IGCSE Biology
Section 4: Ecology and the environment
c. Cycles within Ecosystems
3. Nitrogen Cycle
4.10 describe the stages in the nitrogen cycle, including the roles of nitrogen fixing
bacteria, decomposers, nitrifying bacteria and denitrifying bacteria (specific names of
bacteria are not required).
Ref:
Pg. 163
Pg. 164
15.12 “Few Organisms can use Nitrogen Gas”
15.13 “Fixed nitrogen moves round the nitrogen cycle”
15.14 “Denitrifying bacteria make nitrogen gas”
Nitrogen Cycle Animations
For each animation, you are only interested in finding out about the four words from
the objectives. After you have watched the animations you should be able to describe
in detail the role of each of the flowing in the nitrogen cycle:
1. Nitrogen fixing bacteria
2. Decomposers
3. Nitrifying bacteris
4. Denitrifying bacteria
http://www.classzone.com/books/ml_science_share/vis_sim/em05_pg20_nitrogen/em05_p
g20_nitrogen.html
http://www.nodvin.net/snhu/SCI219/demos/Chapter_3/Chapter_03/Present/animations/3
2_2_1a.html
http://bcs.whfreeman.com/thelifewire/content/chp58/5802004.html
For this website only try the animation and the quiz.
The Nitrogen Cycle – Quick Self Test
In the atmosphere 1. ____________________ is the most abundant gas but plants
cannot absorb it in this form. In order for plants to take 1. ____________________ in
it must be in the form of 2. ____________________.
Nitrogen Fixation
There are four ways in which 2. ____________________ can be added to the soil.
o
The farmer may add 3. ____________________.
o
Naturally in the atmosphere when 4. ____________________ strikes.
o
In the soil by 5. ____________________ bacteria.
o
Some plants such as clover and peas are called 6. ____________________ . They
contain 5. _____________________ bacteria in swellings within their roots called
7. ____________________.
Cycling of Nitrogen in Living Things
Plants absorb the 2. ____________________ from the soil and use them to make
8. ____________________. Animals then 9. ____________________ on plants
obtaining the 8. ____________________. Eventually all living things will die and be
broken down by 10. ____________________. The proteins in plants and animals are
broken down to ammonia and then nitrates by 11. ____________________ bacteria.
Denitrification
Unfortunately for farmers there is also a type of bacteria which can convert
2. ____________________ back into 1. ____________________, they are called
12. ____________________ bacteria. This type of bacteria are often found in
waterlogged soil which lack oxygen, farmers can plough their fields to help aerate them.
The Nitrogen Cycle
1. ____________________
v
in the atmosphere.
Clover and beans are
6._________________
_.
3.
4. _________________
5.__________________ bacteria
in soil and roots of certain plants.
12.
_________________
2. ____________________
in the soil.
The proteins in the dead
plants and animals are broken
down by 11. _____________
bacteria
The bacteria are found in
root 7. ______________
9.
Plants make
8. ___________
Animals and plants eventually
die and are broken down by
10. ___________________
1. The diagram shows the nitrogen cycle.
Use the diagram and your knowledge to answer the following questions.
a. Complete the table to show the source of nitrogen for the different
types of bacteria in the nitrogen cycle.
(3)
Type of Bacteria
Nitrifying
Denitrifying
Nitrogen-fixing
Source of Nitrogen
b. Name two types of micro-organisms which carry out decomposition.(2)