Sec 3 Biology Movement of Substances notes

Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated — that is, down a concentration gradient. It costs the cell nothing: the random jostling of molecules does the work. It stops mattering where the gradient disappears; net movement continues only while a gradient exists.

Life leans on diffusion everywhere. In your lungs, oxygen is thicker in the air than in the blood, so it diffuses into the blood while carbon dioxide diffuses the other way. In a leaf, carbon dioxide diffuses from the air through the stomata into the chloroplast-packed cells, and oxygen escapes the same door. In the gut, digested glucose and amino acids diffuse across the epithelium into the blood once the blood carries less of them than the gut contents.

Three things speed diffusion up: a steeper concentration gradient, a shorter diffusion distance, and a larger surface area. Exchange surfaces are engineered around all three — alveoli, villi, gills and root hairs are thin, numerous and folded exactly because surface-area-to-volume ratio falls as structures grow (see the last section of this chapter).

Osmosis is the net movement of water molecules from a region of higher water potential (a more dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane. The membrane lets water through freely but holds back the dissolved solutes, so the water alone relocates. Think of water potential simply: the more dilute a solution, the higher its water potential — pure water has the highest of all.

In a plant cell, the membrane plus the cellulose wall around it enclose the vacuole’s sap. Placed in water, the cell takes water in; the vacuole swells and presses the cytoplasm against the wall. The wall pushes back, and the swollen, firm cell is turgid — the state that keeps lettuce crisp and seedlings upright. Placed in a concentrated solution, the cell loses water; the vacuole shrinks and the cytoplasm pulls away from the wall. The cell is plasmolysed. Between the two sits the flaccid cell, losing just enough water to go limp.

An animal cell has no wall to lean on. In pure water it swells until the membrane bursts; in a concentrated solution it wrinkles and shrinks. That is why your cells must live in blood and tissue fluid whose concentration matches theirs — and why drinking seawater is a bad idea.

Water never moves randomly across a membrane: it moves from where its water potential is higher to where it is lower. Dissolving anything lowers water potential, so the ordering rule is simple — pure water > dilute solution > concentrated solution. Calculations are not required at O-Level; reading the direction is.

That single rule runs through the syllabus. It keeps a plant cell turgid (soil water outside has a higher water potential than the sap). It plasmolyses the same cell in fertiliser spill or seawater. It wilts a plant in dry soil, when the drying soil’s water potential falls below the root cells’ and water reverses direction. And in the Sec 4 plant topic it drives water’s whole climb from root hair to leaf.

Diffusion and osmosis only run downhill. But cells often need substances that are already scarcer outside than inside. Active transport uses membrane proteins and energy from respiration (ATP) to move particles against their concentration gradient — from lower to higher concentration. It is the only one of the three routes that costs energy, which is why cells packed with it are packed with mitochondria too.

Two syllabus examples matter. Root hair cells pull mineral ions (nitrate, magnesium) out of soil water where those ions are scarcer than in the root — diffusion would leak them out, so active transport hauls them in. The cells of the villi keep absorbing glucose even late in a meal, when the gut lumen holds less glucose than the epithelial cells do. In both cases the cell respires faster to fund the pumping.

Keep the three routes straight by their currency and direction: diffusion moves any particles down their gradient for free; osmosis moves water only down its gradient through a partially permeable membrane; active transport moves substances up their gradient at the price of ATP.

The standard investigation weighs potato strips before and after soaking in solutions of different concentration. Water potential outside the strip decides the direction: strips in water gain mass, strips in strong sugar solution lose mass. Because the strips never start at exactly the same mass, the fair measure is the percentage change in mass, not the raw change.

1. 1Cut and weigh

   Cut strips of identical initial size, blot them dry, and record each starting mass.

2. 2Soak

   Place one strip in each solution (distilled water through concentrated sugar) for the same time, at the same temperature, in the same volume of liquid.

3. 3Blot and reweigh

   Remove each strip, blot its surface dry, and record the final mass.

4. 4Calculate

   Percentage change = (final − initial) ÷ initial × 100. A positive sign means water entered; a negative sign means it left.

For a fair test, control everything but the concentration: identical starting strips, equal solution volumes, equal temperature and timing — and blot every strip before weighing so surface liquid does not masquerade as water uptake.

Every substance a cell gains or loses must cross its surface, but its needs scale with its volume. The surface-area-to-volume ratio therefore decides how easily a structure can feed and breathe itself by diffusion alone. The ratio falls as an object grows.

So every exchange surface in the syllabus follows one recipe: large area (folds, villi, root hairs, alveoli), thin walls (one cell thick), a moist face where gases dissolve, and a rich blood supply or a steep gradient to keep diffusion running. Name the recipe and you can answer any "how is this surface adapted?" question.