Final answer:
Living organisms are primarily composed of carbon, oxygen, nitrogen, and hydrogen because these elements are necessary for the formation of macromolecules and play key roles in biological processes.
Step-by-step explanation:
The most abundant elements in the universe are hydrogen (H) and helium (He). However, living organisms are primarily composed of carbon (C), oxygen (O), nitrogen (N), and hydrogen (H). This is because these elements are necessary for the formation of macromolecules such as nucleic acids, proteins, carbohydrates, and lipids which are the building blocks of life.
For example, carbon forms the backbone of organic molecules and can form long chains and rings, allowing for the diversity of structures and functions in living organisms. Oxygen is essential for respiration and plays a role in energy production. Nitrogen is a key component of proteins and nucleic acids, including DNA and RNA. And hydrogen is involved in many biological processes, including the formation of water and the transfer of energy.
Although hydrogen is the most abundant element in the universe, it is not as abundant in living organisms as carbon, oxygen, and nitrogen. This is because hydrogen is highly reactive and tends to be bound to other elements, such as oxygen in water or carbon in organic compounds. Therefore, while hydrogen is essential for life, it is not as prevalent in living organisms compared to carbon, oxygen, and nitrogen.