The principal structural elements of a living cell are carbon, hydrogen, oxygen, and nitrogen. These elements are essential for the formation and maintenance of the various components of a cell, including biomolecules such as proteins, nucleic acids, carbohydrates, and lipids.

Carbon:
- Carbon is the backbone of organic molecules, as it has the unique ability to form stable covalent bonds with other carbon atoms and a wide range of other elements.
- It is present in all major biomolecules, including proteins, nucleic acids, carbohydrates, and lipids.
- Carbon atoms can form long chains and complex three-dimensional structures, allowing for the diversity and complexity of biological molecules.

Hydrogen:
- Hydrogen is the most abundant element in living organisms and is found in all major biomolecules.
- It forms covalent bonds with carbon, oxygen, nitrogen, and other elements, contributing to the stability and structure of biomolecules.
- Hydrogen bonding between molecules is crucial for the maintenance of the three-dimensional structure of proteins and nucleic acids.

Oxygen:
- Oxygen is necessary for cellular respiration, the process by which cells produce energy.
- It is a component of water, which is essential for various cellular processes.
- Oxygen is also found in biomolecules such as carbohydrates and lipids.

Nitrogen:
- Nitrogen is an essential component of proteins and nucleic acids.
- It is present in the amino acids that make up proteins and the nitrogenous bases that form the building blocks of DNA and RNA.
- Nitrogen is also involved in the synthesis of other important biomolecules, such as ATP (adenosine triphosphate) and nucleotide coenzymes.

In conclusion, carbon, hydrogen, oxygen, and nitrogen are the principal structural elements of a living cell. They are essential for the formation and maintenance of biomolecules and play crucial roles in various cellular processes.