Unit 9. Photosynthesis.

WARM-UP: Think of words and word-combinations you associated with the process of photosynthesis. Share your ideas with the rest of the group. Give the definition to this process.

VOCABULARY: Learn the following words and word combinations.

To conclude to obtain to expose initial substance to converse saturation to deprive to bring about to emit to attain to dissolve to decline to wilt artificial light radiant energy visible spectrum to accumulate ray soil to oxidize

Guess the meaning of the following words.

Source of food; fundamental relations; converse; synthesis; molecule; veins of the leaf; stomata; chlorophyll; chloroplasts; outer atmosphere; deep-sea algae; intensity; plant tissue; daily periodicity; water loss.

Practice the following for pronunciation.

photosynthesis [,fəʋtəʋ'sınɵısıs]

expose [,ıks’pouz]

oxidize [‘ɔksıdaız]

emit [ı’mıt]

synthesize [‘sınƟısaız]

modify [mɔdıfaı]

molecule [‘mɔləkju:l]

algae [‘ældʒı:]

intensity [ın’tensıtı]

saturation [,sætju’reıʃən]

closure [klouʒə]

intercellular [,ıntə’seljulə]

converse [‘kɔnvə:s]

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

Twenty three hundred years ago the philosopher Aristotle concluded that green plants obtain food from the soil. More than 20 centuries passed before the true source of food in the world was discovered. A number of experiments were conducted by different investigators that led to the discovery of certain fundamental relations between carbon dioxide and oxygen and living organisms. When the green parts of plants were exposed to light, the amount of oxygen in the surrounding air increased and the amount of carbon dioxide decreased. During the hours of darkness the converse occurred. The initial substances used in this process are water and carbon dioxide and the final products are sugar and oxygen. The process of making sugar in the green parts of plants is called photosynthesis (Greek, photos, light; and synthesis, putting together), because light is necessary for the synthesis of large molecules from smaller ones.

One of the initial substances is water. It moves up from the soil through the roots and stems into the veins of the leaf and then passes from cell to cell through the leaf. Carbon dioxide enters the leaf from the surrounding atmosphere above the soil. Most of it passes through the stomata inter the intercellular spaces of the mesophyll. It dissolves in the water of the cell and finally gets to the chlorophyll in the chloroplasts. One of the final substances of the photosynthesis – oxygen – passes through the stomata into the outer atmosphere. The second substance – sugar – is transformed to other types of foods. Some is transformed directly into substances of which cells are composed, some of it is oxidized in the plant, and some accumulates within the plant body.

Under natural conditions the energy of photosynthesis is sunlight. No sugar is made from carbon dioxide and water in green plants deprived of light. Radiant energy that we see as light is known as the visible spectrum. Red, orange, yellow and blue light is the most effective in photosynthesis. In land plants most of the photosynthesis is brought about by the radiant energy of the red end spectrum, but the radiant energy that penetrates the water to the depth of deep-sea algae is mainly the blue end. Any artificial light that emits these rays may start photosynthesis. If the intensity is low, plants will grow but without bearing flowers and fruits. The lowest light intensity at which photosynthesis will occur is that of full moon. And only deep-sea algae can synthesize food at such low intensities.

The degree of saturation of plant tissues may modify the rate of photosynthesis. The effects are apparent in the daily periodicity of photosynthesis. In the morning hours the plant tissues are saturated, the stomata are open, and photosynthesis soon attains a maximum rate. Later in the day the rate declines because of water loss from mesophyll cells. Still later photosynthesis is further decreased by the gradual closing of the stomata. When plants begin to wilt, photosynthesis is sharply decreased mainly because the water content of the cell is lower, and to some extent as a result of the closure of stomata.