Transport for organic molecules in the phloem

Translocation

• The movement of solutes (e.g. sucrose) from one part of a plant to another.

‣ Movement is from the source to the sink.

‣ Active process (requires ATP).

Source & sink

‣ Source = where the solute is made → solute is at a high concentration at the source (usually the leaves).

‣ Sink = where the solute is used → solute is at a lower concentration at the sink (e.g. roots, areas of growth in stems/leaves).

Why are carbohydrates transported in the form of sucrose instead of

glucose or starch?

• Instead of glucose because → sucrose is less reactive than glucose because it is a non-reducing sugar, so is less likely to be metabolised/used in respiration as it is transported.

‣ Instead of starch because → starch is not soluble, so wouldn’t affect osmosis i.e. presence of starch in sieve tube elements would not cause water to enter

Mass flow hypothesis

• Glucose converted to sucrose in photosynthesising cells

‣ Sucrose actively transported into the source end of the phloem by

companion cells.

‣ Lowers water potential in sieve tube elements.

‣ Water enters by osmosis from xylem & companion cells.

‣ Creates high hydrostatic pressure at source end.

‣ At sink end, sucrose moves by facilitated diffusion or active transport from phloem to surrounding cells & is converted back into glucose.

‣ Glucose used for respiration or converted to starch for storage.

‣ Loss of sucrose from sink end of sieve tubes, raises the water potential so water leaves by osmosis to surrounding cells → results in lower hydrostatic pressure.

‣ Creates a pressure gradient from the source to the sink, resulting in MASS FLOW of sucrose down the pressure gradient to respiring cells/ storage tissues e.g. roots.

Similarities & differences between transport in xylem & phloem

Similarities:

‣ Both transport solutes in solution.

‣ Both use mass flow.

Differences:

‣ Transport in xylem only takes place UP the plant, transport in phloem can take place in different directions (bidirectional).

‣ Xylem doesn’t transport carbohydrates, phloem does.

‣ Xylem uses capillary action, phloem does not.

How can one part of a plant act as both a source and a sink?

• e.g. a root can store carbohydrates (in the form of starch) but can also release carbohydrates when they are needed for growth.

‣ So, depending on the time of year, roots can act as a source or a sink.

Use of ringing experiments to investigate transport in plants

‣ Ring of bark removed (removes phloem, but xylem are left intact/undamaged).

‣ Bulge forms above the ring.

‣ Evidence that sugars are transported in the phloem, because if xylem were responsible for translocation, there would have been no accumulation of sugars above the ring.

‣ Bulge is larger in summer → greater rate of photosynthesis → greater production of sugars.

Use of tracers to investigate transport in plants

‣ Leaf supplied with radioactive tracer e.g. CO2 containing 14C.

‣ Radioactive carbon is incorporated into organic substances via photosynthesis.

‣ Movement of radioactive organic substances around plant can be tracked using autoradiography:

‣ Plant killed (e.g. using liquid nitrogen).

‣ Sections of plant placed on photographic film.

‣ Film turns black where 14C is present.