SNC 1D/2D
Unit 1: Sustainable Ecosystems
Name: _____________________
Date: _____________________
Video: The Carbon Cycle
A. Why is Nitrogen Important? How do Living Things Get It?
1. All living things must have nitrogen to make _____________ ___________
which are then used to build __________________ .
2. Most plants absorb the nitrogen they need from ___________ or ____________ .
3. Animals absorb the nitrogen they need by eating ______________ or eating other
animals that have eaten ______________ .
B. Nitrogen Fixation – The __________________ or Building Up of Nitrogen
4. Because nitrogen is used in the tissues of all living things, it is vital that nature
does not run out of it. This does not seem likely because the atmosphere is about
__________ nitrogen gas (N2(g)). However, most plants cannot use pure nitrogen
gas. They need nitrogen that has been “_____________” by combining it with
other elements to form compounds. [e.g. Pure nitrogen gas, N2(g) can be
combined with oxygen gas to form nitrates, NO31- and nitrites NO21- which plants
can use.]
5. Two ways to “fix” pure nitrogen gas into compounds plants can absorb and use
are:
a) Lightning – Lightning causes nitrogen gas to mix with ______________ gas
and rainwater to make “fixed” nitrogen that plants can use.
b) nitrogen “fixing” bacteria –
Nitrogen gas from the air is “fixed” by nitrogen fixing bacteria
found in special bulges or _________________ in the roots of
________________ plants like soybeans, p_________,
c_______________ and a________________ .
Nitrogen fixing bacteria have a s________________ relationship
with legume plants. This means that both the bacteria and the
legume benefit from the relationship. The legumes provide
____________________ for food for the bacteria and the bacteria
fix nitrogen for the legume. Extra nitrogen is released into the soil
for other plants to absorb.
There are also free __________________ nitrogen fixing bacteria
in the soil that are not in the root nodules of legume plants.
In aquatic ecosystems, cyanobacteria turn nitrogen gas into
ammonium, NH41+, which can then be used by plants.
C. The D________ or Breaking Down of Nitrogen
 Dead organisms, manure and other wastes are broken down (by decomposers) to
release nitrogen compounds (like ammonium, NH41+, nitrates and nitrites) into
the ______________ for reuse by other organisms.
 Denitrifying bacteria break down nitrogen containing compounds to release
pure nitrogen gas, N2(g), back into the ___________________.
D. Human Practices and the Nitrogen Cycle
 In a sustainable ecosystem, the amount of nitrogen converted into usable forms is
equal to the amount of nitrogen returned to the _______________. To help with this,
excess nitrogen bound up in nitrate and ammonium compounds eventually enters the
________sphere, becoming part of rocks. This nitrogen returns to the atmosphere only
after many centuries when the rock is broken down into smaller pieces.
 Artificial fertilizers introduce forms of fixed nitrogen into the environment for plants
to use. They can cause water pollution and excessive, oxygen-robbing plant growth if
they allow excess amounts of fixed nitrogen enter aquatic ecosystems.
 Exhaust from cars and power generation contain nitrogen gas and nitrogen compounds
that can react with water to form acid ____________________ .
Complete and complete the diagram by inserting the correct labels.
Answers:
1. amino acids, protein
2. soil, water
3. plants, plants
4. ¾ (actually about 78%), fixed
5. oxygen, nodules, legume, peas, clover, alfalfa, symbiotic, carbohydrate
C. Decay
 soil
 atmosphere
D.
 atmosphere, lithosphere
 precipitation