9.2 Transport in the Phloem 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.

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.