Explain the process of phototropism, including the role of auxin in this process.

Phototropism orients certain parts of the plant in the direction that maximizes the amount of light intercepted by its photosynthetic cells. Phototropism in shoots occurs in response to blue light absorbed by nonphotosynthetic pigments called phototropins. In a shoot tip or coleoptile, light-energized phototropins repress transcription of a gene whose product causes auxin efflux carriers to be distributed evenly around the cell membrane. Thus, a cell exposed to a directional light source ends up with efflux carriers positioned mainly on its shady side. The polarization directs auxin flow toward the shaded side of the structure, causing cells on that side to elongate more than cells on the illuminated side. The difference causes the entire structure to bend toward the light as it lengthens.