Chapter 6: Photosynthesis Flashcards
Earliest living organisms were ______
heterotrophs
chemoautotrophs
use energy from inorganic molecules
photoautotrophs
use radiant energy to make organic compounds
electron source of first autotrophs
H2S
Photosynthesis reaction
6CO2 + 6H2O –> C6H1206 + 6O2
Photosynthesis in eukaryotes takes place in the ________
chloroplast
T/F: the inner membrane of the chloroplast is permeable to large molecules.
False. The OUTER membrane of the chloroplast contains PORINS and is permeable to large molecules.
The inner membrane of the chloroplast is folded into flattened sacs called ________, arranged in stacks called __________
The inner membrane of the chloroplast is folded into flattened sacs called THYLAKOIDS, arranged in stacks called GRANA.
Do chloroplasts contain their own DNA?
yes. Chloroplasts are self-replicating organelles.
What makes the thylakoid membrane highly fluid for diffusion of protein complexes?
The large percentage of glycolipids in thylakoid membrane.
Matrix is to mitochondria as _______ is to chloroplasts.
STROMA
Where is ATPase located in the chloroplast?
The thylakoid membrane
Photosynthesis oxidizes _____ to _______
Photosynthesis oxidized WATER to OXYGEN.
In the light-dependent reactions, sunlight is absorbed and converted into _______ and _________
ATP and NADPH
In the light-independent reactions, ATP and NADPH are used to produce ____________
carbohydrates
What are photosynthetic pigments?
molecules that absorb light of particular wavelengths
Purpose of rign and tail on chlorophyll
- PORPHYRIN RING absorbs light.
- Hydrophobic tail is embedding chlorophyll to the photosynthetic membrane.
Conjugated system in porphyrin ring
Conjugated system = alternating single and double bonds along the porphyrin ring form a cloud. These systems absorb energy of a range of wavelengths.
Carotenoids
accessory pigments which absorb blue-green spectrum of light.
Antenna pigment
Pigments that do not directly participate in the conversion of light energy.
Responsible for light ABSORPTION.
The reaction center of PSII is referred to as ____ because…
The reaction center of PSII is referred to as P680 because it is the wavelength where ABSORPTION is STRONGEST.
The reaction center of PSI is referred to as ____
P700
The flow of electrons from H2O to NADP+ is referred to as _________
the Z Scheme
Two proteins that bind to P680 are called ____ and ______.
D1 and D2
What is LHCII?
“Light Harvesting Complex II”
- A pigment-protein complex OUTSIDE of PSII
- LHCII proteins bind both chlorophylls and carotenoids
- excitation energy is passed from the outer-antenna pigments of LHCII to inner-antenna chlorophyll molecules in PSII and then to P680. The excited reaction center pigment then responds by transferring a single excited electron to the primary electron acceptor (pheophytin)
What is the primary electron acceptor of P680 reaction center?
Pheophytin
Pheo- transfers its electron to ________
Pheo- transfers its electron to Plastoquinone (PQa)
T/F: plastoquinone is lipid-soluble
true
3 states of plastoquinone
- Plastoquinone: fully oxidized (0 e-)
- Plastosemiquinone: 1 e-
- Plastoquinol: Fully reduced (2e-)
How are the oppositely charged molecules (P680+ and Pheo-) stopped from interacting?
by moving Pheo all the way to the other side of the membrane
The electron from PQa is transferred to ___ to produce ______
- PQb
2. plastosemiquinone
Once P680 is reduced again and excited by another photon, it transfers another electron from P680 to Pheo, to PQa, and then to PQb to form _______
plastoquinol (when combined with two electrons) (PQH2)
The protons utilized to form PQH2 are derived from the _______
stroma
What happens to the PQH2 molecule once it is fully reduced?
It dissociates from the D1 protein and diffuses into the lipid bilayer, binds cytochrome b6f, and releases protons into the lumen of the thylakoid.
Replaced with a fully oxidized PQ molecule.
how many electrons from water are required to form O2?
4 electrons
2H2O –> 4H+ + O2 + 4e-
The splitting of water during photosynthesis is called ___
Photolysis
Cluster of 5 metal atoms closely associated with the D1 protein of PSII
OXYGEN-EVOLVING COMPLEX.
- 4 Mnganese (Mn) and 1 Calcium (Ca) atom
- stabilized and protected by 4 peripheral proteins
What is the role of the oxygen-evolving complex?
- To split water (remove electrons from water), generating free oxygen.
- Electrons travel to Tyrz (turosine residue on the D1 protein) to P680+ to make it electrically neutral.
- The system catalyses the removal of 4 electrons from two closely bound H2O molecules once it has absorbed 4 photons by the PSII system.
Protons produced in photolysis are maintained in the ________, What happens to oxygen?
Protons produced in photolysis are maintained in the THYLAKOID LUMEN.
Oxygen is released into the environment as a waste product.
How many PQH2’s are formed in the production of one O2 molecule?
