Photosynthesis Flashcards
What organisms have chloroplasts?
all plants
unicellular plant-like protists
multicellular plant-like protists (like kelp)
some photosynthetic prokaryotes (cyanobacteria)
In what 2 ways do chloroplasts arise?
by fission from other mature chloroplasts
from their non-photosynthetic precursors, proplastids
in higher plants, where are chloroplasts located?
in the mesophyll layer of leaves
Approximately how many chloroplasts are there per cell in leaves of higher plants?
20-40
How many membranes do chloroplasts have?
2
What separates the double membrane envelope of chloroplasts?
an intermembrane space
Describe the outer envelope membrane of chloroplasts
not very selective, it allows many molecules to pass through its large porins
Describe the inner envelope membrane of chloroplasts
highly impermeable and substances require a specific transporter to enter
Aside from the double membrane, what are the two components of chloroplasts?
thylakoids
stroma
Describe thylakoids
flattened sacs separated from the envelope with their own distinct membrane
T or F: thylakoids have their own distinct membrane
true
How are thylakoids arranged?
in stacks called grana (pl.) or granum (s.)
What do thylakoid membranes contain?
all the protein components for photosynthesis
What occurs in the thylakoid lumen?
it is where H+ is pumped to create a gradient for ATP synthesis
T or F: all thylakoids are stacked as grana
false! some are singular and not stacked
What are thylakoids that are singular and not stacked called?
stroma thylakoids
What is the purpose of stroma thylakoids?
they usually connect stacks of grana thylakoids
Describe the stroma
the fluid outside the thylakoid in chloroplasts
What 5 things does the stroma of chloroplasts contain?
a single, small, circular DNA chromosome
ribosomes
enzymes
lipids
starch granules
T or F: the stroma of chloroplasts contains DNA
true
Describe the structure of the DNA found in the stroma of chloroplasts. What is its purpose?
it is a single, circular DNA chromosome
it codes for about 65 synthesizing chloroplast proteins
What are the ribosomes found in stroma of chloroplasts associated with?
the surface of thylakoids
What are the enzymes of the stroma of chloroplasts for?
the Calvin Cycle
What purpose do the lipids and starch granules in the stroma of chloroplasts have?
they are for stored energy
What is an example of a protein the stroma DNA chromosome codes for?
RUBisCO
Where are most of the chloroplast proteins synthesized and translated?
synthesized in the nucleus
translated in the cytoplasm
What do the proteins synthesized in the nucleus but bound for the chloroplasts require to be directed to the chloroplasts?
they require import into the chloroplast with a specific N-terminal stroma targeting signal
What kind of signal do chloroplast-bound proteins synthesized and translated outside of the chloroplast have?
an N-terminal stroma signal
What are the 2 large translocation complexes in chloroplasts and where are they located?
Toc = outer envelope membrane
Tic = inner envelope membrane
Which of the 2 translocation complexes will proteins encounter first on their journey to the chloroplast stroma?
TOC
What happens to proteins when they encounter the TOC complex?
they are unfolded by cytosolic chaperone proteins
What chaperone protein associates with the unfolded polypeptide as it is translocated through TOC and TIC into the stroma? What is its purpose?
Hsp70
encourages refolding in the stroma
What happens to the protein once its in the stroma?
an enzyme cleaves the N-terminal stroma targeting sequence and
a barrel-shaped chaperone (Hsp60) folds the proteins
What do proteins bound for the thylakoid membrane or lumen have in addition to the signal sequence for the stroma?
they will have an additional thylakoid transfer domain sequence that targets them for either the thylakoid lumen or membrane
Why are ribosomes usually bound to the thylakoid surface?
so they can directly translocate a protein that was synthesized from genes within the chloroplast DNA through the thylakoid membrane
What happens to proteins synthesized from genes within the chloroplast DNA?
the chloroplast ribosome will directly translocate the protein through the thylakoid membrane
What is the overall purpose of the light reactions of photosynthesis?
