Final answer:
Carbon is the molecule present in all living organisms, essential for the structure of macromolecules and accounting for about 50% of a cell's composition.
Step-by-step explanation:
The molecule that is present in all living organisms is carbon (C). Carbon is a fundamental component for all macromolecules, including nucleic acids, proteins, carbohydrates, and lipids, which serve as the building blocks of life. It is part of the unique molecular structures that enable living organisms to form cells, tissues, and organ systems. Carbon constitutes roughly 50% of the total composition of a cell and is vital for the structure of macromolecules, critical to living organisms.