The pressure-flow model of phloem transport states that
a. phloem content always flows from the leaves to the root.
b. phloem content always flows from the root to the leaves.
c. water flow takes sucrose from a source to a sink.
d. water pressure creates a flow of water toward the source.
e. Both c and d are correct.
The Correct Answer and Explanation is :
The correct answer is e. Both c and d are correct.
Explanation:
The pressure-flow model of phloem transport describes the movement of nutrients, primarily sugars (sucrose), through the phloem tissue in plants. This process occurs through a mechanism of pressure differences that cause a flow from regions of higher pressure (sources) to lower pressure (sinks). Here’s a detailed explanation of why e is the correct choice:
- Water flow takes sucrose from a source to a sink (Answer c): In the phloem, sucrose, which is a major product of photosynthesis, is actively transported into the phloem sieve tubes in source regions, usually the leaves. This process creates a high concentration of solutes (sucrose) in these sieve tubes. As a result, water from nearby xylem vessels moves into the phloem due to osmosis, increasing the turgor pressure inside the phloem. This increased pressure pushes the phloem sap, containing sucrose, from the source regions (like the leaves) to sink regions (such as the roots or growing tissues). Therefore, the flow of sucrose from the source to the sink is driven by this water movement.
- Water pressure creates a flow of water toward the source (Answer d): The pressure-flow model is based on differences in water pressure. In the source, where sucrose is actively loaded into the phloem, water from the surrounding tissues (like xylem) moves into the phloem due to the increased solute concentration. This influx of water generates a high turgor pressure at the source. In the sink regions, where sucrose is unloaded from the phloem for storage or utilization, the pressure is lower. This difference in pressure drives the flow of the phloem sap from source to sink.
Thus, both the movement of sucrose from source to sink and the creation of water pressure gradients are critical to the pressure-flow model of phloem transport. Therefore, the answer e. Both c and d correctly explains the mechanism behind phloem transport.