings and shrivel in concentrated ones. Their surroundings must be kept balanced.
Worked Example: Burst vs BulgeE3.2
Problem A red blood cell and a plant cell are both placed in distilled water. The red blood cell bursts but the plant cell doesn't. Why?
Step 1 Distilled water has a higher water potential than both cells, so water enters both by osmosis. So far, same situation.
Step 2 The plant cell has a strong cellulose cell wall. As it swells, the wall pushes back (turgor pressure) and stops it bursting.
Step 3 The red blood cell has no wall, so nothing resists the swelling, water keeps entering and it bursts. Answer: the plant cell's wall saves it; the animal cell has no wall.
Plant Cells: Turgid, Flaccid, PlasmolysedE3.2
Plant cells have that cell wall, so instead of bursting they get firm. Three states, learn the exact words: turgid (in water/dilute solution, water enters, the cell fills up and pushes against the wall, so it's firm and full, this keeps plants standing up), flaccid (short of water, water leaves, the cell goes soft, and the plant wilts), and plasmolysed (in a really concentrated solution, so much water leaves that the membrane actually pulls away from the cell wall).<br><br>Order of water loss: turgid → flaccid → plasmolysed. And remember, during plasmolysis the wall doesn't shrink, only the living contents inside it pull inward.
> 🔑 Key idea: turgid = firm (took in water), flaccid = soft (lost some water, plant wilts), plasmolysed = membrane pulled off the wall (lost lots of water). The wall never shrinks.
Worked Example: Define PlasmolysisE3.2
Problem A plant cell is placed in a very concentrated sugar solution. What is meant by plasmolysis?
Step 1 The sugar solution has a much lower water potential than the cell, so water leaves the cell by osmosis.
Step 2 The cell loses so much water that the cell membrane and cytoplasm pull away from the cell wall.
Step 3 That pulling-away is plasmolysis. Two easy marks to grab: say "membrane pulls away from the wall", and don't say "the wall shrinks" (it doesn't).
Investigating Osmosis: The Potato ExperimentE3.2
The classic experiment: cut equal-sized potato cylinders, weigh them, pop them in different solutions (plus distilled water), wait, blot them dry, and weigh again. A potato in distilled water (higher water potential) gains mass; a potato in a concentrated solution (lower water potential) loses mass; and the solution that gives no change matches the potato's own cell sap. Dialysis (Visking) tubing does the same job as an artificial partially permeable membrane, a model cell.<br><br>Because the pieces don't all start the same, you compare them fairly using percentage change in mass = (final − start) ÷ start × 100. Plus means it gained water, minus means it lost water. Pro moves: blot dry before weighing (surface water = fake extra mass), and measure several pieces to shrink your error.
> 🔑 Key formula: percentage change in mass = (final mass − starting mass) ÷ starting mass × 100. Plus = gained water; minus = lost water.
Worked Example: Potato Percentage ChangeE3.2
Problem A potato cylinder starts at 5.0 g. After soaking it weighs 4.4 g. What is the percentage change in mass?
Step 1 Find the change: final minus start. $$4.4 - 5.0 = -0.6 \text{ g}$$
Step 2 Divide by the starting mass and multiply by 100. $$\frac{-0.6}{5.0} \times 100 = -12\%$$
Step 3 The mass fell by 12%. The minus sign tells you the potato lost water, so the solution had a lower water potential than the potato cells. Always divide by the start mass, and keep the sign.
Why Water Is a Big DealE3.2
Two quick "importance of water" marks. First, water is the body's solvent, glucose, mineral ions, amino acids, and waste like urea all get carried around dissolved in water (in your blood plasma, and in plant sap). Stuff usually has to be dissolved before it can move or react, so water quietly underpins transport, digestion, and excretion.<br><br>Second, water gives non-woody plants their support through turgor. When plant cells are turgid they press against each other and keep soft stems upright and leaves flat to catch light. Let the plant dry out and the cells go flaccid, and the whole thing wilts. Turgor is basically a plant's water-powered skeleton.
> 🔑 Key idea: water is the main solvent (carries dissolved glucose, ions, urea) AND, through turgor, the support of soft plants.
