1.10 The Nitrogen Cycle

(Sec 4.3 pg 92 – 95)CRASH COURSE VIDEO:

http://www.youtube.com/watch?v=leHy-Y_8nRs&feature=c4-overview-vl&list=PL8dPuuaLjXtNdTKZkV_GiIYXpV9w4WxbX

• 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.

• 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 (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

• 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.

• 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).

• 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!