8.1.7Transport in Plants
Translocation — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 8 of 8
Part 1 of 7What is translocation?
Translocation is the movement of dissolved food, mainly sucrose, from the leaves where it is made to other parts of the plant.
- but it is converted to sucrose for transport because sucrose is more soluble and less likely to be used in reactions on the way.
Part 2 of 7Sources and sinks
The phloem moves substances from a source, a part of the plant that makes or releases sugar, to a sink, a part that uses or stores it. Leaves are the main source. Sinks include:
The roles can change during the year. A potato tuber is a sink while the plant is growing, but in spring it becomes a source when stored food is used to grow new shoots.
Part 3 of 7The structure of phloem
Phloem is made of living cells. Sieve tube elements are joined end to end, with perforated end walls called sieve plates through which dissolved food can flow.
- so it carries out the metabolism for both cells and supplies the energy to load sucrose into the sieve tube by active transport.
Part 4 of 7How translocation works
Because translocation needs energy, it is an active process, unlike transpiration. In outline:
This is the mass flow idea. The sap can move in both directions in a plant, but in a given tube it flows in one direction at a time.
Part 5 of 7Comparing the two transport streams
| Feature | Transpiration (xylem) | Translocation (phloem) |
|---|---|---|
| Substance | Water and mineral ions | Sucrose and amino acids |
| Direction | Upwards, from roots to leaves | From sources to sinks, up or down |
| Tissue | Dead xylem vessels | Living sieve tubes and companion cells |
| Energy needed | No energy from the plant; driven by evaporation | Yes, energy needed for loading |
Part 6 of 7Evidence and investigation
Scientists have shown that sap flows in the phloem using aphids.
- which shows that phloem carries sugar downwards.
Uses of the sugar that arrives
What happens to sucrose at the sink depends on the cell.
- which does not affect water potential in the cells,
- so it can be stored in large amounts without drawing in water.
- which is why translocation can go upwards as well as downwards.
Part 7 of 7Exam-style questions
Describe the function of the phloem and explain how it is adapted for this function. [4]4 marks
The phloem carries sucrose (and amino acids) from the leaves to other parts of the plant [1]. Sieve plates allow the sap to flow from cell to cell [1]. Sieve tube elements have little cytoplasm and no nucleus, so there is little to obstruct the flow [1]. Companion cells have many mitochondria and supply energy for loading [1].
Give two differences between transpiration and translocation. [2]2 marks
Transpiration moves water, translocation moves sucrose [1]. Transpiration occurs in dead xylem and uses no energy from the plant, translocation occurs in living phloem and needs energy (or: translocation can move in both directions) [1].
A gardener removes a complete ring of bark from a tree trunk. Explain why the tree eventually dies. [3]3 marks
The ring of bark contains the phloem, so sucrose can no longer move from the leaves to the roots [1]. The roots are starved of sugar and cannot respire to take up water and mineral ions by active transport [1], so the whole tree dies [1].
Key terms
| Term | Meaning |
|---|---|
| Translocation | The transport of dissolved sugars in the phloem. |
| Source | A part of the plant that makes or releases sugars. |
| Sink | A part of the plant that uses or stores sugars. |
| Sieve plate | A perforated end wall between phloem sieve tube elements. |
| Companion cell | A cell next to a sieve tube that provides energy and carries out its metabolism. |
| Sucrose | The sugar in which food is transported through the phloem. |
| Mass flow | The movement of fluid along a pressure gradient. |
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