Unit 2. Cells as biological units.

WARM-UP: Write down the parts of the cell you know. Share your ideas with the partner.

VOCABULARY: Learn the following words and word combinations.

medium (pl. media) recognize behave multiply extend nucleus (pl. nuclei) sap strand cavity minute successive to be similar on the account hereditary factors outermost layer granular accumulation starch minute bodies enlarge solution acid soluble compounds layer penetrate particles

Guess the meaning of the following words.

Vacuole, cytoplasm, plastid, phosphorus, chromosome, structural, organism, physical basis, protoplasm, chlorophyll, protein, pigment, cellulose.

Practice the following for pronunciation.

vacuole – [‘vækjuəul] essential – [i'sentʃəl]

medium – [‘mi:djəm] numerous – [‘nju:mərəs]

cytoplasm – [‘saitəplæzm] plastid – [‘plæstid]

nucleus – [‘nju:kliəs] nuclei – [‘nju:kliai]

granular – [‘grænjlə] phosphorus – [‘fosfərəs]

chromosome – [‘krouməsoum] hereditary – [hi’reditəri]

READING: Read the text and do the tasks that follow.

Cells were seen and recognized as structural parts of plants only in the 18^th century. Scientists came to the conclusion that the bodies of all living organisms are composed of cells, or products of cells, that in certain features the cells of plants and animals are alike, and that protoplasm is the physical basis of life phenomena. To these particles we may add that when plants are similar it is because their protoplasms are similar, that when plants develop or behave differently under the same environment it is because their protoplasms are unlike and that life is a very complex physical-chemical system.

The protoplasm, the vacuole and the cell wall are the essential parts of a plant cell. Protoplasm is a medium in which the numerous chemical and physical processes of the cell occur. In most plant cells the protoplasm is organized into cytoplasm, plastids and nucleus. Cytoplasm is usually granular in appearance because of the presence of particles of various foods and other substances. Plastids are protoplasmic bodies in the cytoplasm. Special reactions take place in them resulting in the accumulation of particular substances such as starches and chlorophyll. As the cells multiply by division, the plastids also increase in number by division.

The nucleus is denser than the cytoplasm. Chemically it differs from the cytoplasm in that the proportion of proteins is less and its proteins are far more complex. They are relatively high in phosphorus and on this account more reactive. Nucleus is a center of many cell activities. It contains very minute bodies called chromosomes that are the carriers of many of the hereditary factors of the plant.

As the cell enlarges microscopic droplets of liquid become visible in the cytoplasm. They form large cavities called vacuoles. The vacuoles are filled with “cell sap”, a solution of sugar, salts, acids, and other soluble compounds. Some of the substances in solution readily pass from the vacuole into the surrounding cytoplasm or from the cytoplasm to the vacuole. Sometimes the cell sap may become colored with pigments as in the cells of many flowers, fruits and red autumn leaves.

The outermost part of a plant cell is the cell wall. It covers the protoplasm. During its development the plant cell becomes surrounded by wall composed of pectic material, later by successive cellulose layers. The cell wall has a high water content during the active life of the cell. It is composed of small submicroscopic particles between which molecules of water may penetrate and be held there. Fine strands of protoplasm sometimes extend through the wall from one cell to another.