1. (Sec 4.3 pg 92 – 95) CRASH COURSE VIDEO: watch only the first half
The Nitrogen Cycle
(Sec 4.3 pg 92 – 95)
CRASH COURSE VIDEO:
watch only the first half

2. Even though nitrogen (N) is the most abundant gas in the atmosphere, organisms cannot use it in atmospheric form (N2).
N2 is very stable and will not break down.

3. N is an essential component of biological molecules (DNA, proteins, etc).

4. For this reason, the N cycle is extremely important.
There are 3 key stages in the cycle (please refer to Fig.2 p.93 and your Data Pages for diagrams of the N cycle):

5. Nitrogen fixation:
Nitrogen gas (N2) is combined with hydrogen (H2) to form ammonia (NH3).
the process of N fixation is carried out by bacteria, many of these bacteria live in the soil, others live on the nodules of plants called legumes (Fig 3 p.93).
Extra NH3 dissolves in the soil to become NH4+, ammonium

6. Nitrification:
Turns ammonium NH4+ into NO3- (nitrate ions); nitrate is required by many plants for optimal growth.
process done by bacteria in the soil; the process is called nitrification.
Nitrate ions are absorbed by plants to make amino acids, DNA, proteins, etc.

7. Decomposition and Denitrification:
When organisms die, nitrates and ammonium leave the body. It is converted back into N2 gas by decomposing bacteria (this is called denitrification).

8. Animals obtain their N by consuming plants, or by consuming organisms that have consumed plants; plants are essential for providing N to the food chain!