9.4 - Translocation Flashcards
Why do plants produce large amounts of glucose?
Glucose is produced during photosynthesis and is essential for respiration, providing energy for all plant cells.
Why is glucose converted to sucrose for transport in plants?
Sucrose is less reactive than glucose, preventing unwanted chemical reactions during transport.
Sucrose is more soluble, making it easier to transport in the phloem.
What happens to sucrose when it reaches cells where it is needed? (3)
It can be:
- Converted back to glucose for respiration
- Converted to starch for storage
- Used to produce amino acids and other essential compounds
Why is glucose stored as starch instead of being kept as glucose?
Starch is insoluble, so it does not affect water potential in cells.
Starch is a compact storage molecule, allowing large amounts of energy to be stored efficiently.
What is translocation in plants?
Translocation is the active transport of organic compounds (assimilates) in the phloem from sources (where they are produced) to sinks (where they are needed).
Why is translocation an active process?
It requires energy (ATP) to load assimilates into the phloem and move them both up and down the plant to the areas where they are needed.
What are assimilates in plants?
Assimilates are organic molecules transported in the phloem, primarily sucrose, which is the main product of photosynthesis that moves through the plant.
Why is sucrose the main assimilate transported instead of glucose? (3)
- Sucrose is less reactive, preventing unwanted chemical reactions.
- It is more soluble, making transport in the phloem more efficient.
- It allows for higher concentrations in the phloem (20-30%) compared to cell sap (0.5%).
What are the main sources of assimilates in a plant? (3)
- Green leaves and green stems (photosynthesis sites).
- Storage organs (e.g., tubers, taproots) when they release stored nutrients at the start of a growth period.
- Food stores in seeds during germination.
What are the main sinks in a plant? (3)
- Growing roots actively absorbing minerals.
- Meristems undergoing active cell division.
- Developing seeds, fruits, or storage organs that need resources to grow.
Can translocation move assimilates both up and down a plant? Why or why not?
Yes! Unlike water movement in the xylem (which is only upward), translocation in the phloem moves assimilates in both directions depending on where sinks and sources are.
What is translocation in plants?
Translocation is the movement of assimilates (mainly sucrose) in the phloem from sources (where they are produced) to sinks (where they are needed).
How fast can substances move in the phloem?
Substances can move at speeds between 0.15 - 7 meters per hour, and a large tree can transport around 250kg of sucrose down its trunk in a year.
What are the two main ways assimilates enter the phloem?
- Symplast Route (passive)
- Apoplast Route (active)
What is the symplast route in phloem loading? What type of process is this?
- Sucrose moves from mesophyll cells into sieve tube elements via plasmodesmata (small cytoplasmic channels).
- Water follows by osmosis, generating pressure that pushes sucrose through the phloem by mass flow.
- This process is passive.
What is the apoplast route in phloem loading?
- Sucrose moves through cell walls and intercellular spaces to the phloem by diffusion.
- Active transport moves sucrose into the companion cells and sieve elements using ATP.
- Hydrogen ions (H⁺) are actively pumped out of companion cells, and they return via co-transport proteins, bringing sucrose in with them.
- This creates a high concentration of sucrose in the phloem, drawing in water by osmosis, generating high turgor pressure.
Draw the apoplast route on a piece of paper and describe the process.
Why do companion cells have many mitochondria and membrane infoldings?
Mitochondria provide ATP for active transport.
Membrane infoldings increase the surface area for sucrose transport.
How does water movement contribute to translocation? (3)
The high sucrose concentration in the phloem causes water to enter by osmosis.
This builds up turgor pressure, pushing assimilates through the sieve tubes via mass flow.
Solutes move up or down the phloem, depending on the location of the source and sink.
How does pressure in the phloem compare to human arteries?
Phloem pressure is 2 MPa (15,000 mmHg).
Human arteries have a much lower pressure of 0.016 MPa (120 mmHg).
What happens during phloem unloading? (3)
- Sucrose diffuses from the phloem into surrounding sink cells.
- It is quickly converted into other substances (e.g., glucose for respiration, starch for storage).
- This maintains a sucrose concentration gradient, keeping the process efficient.
How does water exit the phloem after unloading?
- The loss of solutes raises the water potential in the phloem.
- Water moves out by osmosis into surrounding cells.
- Some of this water is drawn into the xylem, joining the transpiration stream.
How has microscopy contributed to our understanding of translocation?
Advances in microscopy have allowed scientists to observe the adaptations of companion cells, confirming their role in active transport.
What happens when mitochondria in companion cells are poisoned?
Translocation stops, indicating that active transport (which requires ATP) is essential for moving sugars in the phloem.
Why is translocation unlikely to be passive diffusion?
The flow of sugars in the phloem is about 10,000 times faster than what would be expected if diffusion alone were responsible.
How do aphid studies provide evidence for translocation?
- Aphids feed on phloem sap using their stylet, which can be left inserted into the phloem.
- Sap continues to flow out, showing that translocation occurs under positive pressure.
- The flow rate and sucrose concentration are higher near the source and lower near the sink, supporting pressure flow theory.
What are some unresolved questions about translocation?
- Not all solutes move at the same rate in the phloem.
- Sucrose always moves at the same rate, regardless of concentration at the sink.
- The exact role of sieve plates in translocation is still uncertain.
