Unit 3. Cell structure.

WARM-UP: Name all the organelles of a cell.

VOCABULARY: Learn the following words and word combinations.

lifeless living matter fluid substance digestion respiration to be bounded by cell division hereditary traits metabolic processes protein to carry out under the control of to be associated with tissue to manufacture to be responsible for permeable substance impermeable semi-permeable environmental conditions to consist of nutritive excretory products carbon

Guess the meaning of the following words.

Microscope; hydrogen; oxygen; nitrogen; sulphur; fats; mineral salts; absorption of water; reproduction; membrane; mitochondria; physiological activity.

Practice the following for pronunciation.

protoplasm – [‘proutəplæzəm]

protoplasmic – [[‘proutəplæzəmik]

digestion – [di’dʒestʃən]

hydrogen – [‘haidridʒən]

liquid – [‘likwid]

minute – [mai’nju:t]

carbon – [‘ka: bən]

protein – [‘prouti:n]

oxygen – [‘oksidʒən]

sulphur – [‘s۸lfə]

nitrogen – [‘naitridʒən]

homogeneous – [homə’dʒi:njəs]

excretory – [ekskr’:təri]

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

All living matter consists of protoplasm. Protoplasm is the physical basis of life. Nearly every organism contains lifeless as well as living matter. There is no protoplasm in lifeless matter.

Protoplasm when it is examined under the compound microscope is a thing granular, nearly transparent fluid substance. It has the same appearance, chemical composition and the same characteristics in the both animals and plants. It contains a very large amount of water.

The remainder of protoplasmic substance consists of proteins which are composed of carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus. Fats, sugars and mineral salts are also present.

All the processes which go on in the body of the plant, for example, such as the absorption of water and salts, the manufacture of food, digestion, respiration, growth and reproduction, are ultimately directed and controlled by protoplasm. For this reason protoplasm is regarded as the only part of an organism which is truly alive and the knowledge of its structure and activities is therefore essential for an understanding of any living thing.

A typical cell contains the nucleus, the cytoplasm and the cell wall. The protoplasm of a cell is not homogeneous but contains within it a number of specialized structures. The nucleus is chief among them. It is approximately spherical body denser than the rest of the protoplasm and bounded by a definite membrane. The nucleus is evidently the directing center for the more important physiological activities of the cell such as the synthesis, digestion and assimilation of food, formation of the cell wall, cell division, growth and transmission of hereditary traits in reproduction. Cells from which the nucleus has been removed artificially will often live for a time but fail to carry on actively their normal metabolic processes.

Chemically the nucleus is much like the rest of the protoplasm, except the fact that it contains a large amount of protein, nuclein of which phosphorus is an important element.

A typical cell usually contains one nucleus, but there are cells with many nuclei. Among some of the algae and fungi, for example.

The remainder of the protoplasm of the cell is known collectively as the cytoplasm. Different activities of the cell are carried out in the cytoplasm under the control of the nucleus. Some of them are performed by the cytoplasm as a whole, others are associated with definite bodies plastids. Important among these are chloroplasts, small roundish bodies present in the cells of green tissue of leaves and certain other parts of the plant exposed to light. They contain chlorophyll, with the help of which organic food is manufactured, chromoplasts which are related closely to chloroplasts are responsible for most of the red and yellow colors of plants. The change from unripe fruit to the ripe condition in many plants is due to the conversion chloroplasts into chromoplasts.

In addition to the plastids there are usually other bodies in the cytoplasm which are very minute and appear as tiny granules. They are called mitochondria.

On the outside of the cytoplasm a delicate plasma membrane is developed. A similar vacuolar membrane separates cytoplasm and vacuole. Through one or both of these membranes must pass everything that enters or leaves the cell. These membranes are permeable to some substances and impermeable to others. The cytoplasm is not quite but flows from one part of the cell to another. The rate of the flow is affected environmental conditions, esp. temperature and in general is the most rapid when physiological activity is the highest.

Animal cells are bounded by a thin semi-permeable membrane. The membrane consists of two or three layers and is permeated by openings. Through these openings the cytoplasm of the cell communicates with that of the neighboring cells. But the protoplasm of one cell never enters another cell. The protoplasmic body of the cell is encased by a cell wall. This is the main feature distinguishing plant cells from animal cells. The cell membrane and the cell wall play an important part in protecting the cell. All the nutritive substances have to pass through the membrane, all the metabolic waste and excretory products of the cell move across in the opposite direction when ejected.