Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins

Sourav PanOctober 29, 2024

Answered

Companion cells play a crucial role in the transport of assimilates (like sucrose and amino acids) to phloem sieve tubes in plants. Here’s how this process occurs, with reference to proton pumps and cotransporter proteins:

1. Role of Companion Cells

Companion cells are specialized parenchyma cells that are closely associated with phloem sieve tubes. They help in loading and unloading assimilates into and out of the sieve tubes.

2. Proton Pumps

• Function: Companion cells use proton pumps (H+^++ ATPases) to actively transport protons (H+^++ ions) out of the cell into the surrounding apoplast (the space outside the plasma membrane).
• Energy Requirement: This process requires ATP, as it is an active transport mechanism. The movement of protons creates a higher concentration of H+^++ ions outside the companion cells than inside.

3. Formation of a Proton Gradient

• Result: The activity of proton pumps establishes a proton gradient across the plasma membrane of the companion cells. The exterior of the cell becomes more positively charged relative to the interior.

4. Cotransporter Proteins

• Function: The proton gradient created by the proton pumps is used by cotransporter proteins (also known as symporters) to facilitate the uptake of sucrose and other assimilates.
• Mechanism: These cotransporter proteins transport sucrose into the companion cells by utilizing the energy from the flow of protons back into the cell (down their concentration gradient). As protons move back into the cell through these cotransporters, sucrose molecules are co-transported into the companion cells simultaneously.

5. Loading into Phloem Sieve Tubes

• Transfer to Sieve Tubes: Once sucrose and other assimilates accumulate in the companion cells, they are transferred to the phloem sieve tubes. This is facilitated by the high osmotic pressure created due to the presence of these solutes, which causes water to flow into the sieve tubes from the surrounding cells.
• Pressure Flow Mechanism: This influx of water into the sieve tubes increases turgor pressure, driving the movement of the dissolved assimilates from areas of high concentration (sources) to areas of lower concentration (sinks) throughout the plant.