Look at Fig. 3-1

The cell is the basic unit of life. It is the simplest structure that shows all the characteristics of life: growth, metabolism, reproduction, responsiveness and homeostasis. The cell is invisible to the naked eye. The scientific study of cells began with the invention by Anton van Leeuwenhock of microscope some 350 years ago. Today this single-lens microscope has been replaced by the modern electronic microscope.

Fig. 3-2. Diagram of a typical cell showing the main organelles

Fig. 3-1. A simplified comparison of an optical (compound)

Fig. 3-3. Mitosis. Two centrioles migrate, the genetic material of the nucleus coifs into threadlike chromosomes, and two daughter cells form within the cell membrane. (The cell shown is for illustration only. It is not a human cell, which has 46 chromosomes)

3. Read the text and translate it.

Cells and Their Functions

Look at the cell (Fig. 3-2). The outer covering of the cell is the cell membrane. It is very important in regulating what can enter and leave the cell. The main substance that fills the cell and holds the cell contents is the cytoplasm. This is a colloidal suspension of water, food, minerals, enzymes and other specialized materials.

The cell contents include organelles. All the organelles and their functions in the cell are demonstrated in Table 3—1.

Table 3 -1. Cell Structure

Protein plays a very important part in the organism and its production takes place in the cell. There are two nucleic acids in the cytoplasm and nucleus, which are important in protein production:

- deoxyribonucleic acid or DNA is mostly found in the nucleus and makes up chromosomes. DNA is double stranded;

- ribonucleic acid or RNA is single stranded, it is located in the cytoplasm.

Cell reproduction occurs by the dividing process called mitosis. In this process each original parent cell divides to form two identical daughter cells.

Various physical processes take place in the cell:

1) diffusion - molecules move from an area of higher concentration to an area of lower concentration;

2) osmosis - diffusion of water through the semipermeable membrane;

3) filtration - movement of materials through the cell membrane under mechanical force;

4) active transport - movement of molecules from an area of lower concentration to an area of higher concentration;

5) phagocytosis"- engulfing of large particules by the cell membrane;

6) pinocytosis - intake of droplets of fluid; the word pinocytosis means "cell drinking".
Each of these processes is necessary for the functioning of the cells.

Cells develop different forms, these forms of the same kind congregate to form one of the basic tissues. These tissues, in turn, become specialized organs. So, every cell and formation of cells has its purpose.

But sometimes in the DNA or genetic material a mutation or change occurs. If such a cell doesn't die naturally or get destroyed by the immune system, it may begin to multiply out of control and spread to other tissues, producing cancer. Cancer cells form tumors, which interfere with normal functions, crowding out normal cells and robbing them of nutrients.

Because cancer may take a long period to develop, it is often difficult to identify its causes. Certain factors increase the changes of developing the disease and are considered risk factors.

Risk factors include heredity, carcinogens or chemicals (in food, environment, drugs), ionising radiation (X-rays, ultraviolet rays), physical irritation (intake of hot food), diet (high in fats), viruses.

Date: 2015-09-18; view: 762; Нарушение авторских прав