Transport Across the Cell Membrane
Senior 2 · Biology
Transport Across the Cell Membrane
Lesson 5 of 5
20 min
Let's begin
Soil water around a bean root may contain far fewer nitrate ions than the root cells already hold. By diffusion, nitrate would leak out of the root. Yet healthy roots keep absorbing nitrate from the soil.
Cells can move substances the "wrong" way, against the concentration gradient, but only by spending energy. They also swallow whole bacteria and release enzymes in bulk. This lesson covers the processes that use energy.
Active transport is the movement of particles across a membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration (released as ATP).
Examples: root-hair cells taking up mineral ions (nitrate, magnesium) from dilute soil water; absorption of the last glucose and amino acids from the small intestine into the blood; reabsorption of glucose in the kidney.
Anything that affects respiration affects active transport, because respiration supplies the energy.
Question: Barley roots were kept in a solution of potassium ions. Air was bubbled through one beaker and nitrogen gas through another. After 3 hours, the roots with air had taken up 10 times more potassium. When cyanide was added to the air beaker, uptake stopped. What kind of transport is this? Explain.
Solution: It is active transport. Bubbling nitrogen removes oxygen, so aerobic respiration nearly stops, and uptake fell sharply. Cyanide, a respiratory poison, stopped uptake completely. Uptake therefore depends on energy from respiration. Diffusion would not be affected by oxygen or cyanide.
Some particles are far too large for any carrier. The cell moves them wrapped in pieces of membrane, which also uses energy.
| Diffusion and osmosis (passive) | Active transport | |
|---|---|---|
| Direction | down the concentration gradient (high → low) | against the gradient (low → high) |
| Energy from respiration | not needed | needed (ATP) |
| Carrier proteins | not needed (except in facilitated diffusion) | always needed |
| Affected by oxygen or cyanide? | no | yes |
| Stops at equilibrium? | yes, no net movement once concentrations are equal | no, it can keep piling up a substance |
| Example | oxygen entering blood in the lungs | root hairs absorbing nitrate |
Tip
Memory hook: diffusion is like rolling a ball downhill, and it costs nothing. Active transport is pushing it uphill, and it costs energy (ATP).
Exam technique: if uptake stops without oxygen or with cyanide, the answer is active transport, and you must mention energy from respiration.
Define active transport.
Solution: The movement of particles across a membrane from a lower to a higher concentration (against a concentration gradient), through carrier proteins, using energy from respiration.
Explain why crops in waterlogged soil may show signs of mineral deficiency.
Solution: Water fills the air spaces in the soil, so roots get little oxygen. Their aerobic respiration slows, less energy is available, and active transport of mineral ions into the root hairs falls.
Name the process by which a white blood cell engulfs a bacterium.
Solution: Phagocytosis, a type of endocytosis.
Give two differences between diffusion and active transport.
Solution: Any two: diffusion goes down a concentration gradient and active transport against it; diffusion needs no energy from respiration and active transport does; active transport always uses carrier proteins; active transport is stopped by lack of oxygen or by respiratory poisons.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
