3.  Nitrogen is essential to living things for the production of proteins and DNA which are used to pass on the hereditary information from parent to offspring.  Even though the atmosphere is about 78% nitrogen gas, plants and animals are unable to use nitrogen gas directly as a source of nitrogen to make organic nitrogen compounds.

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5.  The nitrogen cycle can occur in both terrestrial and aquatic ecosystems.

6. Path 1: Nitrogen Fixation by Lightning  The electrical energy of lighting causes nitrogen gas (N2) to react with oxygen (O2) in the atmosphere to produce nitrate ions (NO3-) which reach the soil dissolved in precipitation. Path 2: Nitrogen Fixation by Bacteria  Bacteria in the soil can change nitrogen gas (N2) into ammonia (NH3) which dissolves in water to form ammonium ions (NH4+)

7.  It is a bacterial process in which ammonium (NH4) ions are converted into nitrate ions.  They are first changed into nitrites (NO2-) by bacteria, and then converted to Nitrates (NO3-) by a different group of bacteria.

8.  Assimilation is the process by which plants use the nitrate ions (NO3-) to make amino acids, proteins, and DNA. Only plants and bacteria can carry out the process, all other living organisms receive their nitrogen compounds from the food they eat.

9.  Consumers generally produce wastes throughout their lives. When consumers die their body contain nitrogen compounds such as protein and DNA.  Ammonification is when bacteria and some fungi break down these nitrogen compounds to make ammonia. The ammonia immediately dissolves in soil water to form ammonium ions. (NH4+)

10.  During this step, nitrites are changes to Nitrogen gas (N2) which returns to the atmosphere.  It is basically the reverse of nitrogen fixation and nitrification.  Done by bacteria!

11.  During this step, nitrites are changes to Nitrogen gas (N2) which returns to the atmosphere.  It is basically the reverse of nitrogen fixation and nitrification.  Done by bacteria!

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14.  As a result of human activities most ecosystems have been either totally destroyed or have components such as tropic structure, energy flow and chemical cycling disrupted.  Most effects are local or regional such as agriculture effects on nutrient cycling and introduction of toxic compounds in food chain.

17.  Phosphate is important for cell membranes, it helps release energy that makes up DNA and calcium in bones.  Phosphate in rocks and fossils will weather (break down into tiny pieces).  The inorganic phosphate dissolves in soil and rivers. Plants absorb phosphates and consumers receive phosphate from eating plants. Organic phosphate returns to the soil when decomposition occurs. Bones, teeth and waste, as well as river runoffs go to the ocean. Here the Phosphate is store until there is a geological uplift.

19.  Inorganic nutrients (Carbon, Oxygen, Nitrogen) are recycled continually through ecosystems.  Plants and animals build structures from nutrients and inorganic material.

20.  Energy flows through ecosystems: it enters the ecosystem via sunlight, is stored temporarily in complex molecules, and ultimately leaves in the form of heat.  Nutrients cycle within ecosystems: they are atoms that stay within the ecosystem and are found at different times in different pasrts of the system.

21.  Nutrients are recycled in a living system.  The key nutrients are carbon, hydrogen Oxygen, Nitrogen and Phosphorus. These constitute 95% of all living matter

24.  The Earth's atmosphere contains about 21% oxygen  As you know from the previous lesson, oxygen gas (O2) is recycled as part of the carbon, hydrogen, and oxygen cycles.  Oxygen gas is cycled between the atmosphere and the living organisms of both aquatic and terrestrial ecosystems.  Oxygen gas from the atmosphere is absorbed by the water in aquatic ecosystems.  Oxygen is also produced as a byproduct of the photosynthetic organisms that live in the aquatic ecosystems.  Heterotrophs (consumers) living in aquatic ecosystems require oxygen for cellular respiration but they receive their oxygen from the dissolved oxygen in the water

25.  During winter, ponds and lakes may freeze.  there is little or no light and photosynthesis ceases. As a result, aquatic algae and plants can no longer produce oxygen. Therefore it is possible that all fish within the lake or pond will die off. oxygen may be a limiting factor in aquatic ecosystems  However Oxygen is rarely, if ever, a limiting factor in terrestrial ecosystems.