Phloem Transport (9.2)

Question 1

Define translocation.

Answer 1

The movement of organic compounds (e.g. sugars, amino acids) from sources to sinks via the phloem. The source is where the organic compound is synthesised (eg. the photosynthetic tissue - leaves) and the sink is where the compounds are delivered to for use or storage (eg. roots, fruits and seeds).

Question 2

Describe the structure of phloem.

Answer 2

Phloem are composed of sieve element cells and companion cells. Sieve elements are long and narrow cells that are connected together to form the sieve tube. Companion cells provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink. *Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate).

Question 3

How is the structure of sieve tube cells related to their function?

Answer 3

• Sieve element cells are connected by sieve plates which are porous to enable continuous movement of the organic compounds between cells.- They have no nuclei and a reduced number of organelles to maximise space for the translocation of materials.- They also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow.

Question 4

Describe translocation.

Answer 4

The loading and unloading of sucrose and other organic compounds from the source to the phloem, and from the phloem to the sink is an active process, meaning that it requires energy in the form of ATP. The organic compounds are actively loaded into the phloem vessels. The high concentrations of solutes in the phloem at the source cause water to move into the vessels by osmosis. This results in a raised hydrostatic pressure, which causes the contents of the phloem to flow towards sinks.

Question 5

How are aphids used to measure phloem transport rates?

Answer 5

A plant is grown within a lab containing radioactively-labelled carbon dioxide. The leaves will convert the carbon dioxide into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem. The aphids are encouraged to collect sap at various sites along the plant’s length. The sap is then analysed for the presence of the radioactively-labelled sugars. The rate of phloem transport can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length.