Exercises

I. Reading comprehension

1. What is the main idea of the article?

a. The waste, or byproducts, of each is the food or essential nutri­ ents for the other.

b. The pathways of three key elements: carbon, phosphorus, ni­ trogen.

с Natural ecosystems recycle the same atoms again and again.

2. The first basic principle of ecosystem sustainability says:

a. the carbon dioxide and other wastes are exactly the nutrients needed by green plants.

b. ecosystems dispose of wastes and replenish nutrients by recy­ cling all elements.

с atoms can be reused indefinitely.

3. According to paragraph 2, carbon atoms from carbon dioxide be­ come the carbon atoms of all the organic molecules through

a. the plant's body

b. photosynthesis

с further metabolism

4. As rock gradually breaks down, phosphate and other nutrient ions (in paragraph 3)

a. are locked in the material

b. are released

с are dissolved in water

5. Plants absorb phosphate from

a. organic compounds

b. water solution с the soil

6- Organic phosphate is transferred from producers to the rest of the ecosystem through

a. plants

b. food chains

с cell respiration '• In paragraph 4, no matter where carbon dioxide is released, it will a. dissolve in water but not enter air

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Unit Five

Ecosystems: How They Work

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b. be bonded into organic compounds by the plant с mix into and maintain the concentration of carbon dioxide in the air

8. A very serious case of humans interfering with the phosphorus cy­ cle is seen in

a. recycling of nutrients

b. releasing inorganic phosphate in urine с the cutting of tropical rainforests

9. In the human system phosphate from agricultural croplands makes its way into waterways

a. directly by mining phosphate rock

b. directly by way of runoff from croplands

с indirectly by way of discharge of sewage effluents

10. According to paragraph 5, plants can utilize nitrogen

a. directly from the air

b. in a mineral form

с as ammonium or nitrate ions

11. Nitrogen gas can be converted to the ammonium form by

a. a number of bacteria

b. certain blue-green algae

с members of the pea-bean family of plants

12. Put the following statements in the correct order.

a. Only humans have been able to bypass the necessity for legumes.

b. Consequently, nitrogen will not accumulate in the soil.

с All natural ecosystems, then, depend on nitrogen-fixing organ­isms.

d. The nitrogen cycle in aquatic ecosystems is similar, but there blue-green algae are the most significant nitrogen fixers.

II. Vocabulary

From the list of words and expressions below, choose the correct word or expression for each blank space. Use each word or expression only once.

humans interfering food chains dissolves depletion

desert shrubs croplands accumulate recycling

concentration utilize symbiosis tissues

nitrogen-fixing organisms replenish byproducts nutrients

. soil minerals dispose diversity legumes

1. The waste, or , of each is the food or essential for the

other.

2. For sustainability, ecosystems of wastes and nutri­ents by all elements.

2. Through , the carbon atoms then move into and become part

of the of all the other organisms in the ecosystem.

4. Phosphorus exists in various rock and as inorganic phos­phate ion (P0₄³-).

4. Phosphate in water but does not enter air.

4. No matter where carbon dioxide is released, it will mix into and maintain the of carbon dioxide in the air.

4. A very serious case of with the phosphorus cycle is seen in

the cutting of tropical rainforests.

8. Meanwhile, phosphorus is replaced on by mining phosphate

rock — a process that will ultimately result in .

9. Plants cannot nitrogen gas directly from the air.

9. Fixed organic nitrogen is passed from the to other organ­isms in the ecosystem through food chains.

9. This is another example of .

9. Consequently, nitrogen will not in the soil.

9. All natural ecosystems, then, depend on .

9. The legume family includes a huge of plants, ranging from

clovers through to many trees.