Although fundamental molecular and cellular processes are conserved, biological systems and organisms are extraordinarily diverse. Unlike atoms and simple molecules studied in chemistry and physics, no two cells are identical.
Biological systems maintain homeostasis by the action of complex regulatory systems. These are often networks of interconnecting partially redundant systems to make them stable to internal or external changes.
Cells are fundamental units of living systems. Three fundamental cell types have evolved: bacteria, archea, and eukaryotes.
Living organisms have behavior, which can be altered by experience in many species.
Information encoded in DNA is organized into genes. These heritable units use RNA as informational intermediates to encode protein sequences, which become functional on folding into distinctive three-dimensional structures. In some situations RNA itself has catalytic activity.
Most biological processes are controlled by multiple proteins, which assemble into modular units to carry out and coordinate complex functions.
Lipids assemble with proteins to form membranes, which surround cells to separate them from their environment. Membranes also form distinct compartments within eukaryotic cells.
Communication networks within and between cells, and between organisms, enable multicellular organisms to coordinate development and function.
In multicellular organisms, cells divide and differentiate to form tissues, organs, and organ systems with distinct functions. These differences arise primarily from changes in gene expression.
Many diseases arise from disruption of cellular communication and coordination by infection, mutation, chemical insult, or trauma.
Groups of organisms exist as species, which include interbreeding populations sharing a gene pool.
Populations of species interact with one another and the environment to form interdependent ecosystems with flow of energy and materials between multiple levels.
Humans, as well as many other species, are members of multiple ecosystems. They have the capacity to disrupt or preserve the ecosystems upon which they depend.