The Nitrogen Cycle

Section 4.3BC Science Probe 10

Page 92

The Importance of Nitrogen

• The element nitrogen is extremely important for all organisms.

• Living things need nitrogen to do the following things…

The Importance of Nitrogen

• 1. Make DNA and RNA (nucleic acids)– DNA is made of nucleotides. Nitrogen is found in

the bases of the nucleotides.

The Importance of Nitrogen

• 2. Forms amino acids that bond together to form proteins.– Nitrogen is found in all 20 amino acids.

The Importance of Nitrogen

• Nitrogen is also needed by plants. They cannot make chlorophyll for photosynthesis without it!

Sources of Nitrogen

• Just like carbon, nitrogen is very easy to find.– 78% of Earth’s atmosphere is nitrogen gas.– We actually breathe in 4 times more nitrogen than

oxygen!

Sources of Nitrogen

• But this gaseous nitrogen (N2) is not a usable form for living things because…

Sources of Nitrogen

• In order to be able to use nitrogen, it must be changed into a usable compound form.– Usable nitrogen compounds:• Ammonia – NH3 - compound• Ammonium – NH4+

• Nitrate – NO3- polyatomic ions• Nitrite – NO2-

Sources of Nitrogen

• The process of changing gaseous nitrogen (N2) into a usable compound is called nitrogen fixation.

• Nitrogen fixation is one part of the nitrogen cycle.– The cycling of nitrogen back and forth between

the abiotic and biotic environments.

The Nitrogen Cycle

• Nitrogen, just like carbon, is cycled back and forth between the biotic and abiotic parts of the biosphere in a series of steps.

The Nitrogen Cycle• Step 1: Nitrogen fixation– N2 from the atmosphere combines with hydrogen

N2 + H2 → NH3 (ammonia)

– This happens through bacteria on the roots of legumes (beans, peas, peanuts, alfalfa, clover, alders…)

– Excess NH3 dissolves in H2O to become NH4+ and is used by plants.

The Nitrogen Cycle

• Step 2: Nitrification– NH4+ is converted to nitrates and nitrites by

nitrifying bacteria in the soil.– The nitrates and nitrites get taken up by plant

roots from the soil for growth.• They use them to make up DNA, RNA and protein.• Animals get their nitrogen by eating plants, or by eating

other organisms that ate plants.

The Nitrogen Cycle

• Step 3: Decomposition– Animals and plants die, then decompose– Decomposers (nitrifying bacteria) break down

nitrogen in DNA and proteins into ammonia and nitrates.

The Nitrogen Cycle

• Step 4: Denitrification– Denitrifying bacteria change nitrates and

ammonia back into N2 gas.– This causes the soil to lose nitrates.– Adding oxygen to the soil reduces denitrification

because the denitrifying bacteria do not like oxygen (anaerobic)

Assignment

• CYU 4.3– Page 95• # 1-11, 13, 15, 17