Autotrophs are organisms that convert light energy to chemical energy through photosynthesis. In light-dependent reactions, ATP and NADPH are produced through photosystems of electron transport in the thylakoid inner membranes (where molecules of water are stripped to provide electrons to these ETC systems!), then in the light-independent reactions that ATP and NADPH are used to produce sugars from CO2 in the atmosphere! If we weren’t already so habituated to the facts of photosynthesis, it would seem like an impossible fantasy of science fiction.
Here are a few questions to help you test your understanding. Post your responses to the comments.
- Use the following labels for the diagram below: NADPH, CO2, LIGHT, O2, ATP, SUGAR/CH2O, LIGHT REACTIONS IN THYLAKOID MEMBRANES, CALVIN CYCLE IN STROMA, H20
- Below is an image of an action spectrum (relative rates of photosynthesis under different wavelengths of light) and an absorption spectrum (the different wavelengths of light absorbed by chlorophylls a). Indicate which line (solid or dotted) represents the absorption spectrum and which represents the action spectrum of a plant performing photosynthesis. Why are these lines different?
- How are the electron “holes” filled in chlorophyll P680+? and, P700+?
- Specify the terms for the blanks of the diagram below. Which of these provide chemical energy and reducing power to the Calvin cycle?
- Why is neither O2 nor NADPH generated by cyclic electron flow?
- How is ATP produced by cyclic electron flow? (be specific by referring to elements of the ETC).
- What does rubisco do?
- Refer to the diagram below to answer these questions: What are the three phases (a, b, and c) of the Calvin cycle? What are the key molecules (d to o)?
- What is the cause of photorespiration and what are the consequences?
HTK General Biology 2 
Answers to Questions 4:
a. Photosystem II
b. Photosystem I
c. H2O
d. 1/2 O2
e. P680, reaction centre
f. primary electron acceptor (pheophytin)
g. electron transport chain
h. photophosphorylation by chemiosmosis
i. ATP
j. P700, reaction centre
k. primary electron acceptor
l. NADP+ reductase
m. NADPH
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NADPH and ATP provide chemical energy and reducing power to the Calvin cycle
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Answers to Question 8:
The three phases are
a. carbon fixation
b. reduction
c. regeneration of RuBP (the CO2 acceptor)
The key molecules are
d. 3 CO2
e. RuBP (ribulose bisphosphate)
f. rubisco
g. 3-phosphoglycerate (3PG)
h. 6 ATP –> 6 ADP
i. 1,3-bisphosphoglycerate (don’t worry about remembering this intermediate)
j. 6 NADPH –> 6 NADP+
k. 6 Pi
l. glyceraldehyde 3 phosphate (G3P)
m. G3P
n. glucose and other organic compounds
o. 3 ATP –> 3 ADP
9 ATP and 6 NADPH are required to synthesize one G3P
and, 2 G3P are required to produce one glucose
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#1. a) light b) H2O c) Light rections in the thylakoid membrane d) O2 e) ATP f) NADPH g) CO2 h) Calvin cycle in the sttroma i) Sugar/CH2O
#2. a) The solid line represents the absorption spectrum and the dotted line represents the action spectrum.
b) These lines differ because light can only be absorbed in discrete quantities by the chlorophyll, hence explaining the well-defined peaks at wavelengths of red and blue light. On the other hand, the relative rate of photosynthesis does not decrease as much as the absorption spectrum does when light other than red or blue hits the chlorophyll. This can be explained by the fact that carotenoids (accessory pigments) absorb these different wavelengths (that chlorophyll cannot absorb) and then transfer the energy gained to chlorophyll. In other words, these accessory pigments increase the range of wavelengths that can drive photosynthetic reactions, hence explaining why the action spectrum does not decrease as much as the absorption spectrum to zero light other than red or blue hits the chlorophyll.
#3. In P680, the electron holes are filled by the electrons released during H2O splitting that occurs in the thylakoid lumen. In P700, the electron holes are filled by the electrons that are transported from PSII to PSI by plastocyanin.
#5. NADPH is not generated by cyclic electron flow because ferredoxin does not give the electrons to NADP+, but rather transports them back to plastoquinone, which causes the electrons to go through the ETC of PSII and drives the synthesis of ATP through photophosphorylation. O2 is not generated by cyclic electron flow because a constant flow of electrons is provided to plastoquinone from ferredoxin, hence making the splitting of H2O molecules releasing H+ ions, electrons, and O2 unnecessary.
#6. ATP is produced by cyclic electron flow because ferredoxin provides electrons to plastoquinone, who then transports these electrons to the cytochrome complex. As the electrons progress through the ETC, redox reactions occur and release energy. This energy is then used by plastoquinone to pump H+ ions into the thylakoid lumen. A proton gradient is therefore created. The H+ ions in the thylakoid lumen then go through ATP synthase (going down their concentration gradient), which generates the production of ATP in the chloroplast stroma by photophosphorylation.
#7. It catalyzes the fixation of CO2 to RuBP.
#9. Photorespiration occurs when an O2 molecules bind to rubisco instead of CO2 molecules. This triggers the release of 1 3PG, but also of one 2-phosphoglycolate, which uses ATP and releases CO2 when it is processed. In essence, photorespiration reverses photosynthesis because it produces CO2 and consumes O2, hence decreasing the rate of photosynthesis.
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