Lesson 4 of 7 / Movement of substances
Active transport
How can a root hair cell take in ions from a dilute solution?
I can explain movement against a concentration gradient using energy from respiration.
I can explain energy-dependent uptake in roots and villi.Syllabus
Syllabus K325, Topic 2(c). Energy-dependent uptake in roots and villi.
About 4 min
Make a guess. You are not marked.
A root hair cell already has more nitrate ions inside it than the soil water around it. Can it still take in more nitrate?
Show the answer
Yes, by using energy from respiration.
Active transport moves ions against their gradient. Carrier proteins in the membrane do this using energy from respiration.
The key idea
Active transport moves substances across a cell membrane from lower to higher concentration, against their concentration gradient, using energy from respiration.
Explore the idea
Can a cell take in ions when there are already more inside?
Energy allows transport against the gradient.A carrier protein uses energy from respiration to move the ion from the lower concentration outside to the higher concentration inside.
- The ion being transported
- Carrier protein
- Cell membrane
A carrier binds a substance and changes shape; it is not a permanently open pore. This is a simplified transport step, not a molecular simulation. The concentration difference remains after one ion moves.
Explanation
Diffusion cannot produce a net movement up a concentration gradient. A cell can move some substances in that direction by active transport. Specific carrier proteins in the membrane help move them across, using energy supplied by respiration.
A root hair cell can take up mineral ions from soil water even when their concentration is lower in the soil water than inside the cell. Active transport allows uptake to continue against that concentration gradient.
Active transport also helps cells on the villi of the small intestine absorb glucose when its concentration is lower in the intestine than inside those cells. The uptake depends on energy from respiration. Always use the gradient given in the question: movement into a cell is not automatically active transport.
If less energy is available from respiration, active transport is reduced. This does not mean every kind of movement across the membrane stops. Diffusion and osmosis do not directly require that energy supply.
Step by step
- 1
Read the concentration values
Check whether the substance is moving from lower to higher concentration.
- 2
Identify the process
Movement against the concentration gradient across a membrane requires active transport in these examples.
- 3
Explain the energy supply
Link active transport to energy released by respiration in the cells.
Worked example
Mineral ions entering roots
Mineral ion concentration is lower in the soil water than inside a root hair cell, but the cell continues taking up the ions. Explain how.
One way to explain it
The mineral ions enter the root hair cell by active transport, from a lower concentration in the soil water to a higher concentration inside the cell. This movement is against their concentration gradient and uses energy released by respiration.
Why this answer works
- Name mineral ions and the root hair cell.
- State lower to higher concentration.
- Connect movement against the gradient to energy from respiration.
Watch out for this
A student says: "Any substance moving into a cell enters by active transport." What is wrong with this?
Show the answer
Direction relative to the concentration gradient matters. Oxygen can diffuse into a cell down its gradient; mineral ions can be actively transported into a cell against theirs.
Check your understanding
Glucose concentration is lower in the small intestine than inside its villus cells. How can glucose uptake into those cells continue?
Show the answer
By active transport, supported by energy from respiration.
Glucose is taken up against its concentration gradient, from lower concentration in the intestine to higher concentration in the villus cells. This uptake depends on energy from respiration.
Quick recall
How can a root hair cell take in ions from a dilute solution?