Which statement correctly describes the flow of energy in photosynthesis?
A. Photosynthesis releases energy when the reactants rearrange and form the bonds in the products.
B. Photosynthesis releases energy when carbon dioxide and water combine to form glucose molecules.
C. Photosynthesis stores energy because more energy is stored in glucose than it is in carbon dioxide or water.
D. Photosynthesis stores energy because energy is released when glucose is formed from carbon dioxide and water.
The Correct Answer and Explanation is:
Correct Answer: C. Photosynthesis stores energy because more energy is stored in glucose than it is in carbon dioxide or water.
Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in glucose, a type of sugar. This process takes place primarily in the chloroplasts of plant cells and can be summarized by the following chemical equation:
$$
6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2
$$
In this process, carbon dioxide (CO₂) from the atmosphere and water (H₂O) from the soil are used to produce glucose (C₆H₁₂O₆) and oxygen (O₂). The energy required to drive this reaction comes from sunlight, which is absorbed by chlorophyll, the green pigment in plants.
The key concept to understand here is the energy transformation. The bonds in carbon dioxide and water molecules are relatively low in stored chemical energy. In contrast, glucose molecules have high-energy bonds, particularly in their carbon-hydrogen bonds. To convert low-energy molecules (CO₂ and H₂O) into a high-energy molecule (glucose), energy must be input into the system. This energy comes from sunlight.
Therefore, photosynthesis is an endothermic process — it stores energy. This stored energy can later be released through cellular respiration, where organisms break down glucose to produce usable energy in the form of ATP.
Let’s break down why the other options are incorrect:
- A and B suggest that energy is released during photosynthesis, which is false.
- D wrongly implies energy is released when glucose is formed, contradicting the actual need for energy input.
Thus, option C is correct because it accurately states that photosynthesis stores energy by creating glucose, which contains more energy than the original reactants, carbon dioxide and water.
