the atmosphere during photosynthesis. In this process, the pro-ducers use carbon from the carbon dioxide to produce othercarbon-containing molecules. These molecules include sugarsand starches. To obtain energy from these molecules, consumersbreak them down into simpler molecules. Consumers releasewater and carbon dioxide as waste products.

At the same time, oxygen is also cycling through the ecosys-tem. Producers release oxygen as a result of photosynthesis.Other organisms take in oxygen from the atmosphere and use itin their life processes.

wf e~ How is oxygen returned to the environment?

The Nitrogen CycleLike carbon, nitrogen is a necessary building block in thematter that makes up living things. Since the air around you isabout 78 percent nitrogen gas, you might think that it would beeasy for living things to obtain nitrogen. However, most organ-isms cannot use the nitrogen gas in the air. Nitrogen gas iscalled "free" nitrogen, meaning it is not combined with otherkinds of atoms. Most organisms can use nitrogen only once ithas been "fixed," or combined with other elements to formnitrogen-containing compounds. You can follow this process inFigure 10 below.

Figure 10 In the nitrogen cycle,nitrogen moves from the air to thesoil, into living things, and backinto the air.Interpreting Diagrams How doconsumers obtain nitrogen? ~ Free nitrogen in air

Bacteria releasesome free nitrogenback to air

Bacteria in root nodulesfix free nitrogeninto compounds

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: ~~~fil~/.lf Check Your Progress. Once your teacher has: approved your plan, place the: waste into your compost chambers.: Record your hypothesis about the effectf of the variable you are investigating.: Observe the two containers daily. (Hint: If: there are no signs of decomposition after: several days, you may wish to stir the

contents of each chamber. Stirring allowsmore oxygen to enter the mixture.)

Nitrogen Fixation The process of changing free nitrogen gasinto a usable form of nitrogen is called nitrogen fixation. Mostnitrogen fixation is performed by certain kinds of bacteria. Someof these bacteria live in bumps called nodules (NAHJ oolz) on theroots of certain plants. These plants, known as legumes, includeclover,beans, peas, alfalfa, and peanuts.

The relationship between the bacteria and the legumes is anexample of mutualism. As you recall from Chapter 1, a symbioticrelationship in which both species benefit is called mutualism.Both the bacteria and the plant benefit from this relationship:The bacteria feed on the plant's sugars, and the plant is suppliedwith nitrogen in a usable form .

..Jt:'!.g~.~1!!.~~ Many farmers make use of the nitrogen-_ ~} TECHNOLOGY fixing bacteria in legumes to enrich their

fields. Every few years, a farmer may plant a legume such asalfalfa in a field. The bacteria in the alfalfa roots build up a newsupply of nitrogen compounds in the soil. The following year,the new crops planted in the field benefit from the improved soil.

, Return of Nitrogen to the Environment Once the nitro-gen has been fixed into chemical compounds, it can be used byorganisms to build proteins and other complex substances.Decomposers break down these complex compounds in animalwastes and in the bodies of dead organisms. This returns simplenitrogen compounds to the soil. Nitrogen can cycle from the soilto producers and consumers many times. At some point, how-ever, bacteria break down the nitrogen compounds completely.These bacteria release free nitrogen back into the air. Then thecycle starts again.

1. Name and define the three major processes thatoccur during the water cycle.

2. Explain the role of plants in the carbon cycle.3. How is nitrogen fixation a necessary part of the

nitrogen cycle?4. Where do nitrogen-fixing bacteria live?5. Thinking Critically Comparing and

Contrasting Explain how the movement ofmatter through an ecosystem is different thanthe movement of energy through an ecosystem.

Figure 11 Lumpy nodules areclearly visible on the roots of thisclover plant. Bacteria inside thenodules carry out nitrogen fixation.

Chapter 2 E. 55

~C.TI~,.,

"~ Earth's BiomesDISCOVER ........................................... ~~.How Mu(h Rain Is That?The table shows the average amount of precip-itation that falls each year in four differentregions. With your classmates, you will create afull-size bar graph on a wall to help you visualizethese amounts of rain.

2. Repeat Step 1 for the other three locations. Ifnecessary, tape strips of paper together tomake the correct length. Label each strip.

3. Now find a place where you can display thefour strips vertically. If the wall of yourclassroom is not tall enough, you may needto use another wall in your school building.Follow your teacher's instructions to hangyour precipitation strips.

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Blome

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Rainfall (em)Mojave Desert 15Illinois prairie 70

Smoky Mountains 180Think It OverDeveloping Hypotheses Which ecosystemreceives the most precipitation? Which receivesthe least? What effect do you think the amountof rainfall might have on the types of species thatlive in these ecosystems?

Costa Rican rain forest

1. Using a meter stick, measure a strip of adding-machine paper 15 centimeters long. Label thispiece of paper "Mojave Desert."

Congratulations! You and your classmates have been selectedas the student members of an around-the-world scientificexpedition. Your mission is to study the major types of

ecosystems on Earth. You will be collecting data on the climateconditions and typical organisms found in each of these ecosys-tems. The result of this expedition will be a database of informa-tion on the biomes you visit. A biome is a group of ecosystemswith similar climates and organisms.

Classifying ecosystems into biomes helps ecologists describethe world. As you might expect, not all ecologists agree on theexact number and kinds of biomes. The scientists guiding yourexpedition have chosen to focus on six major land biomes and twomajor water biomes.

Be sure to pack a variety of clothing for your journey.During your trip, you will visit places ranging fromfrozen, windy Arctic plains to-steamy tropical jungles.In fact, it is mostly the climate conditions-temper-ature and rainfall-in an area that determine itsbiome. This is because climate limits the distributionof plants in the area. In turn, the types of plantsdetermine the kinds of animals that live there.

• What determines the typeof biome found in an area?

• Where can photosynthesisoccur in water biomes?

III

Reading Tip As you read,make a list of the biomesdescribed in this section.Under each biome name,take notes on the character-istics of that biome.

Rain Forest Biomes

Tropical rain forest Temperate rain forest

Rain Forest BiomesThe first stop on your expedition is atropical rain forest close to the equator.The rain forest is warm and humid-in fact, it's pouring rain! Fortunately,you remembered to pack a poncho.After just a short shower, the sun re-appears. But even though the sun isshining, very little light penetrates thethick vegetation.

Plants are everywhere in the rainforest.Some, such as the ferns, orchids,and vines you observe hanging fromtree limbs, even grow on other plants.Among the plants are many species ofbirds as bright as the numerous flowersallaround you.

Tropical Rain Forests Tropical rainforests are found in warm regionsclose to the equator. Tropical rainforests typically receive a lot of rain.The warm temperatures do not varymuch throughout the year, and thesunlight is fairly constant all year.

Tropical rain forests contain an astounding variety ofspecies. For example, scientists studying a 100-square-meterarea of one rain forest identified 300 different kinds of trees!These trees form several distinct layers. The tall trees form aleafy roof called the canopy. A few giant trees poke out abovethe canopy. Below the canopy, a second layer of shorter treesand vines form an understory. Understory plants grow well inthe shade formed by the canopy. Finally, some plants thrive inthe near-darkness of the forest floor.

Figure 16 Tropical rainforests contain an amazingvariety of plants and otherorganisms. In the large photo,a river winds through the lushIndonesian rain forest. The topcloseup shows a youngorangutan swinging from treelimbs. In the bottom closeup,a tarantula climbs over abrightly colored bracketfungus on the forest floor.

Chapter 2 E. 63