Transport of organic substances in the phloem 2 Flashcards
(18 cards)
What is the mass flow?
The bulk movement of a substance through a given channel or area in a specified time
How does sucrose enter the sieve tube elements?
It is actively transported into the sieve tubes from photosynthesising cells (the source)
What happens to the water potential in sieve tubs when sucrose enters?
It becomes more negative (lower water potential)
Why does water move from the xylem into the sieve tubes?
The xylem has a higher (less negative) water potential, so water moves by osmosis into the sieve tubes
What happens to sucrose at the sink?
It is used in respiration or converted into starch for storage
How does the movement of sucrose into sink cells affect their water potential?
It lowers the water potential, causing water to move into sink cells by osmosis
How is high hydrostatic pressure created in the sieve tubes?
Water moving in from the xylem increases the pressure inside the sieve tubes
How does hydrostatic pressure change from the source to the sink?
The source has high hydrostatic pressure, and the sink has low hydrostatic pressure
What causes the mass flow of sucrose solution in the sieve tubes?
The hydrostatic pressure gradient between the source (high pressure) and sink (low pressure)
Why is translocation considered an active process overall?
This is because it relies on the active transport of sucrose into sieve tube elements, which requires ATP
Why does temperature affect translocation?
This is because active transport relies on enzymes, which are temperature - sensitive
Is mass flow n active or passive process?
Mass flow itself is passive, but it occurs as a result of the active transport of sugars
What factors can affect the rate of translocation?
Temperature and metabolic poisons
How do metabolic poisons affect translocation?
They inhibit ATP production, which is needed for active transport in translocation
How is sucrose transferred from sieve tubes elements into sink cells?
By active transport through companion cells
What role do companion cells play in the transfer of sucrose to sink cells?
They actively transport sucrose out of the sieve tubes into sink cells
Why does the transfer of sucrose from sieve tubes to sink cells require energy?
This is because it is an active process that requires ATP
What happens to sucrose after it is transferred into sink cells?
It is either used for respiration or stored as starch
