June 2011 essay - A cycle is a biological pathway or process in which the end product of one cycle becomes the starting point for the next cycle. Write an essay about cycles in biology.

Plan:
Nutrient cycle – carbon and nitrogen
Respiration – krebs cycle
Photosynthesis – calvin cycle
Heart – cardiac cycle
Lungs
Action potential
Menstrual cycle – feedback mechanisms
Cell division

Nutrients are essential for organisms to survive, they are recycled so that they can be used over and over again. Two major nutrient cycles are the carbon and nitrogen cycles. The carbon cycle starts in the form of CO2 and is absorbed by plants in photosynthesis to make glucose. This becomes carbon compounds in plant tissue, which is then passed onto primary consumers and then secondary and tertiary consumers. When these organisms die the carbon containing compounds are broken down by decomposers which respire, releasing CO2 back into the atmosphere for the cycle to repeat.

The nitrogen cycle is also very important as nitrogen is needed to make proteins. Nitrogen in the atmosphere is turned into ammonia by bacteria, which can then be used by plants. These nitrogen compounds in plants are then turned in ammonium compounds by decomposers. The ammonium compounds are turned into nitrites and nitrates by nitrifying bacteria. The nitrates are then converted back into nitrogen gas, ready for the cycle to repeat.

Aerobic respiration also includes a cycle; the krebs cycle. The krebs cycle is a series of oxidation-reduction reactions which take in the matrix of the mitochondria. It involves a 4 carbon compounds undergoing different reactions, forming a six carbon molecules, then being broken back down again into a 4 carbon molecule. The steps involved release lots of ATP and Reduced NAD, needed for the electron transport chain.

Oxygen needs to carried around the body, and