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Biology Praxis II 5235 study questions with 100% Correct Answers | Verified | Updated 2024
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Biology Praxis II 5235 study questions with100% Correct Answers | Verified | Updated
2024
purpose of light reactions of photosynthesis? - ✔✔Make ATP and NADPH+ that will energize the
Calvin cycle (dark reactions) which is when glucose is formed from carbon dioxide molecules
where do light reactions take place in the chloroplast? - ✔✔on the thylakoid membrane of
the chloroplast
light reactions summary? - ✔✔1.Light reactions occur on the membranes of thylakoids
2.Photon absorbed by psII (P680 - chlorophyll b)
3.E excites an e' that is passed from e'-accepting molecule to molecule (ETC) until it reaches PSI
4.The lost e' in PSII is replaced by photolysis - splitting of water into H+ and O - the H+'s are kept inside
the thylakoid and the O bonds with another O and is released as O2
5.As the e' travels down the ETC (or electron transport system) H+'s are pumped into the thylakoid
6.Electron from PSII eventually reaches PSI (P700 - chlorophyll a) where it is excited by another photon;
the e' travels down the ETC and converts NADP+ to become NADPH
7.Also, H+ from the photolysis of water are used to generate ATP
Photolysis of water in light reactions? - ✔✔2H2O --> 4 e' (replace lost ones in PSII) + 4H+ + 2O
,Oxidation and reduction of PSII? - ✔✔PSII is oxidized when it loses its e' and then PSII is reduced when it
retrieves an electron from water which returns PSII to its unexcited state.
Relation btw ETC and Redox? - ✔✔The ETC is a series of oxidation-reduction reactions --> molecule
is reduced by gaining the e' and then the molecule is quickly oxidized by it losing the e'; the reduced
molecule becomes the reducing agent for the next molecule.
Role of plastoquinone (PQ) in the light reactions? - ✔✔PQ's job is to bond with 2H+ in order to move
the 2H+ ions from the stroma to the lumen inside the chloroplast. The e' from PSII is passed to PQ and
PQ bonds with the 2H+ and then this bigger molecule diffuses through the thylakoid membrane and
passes the e's to the next acceptor in the ETC (cytochrome b6f complex); the 2H+ are released into the
lumen (cavity of the thylakoid membrane - think of thylakoid as flattened penne pasta)
So...PQ moves H+'s from the stroma into the lumen - space within the thylakoid, AKA H+ pumping!
What molecule transfers e's from PSII to PS1? - ✔✔Cytochrome b6f complex, a pigment embedded
in the thylakoid membrane, accepts the 2 e's from PQ; this occurs after PQ releases the 2H+'s into
the lumen that it pulled out of the stroma. Cytochrome b6f complex passes the e's to plastocyanin.
What is the ONLY biological molecule capable of oxidizing water? - ✔✔PSII - the e's produced by
the oxidation of water supply a steady source of e's for PSII!
What is the antenna complex of the light reactions? - ✔✔The conglomeration of pigment molecules
and the "reaction center" which is where the photon is ultimately absorbed by an e'
Role of NADP in light reactions? - ✔✔NADP is the final e' acceptor in PS1 and is a hydrogen ion
acceptor - the stored E in NADPH will be used in the Calvin cycle.
Why build up H+ concentration in the lumen? - ✔✔To generate ATP - the H+ pass thru ATP
synthase embedded in the thylakoid membrane which results in the photophosphorylation of ADP
,proton motive force? - ✔✔build up of H+ in the lumen (created by the movement of H+ from the stroma
into the lumen by PQ) and then the movement of the H+'s thru ATP synthase for the generation of ATP
from ADP - the ATP will be used to energize the dark reactions
How many ATP and NADPH formed in light reactions? - ✔✔2 of each
Goal of the Calvin-Benson Cycle, aka dark reactions or light-INdependent reactions? - ✔✔Take the
recently created NADPH and ATP and store their energy by constructing sugars from CO2
•Where: in the stroma of the chloroplast
•Where does the CO2 come from? ◦The atmosphere - the leaf opens up its stoma and lets CO2 in
◦When this happens, H2O is inadvertently released
phototaxis? - ✔✔directional movement in response to light
summary of dark reactions (calvin cycle)? - ✔✔1.CO2 is combined w/ RuBp (5C) to form a 6C sugar
via the enzyme RuBisCo.
2.6C immediately breaks down to form 2 PGA's (each 3C)
3.NADPH and ATP are used to convert PGA's into PGAL's (aka G3P)
4.PGAL's can form a 6 carbon sugar (glucose)
5.But... it takes 6 turns to generate enough carbon so that 12 molecules of PGAL are formed 10
PGAL's are converted back to RuBP via E from ATP and 2 PGAL's combine to make glucose
What does RuBP stand for and why is it called a CO2 acceptor? - ✔✔Ribulose biphosphate; called
this because it bonds with CO2 - RuBP is carboxylated which is catalyzed by Rubisco enzyme
Why is this called C3 carbon fixation? - ✔✔Because the 1st products formed are 2, 3-C
compounds (PGA)
Performed by more than 90% of angiosperms
Photorespiration is a problem
, C4 carbon fixation pathway? - ✔✔A 4-C compound is formed instead of a 3-C compound; the 4-C
compound is oxaloacetate. Adaptation to circumvent photorespiration.
Plant leaf has Kranz anatomy - has bundle sheath cells which fix CO2 and have rubisco but mesophyll
cells don't so that photorespiration can be avoided.
what is Kranz anatomy? - ✔✔plant leaves w/ bundle sheaf cells for photosynthesis that
avoids photorespiration
How does CO2 get to the bundle sheath cells in C4 plants? - ✔✔CO2 bonds with PEP to form OAA
(oxaloacetate - a 4C compound) which converts to malate which is shuttled to the bundle sheath cell
where the CO2 is removed and used in the calvin cycle. The remaining molecule is pyruvate which
goes back to the mesophyll cell to be converted to PEP.
Mostly in plants in hot, dry climates where C4 pathway is more efficient because CO2 has higher affinity
for PEP carboxylase than rubisco at high temperatures; also corn and sugar cane
What is photorespiration? - ✔✔A process in plants in which oxygen reacts with RuBP in the dark
reactions; oxygen can react with RuBP in the same way that CO2 can, but no energy is produced so this
is a wasteful process the plant needs to avoid.
Plants use up to 40% of the stored E to deal with this oxygen fixing problem.
Plants go through a complex series of reactions to remove O2 from phosphoglycolate and reclaim the
2 carbons needed to make PGA.
When does cyclic photophosphorylation take place? - ✔✔In Anaerobic photosynthetic bacteria - ones
that photosynthesize in environments devoid of oxygen
What organisms can carry out non-cyclic photophosphorylation? - ✔✔Plants, algae, cyanobacteria
(formerly called blue-green algae but they are prokaryotes, not plants or algae) and some other types of
aerobic, photosynthetic bacteria - only have PSI.
Why are cyanobacteria important? - ✔✔They play a predominant role in photosynthesis in fresh
and marine waters
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