Optometry Admission Test (OAT) Practice

What is released as a by-product of the light reactions in photosynthesis?

1. Carbon dioxide

2. Glucose

3. Water

4. Oxygen

The correct answer is: Oxygen

During the light reactions of photosynthesis, which take place in the thylakoid membranes of chloroplasts, light energy is harnessed to split water molecules in a process known as photolysis. This reaction generates several key products: ATP and NADPH, which are used in the Calvin cycle for the synthesis of glucose, as well as oxygen. The oxygen produced is released into the atmosphere as a by-product. This release occurs because water molecules are broken apart to provide electrons needed for the photosystem, and oxygen is a by-product of this splitting process. This is a crucial element of photosynthesis, as it not only allows plants to convert light energy into chemical energy but also contributes to the atmospheric oxygen that is essential for the survival of aerobic organisms. The other choices represent products or components not generated from the light reactions. Carbon dioxide is not produced; rather, it is taken in during the process of photosynthesis. Glucose is a product of the subsequent Calvin cycle, which utilizes the energy carriers produced during the light reactions. Water, while a necessary reactant of the light reactions, is not released but instead is consumed in the process. Thus, the release of oxygen is the defining characteristic of the light reactions of photosynthesis.