Chapter 6 Flashcards
____ manufacture organic nutrients from CO2 and H2S
Autotrophs
________autotrophs use energy from inorganic molecules
Chemoautotrophs
______autotrophs using radiant energy to make organic compounds
Photoautotrophs
The first photoautotrophs used _____ as an electron source
H2S
What are the main principles of photosynthesis?
Low energy electrons removed from donor compound, converted into high energy electrons (using light energy). First autotrophs used H2S but as cyanobacteria evolved they used H2O to produce O2 as an end product
Photosynthesis in eukaryotes takes place in the _____
chloroplast
Describe the structure of the chloroplast membrane
Outer membrane contains porins and is inpermeable to large molecules (due to large percentage of glycolipids), inner membrane contains light absorbing pigment, electron carriers, ATP-synthesizing enzymes. Inner membrane is folded into flattened sacs called THYLAKOIDS, arranged into stacks called “grana”
Lay out a photosynthesis reaction
6CO2 + 12H2O > C6H12O6 + 6H2O + 6O2
How was it shown that the two O2 molecules produce from photosynthesis are from H2O, not CO2? Why was proving this necessary?
Using an 18O isotope of oxygen, oxygen atoms in water were tagged.
It was necessary to prove this because it was originally thought that CO2 was split and produced CH2O
The intermembrane space of chloroplasts is referred to as _____
Stroma
diagram to label chloroplast vs mitochondrion
nflakd
Where do chloroplasts synthesize ATP?
Chloroplasts have ATP synthases on the thylakoid membrane, which use proton motive force generated by light-driven electron transport to make ATP (AKA PHOSPHORYLATION)
Photosynthesis _____ water to oxygen; respiration _____ oxygen to form water
Oxidizes, reduces
Photosynthesis uses low energy electrons to form ATP and ____, which are then used to reduce _____ to carbohydrate
NADPH,
CO2
Respiration removed high energy electrons from reduced organic substrates to form ____ and _____
ATP and NADH
label slide 38 diagram
ahwlehl
What are the two stages of photosynthesis? Briefly describe each.
Light-dependent reactions - Light reactions in which sunlight is absorbed, converting it into ATP and NADPH
Light-independent reactions - Dark reactions use the energy stored ATP and NADPH to produce carbohydrate
Absorption of ______ from the sun by molecules makes them go from ground state to excited state
Photons
Describe the basic facts pertaining to absorption of photons by molecules
Energy of the photon depends on wavelength of light. Energy required to shift electrons varies for different molecules.
Molecules absorb specific wavelengths of light
What purpose do photosynthetic pigments serve?
Absorb light of particular wavelengths
What is the structure of chlorophyll?
Contains a porphyrin ring that absorbs light and a hydrophobic Phytol tail embedding it into the photosynthetic membrane
How does the porphyrin ring in Chlorophyll compare to that in the heme group used in cellular respiration
This porphyrin ring has Mg in the center (MAGNESIUM!!!)
What does it mean to say that the cloud around the porphyrin ring is a conjugated system?
The alternating single and double bonds along the porphyrin ring form a cloud that absorbs energy of a range of wavelengths
Beside chlorophyll, there are accessory pigments called _____
carotenoids
What kind of light do carotenoids absorb?
Blue-green spectrum
____ photons are needed for 1 oxygen molecule
8
The _______ cholorophyll transfers electrons to an electron acceptor
Reaction-center chlorophyll
What are antenna pigments?
Pigments that do not participate directly in the conversion of light energy and are responsible for light absorption
What are the two large pigment-protein complexes and what are their functions?
Photosystem II (PSII) - Boosts electrons from below energy level of water to a midpoint. Photosystem I (PSI) - boosts electrons to a level above NADP+
What is “photolysis”?
When water is split using light energy
The reaction center of PSII is referred to as _____, and PSI is referred to as ____
P680
P700
The flow of electrons from H2O to NADP_ is referred to as the ____
Z scheme
How does PSII obtain electrons by splitting water
Two proteins, D1 and D2, bind the P680 cholorophyll and perform reactions to oxidize H2O. Light is harvested by light-harvesting complex II (LHCII)
How do electrons flow from PSII to Plastoquinone
PAssed from LHCII to inner-antenna molecules within PSII. Excited P680* transfers energy to an electron acceptor, generating P680+ and Pheo-. They are transferred to opposite sides of the thylakoid membrane where Pheo- passes its electron to Plastiquinone (PQ). PQ passes it to another PQ, which moves it to the stromal side of the membrane. This creates a proton gradient
How are the four electrons from H2O transferred in order to form O2?
