Transport in plants - phloem

In a nutshell

The phloem are tubes that transport solutes around the plant to where they are needed. Solutes are dissolved substances that are transported through the plant. This summary details the mechanism of translocation and evidence supporting the theory.

Adaptations

Phloem tubes have certain adaptations that make them better suited to transport solutes around plants.

Adaptation	Description
Sieve tube elements	The sieve tube elements are living cells with no nucleus and very few organelles. They form a tube to transport the solutes.
Companion cells	Each sieve tube element has a companion cell. This is the cell that carries out the metabolic processes to support the sieve tube element. Example Providing energy required for the active transport of solutes.
Plasmodesmata	The companion cells are attached to the sieve tube elements through plasmodesmata.

Translocation

Definition

Translocation is the movement of solutes through a plant to where they are needed. The solutes start at their source, which is where they are made, and they are transported to their sink. The sink is where they are used.

There is a clear concentration gradient with a higher solute concentration at the source than the sink.

Example

Glucose is a sugar and the source of glucose is the leaves as it is made during photosynthesis. The solute will be transported to other parts of the plant such as the meristems (areas of active growth). The meristems will be the sinks.

Enzymes play an important role in translocation as they convert the solutes into other products at the sink. This ensures that there is always a lower concentration of solutes at the sink than at the source.

Example

Sucrose is often converted to starch at the sink so there is a lower concentration of sucrose at the sink than inside the phloem.

Mass flow hypothesis

The current accepted theory of how translocation occurs is the mass flow hypothesis. This process is explained below.

Biology; Exchange and transport; KS5 Year 12; Transport in plants - phloem

A.	Companion cell
B.	Solutes are produced at the source
C.	Solutes are removed at the sink
1.	Chloroplast
2.	Nucleus
3.	Starch grain

Procedure

1.	Solutes are produced. Example Sucrose is produced during photosynthesis in leaf cells.
2.	The solutes are transported through active transport from the companion cells into the sieve tubes at the source. This process uses ATP from respiration.
3.	The high solute concentration lowers the water potential inside the phloem so water travels through osmosis from adjacent xylem tubes.
4.	This increased water increases the hydrostatic pressure in the lumen of the phloem at the source end of the sieve tube.
5.	The fluid in the sieve tubes flow from high to low pressure.
6.	The solutes are removed from the phloem at the sink.
7.	This increases the water potential inside the phloem so water will leave through osmosis back into the xylem.
8.	This will lower the hydrostatic pressure inside the sieve tube.
9.	The pressure gradient that is generated will push the solutes from their source to their sink where they are used.

Note: The higher the concentration of the solute, the greater the rate of translocation.

Evidence supporting and against the hypothesis

Supporting	Against
Sap is released when some plants are cut. This proves there is pressure within the sieve tubes.	The mass flow hypothesis assumes that all solutes move at the same speed and this is not always the case.
Radioactive tracers (like ${^1}{^4}C$ ) can be used to track the movement of solutes. These have supported the mass flow hypothesis.	Sieve plates should act as a barrier to mass flow. However, they do not.
The concentration of solutes such as sucrose are higher in leaves (source) than in roots (sink).	Solutes travel to many different sinks at roughly the same rate. However, the mass flow hypothesis suggests that the solutes should move more quickly to the sinks with the lowest solute concentration.

Investigating translocation

Radioactive tracers like ${^1}{^4}C$ can be used to investigate the translocation of solutes around a plant.

1.	Surround a leaf on a plant with a container and fill the container with carbon dioxide containing radioactive carbon ( ${^1}{^4}C$ ).
2.	When the leaf photosynthesises, the organic compounds will contain the radioactive carbon.
3.	The movement of the solutes can be tracked by autoradiography.
4.	The plant is killed and placed on photographic film. The film will turn black where there are radioactive solutes present.
5.	Autoradiographs can be taken of plants killed at different times to show the movement of solutes from the source to the sink.