Cycles (carbon + nitrogen)

Decomposition (2)

Breakdown of the dead remains/material of other organisms

by organisms such as bacteria or fungi

Carbon cycle: why is carbon important?

Is a component of all major biological molecules. Carbohydrates, proteins, lipids, DNA, vitamins and other molecules all contain carbon.

Carbon cycle diagram

Processes during the carbon cycle

• Photosynthesis: ‘fixes’ carbon atoms from CO2 into organic compounds

• Feeding and assimilation: pass carbon atoms along food chains

• Respiration: releases CO2 as organic compounds (glucose) is broken down to release energy

• Fossilisation: sometimes they do not decay fully (soil is too acidic) and fossil fuels (coal, oil, natural gas) are formed

• Combustion: burning fossil fuels releases CO2

Nitrogen cycle diagram

Here excretion is about urea not faeces. Faeces mainly contain undigested food while most of the nitrogenous matter is excreted in urine (with the urea).

Plant roots can absorb the nitrates

Bacteria involved in nitrogen cycle

Nitrogen fixing bacteria, decomposers (bacteria), nitrifying bacteria, denitrifying bacteria

Nitrogen fixing bacteria

Convert nitrogen (N2) into ammonia (NH3). Ammonia gas dissolved in water forms ammonium ions (NH4+) which are the ones used by the plant

Textbook:

• Free in soil: Ammonia is used by nitrogen fixing bacteria to make proteins and amino acids. When they die NH3 is released back into soil.

• In root nodules: also make ammonia but this is converted by the plant into amino acids and other organic nitrogen compounds. Death and decomposition of plant returns the nitrogen into soil as ammonia.

Decomposers

Break down dead remains/material containing protein, DNA, vitamins and excreted nitrogenous waste products (urea) and turn them into ammonia. The ammonia dissolves in water to form ammonium ions

Nitrifying bacteria

Convert ammonium ions (in decaying matter) into nitrite (NO2^-) and then into nitrate (NO3^-)

Denitrifying bacteria

Convert nitrate (NO3^-) into nitrogen (NO2). Denitrification reduces amount of nitrate in soil and does no benefit to living organisms

Textbook: use nitrates as an energy source and convert them into nitrogen gas.

What are the two ways which nitrogen fixation can occur?

Lightning: there is so much energy in a bolt of lightning that it’s enough to make nitrogen react with oxygen in the air to give nitrates

Nitrogen fixing bacteria in soil and the roots of legumes

Explain how burning biomass returns nitrogen to the atmosphere (3)

• (biomass) contains proteins / amino acids / DNA / RNA

• They are nitrogen containing compounds (1)

• produce nitrous oxides / oxides of nitrogen when burned / eq (1)