Transferred through P680+ to four Mn ions and one Ca ion (which form the oxygen-evolving complex)
How was it found that 8 photons were needed in photosynthesis?
Found that an oxygen was released every 8 flashes of the thylakoid
How do electrons move from photosystem II to Photosystem I
Two molecules of PQH2 diffuse through thylakoid membrane and bind cytochrome b6f and release protons into the lumen of the thylakoid. Electrons from cytochrome b6f are passed to carrier plastocyanin, which brings them to P700+.
How is NADPH produced in PSI
PSI has a reaction core LHCI (protein-bound pigment complex). Photons harvested by the antenna pigments in this complex oxidize chlorophyll a, orming P700*. Absorption of light leads to production of P700+ and Ao-. This redox potential reduces NADP+ to NADPH.
The enzyme ferredoxin-NADP+ reductase serves what function in photosynthesis?
Catalyzes the reduction of NADP+ to NADPH, but also can use electrons to reduce nitrate, ammonia, sulfate into usable forms
What, in summary, happens during photosynthetic electron transport?
For every 8 photons absorbed:
2H2O + 2NADP+ > O2 +2NADPH
Electron transport also produces a proton gradient across thylakoid membrane
How do herbicides inhibit electron transport?
Bind to a core protein of PSII
Describe the machinery for ATP synthesis in the chloroplast
Consists of head (CF1) and base (CF0). Head projects outward into the stroma, keeping with the orientation of the proton gradient.
How does photophosphorylation occur?
protons move into the lumen at the CF0 base of the synthase, causing ATP production
The movement of electrons during the formation of oxygen is _____
Noncyclic photophosphorylation
Where is cyclic photophosphorylation performed and what is the purpose?
PErformed in PSI independently of PSII. Thought to provide additional ATP required for carbohydrate synthesis
How can a scientist track the movement of carbon in the cell during photosynthesis?
using [14C]O2 as a tracer and analyzing cell extracts using autoradiography
How is carbohydrate synthesis different in C3 plants?
C3 plants produce the three-carbon intermediate “3-phosphoglycerate (PGA)” as first compound during CO2 fixation.
It starts by CO2 being condensed into five carbon “ribulose 1,5-biphosphate (RuBP)”, which is catalyzed by “ribulose biphosphate carboxylase (Rubisco)” to form a six carbon molecule, and then it splits into two 3-phosphoglcerate
RuBP > PGA > GAP (using NADPH and ATP) > Regeneration of RuBP
The C3 pathway is otherwise known as _____
The Calvin-Benson cycle
What purpose do the “GAP” molecules serve at the end of the Calvin cycle?
GAP molecules can either be used to synthesize sucrose or remain in the chloroplast to be converted to starch
Why is carbohydrate synthesis in plants such an “expensive” process energy wise?
Conversion of 6 molecules of CO2 to 1 6-carbon sugar requires 12 NADPH and 18 ATP.
How is redox control light dependent?
The key enzymes in the Calvin Cycle are only active when ATP and NADPH are present during photosynthesis.
Thioredoxin carries some of the electrons needed to reduce NADP+, and thiredoxin becomes deactivated in lack of light
Why is the enzyme Rubisco so important to photosynthesis?
Catalyzes attachment of O2 to RuBP to produce 2-phosphoglycolate
Rate pf photorespiration depends on ____ and ____ ratio
CO2 and O2
What is the function of peroxisomes in photorespiration?
Shuttle glycolate and convert it to glyoxylate and then glycine (losing a CO2)
Describe carbohydrate synthesis in C4 plants
Involves production of phosphoenolpyruvate (PEP), which combines with CO2 to produce 4-carbon oxaloacetate or malate. IT starts with a lot of CO2 to synthesize more carbohydrates
Structurally, how do C4 plants function in relation to their carbohydrate synthesis?
They keep CO2 levels high by keeping stomata partially closed (to prevent water loss and keep CO2 in). They transport C4 products into bundle sheath cells, where the fixed CO2 can be split from 4-carbon carrier producing the high CO2 level necessary for Rubisco to fixate stuff
How do CAM plants carry out carbohydrate synthesis?
Carry out light reactions and CO2 fixation at different times of the day using PEP decarboxylase. CAM plants keep their stomata closed during the day to reduce water loss
Rubisco is otherwise known as
Ribulose bis phosphate carboxylase
What is the “GAP” molecule?
Glyceraldehyde 3 phosphate
