nitrogen cycle in ecosystems

up the food chain

Animals eat plant tissue and create animal tissue. After a plant or animal dies or an animal excretes waste, bacteria and some fungi in the soil fix the organic nitrogen and return it to the soil as ammonia. Nitrifying bacteria oxidize the ammonia to nitrites, while other bacteria oxidize the nitrites to nitrates, which can be used by the next generation of plants. In this way, nitrogen does not need to return to a gas. Under conditions when there is no oxygen, some bacteria can reduce nitrates to molecular nitrogen. Click image to the left or use the URL below. URL:

nitrogen as a nutrient

Nitrogen (N2 ) is vital for life on Earth as an essential component of organic materials, such as amino acids, chloro- phyll, and nucleic acids such as DNA and RNA (Figure 1.1). Chlorophyll molecules, essential for photosynthesis, contain nitrogen.

nitrogen fixing

Although nitrogen is the most abundant gas in the atmosphere, it is not in a form that plants can use. To be useful, nitrogen must be fixed, or converted into a more useful form. Although some nitrogen is fixed by lightning or blue-green algae, much is modified by bacteria in the soil. These bacteria combine the nitrogen with oxygen or hydrogen to create nitrates or ammonia (Figure 1.2). (a) Nucleic acids contain nitrogen (b) Chlorophyll molecules contain nitrogen



instructional diagrams

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which of the following have bacteria in their root nodules that fix nitrogen?

-->  a) legumes

b) meat

c) trees

d) algae

nitrogen gas found in the atmosphere is useful to organisms.

a) true

-->  b) false

living things need nitrogen for

a) amino acids

b) nucleic acids

c) photosynthesis

-->  d) all of these

nitrogen is only useful to animals and humans in which form

a) plant

b) lightning

c) gas

-->  d) none of the above

nitrogen fixing bacteria convert nitrogen to

a) nitrogen oxide.

-->  b) ammonium.

c) water.

d) plants.

nitrogen can be recycled in plants and soil and does not need to return to the atmosphere as a gas.

-->  a) true

b) false

nitrifying bacteria convert

a) nitrogen oxide to nitrogen gas.

b) nitrogen to nitrates.

-->  c) ammonium to nitrites.

d) water to nitrogen gas.

leguminous plants

a) include peas, beans and peanuts

b) use ammonia produced by nitrogen-fixing bacteria.

c) supply carbohydrates to nitrogen-fixing bacteria.

-->  d) all of these.

nitrogen from animal wastes or plant and animal tissue

a) must be fixed near leguminous plants.

b) is lost from the system.

-->  c) is fixed by bacteria and fungi in the soil.

d) is already fixed and can be used.

modern fertilizers supply fixed nitrogen to the soil, eliminating the need for nitrogen-fixing bacteria.

-->  a) true

b) false

diagram questions

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