Explain how Auxin is used to cause phototropism in plants.

Auxin is a growth hormone released from the tip of a shoot. It is distributed uniformly along the shoot so it can diffuse down the entirety of the stem of the plant. Normally the auxin is broken down in sunlight so it does not affect the growth of the plant, however when part of the plant is shaded, the auxin is not broken down, so it diffuses down this shaded part, causing this side of the shoot to grow much faster than the other sides. This therefore causes the plant to start bending towards the light, until the entirety of the tip is once again in the sunlight, when all the auxin will be broken down again. Overall, therefore, auxin has caused the shoot to grow towards the sunlight, causing phototropism.