BRAINCAKEEvidence for Active Transport
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:
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Key terms in this lesson
- active transport
- The movement of molecules or ions across a membrane against their concentration gradient, using energy from ATP.
- concentration gradient
- The difference in the concentration of a substance between two regions, which determines the direction of net diffusion.
More in Cell Transport Mechanisms
- Osmosis in Animal Cells
- Water Potential in Plant Cells
- Osmosis in Plant Cells
- Turgor Pressure (P)
- Osmotic Potential (π)
- Water Potential, Turgor Pressure and Osmotic Potential
- Active Transport
- Endocytosis and Exocytosis
All 18 lessons in Cell Transport Mechanisms · All Edexcel AS-level Biology topics