Assessment and Qualifications Alliance (AQA) GCSE Biology Paper 1 Practice Exam

Most photosynthesis occurs in the palisade mesophyll tissue due to its structural characteristics and location within the leaf. This tissue is found just beneath the upper epidermis of the leaf and is composed of tightly packed cells that contain a high density of chloroplasts. Chloroplasts are organelles responsible for capturing light energy and converting carbon dioxide and water into glucose and oxygen during the photosynthesis process.

The alignment of these cells maximizes light absorption, as they are vertically oriented, which allows for more exposure to sunlight. Additionally, the palisade mesophyll is situated in an area with optimal access to sunlight, enhancing the overall efficiency of photosynthesis.

In contrast, other tissues such as the spongy mesophyll do contribute to photosynthesis, but to a lesser extent, given that they are more involved in gas exchange due to their air spaces. Epidermal tissue primarily serves a protective function and has fewer chloroplasts, while root tissue is not involved in photosynthesis as it is primarily responsible for water and nutrient absorption from the soil. Thus, the palisade mesophyll is specially adapted for the primary function of photosynthesis, making it the most significant location for this vital process.