Explain companion-cell loading of phloem
Companion cells use proton pumps and cotransporter proteins to transfer assimilates into phloem sieve tubes.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the role of the companion-cell proton pump?
What you will learn
- Explain proton pumping by companion cells
- Explain how proton-gradient-driven cotransport moves sucrose into sieve tube elements
A proton pump establishes an electrochemical gradient
A proton pump in the companion-cell surface membrane uses energy from ATP to move H⁺ ions out of the cell. This creates a higher H⁺ concentration outside the companion cell and stores electrochemical potential across the membrane.
The pump does not carry sucrose and does not make sucrose from protons. It establishes the H⁺ gradient that can drive a second transport step.
Cotransport couples proton return to sucrose entry
H⁺ ions move back into the companion cell down their electrochemical gradient through a cotransporter protein. The same protein carries sucrose into the companion cell with the H⁺, even when sucrose moves against its concentration gradient. This is secondary active transport because the cotransporter uses a gradient established using ATP by the proton pump.
Sucrose is then transferred from companion cells into sieve tube elements. Keep the roles in order: ATP-powered proton pump establishes the gradient; cotransporter uses H⁺ return to bring sucrose in; sucrose enters the sieve tube for translocation.
Pause and explain
How does the cotransporter move sucrose into the companion cell?
Worked example
Explain how a companion cell can load sucrose into the phloem using a proton pump and cotransporter.
Show the worked solution
- ATP powers proton pumps moving H⁺ out of the companion cell.
- H⁺ returns through a cotransporter carrying sucrose into the cell; sucrose then passes into the sieve tube.
Answer ATP powers proton pumps that move H⁺ out of the companion cell, establishing an electrochemical gradient. H⁺ then moves back down that gradient through a cotransporter, carrying sucrose into the companion cell; sucrose is transferred into the sieve tube element.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how a proton pump prepares a companion cell for sucrose cotransport.
Give me a hint
Name the energy source, ion and direction.
Compare my reasoning
- ATP powers a proton pump.
- The pump moves H⁺ out, establishing an electrochemical gradient.
ATP powers proton pumps in the companion-cell membrane to move H⁺ out of the cell. This establishes an electrochemical gradient with a higher H⁺ concentration outside.
Look for these in your work
- ATP named
- H⁺ direction correct
- Gradient described
Explain why sucrose can enter a companion cell through a cotransporter even when its concentration is already higher inside.
Give me a hint
Follow the coupled H⁺ movement.
Compare my reasoning
- H⁺ moves back into the cell down its electrochemical gradient.
- The cotransporter couples that movement to sucrose entry against its concentration gradient.
H⁺ returns into the companion cell down the electrochemical gradient established by proton pumps. The cotransporter couples this favourable H⁺ movement to sucrose entry against its concentration gradient; ATP was used by the pump to establish the gradient.
Look for these in your work
- H⁺ movement explained
- Sucrose gradient direction stated
- Primary/secondary energy roles distinguished
A student writes: ‘ATP carries sucrose directly through the phloem membrane, while the proton pump brings H⁺ into the companion cell.’ Correct both errors.
Give me a hint
Separate the pump from the cotransporter and preserve ion directions.
Compare my reasoning
- The pump uses ATP to move H⁺ out.
- H⁺ then returns through a cotransporter carrying sucrose into the companion cell.
ATP powers a proton pump that moves H⁺ out of the companion cell. H⁺ returns down its electrochemical gradient through a cotransporter, which carries sucrose into the companion cell; sucrose is then transferred into the sieve tube element.
Look for these in your work
- Pump direction corrected
- Cotransporter direction corrected
- Sucrose route stated
Common mistakes
- Saying ATP directly powers the H⁺/sucrose cotransporter
- Reversing the direction of proton pumping and cotransport
- Calling cotransport simple diffusion of sucrose
What can you explain now?
What is the role of the companion-cell proton pump?
Compare with the explanation
Use ATP to move H⁺ out and establish a gradient
The pump establishes the gradient used by cotransport.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What does the proton pump establish, and how does the cotransporter use that gradient to load sucrose?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.