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

Which substance is produced as a byproduct of photosynthesis?

1. Glucose

2. Hydrogen ions

3. Oxygen

4. Carbon dioxide

The correct answer is: Oxygen

The production of oxygen as a byproduct of photosynthesis is a fundamental aspect of this process. During photosynthesis, plants, algae, and some bacteria convert light energy—typically from the sun—into chemical energy stored in glucose. This takes place in the chloroplasts of plant cells, where chlorophyll captures light energy. As part of the photosynthetic process, water molecules are split in a reaction known as photolysis. This process releases oxygen as a waste product. The overall equation for photosynthesis can be summarized as: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂ Here, carbon dioxide and water are converted into glucose and oxygen, with oxygen being released into the atmosphere. This release is crucial for aerobic life on Earth, as it contributes to the oxygen needed for respiration in most living organisms. While glucose is a primary product of photosynthesis, it is not a byproduct; it is the main goal of the process. Hydrogen ions play a role in the reactions that lead to the formation of glucose but are not considered a byproduct in the same sense as oxygen. Carbon dioxide, on the other hand, is one of the substrates used in