What is the difference between active and passive absorption?

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gsenviro eNotes educator| Certified Educator

Plants absorb water from the soil through the process of water absorption. During conditions of low transpiration (the movement of water from the roots of the plant to the extremities of the plant) and high soil water content, the plant roots absorb water through root hair cells. This type of water absorption, where root hair cells play an active role, is known as active absorption. When transpiration rate is high, cells of root hair remain passive and upper plant parts (such as leaves) play a more dominant role. This type of water absorption is called passive absorption. Transpiration creates a suction pressure that pulls the water into roots. Thus, passive absorption creates negative pressure in the xylem channels (tissues in plants responsible for the transport of water). On the other hand, positive pressure is created in xylem channels due to active absorption. Active absorption is caused by the gradient of the diffusion pressure deficit (DPD) between roots and soil particles, or the forces pushing water to osmose in either direction (whichever has a lower water concentration). In comparison, transpirational pull is responsible for water absorption in passive absorption.

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