In photosynthesis, the light reactions primarily supply the Calvin cycle with which useful products?

Introduction / Context:
Photosynthesis occurs in two main stages: the light reactions and the Calvin cycle (also called the light independent reactions). This question checks whether you understand how these two stages are linked, and specifically what useful chemical products generated in the light reactions are used by the Calvin cycle to synthesize sugars. This is a fundamental topic in plant physiology and bioenergetics.

Given Data / Assumptions:

Photosynthesis is divided into light reactions and the Calvin cycle.
The light reactions capture light energy and convert it to chemical energy.
The Calvin cycle uses that chemical energy to fix carbon dioxide into sugars.

Concept / Approach:
In the light reactions, which occur in the thylakoid membranes, chlorophyll and other pigments capture light energy. This energy is used to split water, release oxygen, and generate two main products: ATP and NADPH. These molecules carry chemical energy and reducing power. The Calvin cycle, occurring in the stroma of the chloroplast, uses ATP as an energy source and NADPH as a reducing agent to convert carbon dioxide into glucose and other carbohydrates. Therefore, the correct answer must include both ATP and NADPH together, because these are the specific outputs required by the Calvin cycle.

Step-by-Step Solution:
Step 1: Recall that the light reactions convert light energy into chemical energy stored in ATP and NADPH. Step 2: Remember that the Calvin cycle uses ATP and NADPH to fix CO2 into organic molecules. Step 3: Examine the options and identify which one lists both ATP and NADPH together. Step 4: Select the option that correctly states ATP and NADPH as the products supplied to the Calvin cycle.

Verification / Alternative check:
Another way to verify is to recall the simplified reaction of the light reactions: light + water produce oxygen, ATP, and NADPH. Oxygen is released as a byproduct, while ATP and NADPH move into the stroma for the Calvin cycle. Carbon dioxide does not come from the light reactions; it enters from the atmosphere. Therefore, any option that includes CO2 as a product of the light reactions is wrong.

Why Other Options Are Wrong:
Carbon dioxide (CO2) and ATP: CO2 is not produced by light reactions; it is a substrate for the Calvin cycle.
Light energy: Light energy is captured, not directly supplied to the Calvin cycle; the cycle uses chemical energy, not photons.
Water (H2O) and NADPH: Water is a reactant in the light reactions, not a product supplied to the Calvin cycle.

Common Pitfalls:
Students sometimes confuse inputs and outputs of different stages. A typical error is to think that light energy itself is directly used by the Calvin cycle or that CO2 is produced rather than consumed. Keeping a clear mental map of what goes into and comes out of each stage prevents such confusion.

Final Answer:
The light reactions provide the Calvin cycle with ATP and NADPH as essential energy and reducing power sources.