Question

How does the pressure flow hypothesis explain the movement of sugars in the phloem?

• A. Sugars move from areas of low pressure to high pressure
• B. Sugars move from areas of high pressure to low pressure
• C. Sugars move in response to the force of gravity
• D. Sugars move passively through diffusion

Hint:

Remember that phloem transport is active at the loading and unloading ends (requiring ATP to set up the concentration gradient), but the physical movement through the sieve tubes is a passive bulk flow driven by the resulting pressure gradient.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The long-distance transport of organic nutrients (primarily sucrose) in the phloem is explained by the Pressure-Flow (or Mass-Flow) hypothesis, first proposed by Ernst Münch in 1930.

Step 2: Detailed Explanation:
According to the Pressure-Flow model, phloem sap translocates through sieve tubes along a hydrostatic (turgor) pressure gradient generated between source and sink tissues:
1. At the Source (leaves): Photosynthetically produced sugars are actively loaded into the sieve tube elements (phloem loading). This active accumulation of solutes lowers the water potential ($\Psi_w$) inside the sieve tubes. As a result, water enters osmotically from the adjacent xylem, generating a high turgor (hydrostatic) pressure at the source end.
2. At the Sink (roots, fruits, storage organs): Sugars are actively or passively unloaded from the phloem (phloem unloading). The loss of solutes raises the water potential inside the sieve tubes, causing water to exit osmotically. This reduces the turgor pressure, resulting in a low turgor pressure at the sink end.
3. Bulk Flow: This pressure difference ($\Delta\Psi_p$) between the source (high pressure) and sink (low pressure) drives the bulk flow of phloem sap through the sieve tube system.
- Simple diffusion (Option 4) is too slow to account for long-distance transport rates, and gravity (Option 3) does not dictate the direction of transport, which can occur upward (to developing buds) or downward (to roots).

Step 3: Final Answer:
The pressure-flow hypothesis states that sugars move from areas of high pressure to low pressure.