Title: The Nitrogen Cycle
Discipline: Life Science
Ability: 7th Grade and 8th grade
NJ Core Curriculum Content Standards:
5.1 a, b 5.6 5.8 a, b
Objective:
The students will be able to explain how nitrogen cycles through their fish tanks from a toxic form as ammonia to a non-toxic form as nitrate. The students will be able to explain how aerobic and anaerobic bacteria convert nitrogen from one compound to another.
Note:
This lesson was done in the 7th grade Life Science class and the 8th grade Physical Science class. In the Life Science class, the concepts were simplified since the students did not have a chemistry background.
Discussion:
Why are cycles important to living things?
Do Now: Explain the different parts of the water cycle.
What would happen if one of these steps were lost? Ex. no precipitation
Organisms usually have very important roles in keeping the cycles going.
Chemical elements like Nitrogen, carbon and phosphorus are available in only limited amounts. So these essential elements need to be recycled. This is why decomposition is so important.
Nitrogen is an element found all over the earth.
What is an element? Pure substance that cannot be broken down into simpler substance.
What is a compound? Pure substances composed of more than
one element combined together
Nitrogen is an important element for plant growth and development.
80 % of the atmosphere is Nitrogen gas. Plants cannot use this Nitrogen gas. Its use must be converted into another form in order for plants to use it.
We will use the aquariums as model system. Same cycles going on it stream and pond as aquarium. Understanding the nitrogen cycle is extremely important for fish tank maintenance. Nitrogen changes forms in the tank and some forms are more toxic to fish than others.
2 questions we will answer today-
How does nitrogen get into the tank? How does nitrogen change forms?
Draw a cycle-start with plants and fish
How does nitrogen get into the tank?
Fish waste- Ammonia (toxic!!) NH3
If there is all this toxic ammonia in the tank, why don’t the fish die? NITRIFICATION
Aerobic bacteria- nitrifying bacteria oxidize nitrogen. Convert into another Nitrogen compound=add oxygen and take away hydrogen. Oxidation- nitrogen loses electrons to oxygen. Where would most of these bacteria be found? In the filter because that is where the most oxygen would be which is needed by bacteria to convert nitrogen.
1st nitrifying bacteria- ammonia (toxic!) into Nitrate (toxic!)
2nd nitrifying bacteria- Nitrate to Nitrate.
Nitrate can be taken up by plants or can be converted into another form- DENTRIFICATION
Anaerobic process. Nitrate in converted back into N2 returns to the atmosphere.
If this is an anaerobic process where does it occur in the tank?
Beauty of the cycle: Organisms converts a toxic substance to a non-toxic substance that can be used to make food for fish
Activity:
Each student should get a copy of the handout. The activity should be done in groups. Each group should get a Nitrogen cycle board.
It is helpful if the teacher goes through the first example with the students. Each row is an example of a different tank. In each tank there are different numbers of fish and different numbers of bacteria. For the purposes of this activity each fish produces 1 ammonia, one “1st nitrifying bacteria” converts 1 ammonia to 1 nitrate, one “2nd nitrifying bacteria” converts 1 nitrate to 1 nitrate and one plant takes up one nitrate. In other words it is an even exchange. Obviously in a real tank there would be millions of bacteria converting nitrogen from one compound to another.
Example from handout- tank #1
Start with 6 fish. These fish produce 6 ammonia. The students put down 6 ammonia cards on the board.
There is 1 of the 1st nitrifying bacteria in the tank. Only 1 ammonia gets converted to nitrate. Students should take 1 ammonia card off the board and put down 1 nitrate.
There is 1 of the 2nd nitrifying bacteria in the tank. Only 1 nitrate gets converted to nitrate. Students should take the 1 nitrate card off the board and put down one nitrate card.
There is 1 plant in the tank. The plant uses only 1 nitrate. Students should remove the 1 nitrate from the board.
You should have 5 ammonia cards left on the board once you have gone through the cycle with tank #1. The nitrogen compound left in the tank is ammonia and since ammonia is toxic to fish the fish in tank #1 would die. The cycle is therefore not balanced.
Assessment:
Feedback from Mr. Wares: Instead of using the nitrogen compounds as the cards we should make fish and bacteria cards so the students visually see how many of each are in the tank. Have them figure out how many ammonia are produced.
Make sure we go over the importance of cycles in the beginning. Also, go over the spike in the tank. What happens if the number of fish or bacteria changes in the tank? How does that affect the nitrogen cycle?