In humans which of the following is not a step in respiration?

Options

A. Alveolar diffusion of `O_(2)` and `CO_(2)`
B. Transport of gases by blood
C. Diffusion of `O_(2)` and `CO_(2)` between blood and tissues
D. Utilisation of `CO_(2)` by cells for catabolic reactions

Answer

To answer the question "In humans which of the following is not a step in respiration?", we need to analyze the given options related to the respiration process in humans. Here are the steps to arrive at the solution:

1. **Understanding Respiration**: Respiration in humans involves several key processes that facilitate the exchange of gases (oxygen and carbon dioxide) and the utilization of these gases by cells.

2. **Identifying the Steps of Respiration**:
- **Alveolar diffusion of oxygen and carbon dioxide**: This is the process where oxygen from the air in the alveoli diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled. This is a crucial step in respiration.
- **Transport of gases by blood**: Once oxygen is in the blood, it is transported to various tissues throughout the body. Similarly, carbon dioxide produced by cells is transported back to the lungs. This is also a vital step in respiration.
- **Diffusion of oxygen and carbon dioxide between blood and tissues**: This refers to the exchange of gases at the cellular level, where oxygen is delivered to cells and carbon dioxide is picked up. This is another essential step in the respiration process.

3. **Analyzing the Last Option**:
- **Utilization of carbon dioxide by cells for catabolic reactions**: This statement suggests that cells use carbon dioxide for energy production or metabolic processes. However, in cellular respiration, cells primarily utilize oxygen to produce energy, and carbon dioxide is a waste product of this process. Therefore, this option does not describe a step in respiration.

4. **Conclusion**: Based on the analysis, the correct answer to the question is that the "utilization of carbon dioxide by cells for catabolic reactions" is not a step in respiration.

**Final Answer**: The correct option is D: Utilization of carbon dioxide by cells for catabolic reactions.

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