Carbon-based life is the concept that carbon is the core element for all known forms of life, and a core component of all complex biological molecules with carbon representing approximately 45–50% of all dry biomass. Complex biological molecules consist of carbon atoms bonded with other elements, in particular oxygen and hydrogen but frequently also nitrogen, phosphorus, and sulfur. In these molecules carbon's four valence electrons and subsequent propensity to form four bonds lends to its function as the "skeleton" of organic molecules. In addition to molecular level properties of carbon, life on Earth also relies on large scale processes known as the carbon cycle, involving geological and atmospheric processes.