How do auxins bring about positive phototropism in plants?

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1) Phototropins are enzymes found in the plasma membrane of particular cells.  They are phosphorylated when hit by blue light.

2) This phosphorylation triggers the movement of auxin across the plant shoot by active transport as transporter proteins pump the hormone out of cells.

3) As a result, there is more auxin on the shaded side of the plant, producing a concentration gradient.

4) Auxin promotes active transport of hydrogen ions into the cell wall by ATPase action.  Where there is more auxin, more ions are brought in.

5) The hydrogen ions lower the pH of the cell wall, reaching the optimum for the enzymes that hydrolyse bonds withincellulose fibres.  The ions also disrupt hydrogen bonds between cellulose fibres.  This makes the cell wall less rigid.

6) As the cell absorbs water by osmosis, it swells and elongates.  Because of the reduced rigidity of the cell wall, the cell can stretch more than cells with lower auxin concentrations and therefore more restrictive cell walls.

7) Consequently the cells on the shaded side of the plant grow longer than those facing the light due to the difference in auxin concentration.  This causes the plant to bend towards the light.

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