Cycles in Nature

• Law of Conservation of Matter - Matter cannot be created nor destroyed in any chemical reaction. It can only be transformed from one form to another.
• Cycle - A cycle shows the reusing of certain elements and compounds (e.g. water, carbon, oxygen, nitrogen, phosphorus) in different forms in ecosystems.

Water Cycle

• The sun provides the heat energy required for the cycle to continue.
• The oceans and rivers are the main reservoirs of water.
• Evaporation - Water evaporates into the atmosphere.
• Condensation - Water condenses into droplets in the clouds.
• Precipitation - As the water droplets in clouds enlarge, water falls to earth in the form of rain, snow or hail.
• Transpiration - Water is lost through the leaves of plants ('trees perspiring').

Carbon and Oxygen Cycles

• Carbon occurs in all living organisms in the forms of carbon dioxide, carbohydrates (sugars and starches), proteins and fats.
• Oxygen occurs in all living organisms in the forms of oxygen gas, water, carbon dioxide, carbohydrates, proteins and fats.
• Photosynthesis - Photosynthesis is the process used by plants containing chlorophyll to utilise sunlight, carbon dioxide and water to form sugar (as glucose) and oxygen.

Carbon Dioxide	+	Water	Sunlight	Glucose Sugar	+	Oxygen
			Chlorophyll
CO2	+	H2O		C6H12O6	+	O2

• Respiration - Respiration is the process that most living organisms (including animals, plants, fungi and micro-organisms) use to obtain energy from glucose sugar and oxygen. The wastes formed are carbon dioxide and water.

Glucose	+	Oxygen		Carbon Dioxide	+	Water	+	Energy
C6H12O6	+	O2		CO2	+	H2O	+	Energy

Nitrogen Cycle

There are two interrelated parts of the Nitrogen Cycle.

1^st Part of the Nitrogen Cycle

• Nitrogen gas forms about 80% of the atmosphere. However, no animals and few plants can utilise nitrogen in this gaseous form.
• Nitrogen-Fixing Bacteria converts atmospheric nitrogen to amino acids, which are the components of proteins. Examples of nitrogen-fixing bacteria are the cyanobacteria Nostoc, soil bacteria Azotobacter and Clostridium, and the legume root bacteria Rhizobium.
• Atmospheric nitrogen can also be fixed or converted to a more solid form by lightning.
• In reverse, Denitrifying Bacteria in the soil convert ammonia, a nitrogenous waste in urine, back to atmospheric nitrogen.

2^nd Part of the Nitrogen Cycle

• Nitrogen in animals and plants is in the form of amino acids and proteins. As one organism eats another along the food chain, the nitrogen passes from one organism to another.
• Animals also excrete nitrogenous wastes in urine as urea, uric acid or ammonia. This passes to the soil.
• When animals and plants die, Putrefactive Bacteria in the soil converts the body amino acids and proteins to ammonia in the soil. These decay bacteria also convert urea and uric acid to ammonia in the soil.
• Nitrite Bacteria in the soil convert ammonia to nitrites in the soil.
• Nitrate Bacteria in the soil convert those nitrites to nitrates in the soil. These are used by the plants as fertilisers.

Phosphorus Cycle

• Phosphorus is an essential component of many chemicals within living organisms. Examples include the energy-rich compound ATP (Adenosine Triphosphate) and the phosphates used by plants as fertilisers.
• Phosphorus cycles in a complex way which is different from the elements of carbon, oxygen and nitrogen. Phosphorus in the form of phosphates is deposited as faeces and guano on land masses such as the island of Nauru. Rain then erodes the phosphorus-rich guano into the ocean. The guano then is eroded further in the ocean. There is little deposition back onto the land.