Sec 3 Combined Science (Biology) Movement of Substances notes

Dissolved particles and gas particles never stop moving — they jiggle randomly, colliding and ricocheting billions of times a second. Watch any single particle and its path is a random walk with no destination. But watch the crowd: where a substance is crowded (high concentration) more particles happen to leave than arrive, and where it is sparse (low concentration) more arrive than leave. The result is a net movement from high to low concentration — no cell needs to push anything.

Three conditions make this net movement faster: a steeper concentration difference (bigger crowd gap), a shorter distance to travel, and a higher temperature (faster jiggling). Three everyday proofs: a tea bag colours hot water faster than cold; a smell crosses a room in seconds through air but takes far longer through a closed door; and a sugar cube dissolves and spreads quickest in stirred hot tea.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. Three words carry the marks: net (overall drift, not every single particle), higher to lower (the direction), and particles (any dissolved substance or gas — not just water).

Life runs on diffusion. In human nutrition, the end-products of digestion — glucose and amino acids — are in high concentration inside the small intestine after a meal and lower concentration in the blood, so they diffuse across the intestinal lining into the bloodstream. In gaseous exchange in humans, oxygen is crowded in the lungs' air sacs and sparse in the blood, so it diffuses in; carbon dioxide runs the opposite way, out. In plants, gases move in and out of leaves through tiny pores called stomata: carbon dioxide diffuses in for photosynthesis by day, and oxygen diffuses out as a product.

Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. Two upgrades turn diffusion into osmosis: only water moves, and it must cross a partially permeable membrane — a barrier with tiny holes that let small water molecules through but hold back larger dissolved particles. The membrane is what makes osmosis matter to cells, because every cell is wrapped in exactly such a membrane.

Direction matters and is the most-tested idea in the topic: water moves from the dilute (more watery) side to the concentrated (less watery) side. Say it as "water goes where the dissolved substances are more crowded" and the direction can never be reversed by a trick question. A simple model: a tube of 2% sugar solution and a tube of 8% sugar solution joined by a partially permeable membrane — water crosses from the 2% side into the 8% side, so the 8% level rises and dilutes.

A plant cell's cell sap (the solution in its large vacuole) is usually more concentrated than the water around the roots or in a raindrop, so water tends to enter by osmosis. As the vacuole swells it pushes the cytoplasm against the rigid cellulose wall, and the cell becomes firm. That internal push, cell after cell, is what keeps stems upright and lettuce crisp. The wall is the plant's safety device: it is strong, so a firm plant cell does not burst even in pure water.

Reverse the surroundings and the plant loses. In a concentrated solution — salty or sugary water, or a plant left without water so its cell sap becomes relatively concentrated — water leaves the cells by osmosis. Vacuoles shrink, the cytoplasm pulls away from the wall, and the tissue goes limp: wilting leaves, drooping stems. The wall itself never disappears; only the water content changes.

Animal cells have no cell wall, so osmosis has free rein over their size. Place a red blood cell or a cheek-lining cell in pure water: water floods in, the cell swells — and because nothing resists the inflow, it may burst. Place the same cell in a concentrated salt solution: water leaves, and the cell shrinks into a crinkled shape. Only in a solution matching the cell's own concentration — as body fluids normally do — does the cell keep its size.

This is why the fluids used in medicine and sport are matched to body concentration: drips and rehydration drinks are formulated so that cells neither swell nor shrink. It also explains kitchen observations — vegetables sprinkled with salt lose water and soften, because the salt solution outside is now more concentrated than the cell sap inside.