II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

Anyone who has eaten yogurt, cheese, or bread, smelled spoiled milk or suffered from strep throat has encountered Monera, more commonly called bacteria. Bacteria can be found in all natural environments, often in extremely large numbers. As a group, they display exceedingly diverse metabolic capabilities and use almost any organic compound, and even some inorganic salts, as a food source. In a sense, bacteria are the dominant living creatures on Earth, having been present for perhaps three-quarters of Earth history and having adapted to almost all available ecological habitats. Studies of the relationships among different groups of bacteria continue to yield new insights into the origin of life on Earth and the directions of evolution.

Bacteria are classified as the prokaryotic kingdom Monera; all bacterial cells, and only bacterial cells, are prokaryotic in nature. Bacteria are unicellular microorganisms and thus are not organized into tissues. Each bacterium grows and divides independently of any other, although aggregates of bacteria, sometimes containing members of different species, are found. Individual bacteria can assume three basic shapes: spherical (Coccus), rodlike (Bacillus), or curved (Vibrio, Spirillum, or Spirochete). Considerable variation is seen in the actual shapes, and cells are usually stretched or compressed in one dimension.

Monera are grouped by how they acquire energy. The most fundamental distinction reflects carbon source. Autotrophs extract energy from carbon dioxide and heterotrophs use more reduced and complex organic molecules, typically from other organisms. A rare type of bacterium, called mixotroph, combines characteristics of autotrophs and heterotrophs and uses inorganic molecules for energy and organic molecules for carbon. Hetorothrophs are further distinguished by their source of organic compounds. Saprobes obtain nutrients from dead plants and animals. Symbionts live within living organisms and acquire nutrients from them. Finally, monera are classified by the source of hydrogen or electrons, which are important in energy acquisition. Lithotrophs obtain electrons from reduced inorganic compounds. Organotrophs obtain hydrogen or electrons from organic compounds.

Monera are also classified by their oxygen requirements. Obligate aerobes require oxygen and harness it in carrier molecules forming an electron transport chain on the inner face of the cell membrane. Facultative anaerobes use oxygen or not, and they obtain energy from fermentation. For obligate anaerobes, oxygen is toxic, and they live in habitats that lack it. Due to the differences in the structure of the bacterial wall surface scientists distinguish between gram-positive and gram-negative bacteria (named after the Danish physician Hans Christian Gram who developed one of the most useful staining reactions for bacteria).

Bacteria have severaldistinguishing characteristics. Piliare short projections on bacterial cells, resembling hairs, that enable the cells to attach to objects. Another bacterial structure is a flagellum, which is an extension that rotates, moving the cell. A bacterium’s cell wall may have a sticky layer called a glycocalyx, which is composed of proteins and/or polysaccarides. A loose glycocalyx is called a slime layer, and a firm glycocalyx is a capsule. The glycocalyx enables the cell to adhere to various surfaces. Bacteria can form structures called endospores that enable them to survive harsh conditions. An endospore is a walled structure that forms around the nucleus and a small amount of cytoplasm. The normal cellular form returns when environmental conditions improve.

Most bacteria reproduce by a process of binary transverse fission, in which the cell grows in volume until it divides in half to yield two identical daughter cells. Each daughter cell can continue to grow at the same rate as its parent. One group of environmental bacteria reproduces in a different manner, called budding. A small bud forms at one end of the mother cells or on filaments called prosthecae. As growth proceeds, the size of the mother cell remains about constant, but the bud enlarges. When it is about the same size as the mother cell, it separates. This type of reproduction is analogous to that in the budding fungi, such as brewer's yeast (Saccharomyces cerevisiae). One difference between transverse fission and budding is that, in the latter, the mother cell often has different properties than the offspring.

Bacteria and Archaea.It has become clear from studies of bacterial genes that bacteria are not simply primitive cells or precursors to higher organisms. In fact, bacteria have been divided into two major phylogenetic kingdoms, Eubacteria and Archaebacteria (Archaea), based on such differences as chemistry and physiology. All remaining living organisms are eukaryotes. It can be said that members of these two prokaryotic kingdoms are as different from one another as they are from eukaryotic cells; these differences are manifested in almost all observable characteristics, including metabolic pathways, identity of lipids, cell surface structures, and gene sequences.

The archaebacteria have markedly different surface structures from the eubacteria. Their membrane lipids are not fatty acids linked to glycerol by ether bonds, as in eubacteria and eukaryotes, but are branched isoprenoids linked to glycerol by ether bonds. Another difference between bacteria and archaea is in the structure of RNA polymerase, the enzyme that all cells use to transcribe DNA into mRNA. Bacteria use a single type of RNA polymerase consisting of four polypeptide subunits. The archaeal equivalent of this enzyme occurs in several forms that are far more complex than the bacterial RNA polymerase.

Microbial ecology. Monera and other unicellular organisms are essential components of the global ecosystem. Without microscopic life, macroscopic life would not exist, because microorganisms capture energy from the nonliving environment and form the bases of food webs. Photosynthesis harnesses much of this energy and bacteria that can photosynthesize support vast living communities in many habitats. Even in areas far from sunlight, monera support life by harnessing chemical energy from the non-living environment and using it to synthesize compounds that are nutrients for other organisms. Some bacteria cause disease in humans, animals, or plants, but most are harmless or beneficial ecological agents whose metabolic activities sustain higher life-forms. Without bacteria, soil would not be fertile, and dead organic material would decay much more slowly. Some bacteria are widely used in the preparation of foods, chemicals, and antibiotics.

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