Plant Biology: Topic 9.2: Phloem Transport

Question 1

Define translocation

Answer 1

Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

Question 2

Define source and sink

Answer 2

The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)
The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds

Question 3

Why is sucrose the transport. medium and what is phloem sap

Answer 3

Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert
The nutrient-rich, viscous fluid of the phloem is called plant sap

Question 4

Outline structure of sieve elements

Answer 4

Phloem sieve tubes are primarily composed of two main types of cells – sieve element cells and companion cells

Sieve elements are long and narrow cells that are connected together to form the sieve tube

Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow

Question 5

Outline the structure and function of companion cells

Answer 5

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink

Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube

Question 6

Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell, why?

Answer 6

This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites

Question 7

How many ways can organic compounds produced at the source can be loaded into sieve tubes?

Answer 7

Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

Question 8

Outline the apoplastic loading of sucrose into the phloem sieve tubes

Answer 8

Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure

Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source

Question 9

Explain the active translocation of materials via mass flow at the source

Answer 9

At the Source

The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
Hence, the phloem transports solutes away from the source (and consequently towards the sink)

Question 10

Explain the active translocation of materials via mass flow at the sink

Answer 10

The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
Hence, phloem sap will always move from the source towards the sink
When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles