Phototropism is the growth of plants in response to light. It is a fundamental process that allows plants to orient themselves towards the sun, the main source of light for photosynthesis. The mechanism behind phototropism involves a plant hormone called auxin, which is responsible for cell elongation.

When light strikes a plant, it causes auxin to be transported to the shaded side of the stem. This results in the cells on the shaded side to elongate more than those on the lit side, causing the stem to bend towards the light.

The specific mechanism of auxin transport in phototropism is still not fully understood, but it is known to involve several proteins, including phototropin, which is a blue-light receptor. Phototropin triggers a signalling cascade that results in the redistribution of auxin.

Phototropism is an essential process for plants as it allows them to maximise their exposure to light for photosynthesis. It also plays a role in other plant processes, such as the opening and closing of stomata, the regulation of leaf growth, and the formation of flowers.

Here is a simplified step-by-step explanation of the mechanism behind phototropism:

1. Light strikes a plant.

2. Auxin is transported to the shaded side of the stem.

3. The cells on the shaded side elongate more than those on the lit side.

4. The stem bends towards the light.