Transport in Plants

Imagine a massive tree. It doesn’t have a heart to pump fluids, nor does it have muscles to push things around. Yet, it manages to pull water from deep underground all the way to its highest leaves, and it sends food from the leaves down to its roots. It does this using two specialized plumbing systems: the xylem and the phloem.

The upward journey: Water and xylem
Plants need water for photosynthesis and to keep their cells firm. This water is absorbed from the soil by tiny root hairs through a process called osmosis. Once inside the root, the water enters the xylem. You can think of xylem vessels as long, microscopic, dead hollow pipes that run from the roots, up the stem, and into the leaves.

But how does the water travel upward against gravity? The secret is transpiration pull.
When the sun shines on the leaves, water inside the leaf evaporates and escapes into the air through tiny pores called stomata. This evaporation is called transpiration. Because water molecules naturally stick together (cohesion), when one water molecule evaporates from the top of the leaf, it pulls the next water molecule up to take its place. This creates a continuous suction force that pulls a column of water from the roots to the top of the plant. If an air bubble enters the xylem, this column breaks and water transport in that part of the plant stops.

The two-way journey: Food and phloem
While xylem only moves water upward, plants also need to distribute food (sugar) made in leaves to the rest of the body. This is done by the phloem, which consists of living tube-like cells.

This movement is called translocation and is explained by the pressure-flow theory. The leaf acts as a sugar source and roots or fruits act as sinks. Sugar is actively loaded into phloem at the source. Water then enters from nearby xylem, increasing pressure. This pressure pushes the sugary solution through the phloem toward areas where sugar is being used or stored.