Define translocation.

It is the transport of assimilates, mainly sucrose, through the phloem from a source to a sink.

What is meant by a source, and give two examples.

A source is a part of the plant that loads assimilates into the phloem, such as a photosynthesising leaf or a storage organ mobilising its reserves in spring.

What is meant by a sink, and give three examples of harvestable sinks.

A sink is a part of the plant that removes assimilates from the phloem to use or store them, such as a potato tuber, a carrot storage root or a cereal grain.

How does sucrose pass from a companion cell into a sieve tube element?

It diffuses through the plasmodesmata that connect the two cells.

OCR A-Level Biology B H422

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Transport Systems in Plants

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Transport Systems in Plants

Translocation in the Phloem

3 things to know
Components 01, 02 and 03

Overview

Recall questions on sources and sinks, how sucrose is loaded at a source, what drives mass flow, why sucrose rather than glucose is carried, and the evidence from aphids.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Why is translocation described as an energy-requiring process?

Key term

How is sucrose actively loaded into the phloem at a source?

Key term

How is a high hydrostatic pressure generated in the phloem at a source?

Questions from this subtopic

Each one with the model answer underneath.

1
Define translocation.
Model answer
It is the transport of assimilates, mainly sucrose, through the phloem from a source to a sink.
2
What is meant by a source, and give two examples.
Model answer
A source is a part of the plant that loads assimilates into the phloem, such as a photosynthesising leaf or a storage organ mobilising its reserves in spring.
3
What is meant by a sink, and give three examples of harvestable sinks.
Model answer
A sink is a part of the plant that removes assimilates from the phloem to use or store them, such as a potato tuber, a carrot storage root or a cereal grain.
4
Why is translocation described as an energy-requiring process?
Model answer
Sucrose is loaded into the phloem at the source by active transport using ATP, so translocation stops if respiration in the companion cells is prevented.
5
How is sucrose actively loaded into the phloem at a source?
Model answer
The companion cell uses ATP to pump hydrogen ions out into the cell wall, and these ions then re-enter through a co-transporter protein that brings a sucrose molecule in with each one, so sucrose accumulates against its concentration gradient.
6
How does sucrose pass from a companion cell into a sieve tube element?
Model answer
It diffuses through the plasmodesmata that connect the two cells.

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