Plant cells contain a protein called phototropin that is mostly concentrated in the growing tip of the plant shoot. This protein unfolds into an activated state when it absorbs blue wavelengths of light. This sets off a cascade of interactions between different proteins in the cells, which ultimately changes the alignment of cellular scaffolding proteins, called microtubules.

The upshot of this is that the cells on the darker side of the shoot elongate, while those on the light side remain squat and boxy. As the dark side of the plant grows longer, the shoot as a whole bends away from that side and towards the light.