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How do we define “life”? What separates the living organisms from the non-living? Biologists usually define something as living if it can (1) reproduce, (2) respond to external stimuli through internal mechanisms, and (3) metabolize energy from the environment for its own uses. All three qualities are required to be considered alive.

Although for the purposes of definition a clear separation can be made between life and non-life, in truth the distinction is less clear. Viruses, for example, are a problematic group of structures composed of the same organic molecules (proteins and nucleic acids) that typify living organisms. In many ways viruses also behave like living organisms, but they cannot reproduce without using parts of the genetic machinery from another life form. In this technical sense, viruses are not alive. Nonetheless, most biologists consider viruses to be specialized, parasitic life forms that have secondarily lost the ability to reproduce independently.

All other life on Earth fulfills the basic definitions set forth above. Living organisms are composed of at least one cell, often referred to as the “building block” of life. Cells are self-contained engines of life, capable of interacting with the outside world, ingesting food and generating energy from it, and reproducing. Cells are mostly watery protoplasm, which is enclosed within an external cell membrane. The basic functions of the cell are performed by specialized molecules or by whole structures called organelles. Organelles are the “organs” of the cell, and they work in such roles as protein synthesis, food digestion, and energy storage.

The most specialized organelle is the nucleus. Present only in the cells of certain organisms (eukaryotes), the nucleus is often called the “control center” of the cell. It contains the genetic material of the cell, and therefore the code by which all processes in the cell are regulated. In cells without a nucleus (prokaryotes), this genetic material is dispersed throughout the protoplasm.

The genetic code is written in the form of DNA and RNA, molecules found only in living cells. These extraordinarily long molecules are usually tightly coiled into chromosomes, which vary in number depending on the species. The genetic code controls both the way an organism develops and, along with environmental factors, its ultimate external and internal appearance. Through the genes, the nucleus also controls much of the cell’s physiology—its biochemical functions.

The most primitive life forms are single-celled cyanobacteria, archaebacteria, and true bacteria. These organisms lack a nucleus and are described as prokaryotic. The specialized energy-producing organelles (chloroplasts and mitochondria) of eukaryotes are thought to have first appeared through early symbiotic relationships between primitive prokaryotic organisms. In these instances, the organelles were once free-living organisms that were absorbed into other cells where they carried out specialized functions. Single-celled organisms usually reproduce asexually, through the process of mitosis, or cell-splitting. Occasionally they may also engage in conjugation, which allows genetic material to be shared between different individuals.

Eukaryotes are highly diverse and the great majority of them are multicellular forms rather than single celled. Multicellular life forms are capable of great structural diversity because they can specialize in ways unavailable to single-celled organisms. In single celled organisms all biological processes are carried out within that one cell. This cell is limited in size and design because, through evolution, it must make structural and physiological “compromises” to carry out all its functions. In multicellular organisms, however, cells can specialize for certain tasks, permitting cells to work together to make something that is greater than the sum of the parts. Such organisms tend to have organs (such as a liver or kidney) that handle particular biological processes (such as detoxification or filtration). Individual cells no longer need to perform all functions. Rather, as long as the function takes place somewhere within the organism, all cells can benefit. Many of the structural factors that limit the size of a single-celled organism no longer apply to multicellular organisms, which can grow to enormous size.

Eukaryotic organisms reproduce sexually, allowing the exchange of genetic material. Offspring thus contain genetic information from both parents. Because multicellular organisms have specialized parts, they must undergo a process of development in order to permit orderly growth from the initial, single-celled zygote, through the multicelled but mostly cellularly unspecialized embryo, into a complex multicellular adult. This process of ontogeny allows the initial cell to divide into thousands of new cells, which in turn differentiate and acquire specialized functions and structures.

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