Bulk flow is a movement of molecules from an area of high pressure to an area of low pressure. In cell biology, it refers to the transport of fluids or electrolytes between cells through openings, or pores, between the cells. Toilets and faucets employ mechanisms that utilize bulk flow, as well as the transport systems found in plants and animals.
Xylem and phloem in plants carry out a function similar to veins and arteries in animals. In the circulatory system, blood flows through the arteries and veins from areas of high pressure to areas of low pressure. Plants also depend on bulk flow to transport materials to various places. Water in the leaves’ xylem evaporates in a process called transpiration, which creates less pressure in the xylem. The water can then flow up through the xylem into the zone of less pressure.
In general, bulk flow in plants is a quicker process than osmosis or diffusion, both of which involve a passive transport of materials from an area of high to low pressure. But diffusion can transfer materials only over short distances, which is a problem for tall plants. Bulk flow can push water from the roots of a plant all the way to its leaves.