Worked Example: The Comeback PlantE3.2
Problem A wilted houseplant is watered, and within a few hours its leaves and stem are firm and upright again. What changed in the cells?
Step 1 A wilted plant has flaccid cells, they've lost water and turgor, so the soft tissues droop.
Step 2 After watering, the soil water potential rises above the cells' water potential, so water enters the cells by osmosis.
Step 3 The cells fill up and become turgid again, and that returning turgor pressure stiffens the tissues. The chain: osmosis → turgid → firm.
Active Transport: The Uphill RouteE3.3
Diffusion and osmosis are the free, downhill options. But sometimes a cell needs to grab a substance even when there's already more inside than outside, that's moving it against the gradient (low to high), and random motion can't do uphill. So it has to pay, using energy from respiration. That's active transport: the movement of particles through a membrane from a lower concentration to a higher concentration (against the gradient), using energy from respiration.<br><br>How? Carrier proteins in the membrane grab the particle, change shape, and push it through to the crowded side, like a revolving door that only turns because it's plugged in. The "plug" is energy from respiration, which is why active transport only happens in living cells and stops the moment respiration stops.
> 🔑 Key idea: active transport = particles moved against the gradient (low → high) using carrier proteins + energy from respiration. It's the only one of the three that costs energy.
Worked Example: Root Hair Cells Working OvertimeE3.3
Problem A root hair cell already has a higher concentration of nitrate ions than the soil, yet it keeps taking in more nitrate ions. Which process is this, and why does it need energy?
Step 1 Track the direction: ions moving from low concentration (soil) to high concentration (cell). That's against the gradient.
Step 2 Diffusion can only go downhill, so it's ruled out. The only process that goes uphill is active transport. $$\text{soil (low ion conc.)} \longrightarrow \text{cell (high ion conc.), against the gradient}$$
Step 3 Going against the gradient needs a push, so it uses energy from respiration (which is why root hair cells are packed with mitochondria). Answer: active transport, powered by respiration.
Active Transport in Real LifeE3.3
Active transport isn't just root hair cells. Your small intestine uses it too: most glucose is absorbed by diffusion, but near the end of a meal the gut has less glucose than your blood, so the gut lining switches to active transport to grab the last of it, no wasted food.<br><br>The big consequence of "active transport needs energy" is that anything which stops respiration stops active transport, while leaving the free processes untouched. Waterlogged soil (low oxygen) means root cells can't respire much, so they can't take up minerals well, that's why over-watered plants can still look starved. And if a poison blocks respiration, active transport shuts down but diffusion and osmosis carry right on.
> 🔑 Key idea: no respiration = no active transport (but diffusion and osmosis keep going). Low oxygen or a respiratory poison hits active transport only.
Worked Example: The Respiratory PoisonE3.3
Problem A poison is added to living cells so they can no longer release energy from respiration. What happens to movement across the membrane?
Step 1 Sort the three processes by whether they need energy. Diffusion and osmosis are passive, they run on the particles' own motion.
Step 2 Active transport is the only one that needs energy from respiration.
Step 3 So when respiration stops: active transport stops (no more uptake against the gradient), but diffusion and osmosis keep going. Don't say "everything stops", the free processes don't care about the poison.
Key Points to Remember
- Diffusion: net movement of particles from high to low concentration (down the gradient), from random motion; free (no energy).
- Faster diffusion = bigger surface area, steeper gradient, shorter distance, higher temperature, smaller molecules. Small things have high SA:V; big active organisms need exchange surfaces + transport systems.
- Osmosis: net movement of water from higher to lower water potential through a partially permeable membrane; free. Pure water = highest (zero) water potential.
- Animal cells (no wall) burst (lysis) in dilute surroundings and shrivel in concentrated ones. Plant cells (with wall) go turgid (firm), flaccid (soft/wilting), or plasmolysed (membrane pulls off the wall).
- Water is the body's main solvent, and turgor gives soft plants their support.
- Percentage change in mass = (final − start) ÷ start × 100. Plus = gained water, minus = lost water.
- Active transport: particles moved against the gradient (low → high) using carrier proteins + energy from respiration. Examples: root hair mineral ion uptake, gut glucose absorption.