Читать книгу Astrobiology - Charles S. Cockell - Страница 100

Cells are the packages that hold life together. The term “cell” is derived from the Latin word, cella, which means “small room.” In biology, the cell refers to the structure that encloses the apparatus that allows for the growth and reproduction of organisms. Indeed, the compartmentalization of life's major biomolecules within cellular packages is so fundamental that some definitions of life have explicitly included compartmentalization or cellular structure as a feature of living things. Cells were first observed and described by polymath Robert Hooke (1636–1703) in his book Micrographia, in which he documented the structure of plant cells by observing thin slices of cork. He found them to look like honeycomb cells, an indication of compartmentalization.

Broadly, we can recognize two types of cells – prokaryotic and eukaryotic. Prokaryotic cells do not have a nucleus, hence their name, which derives from the two Greek words, pro (before) and karyon (nut or kernel – a reference to the nucleus in the biological context). Prokaryotes are single-celled organisms and include two major domains of life: bacteria and archaea. Domains are the highest hierarchy of life. The bacteria include many species of microorganisms that live in soils, in the oceans, in your gut, and in many other environments. Archaea is the domain that includes many of the species that live in extreme environments, for example high-temperature-loving microbes (hyperthermophiles) that live in deep-sea hydrothermal vents, where hot water from the crust meets the deep ocean. However, the archaea also include many non-extremophile species that have important roles to play in natural environments, including soils, such as species that cycle nitrogen. We encounter both groups more as we proceed through the book. In prokaryotic cells, the DNA is free-floating in the cell fluid or cytoplasm.

By contrast, eukaryotic cells are cells with a nucleus that contains the DNA. They make up the structure of most multicellular organisms, including us, and some single-celled organisms like algae. Eukaryotic cells are usually larger (typically 10–100 μm) than prokaryotic cells (typically 1–10 μm).