State that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes

Sourav PanOctober 29, 2024

Answered

Assimilates, including sucrose and amino acids, dissolved in water, are transported from sources (e.g., mature leaves where photosynthesis occurs) to sinks (e.g., roots, buds, stems, seeds, and fruits) through phloem sieve tubes via the process of translocation.

Key Mechanisms:

1. Loading at Source:
   • Assimilates (sucrose, amino acids) are actively loaded into phloem sieve tubes from companion cells.
   • This increases solute concentration, decreasing water potential, which draws in water from surrounding tissues by osmosis.
2. Translocation Through Phloem:
   • The influx of water creates hydrostatic pressure, driving the bulk flow of phloem sap from source to sink.
   • This process is explained by the Pressure-Flow Hypothesis.
3. Unloading at Sink:
   • At the sink, assimilates are unloaded through diffusion or active transport from an area of high concentration to one of low concentration.
   • This reduces the sugar concentration in the phloem near the sink, increasing water potential, causing water to move out of the phloem by osmosis and potentially recycled via the xylem.

Evidence from Search Results:

• Topic 9.2: Transport in the Phloem of Plants – Describes the movement of organic compounds (e.g., sugars, amino acids) from sources to sinks via phloem tubes.
• # 4.5.2: Translocation (Assimilate Transport) – Biology LibreTexts – Explains the pressure-flow hypothesis and the process of translocation from sources to sinks.
• Quora: How does assimilation in phloem sieve tubes move… – Outlines the steps of translocation, including loading, translocation, and unloading of assimilates.