OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which part of the plant primarily responds to light through phototropism?

1. The roots

2. The stem

3. The leaves

4. The flowers

The correct answer is: The stem

The stem of the plant primarily responds to light through phototropism. Phototropism is the growth response of a plant towards a light source. In stems, this occurs due to the uneven distribution of auxins, which are plant hormones responsible for growth. When light shines on one side of the plant, auxins are produced in greater concentrations on the shaded side. This causes the cells on the shaded side to elongate more than those on the lighted side, resulting in the stem bending toward the light. This growth response is essential for plants as it allows them to maximize their exposure to light, which is critical for photosynthesis. In contrast, roots generally grow downwards due to gravitropism and do not significantly respond to light in the same manner. Leaves and flowers also have various adaptations for light absorption and reproduction, but they do not primarily exhibit the directional growth response known as phototropism as prominently as stems do.