1.10 The Nitrogen Cycle (Sec 4.3 pg 92 – 95) CRASH COURSE VIDEO: vl&list=PL8dPuuaLjXtNdTKZkV_GiIYXpV9w4WxbX

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

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

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):

– Nitrogen fixation: Nitrogen gas (N 2 ) is combined with hydrogen (H 2 ) to form ammonia (NH 3 ). 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 NH 3 dissolves in the soil to become NH 4 +, ammonium

Nitrification: – Turns ammonium NH 4 + into NO 3 - (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.

Decomposition and Denitrification: – When organisms die, nitrates and ammonium leave the body. It is converted back into N 2 gas by decomposing bacteria (this is called denitrification).

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!