Biology XI:11 Transport in plants(8 Pressure Flow or Mass Flow Hypo

Pressure Flow or Mass Flow Hypothesis This is the accepted mechanism used for translocation of sugars from the source to the sink. Source cell: Cells which are the supplier of the food material; mainly leaves Sink cell: Cells which require food material for growth and repair Glucose prepared at the source is converted into sucrose. Sucrose is moved to the companion cells, and then to the living phloem sieve tube cells by active transport. This process of loading creates a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. Osmotic pressure builds phloem sap. As hydrostatic pressure on the phloem sieve tube increases, pressure flow begins and sap moves through the phloem. At the sink, incoming sugars are actively transported out of the phloem as complex carbohydrates. Loss of solute produces a high water potential in the phloem, and water passes out, returning to the xylem. Water moves up in the xylem due to transpiration pull