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

A cell is enclosed by the plasma membrane, which forms a selective barrier allowing nutrients to enter and waste products to leave. The interior is organized into many specialized compartments, or organelles, each surrounded by a separate membrane. Between all organelles is the space in the cytoplasm called the cytosol, which is organized around a framework of fibrous molecules constituting the cytoskeleton. The cytosol contains more than 10,000 different kinds of molecules involved in cellular biosynthesis.

A thin membrane, some 0.005 micrometre across, surrounds every living cell, delimiting the cell from the environment around it. The plasma membrane has two functions: first, to be a barrier keeping the constituents of the cell in and unwanted substances out; and second, to be a gate allowing transport into the cell of essential nutrients and movement from the cell of waste products.

Most plant cells contain one or more membrane-bound vesicles called vacuoles. Within the vacuole is the cell sap, a water solution of salts and sugars kept at high concentration by the active transport of ions through permeases in the vacuole membrane. This high concentration causes the entry, via osmosis, of water into the vacuole, which in turn expands the vacuole and generates a hydrostatic pressure, called turgor, that presses the cell membrane against the cell wall. Turgor is the cause of rigidity in living plant tissue.

Potentially dangerous hydrolytic enzymes functioning in acidic conditions (pH 5) are segregated in the lysosomes, bound by a single phospholipid bilayer membrane, to protect the other components of the cell from random destruction. They probably originate by budding from the Golgi membranes. Enzymes known to be present in the lysosomes include hydrolases, which degrade proteins, nucleic acids, lipids, glycolipids, and glycoproteins; they are most active in the acidity maintained in the lysosomes. Lysosomes fuse with vacuoles containing material from inside or outside the cell to be digested. After the material is broken down, lipids and amino acids are transported across the lysosomal membrane by permeases for use in biosynthesis.

The endoplasmic reticulum (ER) is a system of membranous vesicles extending throughout the cytoplasm. Often, it constitutes more than half of the total membrane in the cell. The endoplasmic reticulum can be classified in two functionally distinct forms, the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER). The morphological distinction between the two is the presence of protein-synthesizing particles, called ribosomes, attached to the outer surface of the RER. The functions of the SER vary considerably from cell to cell. One important role is the synthesis of phospholipids and cholesterol. In liver cells, the SER is specialized for the detoxification of a wide variety of compounds produced by metabolic processes. In cells of the adrenal glands and gonads, cholesterol is modified in the SER at one stage of its conversion to steroid hormones. Finally, the SER in muscle cells, known as the sarcoplasmic reticulum, sequesters calcium ions from the cytoplasm. The RER plays a central role in the synthesis and export of proteins and glycoproteins and is best studied in the secretory cells specialized in these functions.

Ribosomes are particles that synthesize proteins from amino acids. They are composed of four RNAmolecules and about 50 proteins assembled into a large and a small subunit. Ribosomes are either free (i.e., not bound to membranes) in the cytoplasm of the cell or bound to the RER. Lysosomal enzymes, proteins destined for the ER, Golgi, and plasma membranes, and proteins to be secreted from the cell are among those synthesized on membrane-bound ribosomes. Fabricated on free ribosomes are proteins remaining in the cytosol, those bound to the internal surface of the plasma membrane, as well as those to be incorporated into the nucleus, mitochondria, chloroplasts, peroxisomes, and other organelles.

The Golgi complex is the site of the modification, completion, and export of secretory proteins and glycoproteins. This organelle has a characteristic structure composed of five to eight flattened, disk-shaped, membrane-defined cisternae arranged in a stack. The Golgi is thought to be the principal director of protein traffic in the cell. Secretory proteins and glycoproteins, plasma membrane proteins and glycoproteins, lysosomal proteins, and some glycolipids all pass through the Golgi structure at some point in their maturation. In plant cells, much of the cell-wall material passes through the Golgi as well.

The nucleus is the information centre of the cell in all higher organisms. It is separated from the cytoplasm by the nuclear envelope, and it houses the double-stranded, spiral-shaped deoxyribonucleic acid (DNA) molecules. The primary function of the nucleus is the expression of selected subsets of the genetic information encoded in the DNA double helix. The messenger RNA molecules are transported through the nuclear envelope into the cytoplasm, where they are translated, serving as templates for the synthesis of specific proteins.

One type of eukaryotic organelle, called mitochondrion, functions primarily to capture energy from food substances in a form useful for the cells. The process begins in the cytosol, resulting in molecules that are taken up by mitochondria, where they are oxidized, that is, electrons are removed from them. The energy available from the electrons is used to drive the synthesis of adenosine triphosphate (ATP), which stores energy in two special chemical bonds. The oxidative metabolism in the mitochondria, with the associated formation of ATP, is called cellular respiration. The number of mitochondria in a cell is variable, ranging from one contorted giant in some protists to a few hundred in large egg cells. Cells that require the most chemical energy tend to have more mitochondria per unit volume.

The plastids are produced only in plants and certain protists. The most familiar of the plastids are the chloroplasts, which are the site of photosynthesis and contain all the chlorophyll in the cell. The major function of the chloroplast is to carry on the process of photosynthesis, by which light energy is converted into the energy of chemical bonds; the molecules thus formed provide food for the plant itself and for other organisms. Chloroplasts are not the only kinds of plastids found in plants. The presence of legions of plastids called chromoplasts imparts the characteristic red, orange, or yellow colouring to different parts of the plant body. Other types of plastids, leucoplasts, serve a storage depot for starch and fats.

The cytoskeleton is the name given to a fibrous network formed by different types of long protein filaments present throughout the cytoplasm of eukaryotic cells. These filaments create a scaffold or framework that organizes other cell constituents and maintains the shape of the cell. In addition, some filaments cause coherent movements, both of the cell itself and of its internal organelles. Three major types of cytoskeletal filaments are commonly recognized: actin filaments, microtubules, and intermediate filaments. Actin filaments and microtubules are dynamic structures that continuously assemble and disassemble in most cells. Intermediate filaments are stabler and seem to be involved mainly in reinforcing cell structures.

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II. Listen to the following words and practice their pronunciation	\|	Vocabulary notes

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