During photosynthesis in green plants, which of the following molecules is consumed (used up) as a reactant rather than produced as a product?

Introduction / Context:
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, storing it in the bonds of carbohydrates. Understanding which substances are used up and which are produced is fundamental in biology and environmental science. This question asks you to identify which of the listed molecules is consumed as a reactant during photosynthesis rather than formed as a product.

Given Data / Assumptions:

• We consider the overall balanced equation for photosynthesis in green plants.
• The main reactants are carbon dioxide (CO2) and water (H2O).
• The main products are carbohydrates (such as glucose) and oxygen (O2).
• The options list water, carbohydrates, oxygen, and sugar.

Concept / Approach:
A simplified overall equation for photosynthesis in plants is often written as: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 This shows that carbon dioxide and water are reactants used up during the process, while glucose (a carbohydrate sugar) and oxygen are products formed. Carbohydrates and sugars are essentially describing the same type of product, and oxygen is clearly a gaseous product released into the environment. Therefore, among the options, water is the molecule that is consumed as a reactant in the photosynthetic process.

Step-by-Step Solution:
Step 1: Recall that photosynthesis uses light energy to convert carbon dioxide and water into chemical energy stored in carbohydrates. Step 2: Identify water as one of the key reactants that is split during the light dependent reactions to provide electrons, protons, and oxygen. Step 3: Recognise that carbohydrates, including sugars like glucose, are synthesized during photosynthesis and are therefore products, not reactants. Step 4: Note that oxygen gas is released as a by product when water is split, so oxygen is also a product. Step 5: Conclude that water is the molecule among the options that is used up in the process rather than produced, and choose water as the correct answer.

Verification / Alternative check:
Biology textbooks consistently present the photosynthetic equation and explain the two main stages: the light dependent reactions and the Calvin cycle. In the light dependent reactions, water molecules are split (photolysis) to provide electrons to the photosystems, protons to form a gradient for ATP production, and oxygen as a waste product. In the Calvin cycle, carbon dioxide is fixed and used to build carbohydrates. These descriptions clearly show that water is consumed as a reactant, while carbohydrates and oxygen are products. This confirms that water must be the correct choice.

Why Other Options Are Wrong:
Carbohydrates are produced during photosynthesis; plants create sugars such as glucose which can be stored or used for energy. Oxygen is released as a product into the atmosphere and is not consumed. Sugar is simply another name for carbohydrates in this context and again refers to a product, not a reactant. Therefore, choosing carbohydrates, oxygen, or sugar would incorrectly identify a product as if it were being used up. Only water is correctly identified as a reactant consumed during the process.

Common Pitfalls:
Students sometimes confuse respiration and photosynthesis. In respiration, oxygen is consumed, and carbon dioxide and water are produced, while in photosynthesis the roles are reversed. Another pitfall is thinking that because plants need sugar, they must also take it in from the environment; in fact, they synthesize it themselves from simpler molecules. To avoid these misunderstandings, memorise the overall equation for photosynthesis and pay attention to which substances appear on the left (reactants) and which appear on the right (products) of the arrow.

Final Answer:
The molecule that is used up (consumed) during photosynthesis is water.