Explain the role of auxin in phototropism

Phototropism is the growing or turning of an organism in response to a unidirectional light source
Auxins (e.g. IAA) are plant hormones that are produced by the tip of a shoot and mediate phototropism
Auxin makes cells enlarge or grow and, in the shoot, are eradicated by light
The accumulation of auxin on the shaded side of a plant causes this side only to lengthen, resulting in the shoot bending towards the light
Auxin causes cell elongation by activating proton pumps that expel H+ ions from the cytoplasm to the cell wall
The resultant decrease in pH within the cell wall causes cellulose fibres to loosen (by breaking the bonds that hold them together)
This makes the cell wall flexible and capable of stretching when water influx promotes cell turgor
Auxin can also alter gene expression to promote cell growth (via the upregulation of expansins)