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

The basic unit of life. The cell is the basic unit of organization in living things. All organisms are composed of cells, and all cells come from preexisting cells — these two statements constitute the cell theory. Even viruses, which are not cells themselves, are entirely dependent on the presence and chemical machinery of cells for their reproduction. A cell may arise either by the division of another cell or by the fusion of two other cells. In nature, a cell cannot simply be formed by a combination of its component parts, nor has such a feat of synthesis been achieved in the laboratory. This situation raises an important question: Where did the first cells come from?

Some cells are free-living organisms in their own right. Others are parts of a multicellular organism. In general, each cell in an organism is totipotent, that is, it contains all the genetic information required to generate that entire organism.

Many plants reproduce by means of seeds, but this form of reproduction can sometimes be bypassed. With some species it is possible to take a cutting consisting of a bit of stem and a leaf or two, put it in soil and care for it, and end up with an entire plant. Going further, one may isolate single cells; and in special laboratory conditions even these may be induced to develop into intact plants. However, if we try to go to a level of structure below that of an entire cell, we come to the end of the line. Subcellular structures such as nuclei and chloroplasts may be isolated from cells in quantity and caused to carry out their normal functions; but they can never be induced to regenerate whole cells, let alone an entire plant. The inability of even the nucleus to produce a life form is one sense in which the cell is a basic unit of function and reproduction.

Main characteristics of cells. Cells are tiny: Most have a volume of 1-1000 cubic micrometers (μm³). Eggs of some birds are enormous exceptions and individual cells of several types of algae are large enough to be viewed with the unaided eye. Neurons (nerve cells) have volumes that fit within the "normal" range, but they often have fine projections that may extend for meters, carrying signals from one part of a large organism to another. In spite of these special cases, we may generalize and say that cells are very small objects.

The cell's activities depend upon specific component structures and their organization into a coordinated whole. A cell must do many things in order to survive. It must obtain energy from its environment. It must be selective as to what materials enter and leave it. All cells must interpret and use the information contained in their DNA. The chemical reactions essential for life must be kept from interfering with one another, etc.

A comparison of numerous kinds of cells shows that there are two distinct general arrangements, with only a few intermediate forms in evidence. One general arrangement, usually the simpler, is the prokaryotic (prenuclear) type, characteristic of the Monera (the bacteria). Organisms in the kingdom Monera are often referred to as prokaryotes; they lack nuclei. The rest of the living world is eukaryotic: Its cells contain true nuclei. These cells usually include additional internal compartments that are surrounded by membranes, and organisms with this type of cell are known as eukaryotes. Both prokaryotes and eukaryotes are to be regarded as great successes, for they have prospered through billions of years of evolution.

Prokaryotic cells. Prokaryotic cells exhibit great variety of internal structure. What they all have, without exception, is three things. The first is a plasma membrane, separating the cell from its environment and regulating the inward and outward traffic of material. The second is a relatively clear area, as seen under the electron microscope, containing the hereditary material (DNA) of the cell. This region is called the nucleoid. Each cell has at least one of these, and some may contain more than one. The remainder of the material within the cell is called the cytoplasm. At high magnification it is seen to be full of minute, roughly spherical structures called ribosomes, the third of the components found in all prokaryotic cells. The ribosomes are approximately 15-20 nm in diameter and consist of three molecules of RNA and about 50 different protein molecules. Their function is to coordinate the synthesis of proteins: The ribosome controls the interaction of various forms of RNA and the other parts of the protein-synthesizing machinery. The remainder of the cytoplasm is a complicated solution containing many kinds of enzymes and the other chemical constituents of the cell. Structurally, a prokaryotic cell is relatively simple, but functionally it is exceedingly complex. There are literally thousands of kinds of chemical reactions occurring under the precise direction of enzymes, with the cell's DNA serving as the molecular memory that allows successive generations of a given cell to be very much like one another.

Eukaryotic cells. The vast majority of living species, including all animals, plants, fungi, and protists, have cells that are considerably more complex in structure than those of the prokaryotes. The eukaryotic cell is full of membranous structures of wondrous diversity. Many of the structures are completely enclosed by one or two membranes, have distinct and characteristic external and internal forms, and carry on particular biochemical functions. They are, in effect, neatly packaged subsystems, with membranes to control their functions and to regulate what gets in and out. Some of the subsystems are like little factories that make specific products. Others like power plants that take energy in one form and convert it to a more useful one. These membranous structures as well as other structures lacking membranes but possessing distinctive shapes and functions (the ribosomes, for example) are referred to as organelles.

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II. Listen to the following piece of information about cellulose. Fill in the missing words or phrases	\|	Vocabulary notes

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