9.2 Transport in the Phloem of Plants

Name the two main assimilates (products of plants' metabolism) that are carried in the phloem. 

[2 marks]

Define the term 'source' when used in the context of plant biology. 

[2 marks]

The image shows a wild banana, cut lengthways. Inside the banana, its seeds are clearly visible. Seeds are usually regarded as sinks. 

Give an example of when seeds such as these act as a source. 

[1 mark]

Name the features of the symplast pathway of sucrose translocation that are not present in the apoplast pathway. 

[2 marks]

Plasmodesmata found in phloem cells tend to have a wider diameter than those found in cells of the xylem. 

Suggest a reason for this.

[1 mark]

Phloem sieve tubes are adapted to minimise the frictional resistance to the flow of fluid within them.

List two ways by which friction is minimised within a phloem sieve tube. 

[2 marks]

A traditional practice used in forestry to manage tree growth is called girdling. This involves removing a complete ring of bark, as shown in the image below. The tree dies over a period of time, allowing forests to be thinned out as required. 

State how girdling may kill a tree over time. 

[2 marks]

State the role of the parenchyma in phloem tissue. 

[2 marks]

Sucrose enters the phloem companion cells via a co-transporter protein. 

Explain how this co-transporter protein works. 

[2 marks]

The processes referred to in part a) take place in a specialised type of companion cell called a transfer cell. 

State two adaptations of a transfer cell that allow it to carry out sucrose co-transport effectively. 

[2 marks]

The same hydrostatic pressure gradient exists within two different phloem tubes (1. and 2.) in a terrestrial plant. 

These phloem tubes are located in the following parts of the plant: 

1. Root tuber to growing shoot tip
2. Mature leaf on lower branch to the root tip

Suggest with a reason, whether 1. or 2. will result in a higher rate of translocation. 

[2 marks]

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

Phloem sieve tubes, similar to many other specialised cells, have features that allow them to carry out their function. Some of these are listed in the table below:

Outline these features and how they assist the function of phloem by completing the missing information in the table above. 

[6 marks]

List five contrasts between xylem and phloem vessels. 

[5 marks]

A plant was given radio-labelled CO₂ containing the ¹⁴C isotope of carbon. 

(i) Explain how this radio-labelled CO₂ is administered to the plant.

[2 marks]

(ii) An aphid stylet placed 18.9cm underneath the leaf first leaked radioactive assimilates after a period of 159 minutes. Calculate the rate of flow of assimilates in the phloem.

State your answer in mm hr^-1.

[2 marks]