Cell elongation is an important process for plant growth, especially in plant shoots. According to "Biology," plant cell elongation is believed to take place in a manner described by the acid growth hypothesis.
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Plant cells are surrounded by a tough but flexible layer of cellulose and other molecules called the cell wall. The cell wall helps to counteract osmotic pressure created by diffusion of water across the plant's cell membrane; it also, however, can restrict cell growth. To elongate, plant cells must temporarily loosen their cell wall.
The acid growth hypothesis posits that plant cells elongate by pumping hydrogen ions across their membrane. Increased concentration of H+ makes the cell wall more acidic, activating enzymes that cut bonds between molecules in the fibers of the wall. The increasing concentration of H+ also changes the potential across the cell membrane, boosting the uptake of other ions into the cell and thereby increasing the rate at which water diffuses into the cell so that it expands.
Auxin is the plant hormone responsible for cell elongation in shoots. Auxin not only triggers hydrogen ion pumping across the cell membrane but also alters the pattern of gene expression inside the plant cell in a way that encourages plant growth.