Translocation
Source to Sink:
- Translocation is the movement of organic compounds from where they are made at their source, to where they are required at their sink.
- It is an active process which can be used to transport phloem up or down the plant.
- Example sources of assimilates:
- Green leaves and stems
- Storage organs such as tubers, when unloading stores during a growth period
- Food stores in seeds during germination
- Example sinks of assimilates:
- Roots that are absorbing mineral ions via active transport.
- Meristems
- Any part of the plant creating food stores such as tubers.
Key Terms:
Source: Where an organic compound is made.
Sink: Where an organic compound is required and used.
Translocation: Movement of substances in the phloem from source to sink.
Assimilate: A product of photosynthesis.
Meristems: Meristematic tissue, a tissue which is actively dividing and growing.
Mass Transport: Where substances are moved in a large amount of liquid.
Phloem Loading:
- Movement of soluble product into the phloem.
- Assimilates move through the spaces in the loose cellulose fibres of the cell wall, known as the apoplast.
- They move into the phloem by diffusion.
- Active transport is used to maintain a concentration gradient.
- Hydrogen ions (H+) are actively pumped out using ATP.
- Hydrogen ions return down a concentration gradient via a co-transporter protein.
- Sucrose must be co-transported with H+ ion, in order for the H+ ion to get back in, down the concentration gradient.
- This increases sucrose concentration in companion cells, which creates a low water potential.
- Water osmoses into the companion cell, increasing turgor pressure.
- The water carrying the assimilates then moves to an areas of lower pressure - the sinks - in a mass transport system.
- The pressure created by solute accumulation in the source is much higher than that of a human artery, so water can be transported rapidly over many metres.
Symplast Route:
- Assimilates move passively through the symplast pathway - the cytoplasm.
- Sucrose accumulates in the sieve tubes, water osmoses from an areas of high potential to the area of newly lowered potential where the sucrose is. Pressure created causes sucrose to move along the phloem by mass flow.
Phloem Unloading:
- Sucrose is offloaded to cells which need it by diffusing down a concentration gradient.
- Loss of solutes from phloem causes water to osmose to surrounding cells. Some of the water enter the transpiration stream in xylem.
- Microscopy allows us o see adaptations of companion cells for active transport.
- If mitochondria are poisoned, translocation stops, suggesting it is an active process which requires ATP.
- Flow of sugars in phloem is 10000x faster than diffusion alone, suggesting there is an active process driving mass flow.
- Positive pressure from inside the phloem forces sap out through aphid stylets (mouth parts), and the pressure lowers closer to the source.