HardBiologyHigh School (9–10)GeneralLife Processes › Transport in Living OrganismsConceptualBoard ExamA-LevelIB
Aphids are small insects that feed on plants…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Aphids are small insects that feed on plants. An aphid pushes its fine, needle-like mouthpart into a young stem. Scientists cut off the aphid's body, and sap kept flowing out of the mouthpart for many hours. The sap was rich in sucrose. (a) Which tissue did the mouthpart reach? (b) What does the steady flow tell us about this tissue? (c) Why would the flow slow down if the stem were cooled to near ?
Need a reminder? The key ideaShow
Xylem carries water and minerals up from the roots. Root pressure pushes water up, mostly at night. In the day, transpiration from the leaves pulls water up the xylem. Phloem carries food (sucrose) to where it is needed, up or down, using energy from ATP. This is translocation.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The tissue. Sucrose is carried in the phloem, so the mouthpart reached the phloem.
Step 2. The flow. The sap came out by itself, so the phloem sap is under pressure. The pressure comes from sucrose loaded into the tubes, which draws in water.
Step 3. The cold. Loading sucrose uses energy from ATP, made by living cells. In the cold, the cells work slowly, so less sucrose is loaded and the flow slows.
Answer: (a) The phloem. (b) Phloem sap is rich in sugar and under pressure, so it flows. (c) Translocation needs energy from living cells, which work slowly in the cold.
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