BIOL 4160 Exams_2006Exam
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Photosynthesis (SC/BIOL 4061) First Term Test (5 Oct 2006)
Answer the following five questions in the exam booklet provided. When finished, please
insert the question sheet and your crib sheet(s) in the exam booklet (all will be returned to
you after grading). You should be able to answer each question on one to two pages.
Excessive length is not encouraged.
Macrofossil evidence for ‘primordial’ photosynthesis is fairly strong, because living
stromatolites were recently discovered that are similar in structure to fossil stromatolites.
Describe the structure and function of a modern stromatolite and how it relates to changes
in atmospheric conditions over the past 3000 million years or so.
Prokaryotic photosynthesis is divided into anoxygenic and oxygenic mechanisms.
Describe the properties of photosynthesis shared among and/or unique to prokaryotic
anoxygenic photosynthesizers (including the nature and properties of the pigments they
use and unique mechanism(s) of carbon dioxide fixation).
In the context of the various fates of an absorbed photon, how would increased light
intensity affect the fate of an excitation event? Give reasons for your hypothesis.
Contrast the two mechanisms of cyclic tetrapyrrole synthesis leading to heme and
chlorophyll from the basic precursors.
Describe how Emerson and Arnold established the concept of a ‘group’ of chlorophyll
molecules participating in oxygen evolution in photosynthesis. What organism did they
use for their experiments?
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First Term Test KEY
Question One: A description of the stromatolite, noting the layering of
organisms, oxygenic and anoxygenic photosynthesizers as well as aerobic and
anaerobic heterotrophs (75%). The relation between these different organisms
and increasing oxygen levels over ca 3000 million year (25%).
Question Two: The prokaryotic anoxygenic photosynthesizers use
bacteriochlorophylls that absorb at wavelengths much longer than the
chlorophylls of cyanobacteria and eukaryotic oxygenic photosynthesizers (33%).
Electron sources include H2S S SO4
2– and H2 (33%). Carbon dioxide
fixation pathways include two unusual ones: a reverse Krebs and the
hydroxypropionate pathways (33%).
Question Three: High light intensity will cause more excitation events (20%),
saturating photochemistry (20%) so that fluorescence (20%) and heat release
(‘radiationless relaxation’)(20%) will increase, as will triplet formation (and thus
singlet oxygen and photooxidation)(20%)
Question Four: Succinyl-CoA and glycine condensed to form delta-ALA in
proteobacteria, animals (and plants) for heme synthesis. In cyanobacteria and
plant chloroplasts, glutamate is converted to delta-ALA by a t-RNA mediated
pathway (60%). Subsequent dehydration of two delta-ALA molecules to form
porphobilinogen and deaminations to create the tetra-pyrrole are similar in both
pathways (20%), followed by Fe insertion to form heme, and Mg insertion to
form chlorophyll (20%).
Question Five: Under conditions in which oxygen evolution was saturated as a
function of light intensity (40%), they examined the dependence of oxygen
evolution on chlorophyll concentration, and from the slope of oxygen molecules
evolved versus chlorophyll concentration, they determined 2480 molecules
chlorophyll per oxygen evolved (40%). The organism was Chlorella (green
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