Proposed Mechanism
The direction of light is detected at the tip of the shoot.
Blue light is most effective.
It is absorbed by a flavoprotein called phototropin. Flavoproteins contain flavin as a prosthetic group.
Auxin moves from the tip down.
An auxin transporter — one of the PIN proteins — is inserted in the plasma membrane at the lateral face of cells of the shoot.
Auxin is pumped out of these efflux transporters and accumulates in the cells on the shady side.
This stimulates elongation of the cells on the shady side causing the shoot to bend toward the light.