Learning focus
Develop accurate biological vocabulary, explain structure–function relationships, apply ideas to unfamiliar contexts, and use practical evidence.
Definition
Active transport is the movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy released during respiration. Membrane carrier proteins are involved.
Comparison with diffusion
Diffusion moves particles down a concentration gradient and does not use energy released by respiration. Active transport can accumulate substances inside cells even when their external concentration is low. Both processes may involve membrane proteins, but the direction relative to the gradient and the energy requirement differ.
Root hair cells
Mineral ions such as nitrate may be less concentrated in soil water than in root hair cytoplasm. Active transport moves these ions into the cell. Root hair cells have a large surface area and contain mitochondria, supporting rapid uptake. Oxygen is needed for aerobic respiration, so waterlogged soil can reduce ion uptake by limiting air around roots.
Human examples
Active transport contributes to glucose and amino-acid absorption in the small intestine and to selective reabsorption in kidney tubules. The process allows valuable substances to be moved even when diffusion alone would not produce sufficient uptake.
Experimental reasoning
If respiration is inhibited by low temperature, lack of oxygen or a metabolic poison, active transport decreases. Diffusion may continue because random particle movement remains. This difference can be used to interpret experimental data comparing living and non-living tissues.
Practical or data skill
Analyse a data set showing ion uptake at different oxygen concentrations or temperatures. Explain the pattern by linking oxygen or temperature to respiration, ATP supply and carrier-protein activity.
Examination tip
State all three elements: against the gradient, through the membrane, using energy from respiration.
Review questions and suggested answers
Question 1
Why is active transport needed for nitrate uptake?
Suggested answer
Nitrate ions may be at a lower concentration in soil than inside root cells, so they must move against the gradient.
Question 2
What supplies the energy for active transport?
Suggested answer
Energy released during respiration.
Question 3
How would lack of oxygen affect active transport in roots?
Suggested answer
It reduces aerobic respiration and therefore reduces energy available for ion uptake.
