9.2 Transport in the Phloem of Plants

Translocation in the phloem can be regarded as the plant equivalent of the mammalian circulatory system. 

Evaluate this statement. 

Two parameters of the contents of the phloem have an influence on the rate of translocation within that phloem. These are collectively called water potential and given the symbol Ψ.

There are two types of water potential, each given the symbols Ψ_p and Ψ_s.

Suggest what each symbol relates to.

Sugar beet (Beta vulgaris subsp vulgaris) is the second most important crop grown for table sugar (behind sugar cane), accounting for around 30% of worldwide production. The roots are harvested and processed for their high sugar content. 

Yields of sugar beet can be affected by Beet Jaundice Virus, which is spread by aphids and turn crops yellow (see image below). 

Suggest how Beet Jaundice Virus decreases yields for commercial beet farmers.

The following statements summarise the results from experiments designed to discover more about the translocation of organic materials in the phloem.

State all the letters that provide evidence for the following conclusions. 

[2]

[2]

Explain how mass flow of the phloem sap occurs in plants with a vascular system. 

The use of aphid stylets to measure translocation is well documented. This exploits the aphid's behaviour of penetrating the plant stem with its stylet in order to extract sugars and nutrients from the phloem. 

Scientists observing this behaviour were initially puzzled by the fact that an aphid appears to excrete a large proportion of the sugar it ingests via its stylet; it clearly ingests a lot more sucrose than it needs for its own metabolism.

Suggest one explanation for this behaviour. 

Phloem sieve plates have an adaptation whereby a sieve plate closes when the phloem is damaged mechanically eg. by a chewing animal. A polysaccharide called callose builds up in sieve plate pores and can seal the pores within as little as 20 minutes. 

Suggest a reason for this adaptation.

Translocation is the movement of the products of photosynthesis within a plant. 

Translocation occurs in the phloem and involves sources and sinks.

Using the outline below, draw the position of the phloem in the root of a dicotyledonous plant. 

Research using carbon dioxide containing a radioactive label, ¹⁴C, has revealed the following evidence about the mechanism of translocation:

A. Radio-labelled carbon can be observed in the phloem soon after being supplied to a well-lit plant

B. The rate of movement of sugars in the phloem is many times faster than could be achieved by diffusion alone

Other research has revealed that:

C. An insect such as an aphid feeds by inserting its stylet (mouth parts) into the phloem

D. The pH of the phloem companion cells is lower than surrounding cells

E. The phloem companion cells contain many mitochondria

Using the letters A, B, C, D and E, select two pieces of evidence from the list above which support the theory that translocation occurs in the phloem.

The majority of cells in phloem tissue are either companion cells or sieve tube elements. A scientist isolated companion cells and conducted some experiments to investigate the mechanism involved in loading sucrose into the sieve tubes. He recorded the following observations:

From Observation 1, the scientist concluded that the mechanism involved a transfer of charged particles (ions) between the companion cells and their surroundings.

Deduce the conclusions that can be drawn from Observations 2 and 3 about the mechanism.

The image below shows a potato plant. Potatoes grow tubers, which are underground storage organs. 

Actively growing tissues have a high demand for carbohydrates. This means that a lot of phloem sap is directed to these tissues and requires sucrose to be unloaded in large amounts.

In an investigation, potato plants were modified by having a gene for invertase inserted into their DNA so that the gene would be expressed in the tubers. Invertase is responsible for catalysing the hydrolysis of the disaccharide sucrose.

A trial experiment was carried out to compare the properties of the modified plants with those that had not been modified. After harvesting, the tubers of three of each type of plant were compared. The results are shown in the table below. 

In the modified plants, the unloading of sucrose is increased in the tubers compared to those that were not modified.

The transport of sucrose to the tubers was also increased in the modified plants.

Using the data and the information given, deduce a possible mechanism to account for the increased unloading and transport of sucrose in the modified plants.

One mark is available for clarity of communication throughout this question.

Compare and contrast the apoplast and symplast pathways of translocation in plants. 

Outline the method by which aphid stylets can be used to measure the rate of translocation in the stems of plants. 

The diagram below shows a transverse section (TS) of a plant stem.

Identify the structure found at I and state its function.

A scientist used radioactive carbon dioxide to investigate the direction and rate of sucrose transport in a plant. Their results showed that during periods of plant growth, sucrose was mainly transported upwards.

Explain why this occurs.

Although most of the sugars produced by plants are used up rapidly in respiration, plants can sometimes overproduce sugars during photosynthesis. Excess sucrose in sink tissues can be converted to starch.

Suggest the benefits of this for maintaining translocation in the plant.

Translocation occurs from regions known as ‘sources’ to regions known as ‘sinks’.

Describe what the term ‘sink’ refers to and state two examples of ‘sinks’.

Plasmodesmata are present in the cell walls of companion cells.

Explain what these plasmodesmata are and their function in companion cells.

Referring to the movement of sucrose in your answer, explain how a high hydrostatic pressure is generated at ‘source’ sites in plants.

An aphid was allowed to feed on a plant stem. The aphid was anaesthetised and its head and body were then removed, leaving the stylet (mouthparts) still in place. The severed stylet of the aphid, still embedded in the plant tissue, is shown in the diagram below.

Identify tissue B.

Identify tissue A in the diagram above and state one piece of evidence to support your identification.

Describe how the movement of solutes through plant tissues can be studied and measured using aphid stylets.

One mark is available for clarity of communication throughout this question.

Outline the symplast pathway for the loading of sucrose into sieve tubes.

Describe how sugar is transported from the lower leaves of a plant to the growing leaf buds at the top of a plant.

Photosynthesis produces glucose, which is then transported and stored in plants.

Outline how this glucose is transported and stored.