Reading

3. Read the text and find which examples illustrate the importance of the topic.

The Tree Comes of Age

Our tree gradually becomes taller and broader, and in the course of time it reaches maturity. The complicated mechanism functions with the precision of a machine, and its many vital processes are well coordinated. Some of the processes, such as respiration or digestion of fats, are strikingly similar in both plants and animals. Others, as mineral nutrition, are found only in the plants.

Let us consider first the process of photosynthesis — that is, the building with the energy of light. In this process, organic matter is formed literally from thin air and water. The air contains minute amounts of carbon dioxide (0.03 percent by volume or three parts in 10,000 parts of air). Through millions of small pores, or stomata, on the leaf surfaces, air penetrates the leaves and gives up about 10 percent of its meager supply of precious carbon dioxide to the tree. Small particles are found in the leaf cells called chloroplasts; these contain a green substance (chlorophyll) similar in structure to the hemoglobin of the blood. In fact, in reflected light chlorophyll appears not green but blood red.

Carbon dioxide unites with the chlorophyll and in a chain of reactions, regulated by the enzymes, it combines with oxygen and hydrogen of water to form sugar. An excess of oxygen is released in this process. The energy that is needed for transformation of carbon dioxide and water into the organic substance (sugar) is supplied by sunlight. Only about 1 percent of the solar energy that falls on a leaf is used for photosynthesis. The sugar formed in the process of photosynthesis is dextrose. From it 95 percent of the body of the tree is ultimately made by a series of complicated reactions. Dextrose may be converted into other sugars or it may be combined with nitrogen to form the amino acids, the building blocks from which proteins are made and on which all life, both plant and animal, depends. Part of the dextrose is also used for other purposes, such as conversion into starch, fats, and other substances.

Vocabulary

Key Words

4. Check you know the key words.

acid

carbon dioxide

chlorophyll

convert

fat

hemoglobin

nutrition

photosynthesis

purpose

release

respiration

similar

starch

surface

vital

5. Match the words to make phrases.

30) to reach a) nutrition

31) amino b) pores

32) organic c) maturity

33) small d) matter

34) mineral e) acid

35) to penetrate f) into other sugars

36) to convert g) leaves

Поиск по сайту: