During photosynthesis in green plants, the chemical energy stored in glucose molecules is originally obtained from which primary energy source?

Introduction / Context:
Photosynthesis is the fundamental process by which green plants convert light energy into chemical energy. The question asks from where the energy stored in the products of photosynthesis, particularly glucose, is originally derived. This is a core idea in plant physiology and ecology because it underpins almost all food chains.

Given Data / Assumptions:

• We are considering the process of photosynthesis in green plants.
• Glucose is one of the main products in which energy is stored.
• We must identify the primary source of this energy.

Concept / Approach:
In photosynthesis, chlorophyll containing cells capture light energy from the sun. This light energy drives a series of light dependent reactions that generate ATP and reducing power in the form of NADPH. These energy rich compounds are then used in the Calvin cycle to convert carbon dioxide and water into glucose. Therefore, the energy in glucose molecules ultimately comes from sunlight, not from the breakdown of existing molecules.

Step-by-Step Solution:
Step 1: Recall the basic photosynthesis equation, which shows carbon dioxide and water forming glucose and oxygen in the presence of light. Step 2: Understand that chlorophyll pigments absorb photons of light from the sun. Step 3: Recognise that the absorbed light energy is converted into chemical energy in ATP and NADPH during the light reactions. Step 4: Note that this chemical energy is then stored in the bonds of glucose molecules during the Calvin cycle, so the original source is sunlight.

Verification / Alternative check:
If the primary energy source were oxidation of glucose or breakdown of starch, photosynthesis would not be a net energy storing process. Instead, plants would simply be consuming stored energy. However, actual observations show that plants can produce large amounts of biomass using light energy, confirming that sunlight is the basic input that drives photosynthesis.

Why Other Options Are Wrong:

• Option B (Oxidation of glucose in mitochondria): This refers to cellular respiration, which releases energy by breaking down glucose; it is almost the reverse of photosynthesis.
• Option C (Breakdown of proteins): Proteins are not the primary energy source used to build glucose in photosynthesis.
• Option D (Breakdown of stored starch): Starch is formed as a storage product of photosynthesis, not as the starting energy source.

Common Pitfalls:
A frequent confusion is between photosynthesis and respiration. Remember that photosynthesis captures energy and stores it in glucose, while respiration releases energy by breaking down glucose. Another pitfall is to think that plants obtain energy from soil nutrients. Mineral nutrients are essential but they do not provide the primary energy for glucose synthesis; sunlight does.

Final Answer:
The energy stored in glucose during photosynthesis is obtained from sunlight.