Explain how the pressure-flow model can result in "two-way" movement of organic molecules in a plant.

Sugars are actively loaded into companion cells from photosynthetic cells. This uses a proton/sucrose code transport proteins. This causes the Phloem to become hypertonic leading to the entry of water via osmosis. This increases turgor pressure, propelling the sugars through the plant. Then the sugar enters into the sink cells causing the phloem cells to become hypotonic, causing water to exit.