and populations to communities.1 Much of the remainder of this report is organized around these biological scales, articulating examples of the biological problems to be addressed at each scale. These scales may be briefly described as follows:
Molecules. Molecular biology focuses on the chemical components of life and their interactions. These components differ greatly in size and complexity, ranging from atoms and simple ions, through the basic molecular building blocks of life such as nucleic and amino acids, sugars, and fats, to polymers and homogeneous and heterogeneous aggregates of the more basic units, forming macromolecular assemblies and super-molecular structures that carry out many of the fundamental processes in the life of a cell. The structures of these objects, as well as the dynamics of molecules and interactions between them, are central to biological function.
Cells. Cell biology is concerned with the self-replicating units of life, including bacteria, plant, and animal cells, as well as the viruses and other parasites that infect them. The study of the cell also includes consideration of many interconnected units or subcellular structures, such as organelles, which range in complexity from peroxisomes, proteosomes, or lysosomes, to mitochondria and chloroplasts, up to the nucleolus and the nucleus itself for eukaryotic organisms, and other structural components intrinsic to cell function such as the endoplasmic reticulum. The mechanisms and consequences of cell–cell communication are also of primary interest.
Organisms. Organismal biology includes both the properties of whole organisms and the complex multicellular structures of which they are composed—the tissues, organs, organ systems, and integrative processes that create a robust whole out of diverse parts. Organisms sustain health and well-being in the face of considerable insults and environmental disturbances, a process known as homeostasis. Another feature at this scale is the study of the breakdown of this robustness—in other words, the etiology and nature of disease.
Dividing life into levels, or scales, is obvious, is essential for understanding, and reflects an intrinsic feature of biology. Nonetheless, the levels interact, and some of the division is for human convenience or is an artifact of scholarly history. Characterizing any one level requires at least considering its immediately adjacent levels; one could also provide a finer subdivision of some of the scales, but for clarity, the committee used the most commonly employed and obvious distinctions, ones that are important for how biologists think about the object of their study and that provide a means for mathematicians to think about how to engage biology.