Humans are carbon-based lifeforms, a designation highlighting the fundamental role of carbon in our biological makeup. The term “carbon-based” refers not to the element making up the greatest percentage of our body mass (which is oxygen), but to the element forming the structural foundation of all known life on Earth. Carbon atoms form the backbone of complex molecules that carry out nearly all functions necessary for life, such as storing genetic information and providing structural support. This foundational chemistry means the study of life is essentially the study of carbon compounds, a field known as organic chemistry.
The Unique Chemistry of Carbon
Carbon’s atomic structure makes it an ideal building block for the complex molecules required for life. With four electrons in its outermost shell, a carbon atom can form four stable covalent bonds with other atoms, a property known as tetravalency. This allows carbon to serve as a versatile junction point, connecting to elements like hydrogen, oxygen, nitrogen, sulfur, and phosphorus, which form the molecules of life.
The small size of the carbon atom enables it to form short and strong bonds, stable enough to withstand conditions inside a living cell. Carbon atoms bond readily with each other to form long chains, branched structures, and stable rings, a process called catenation. This capacity to create intricate, three-dimensional skeletons allows for the enormous variety and complexity seen in biological molecules. Carbon can also form single, double, or triple bonds, adding diversity to the shapes of the resulting compounds.
Carbon’s Role in Biological Structure
The complex structures built upon carbon backbones are classified into four major categories of macromolecules present in the human body:
- Carbohydrates, such as glucose, are built from carbon rings and chains and function primarily as a source of quick energy for cellular processes.
- Lipids, including fats and oils, are nonpolar molecules essential for long-term energy storage and forming the structural boundaries of cell membranes.
- Proteins are polymers constructed from amino acid subunits, carrying out the majority of the work in the cell by serving as enzymes, providing structural support, and transporting materials.
- Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), rely on a carbon and sugar-phosphate backbone to store and transmit genetic instructions.
Why Carbon Dominates Terrestrial Life
While elements like silicon also possess four valence electrons and can form four bonds, carbon holds a chemical advantage in terrestrial life. Silicon’s larger atomic size results in longer and weaker bonds, especially when bonding with itself. This instability makes the complex, long-chain molecules necessary for life less durable in an aqueous environment.
A primary difference lies in the compounds formed when these elements oxidize. Carbon reacts with oxygen to form carbon dioxide (\(\text{CO}_2\)), a gas that is easily soluble and readily expelled as a waste product. Silicon, however, forms silicon dioxide (\(\text{SiO}_2\)), a solid mineral like sand or quartz, making it difficult to metabolize and excrete in a biological system.