How do high solute concentrations in the phloem at the source lead to water uptake by osmosis?

Sourav PanNovember 9, 2024

Answered

High solute concentrations in the phloem at the source lead to water uptake by osmosis through a series of interconnected physiological processes. Here’s how this mechanism works:

1. Active Transport of Solutes

• At the source (typically the leaves), organic compounds such as sucrose are actively transported into the phloem sieve tubes. This process is facilitated by companion cells, which use ATP to pump sucrose against its concentration gradient into the sieve tube elements (STEs) of the phloem. As a result, the concentration of solutes in the phloem sap increases significantly, often reaching levels around 30% sugar content.

2. Osmotic Potential and Water Movement

• The high concentration of solutes in the phloem creates a low water potential (Ψ) within the sieve tubes compared to the surrounding xylem and other tissues. Water potential is influenced by solute concentration; as solute concentration increases, the solute potential (Ψs) decreases. Consequently, water from adjacent xylem vessels, which typically has a higher water potential, moves into the phloem by osmosis. This movement occurs because water naturally flows from areas of higher water potential to areas of lower water potential.

3. Hydrostatic Pressure Build-Up

• As water enters the phloem due to osmosis, it increases the volume of fluid within the sieve tubes, leading to an increase in hydrostatic pressure. This pressure is essential for driving the bulk flow of phloem sap from sources to sinks. The pressure created in the phloem is what allows for efficient transport over long distances within the plant.

4. Mass Flow Mechanism

• The movement of water into the phloem not only helps maintain turgor pressure but also facilitates a mass flow mechanism where phloem sap (containing dissolved sugars and other nutrients) moves through the sieve tubes toward areas of lower pressure (the sinks). At these sinks (such as roots, fruits, or growing tissues), sugars are either utilized for energy or stored, which decreases their concentration in the phloem and allows for continued flow from sources