Learn and test your biological vocabulary using these 9.2 Transport in phloem flashcards.
Summary list for 9.2 Transport in phloem
Phloem and translocation
- Plants transport organic compounds (like sugars) from sources (leaves) to sinks (flowers, roots).
- Transport occurs along hydrostatic pressure gradients.
- Active transport causes high concentrations of solutes in the phloem sieve tubes at the source.
- This leads to water uptake by osmosis and raised hydrostatic pressure
- The result is that the phloem sieve tube contents flow away from the source.
Skills
- Identification of xylem and phloem in microscope images of stem and root.
- Ability to compare structure and function of phloem sieve tubes.
- Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
Revision mindmap
These diagram summaries cover the main sections of topic 9.2 Transport in phloem.
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Exam style question about phloem
Explaining translocation is an important skill from this topic.
Answer the question below, on a piece of paper, then check your answer against the model answer below.
Describe the transport of sucrose in vascular plants [4]
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Click the + icon to see a model answer.
Exam style question on phloem structure and function.
Answer the question on paper and then compare your answer to the model answer.
The image is of phloem tissue in vertical section through a light microscope.
Describe three ways in which the structure of phloem tissue is related to its function. (3 marks)
Examiner hint: In this 'Describe' question– give details. Link structure to function in each or successive sentences.
Phloem tissue includes vessels and companion cells.
This is a self marking quiz containing questions covering the topic outlined above.
Try the questions to check your understanding.
START QUIZ!
The diagram below shows a phloem sieve tube and its companion cell.
Which of the following statements best describes the special features of phloem sieve tubes?
Phloem sieve tube elements have perforated cell walls and no nucleus.
The companion cells help to keep them alive.
The diagram below shows a phloem sieve tube and its companion cell.
Distinguish between companion cells and sieve tube cells?
Phloem sieve tube elements have perforated cell walls, no nucleus and pale cytoplasm because there is a flow of sap through them.
The companion cells have very metabolically active dense cytoplasm, with lots of mitochondria because they perform many of the metabolic functions for the sieve tube cells.
Which adaptations of sieve tube cells help the flow of cell sap?
The perforated cell walls help the flow of cytoplasm from areas where pressure potential is high (source) to the low pressure at the sinks.
The diagram below shows xylem and phloem tissue.
Which is the correct label for each cell type?
Xylem cells have no cytoplasm and spiral thickening in A, not found in phloem. Companion cells and sieve plates of phloem tissue can be seen in the phloem cells in B.
In which direction does sucrose travel in the phloem sieve tubes?
Sucrose and small proteins travel in the phloem by translocation, from a source (usually leaves) to a sink (growing buds, roots, flowers, fruit).
The image below shows results of an experiment in translocation in a photosynthesising plant.
What can be determined from the results?
Sucrose is translocated from photosynthesising leaves (sucrose sources) to rapidly growing leaves (sucrose sinks).
It is interesting to note that only the rapidly growing leaves on the same side as the leaf given radioactive carbon dioxide have received sucrose from this mature leaf.
The image below shows results of an experiment in translocation in a photosynthesising plant.
A high concentration of sucrose would build up in mature leaves doing photosynthesis, because their need for carbohydrate is lower as they are not growing.
The sucrose is moved by active transport into phloem cells of these leaves.
What is the effect of this high sucrose concentration in phloem of leaf veins?
If there is a high concentration of sucrose in the phloem then water will move into the phloem by osmosis from the xylem. This causes an increase in hydrostatic pressure and sap will flow in phloem towards places with lower pressure.
Phloem and xylem cell in plants are highly specialised.
Which of the following statements best describes these specialisations?
Xylem has thickened cells walls, with sometimes large perforations and no cytoplasm, once they are mature. Phloem cells don't have thickened walls, but always have cytoplasm. Companion cells have a nucleus, but sieve tube cells do not. Sieve tube cells have perforated cell walls at their ends, called sieve plates.
Which statement best explains how the phloem contents flow from source to sink in phloem?
The sink takes sucrose out of the phloem, this reduces its water potential and it's (hydrostatic) pressure. Phloem always flow towards the sink to increase this lower pressure.
Aphids feed on
phloem sap and are sometimes used in studies of the movement of phloem contents.
What use do scientists make of the Aphids?
The contents of the phloem can be collected and measured if an aphid stylet is left in the plant.
Drag and drop activities
Test your ability to construct biological explanations using the drag and drop questions below.
Rate and direction of flow of organic substances in the phloem sieve tubes.
Drag and drop the correct word or phrase into the gap to describe how phloem transport was investigated.
aphid Radioactive sap organic stylet fluorescently single radioactively tongue water points
To investigate transport of organic substances in the phloem, it is necessary to penetrate phloem sieve tubes and also to label individual chemicals to determine rate and direction of transport.
An is a small insect which has a long which penetrates a single phloem sieve tube. The aphid is anaesthetised with carbon dioxide and the stylet cut carefully from the head, the stylet remaining in the vessel and exuding phloem for analysis.
tracers allow chemical types to be identified. Aphids placed at different on the stem allow rates of transport to be determined.
Explanation: The aphid stylet allows the contents of a single phloem sieve tube to be collected for analysis.
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