In animals, fungi, and plants, bulk flow is always an active and ATP-expensive process. This is best evidenced by the need for a heart to beat in order for bulk flow to occur in animals.

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Bulk flow can also occur passively, driven by pressure differences without the direct use of ATP, such as in the movement of water and nutrients in plants through xylem and phloem.

In Animals, Fungi, and Plants, Bulk Flow Is Always an Active