Isotopes used in photosynthesis were

Options

A. PS -I
B. PS-II
C. Calvin cycle
D. None-cyclic ETS

Answer

### Step-by-Step Solution:

1. **Understanding Isotopes**: Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This difference in neutrons gives them different atomic masses.

2. **Role of Isotopes in Photosynthesis**: In the study of photosynthesis, isotopes are used to trace the pathways of carbon and oxygen during the process. This helps scientists understand how these elements are utilized in the photosynthetic reactions.

3. **Identifying Key Isotopes**: The isotopes commonly used in photosynthesis research are Carbon-14 (C-14) and Oxygen-18 (O-18). C-14 is used to trace carbon fixation, while O-18 is used to trace the oxygen produced during photosynthesis.

4. **Photosynthetic Reactions**: Photosynthesis consists of several reactions, including the light-dependent reactions (Photosystem I and II) and the light-independent reactions (Calvin cycle).

5. **Focusing on the Calvin Cycle**: The Calvin cycle, also known as the C3 cycle, is the primary pathway for carbon fixation in plants. It is here that the isotopes C-14 and O-18 are specifically employed to trace the incorporation of carbon dioxide into organic molecules.

6. **Conclusion**: The isotopes used in photosynthesis, particularly in the context of tracking carbon fixation, are Carbon-14 and Oxygen-18, and they are primarily utilized in the Calvin cycle.

### Final Answer:
The isotopes used in photosynthesis are Carbon-14 (C-14) and Oxygen-18 (O-18), and they are primarily used in the Calvin cycle.

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