1. Water (H₂O): Water is the most abundant molecule in living organisms and plays a crucial role in many biological processes, including:
* Solvent: Water dissolves many substances, allowing for chemical reactions to occur within cells.
* Temperature regulation: Water has a high heat capacity, helping to maintain a stable internal temperature.
* Transport: Water is essential for transporting nutrients and waste products within the body.
2. Carbon Dioxide (CO₂): While often viewed as a pollutant, carbon dioxide is a key component of photosynthesis, where plants use it to produce energy. Animals exhale carbon dioxide as a waste product of respiration.
3. Oxygen (O₂): Oxygen is essential for cellular respiration, the process by which living organisms obtain energy from food.
4. Nitrogen (N₂): Nitrogen is a component of proteins, nucleic acids (DNA and RNA), and other vital molecules. Organisms cannot directly use atmospheric nitrogen, so it must be converted into usable forms through nitrogen fixation by certain bacteria.
While these four molecules are essential, living things also need a wide range of other small molecules, including sugars, lipids, and amino acids.
