Advanced Placement (AP) Biology Practice Exam

Phototropism specifically refers to the growth of a plant or part of a plant in response to light. This phenomenon occurs because plant cells on the side of the plant that is less exposed to light elongate more than those on the side that receives direct light. As a result, the plant bends toward the light source, optimizing its ability to absorb sunlight for photosynthesis. This response is crucial for the plant's survival, as it enhances the efficiency of light capture, which is essential for energy production. The process involves the plant hormone auxin, which regulates the differential growth of plant cells.

The other choices refer to different types of growth responses: growth toward gravity (gravitropism), movement away from light (a behavior often seen in certain plant species, but not termed phototropism), and growth in water (which would align more with hydrotropism). These concepts represent different stimuli that affect plant growth, but they do not define phototropism specifically.