Biology is the study of living things. The definition of life, however, has been routinely debated for centuries. A modern list of the characteristics of living things includes several components. The key structural characteristic of a living thing is that it is composed of at least one cell. An organism may consist of a single cell, a few thousand cells, or—in the case of large mammals—trillions of cells. Each cell is separate, enclosed in its own membrane. In multicellular organisms, connections between cells allow them to work together within the organism.Living things use energy to carry out life processes. They take in and metabolize nutrients, and they excrete waste products from chemical reactions. Plants and some bacteria are able to synthesize energy-rich molecules from nonliving sources. Animals, fungi, and most bacteria must consume food produced by other living organisms. While the sources of energy may differ among organisms, many of the chemical reactions carried out by their cells are the same. A living organism, whether single-celled or multicellular, uses energy to keep its internal conditions (temperature, pH, concentrations of various elements and molecules) stable regardless of external conditions, known as homeostasis. One example of homeostasis in the human body is the maintenance of a body temperature within the range of 97°F—99°F, with a typical set point of 98.6°F. While maintaining their internal environment, living things detect and respond to their external environment. Some animals have sophisticated nervous systems to carry out information processing, but even the simplest bacterial cells can respond using chemical-sensing systems. Living things grow and undergo change throughout their lives. Asexually reproducing organisms experience this as they develop from immature clonal stages to their adult forms and continue to develop afterward. Sexually reproducing organisms experience this as they develop from fertilized eggs to mature adults. Even a single-celled bacterium increases in size as a precursor to dividing. The timing and sequence of an organism's developmental stages is controlled by its genes and environmental factors. A gene is a unit of heritable material that codes for a particular trait. Genes are sections of a nucleic acid molecule called deoxyribonucleic acid. Deoxyribonucleic acid (DNA) is an organic molecule containing coded instructions for the life processes of an organism. This DNA consists of nucleotides bonded together in the form of a double-helix molecule that allows for genetic inheritance in all living organisms. All living things reproduce, forming the next generation and passing on their traits to their offspring. Reproduction may be sexual, requiring the contributions of two parent organisms and resulting in genetic variations. Conversely, reproduction may be asexual, or carried out by one parent organism and resulting in genetically identical offspring. Evolution ("change over time") occurs within a species as individuals with the most advantageous traits for a particular environment survive and reproduce to a greater extent than other individuals in that environment. Such selective survival and reproduction produces changes in gene frequency from one generation to the next. Offspring with versions of genes most suited for survival in a given environment survive and reproduce more successfully in that environment. However, significant changes in the environment will favor the survival and reproduction of individuals best suited for those later changes. Thus, having a variety of versions of genes in a population increases that population's chances of surviving and reproducing in a wide range of environments and environmental changes.