Introduction:
In all bio-compounds, the most necessary fundamental element is oxygen. Oxygen is an essential element for the survival and functioning of living organisms. It plays a crucial role in various biological processes, including respiration, metabolism, and energy production.

Explanation:
Oxygen is a highly reactive element that readily forms bonds with other elements, making it an important component of bio-compounds. Here are some key reasons why oxygen is the most necessary fundamental element in bio-compounds:

1. Role in Respiration:
Oxygen is necessary for the process of respiration, which is the fundamental process of obtaining energy from food. During respiration, oxygen combines with glucose to produce energy in the form of ATP (Adenosine Triphosphate). This energy is required for various cellular activities.

2. Role in Metabolism:
Metabolism refers to the chemical reactions that occur in living organisms to maintain life. Oxygen is involved in various metabolic processes, such as the breakdown of carbohydrates, fats, and proteins. These processes release energy and produce waste products like carbon dioxide and water.

3. Role in Aerobic Organisms:
Most organisms, including humans, are aerobic organisms that require oxygen for their survival. Oxygen acts as the final electron acceptor in the electron transport chain, a process that occurs in the mitochondria of cells. This process is essential for the production of ATP and the efficient functioning of cells.

4. Role in DNA and RNA:
Oxygen is a component of nucleic acids like DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). These molecules carry genetic information and are essential for the synthesis of proteins, which are the building blocks of life.

5. Role in Photosynthesis:
Although oxygen is not directly involved in photosynthesis, it is a byproduct of this process. During photosynthesis, plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen using sunlight. This oxygen is then released into the atmosphere, providing a vital source of oxygen for other organisms.

Conclusion:
In conclusion, oxygen is the most necessary fundamental element in bio-compounds due to its crucial role in respiration, metabolism, energy production, DNA and RNA synthesis, and photosynthesis. Without oxygen, life as we know it would not be possible.