Several lines of evidence show that some substances enter cells by active transport rather than by diffusion.1. Movement against a concentration gradient. Some cells build up ions at a much higher concentration than in their surroundings. Diffusion can only move a substance down its gradient, so continued uptake when the inside concentration is already higher means the substance is moving against the gradient. For example, if a root contains 50 mmol dm−3 of potassium ions while the solution around it has 5 mmol dm−3, the inside concentration is 10 times the outside.2. Dependence on respiration. Active transport needs ATP from respiration. Uptake falls sharply or stops when:
• a respiratory inhibitor such as cyanide is added, because it inhibits aerobic respiration so little ATP is made;
• oxygen is removed, for example by bubbling nitrogen instead of air through the solution around roots.
Diffusion is unaffected.3. Effect of temperature. Active transport is much more sensitive to temperature than diffusion. Rising temperature increases the rate of the enzyme-controlled reactions of respiration, but very high temperatures denature the enzymes and carrier proteins, so uptake falls.4. Many mitochondria. Cells that carry out a lot of active transport, such as root hair cells, epithelial cells of the small intestine and kidney tubule cells, contain many mitochondria to supply ATP.5. Selectivity. Cells take up some ions much faster than others of similar size, which shows that specific carrier proteins are involved.A typical experiment: barley roots are kept in potassium chloride solution, with one set aerated and one set bubbled with nitrogen. The aerated roots take up far more potassium, showing uptake depends on aerobic respiration.
• a respiratory inhibitor such as cyanide is added, because it inhibits aerobic respiration so little ATP is made;
• oxygen is removed, for example by bubbling nitrogen instead of air through the solution around roots.
Diffusion is unaffected.3. Effect of temperature. Active transport is much more sensitive to temperature than diffusion. Rising temperature increases the rate of the enzyme-controlled reactions of respiration, but very high temperatures denature the enzymes and carrier proteins, so uptake falls.4. Many mitochondria. Cells that carry out a lot of active transport, such as root hair cells, epithelial cells of the small intestine and kidney tubule cells, contain many mitochondria to supply ATP.5. Selectivity. Cells take up some ions much faster than others of similar size, which shows that specific carrier proteins are involved.A typical experiment: barley roots are kept in potassium chloride solution, with one set aerated and one set bubbled with nitrogen. The aerated roots take up far more potassium, showing uptake depends on aerobic respiration.