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

Which type of light is most effective for photosynthesis?

1. Ultraviolet light

2. Green light

3. Red light

4. Blue light

The correct answer is: Red light

The choice of red light as the most effective for photosynthesis is based on how plants absorb light for the process. Photosynthesis primarily occurs in the chloroplasts of plant cells, which contain chlorophyll, the pigment responsible for capturing light energy. Chlorophyll absorbs light most efficiently in the red and blue wavelengths of the spectrum, while it reflects green light, which is why plants appear green. Red light, specifically, falls within the wavelengths that are highly absorbed by chlorophyll a, the main pigment involved in photosynthesis. This efficient absorption of red light leads to optimal energy availability for the conversion of carbon dioxide and water into glucose and oxygen, facilitating the photosynthetic process. While blue light is also effective and is crucial for stimulating certain growth processes in plants, it does not surpass the efficiency of red light when considering overall contributions to photosynthesis. Ultraviolet light can be harmful to plants, and green light is not effectively used for photosynthesis as it is primarily reflected. Thus, red light emerges clearly as the type of light that provides the best efficiency for photosynthesis.