2
Flow of electrons from PSII to PSI
PSII (P680) –> PQa –> PQb –> Cytochrome b6f —> Plastocyanin –> PSI (P700+)
3 sources of proton transfers into the lumen of the thylakoid:
- Splitting of H2O in the lumen
- Protons from plastoquinone
- Cytochrome b6f pumps protons from the stroma into the lumen
Reaction center of PSI is called
P700. It is a CHLOROPHYLL a DIMER
Light energy is absorbed by the antenna pigments of ________ and passed to the P700 pigment
LHCI
Remember, there is a high concentration of protons in the ______ of the thylakoid
lumen
The excited P700 reaction center transfers its electron to ____
a separate monomeric chlorophyll a molecule (Ao), which acts as the primary electron acceptor.
This generates two charged species (Ao- and P700+)
The positive charge of P700+ is neutralized by _______
an electron donated by PLASTOCYANIN
The reduction of NADP+ to NADPH is catalyzed by
ferredoxin-NADP+ reductase
How many ferredoxin molecules are necessary for the reduction of one NADP+? Why?
Two ferredoxin molecules are necessary because each ferredoxin can only transfer one electron at a time
T/F: all electrons that pass through ferredoxin end up in NADPH
False. Some electrons that pass through ferredoxin end up reducing nitrate, ammonia, or sulfate to form other important biological molecules.
ATP synthase pumps protons from the ________ to the ___________
from the LUMEN to the STROMA
ATP synthase consists of a ______ and a _______.
ATP synthase consist of a HEAD (CF1) and a BASE (CFo)
Does the CF1 head project into the stroma or into the lumen?
CF1 heads project outward into the stroma
Through where do protons move into the lumen on ATP synthase?
through the CFo base
Cyclic photophosphorylation is carried out by _____
PSI, independent of PSII
Noncyclic photophosphorylation
the movement of electrons during the formation of Oxygen. Ions move in a nonlinear path from PSII to PSI
Why do C3 plants have their name?
Because they roduce a 3 CARBON INTERMEDIATE (3-phosphoglycerate, PGA) as the first compound to be identified during CO2 fixation.
in C3 plants, CO2 is condensed with ______ to form _____ which then splits into two molecules of ____
In C3 plants, CO2 is condensed with a FIVE-CARBON COMPOUND (RuBP, ribulose-1,5-bisphosphate) to form a SIX-CARBON MOLECULE which then splits into two molecules of PGA
Rubisco catalyzes…
…the condensation of RuBP and the splitting of the 6-carbon molecule.
It is a carboxylase (fixes CO2) and an oxygenase (transfers oxygen to RuBP)
3 phases of carbon cycle
- carbon fixation
- reduction of PGA into GAP (using NADPH and ATP from light reaction)
- Regeneration of RuBP
where does the NADPH and ATP come from for the reduction of PGA?
from the light reactions
Fates of the 2 GAP molecules that “leave” the Calvin Cycle
- GAP molecules can be exported into the cytosol in exchange for phosphate ions and used to SYNTHESIZE SUCROSE.
- GAP can also remain in the chloroplast where it is CONVERTED INTO STARCH
Conversion of 6 molecules of CO2 to one 6-carbon sugar molecule requires ____ molecules of NADPH and ____ molecules of ATP
12 molecules of NADPH and 18 molecules of ATP
Why is it said that redox control is light-dependent in photosynthesis?
- because key enzymes in the Calvin cycle are only active when ATP and NADP are produced by photosynthesis.
- Some electrons used to reduce NADP+ are transferred to THIOREDOXIN, which are then accepted to reduce disulfide bridges in selected Calvin-cycle enzymes
- In the dark, THIOREDOXIN REDUCTION CEASES and enzymes go back to oxidized state and are INACTIVE.
thioredoxin
- contains cystein amino acids that create disulfide bridges which can be reduced to thiols (SH)
- Reduced by FERREDOXIN
- inactive in the dark
Pathway of electron from P700
photon –> P700 –> Ao –> A1 –> Fx –> Fa and Fb –> ferredoxin
NOTE: “F” molecules are iron-sulfur centers
What happens when two separate ferredoxin molecules accept an electron?
They act together to reduce NADP+ to NADPH.
What accounts for the loss of up to 50% of fixed CO2?
PHOTORESPIRATION: the process of uptake of O2 and release of CO2.
What happens in photorespiration?
- RUBISCO catalyzes the attachment of O2 to RuBP to produce 2-phosphoglycolate.
- glycolate is then transferred to the peroxisome and CO2 is released.
what does the rate of photorespiration depend on?
-rate of photorespiration depends on the CO2/O2 ratio
Photorespiration reaction vs. carbon fixation reaction
Photorespiration:
O2 + RuBP –> oxygenase intermediate (5-carbon) –> 3-PGA + 2-PHOSPHOGLYCOLATE
Carbon Fixation:
CO2 + RuBP –> Carboxylase intermediate (6-carbon) –> 2 3-PGA
Why are C4 plants called C4 plants?
Because their first stable intermediate is a 4-carbon compound (mal or oxa). This is derived from PEP and CO2
Difference between stomata in C3, C4, and CAM plants?
C3 and C4 maintain stomata open and fix CO2 during the day.
CAM plants keep stomata closed during the day and open at night.
CAM plants carry out light reactions and CO2 fixation at different times of the day using the enzyme ___________
PEP carboxylase
