Active transport: root hair ion pump sim

What the sim shows

Young barley seedlings stand in a support disc over a beaker of potassium-ion (K⁺) solution, which plays the part of the soil water. An air stone bubbles air, or nitrogen, through it; a three-way tap chooses which. Active transport moves particles against the concentration gradient, from lower concentration to higher, using energy from respiration. Three things follow. It needs carrier proteins in the membrane. It needs ATP, so it stops if respiration stops, which is why cyanide or lack of oxygen halts active transport but not diffusion. And cells that do a lot of it are packed with mitochondria. That is how a root hair takes up mineral ions from soil where they are more dilute than inside the root. Diffusion could never do that, because it would run the other way. Model (illustrative numbers): uptake = active part + passive part. The active part = E × 8 × C ÷ (2 + C) µmol per g root per hour, where C is the soil concentration in mmol/dm³: the carriers saturate, so uptake levels off at high C. E = 1 with air. With nitrogen, or with the inhibitor, aerobic respiration stops and only anaerobic respiration is left, about 2 ATP per glucose instead of about 32, so E = 2/32. The passive part is a tiny diffusion leak, 0.03 × (soil − cell), only while the soil is more concentrated than the cell. The ions stay in the cells' water (0.9 cm³ per g of root), so the cell concentration rises. Vmax, Km and the leak are illustrative (the real Km for K⁺ is much lower); 2 and 32 ATP are approximate; the soil concentration is held fixed; outward leakage and slowing as the cell fills are not modelled. The roots start "low-salt" (grown in calcium sulfate solution), at 5 mmol/dm³. The close-up shows 1 dot for each mmol/dm³ in the soil water and in the cell. Time moves only when you press Wait 10 min.

• The tap's pointer shows which gas bubbles through: right air, left nitrogen
• Close-up: each orange dot is a K⁺ ion; water is not drawn
• Mitochondria glow while the cells respire aerobically

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