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Auxins activate proton pumps, decreasing the pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as expansins which break bonds in the cell wall structure, making the cell walls less rigid. The acidic environment also causes disruption of hydrogen bonds in the cellulose that makes up the cell wall. In addition, increased proton pump activity leads to more H+/solute cotransport into the plant cells on the dark side of the plant, which increases the osmotic gradient between the symplast and apoplast of these plant cells. Water then enters the cells along its osmotic gradient, leading to an increase in turgor pressure. The decrease in cell wall strength and increased turgor pressure causes cells to swell, exerting the mechanical pressure that drives phototropic movement.