ECOLOGY QUESTION 1993: L. PETERSON/AP BIOLOGY
Living organisms play an important role in the recycling of many elements
within an ecosystem. Discuss how various types of organisms and their
biochemical reactions contribute to the recycling or either carbon or
nitrogen in an ecosystem. Include in your answer one way in which human
activity has an impact on the nutrient cycle you have chosen.
This question required students to integrate material typically presented at
widely separated times in their course wor, namely ecology, biochemistry,
and organismal diversity. Conceptual understanding was extremely important;
simple recall of words or phrases did not earn points. Students were required
to choose one cycle, and to trace the pathway(s) of the chosen element through
appropriate organisms and metabolic sequences to return to the starting
material. Description of the carbon cycle involves trophic structure (energy
transfer) of the ecosystem, and depends heavily on processes of autotrophy
(usually photosynthesis) and heterotrophy (usually aerobic respiration).
The nitrogen cycle is somewhat more complex; and bacteria play key roles.
Nitrogen compounds play important structural roles in all organisms but are
less important in energy transfer. Finally, students described an impact of
human technology and related this impact ot the chosen cycle.
NUTRIENT CYCLING
(8 POINTS MAXIMUM)
__ OVERVIEW - Why is recycling necessary?
SPECIFIC CYCLE CHOICE
(GRADE FIRST CYCLE SPECIFICALLY MENTIONED)
CARBON NITROGEN
__ Demonstrates closed C cycle __ Demonstrates closed N cycle
__ Carbon sources/pools, locations __ Nitrogen sources/pools (availability)
__ Basic process of carbon fixation __ N2 conversion (N2 -> "usable" form)
(CO2 -> sugar, biomass or organic form)
__ Elaboration of autotrophy __ Elaboration of fixation
__ Role of respiration (organic C -> CO2) __ Nitrification (see addendum)
__ Elaboration of respiration __ Denitrification (see addendum)
__ Uniqueness of autotrophy __ Uniqueness of bacteria to processes
(universality of respiration) __ Plant assimilation of N compounds:
inorganic -> organic
__ Storage forms/products __ Storage forms/products
__ Food chain sequence __ Food chain sequence
__ Role of decomposer __ Role of decomposer
__ Unusual cases: (see addendum) __ Unusual cases: (see addendum)
methanogenesis deamination
photorespiration ammonification
chemosynthesis re-reduction (reductases)
Urea <-> Uric Acid
HUMAN IMPACT ON CHOSEN CYCLE
(3 POINTS MAXIMUM)
__ Identify single human technological impact on cycle
__ Explanation of impact/consequence
__ Additional details
Nutrient Cycling
ADDENDUM
The intent of this addendum is to clarify details of the scoring standards by providing
additional technical information, names of organisms, enzymes, etc. which may be
mentioned by students. It does NOT replace the standards which should be referred to
for distribution of points, maximum points for certain areas, etc.
CARBON CYCLE
Elaboration of photosynthesis:
Calvin Cycle biochemistry: complete correct equation
(RuBP + CO2 -> 2 PGA or 6 RuBP + 6CO2 + 18 ATP + 12 NADPH2 ->
6 RuBP + hexose + 18 ADP + 12 NADP, etc.) or mention of enzymes involved
(ribulose bisphosphate carboxylase/oxygenase or rubisco is most likely) or
mention that these reactions occur in chloroplasts, in stroma, etc.
mention of distinction between C3 vs C4-CAM pathways or groups of plants.
mention of seasonal fluctuations.
Elaboration of respiration:
mention of specific steps in respiration where CO2 is released:
pyruvate -> acetyl-CoA + CO2, Krebs cycle reactions, etc.
fermentation reactions: pyruvate + NADH -> EtOH + CO2, etc.
Storage forms/products: standing trees, peat, coal, oil, natural gas, bones, shells, coral,
chalk, limestone of biogenic origin, etc.
Unusual or special cases:
methanogenesis: CO2 + 4 H2 -> CH4 + 2 H2O only bacteria actually do this, but
students are more likely to describe the environment in which the bacteria grow,
such as ruminants (belching cows), swamps (waterlogged, anaerobic soils), etc.
Photorespiration: in C3 palnts if CO2/O2 ratio is low, RuBP + O2 -> PGA + P-glycolate
P-glycolyate -> glyoxylate, glyoxylate + NH2 -> glycine, glycine + NAD ->
CO2 + serine + NH3 + NADH
Chemosynthesis: incorporation of CO2 using an energy source other than light
(H2, highly reduced organic compounds, etc.)
IMPACT ON CARBON CYCLE:
Burning fossil fuels -> raises CO2 levels
may produce greenhouse effect, may stimulate autotrophs
Clearing/burning forests -> raises CO2 levels or removes autotrophs
may produce greenhouse effect, may increase CO2 dissolved in oceans, etc.
Acid rain -> dissolves limestone, releases CO2 or kills autotrophs
may contribute to greenhouse effect, increase CO2 content of oceans, etc.
Intensive agriculture/desert irrigation -> increase in autotrophs
removes CO2 from sources, ties up carbon in storage forms
Nutrient Cycling
ADDENDUM
NITROGEN CYCLE
Elaboration of N2 fixation ("conversion")
Equation: N2 + 6 H+ + 6e- -> 2 NH3
Enzyme complex: nitrogenase or N2ase (strictly anaerobic)
Specific organisms: Rhizobium, Azotobacter, Klebsiella, Clostridium,
Nostoc, Anabaena, Gloeotheca, Trichodesmium, etc.
Distinction between free-living and symbiotic nitrogen-fixing organisms.
Association of bacteria with plants, legumes, nodulations, leghemoglobin, etc.
Nitrification: in bacteria, NH4+ + 2 O2 -> NO2- + 2 H2O,
NO2- + O2 -> NO3-
this is done by Nitrosomonas, Nitrobacter, etc.
Denitrification: in bacteria, NO3- -> NO2- -> N2O -> N2
this is true anaerobic respiration; the nitrogen compound acts as an
alternative electron acceptor, replacing O2. Pseudomonas can do this,
also Paracoccus, Bacillus, Thiobacillus, etc. Typically occurs in
water-logged soils depleted of O2.
Unusual or special cases:
Ammonification: usually by bacteria/fungi
proteins -> amino acids, amino acids -> NH3
Re-reduction: many organisms can use NO3 or NO2 but must convert these
back to NH3 first. This is done by a pair of enzymes, namely nitrate reductase
and nitrite reductase.
Some organisms excrete urea and/or uric acid.
Insectivorous plants (Pitcher Plants, Venus Flytrap, etc.) obtain nitrogenous
compounds by digesting animals.
IMPACT ON NITROGEN CYCLE:
Burning fossil fuels -> releases NOx
contributes to acid rain
Burning/clearing forest -> increases leaching of N compounds from soil
decrease in fertility, increased NO3 in runoff water, etc.
Use of N fertilizers -> kills nitrogen fixers, other soil microbes or ->
increased NO3 in runoff water.
decrease in fertility, possible health effects, eutrophication,
algal blooms, possible O2 depletion, etc.
Pesticides -> kill N2 fixers.
decrease in fertility
Genetic Engineering of N2ase -> higher plants can fix their own N2?