How a Leaf Turns Sugar Into a Pressure Gradient
A leaf does not push sap down a phloem tube with a pump. Instead, it creates a difference in pressure between places where sugar is loaded and places where it is used or stored.
After photosynthesis, a leaf may move sucrose—the main sugar carried in phloem—into nearby tubes. The concentrated sugar makes the fluid inside more dissolved than the surrounding tissues. Water then moves in by osmosis, the movement of water across a membrane toward a region with more dissolved material. This added water raises pressure in the tube near the leaf.
The Destination Keeps the Flow Going
At a root, growing shoot, fruit, or storage tissue, sugar is removed from the phloem for energy or stored for later. As sugar concentration falls, water can leave the tube, lowering pressure there. The higher pressure at the leaf end pushes sap toward the lower-pressure destination.
This is why “downhill” is a useful picture but not the whole rule. Phloem flow follows the pressure gradient from a sugar source to a sugar sink; depending on where those are, transport can go upward as well as downward. A mature leaf can supply sugar, while a developing root or fruit draws it in. The tree’s delivery route is therefore powered not by tiny couriers, but by loading and unloading sugar at opposite ends of a living tube.
