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Current theories describe differences in the movement of substances through plants - HSC - SSCE Biology - Question 23 - 2004 - Paper 1
Question 23
Current theories describe differences in the movement of substances through plants. Compare the movement of substances in xylem and phloem tissues. | Feature ... show full transcript
Worked Solution & Example Answer:Current theories describe differences in the movement of substances through plants - HSC - SSCE Biology - Question 23 - 2004 - Paper 1
Step 1
Material transported
Answer
Xylem primarily transports water and dissolved minerals from the roots to the rest of the plant. In contrast, phloem transports organic substances, primarily sugars (mainly in the form of sucrose), produced during photosynthesis from the leaves to other parts of the plant.
Step 2
Process of movement
Answer
In xylem, the movement of water and minerals is largely driven by transpiration pull, which creates negative pressure as water evaporates from the stomata of leaves. This process relies on cohesion between water molecules. Conversely, in phloem, the process of movement is explained by the pressure flow mechanism where the accumulation of sugar at source leaves creates a pressure that drives the movement of sap toward sinks like roots and fruits.
Step 3
Name of relevant current theory
Answer
The relevant current theory for xylem movement is the cohesion-tension theory, which describes how water molecules stick together and pull each other upward. For phloem, the mass flow hypothesis explains how sugars are pushed through phloem tissue due to pressure differences.