Questions: Phloem Translocation and the Pressure-Flow Hypothesis
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
In its second year, a carrot plant uses sugar stored in its taproot to grow a flowering stalk. During this time the taproot acts as:
AA sink
BA source
CNeither a source nor a sink
DPart of the xylem only
A source is any organ that supplies sugar to the phloem. In its first year the taproot was a sink that filled with stored sugar. In the second year it releases that sugar to the growing stalk and flowers, which are now the sinks.
Question 2 Multiple Choice
According to the pressure-flow hypothesis, why does water enter the sieve tubes at a source?
AWater is actively pumped into the sieve tubes by ATP
BSugar loaded into the sieve tubes lowers their water potential, so water enters from nearby xylem by osmosis
CGravity pulls water into the sieve tubes
DEvaporation from the leaf pulls water into the phloem
Loading sucrose raises the solute concentration inside the sieve tube. Its water potential falls below that of the nearby xylem, so water flows in by osmosis. The added water raises the pressure at the source end of the tube.
Question 3 True / False
Phloem sap can move upward in a plant as well as downward.
TTrue
FFalse
Answer: True
The direction of phloem flow is set by where sources and sinks are. Sap moves up from leaves to a developing fruit at the top of a stem and down from the same leaves to the roots.
Question 4 Short Answer
Explain why removing a ring of bark from a tree trunk (girdling) can kill the tree, even though water still reaches the leaves.
Think about your answer, then reveal below.
Model answer: Girdling removes the phloem, which lies in the bark, but leaves the xylem in the wood. Water still moves up, but sugars from the leaves can no longer travel down to the roots. The roots use up their stored food and die, and then the tree dies.
This classic observation shows that sugar transport and water transport use different tissues. Swelling of the bark just above the girdle, where sugar collects, also supports the idea that phloem flow from the leaves is blocked there.