HUMAN BIOLOGY

468 Chapter 24

What is the nitrogen cycle?

• Nitrogen cycles from the atmosphere, through nitrogen-fixing
  organisms in soil and water, into plants and then to consumers,
  and back into the atmosphere.

taKe-Home message

nitrogen cycle Move-

ment of nitrogen from

the atmosphere, through

nitrogen-fixing organisms,

into plants, then back to the

atmosphere.

nitrogen fixation Process

in which certain bacteria

and fungi convert nitrogen

into forms that can be taken

up by plants.

Bacteria and some fungi also break down nitrogen-

containing wastes and remains of organisms. The decom-

posers use part of the released proteins and amino acids

for their own life processes. But most of the nitrogen is still

in the decay products, in the form of ammonia or ammo-

nium, which plants take up. In a process called nitrification,

bacteria convert these compounds to nitrite (NO 22 ). Other

bacteria use the nitrite in metabolism and produce nitrate

(NO 32 ), which plants also use.

Certain plants are better than others at securing nitro-

gen. The best are legumes such as peas and beans, which

have mutually beneficial associations with nitrogen-fixing

bacteria. In addition, most land plants have similar

associations with fungi, forming specialized roots that

enhance the plant’s ability to take up nitrogen.

Some nitrogen is lost to the air by denitrification, when

bacteria convert nitrate or nitrite to N 2 and a bit of nitrous

oxide (N 2 O). Much of the N 2 escapes into the atmosphere,

completing the nitrogen cycle.

the nitrogen Cycle

Nitrogen-Rich Wastes,

Remains in Soil

Nitrate

in Soil

Nitrite

in Soil

Food Webs

on Land

Loss by

Denitrification

Fertilizers

loss by

leaching

loss by

leaching

uptake by

autotrophs

uptake by

autotrophs

excretion, death,

decomposition

Nitrogen Gas

in Atmosphere

Consumers

Nitrogen

Fixation

by industry

for agriculture

Nitrogen
Fixation
by bacteria

Ammonia,

Ammonium in Soil

Nitrification

Nitrification

Ammonification

bacteria, fungi convert

residues to NH 3 ; this

dissolves to form NH 4 +

n Nitrogen, a component of our proteins and nucleic acids,

moves in both an atmospheric and a sedimentary cycle

called the nitrogen cycle.

n Link to Proteins and nucleic acids 2.11–2.13

Gaseous nitrogen (N 2 ) makes up about 80 percent of the

atmosphere, the largest nitrogen reservoir. Among organ-

isms, only a few kinds of bacteria can break the triple

covalent bonds that hold its two

atoms together.

Figure 24.12 shows the nitrogen

cycle. As you can see, bacteria play

key roles. They convert nitrogen to

forms plants can use, and they also

release nitrogen to complete the

cycle. Land ecosystems lose more

nitrogen through leaching of soils,

although leaching provides nitrogen

inputs to aquatic ecosystems such as

streams, lakes, and the oceans.

Nitrogen fixation is the process in which certain

bacteria convert N 2 to ammonia (NH 3 ), which then dis-

solves to form ammonium (NH 41 ). Plants assimilate and

use this nitrogen to make amino acids, proteins, and

nucleic acids. Plant tissues are the only nitrogen source for

humans and other animals.

24.6

Figure 24.12 Animated!

In the nitrogen cycle

bacteria play key roles.

The atmo sphere is the

largest nitrogen reservoir.

(© Cengage Learning)

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).