1. ammonification
2. assimilation
3. denitrification
process by which green plants
convert carbon dioxide, water, and
energy from the sun into sugars
process by which certain bacteria in
the soil convert nitrogen gas into
ammonia and ammonium
process by which certain bacteria
convert organic nitrogen into ammonia
and ammonium salts during
decomposition
process by which living cells
produce usable energy from food
process by which certain bacteria
convert ammonium into nitrates
process by which certain bacteria
convert nitrates into nitrogen gas
process by which plants or animals
convert nutrients into usable organic
compounds, such as proteins
4. nitrification
5. nitrogen fixation
6. photosynthesis
7. respiration

The kidney filters potentially toxic substances in the blood, and thus “clears” the blood of those substances. this clearance function is dependent upon and proportional to the diffusion gradient of the substance across filtering capillaries, i.e. if the concentration of the substance is doubled, twice as much will be cleared from each ml of blood that is filtered. suppose that the body produces a constant amount of a substance x per unit of time. the kidneys eliminate substance x at a rate directly proportional to the concentration of the substance and the volume of blood cleared each minute (c): elimination = c × [x], where [x] is the steady-state concentration of substance x. imagine an individual with an initial concentration of x equal to [x]0 who develops kidney disease. her baseline clearance c0 drops to one half of the original (½c0). what is the new steady state concentration of x? (for simplicity, assume that substance x is 100% filtered by the kidney).