Karl Kjer - Concepts of Biology

 All living things have evolved from a common ancestor, through processes that include natural selection and genetic drift acting on heritable genetic variation. Biological systems obey the laws of chemistry and physics. Structural complexity and information content are built up by combining simpler subunits into multiple complex combinations. Understanding biological systems requires both reductionist and holistic thinking because novel properties emerge as simpler units assemble into more complex structures. Living systems are far from equilibrium. They utilize energy, largely derived from photosynthesis, which is stored in high-energy bonds or ionic concentration gradients. The release of this energy is coupled to thermo dynamically unfavorable reactions to drive biological processes. 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, 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 tissues, organs, and organ systems with distinct functions. These 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.