Why is active transport used to load organic compounds into phloem sieve tubes at the source?

Sourav PanNovember 9, 2024

Answered

Active transport is used to load organic compounds, particularly sucrose, into phloem sieve tubes at the source for several key reasons:

1. Concentration Gradient

• Against the Gradient: Sucrose is often present in higher concentrations in the phloem than in the surrounding mesophyll cells where it is produced. Active transport is necessary to move sucrose against this concentration gradient from the source (e.g., leaf mesophyll) into the phloem sieve tubes. This process ensures that sucrose can accumulate to high levels in the phloem for efficient long-distance transport to sinks (areas of growth or storage).

2. Mechanism of Loading

• Proton-Coupled Transport: The active loading of sucrose involves a proton (H⁺) gradient created by proton pumps in companion cells. These pumps use ATP to actively transport H⁺ ions out of the cell, creating a high concentration of protons in the apoplast (the space outside the plasma membrane). As H⁺ ions diffuse back into the companion cells through a co-transporter protein, they bring sucrose along with them, effectively loading it into the phloem .

3. Efficiency of Transport

• High Growth Potential: Active transport allows for a rapid accumulation of sugars in the phloem, which can enhance the plant’s growth potential. By maintaining high concentrations of assimilates in the phloem, plants can efficiently distribute energy-rich compounds to various sinks, supporting metabolic processes and growth.

4. Control Over Phloem Composition

• Selective Loading: Active transport mechanisms enable plants to selectively load specific organic compounds into the phloem. This selectivity is crucial for regulating nutrient distribution based on plant needs and environmental conditions .

5. Facilitation of Bulk Flow

• Pressure Generation: The active loading of sucrose increases osmotic pressure within the sieve tubes, drawing water from adjacent xylem vessels into the phloem. This influx of water generates turgor pressure that facilitates bulk flow of phloem sap from sources to sinks