they capture energy from light and use it to remove and energize electrons from water
in the light reactions, where do the high-energy electrons move to once removed from water?
through redox reactions in the ETC
What happens in the ETC?
energy is used to pump protons and create an electrochemical gradient
What is the electrochemical gradient produced by the ETC? What is its purpose?
a proton gradient that is used to drive ATP synthase (photophosphorylation)
What is the final electron acceptor at the end of the ETC?
the carrier NADP+
What is the BASIC flow of energy through the light reactions?
light energy –> electrical energy –> chemical energy
What are the inputs of the light reactions?
H2O
light
NADP+ (oxidized electron carrier)
ADP + Pi
What are the outputs of the light reactions?
NADPH (reduced e. carrier)
ATP
O2 (waste)
Is NADP+ the reduced or oxidized version?
oxidized
Is NADPH the reduced or oxidized version?
reduced
What two things are used to drive the Calvin Cycle?
ATP + the final electron carrier NADP+ (NADPH when it has accepted the electron)
What is the purpose of the Calvin Cycle?
carbon fixation
Describe carbon fixation
the conversion of CO2 into simple sugars (CH2O)
What is the purpose of the dark reactions?
to fix carbon by converting CO2 into simple sugars (CH2O), chemical energy that can be stored for when there’s no light for the light reactions to occur
What is the BASIC flow of energy through the dark reactions?
transient chemical energy –> stored chemical energy
What are the inputs of the dark reactions?
CO2
NADPH (reduced version)
ATP
What are the outputs of the dark reactions?
C3H6O3 (simple sugar, later converted to glucose)
NADP+ (oxidized)
ADP + Pi
Where in the chloroplast do the light reactions occur?
in the thylakoid membranes
Where does the calvin cycle/dark reactions occur in the chloroplast?
the stroma
What are some major similarities between the mitochondria and chloroplasts/cellular respiration and photosynthesis?
both have energy-harvesting electron transport chains
both pump H+ to make proton gradients
both have compartments in organelles where pH is different
the reactions are almost exactly the reverse
What is the reaction for photosynthesis?
light energy + 6 H2O + 6 CO2 –> C6H12O6 + 6 O2
What 3 things does the inner membrane system surrounding thylakoids contain?
light-absorbing pigments (organized in protein photosystem complexes)
electron carriers
proteins related to ATP synthesis + ATP synthase
What are photosystems? where are they located?
protein complexes that contain light-absorbing pigments
located in the inner membrane system that surrounds the thylakoids
What are the 2 basic parts of photosystems?
light harvesting complexes
reaction centre
T or F: only the light harvesting complexes have pigments
false! the LHC and the reaction center have pigments
What are pigments?
special chemicals that can absorb certain wavelengths of visible light
What light is harnessed to undergo photosynthesis?
only light that is absorbed by pigments
What is unique about the molecular structure of pigments?
they usually have a metal ion (ex. Mg2+) fixed in the center of a carbon ring with conjugated double bonds
What is the key photosynthetic pigment?
chlorophyll a
What are the two antenna pigments?
chlorophyll b
carotenoids
Why do plants appear green?
because the pigments are absorbing a specific wavelength of light and reflecting green wavelengths
What wavelengths of visible light do pigments absorb? what colours are associated with these?
~450 nm
purple-blue
What happens when a pigment molecule absorbs light?
when a photon of light is absorbed by a pigment molecule, one or more electrons in the pigment is excited from its ground state to an unstable excited state with higher potential energy
What are the 3 strategies for an excited electron to return to ground state?
- release of heat and a photon of a different wavelength (fluorescence)
- transfer of energy to a neighbouring chlorophyll
- transfer of the excited electron to a neighbouring electron acceptor
Which of the 3 strategies for an excited electron to return to ground state are the most common in photosynthesis?
transfer of energy to a neighbouring chlorophyll
transfer of the excited electron to a neighbouring electron acceptor
What happens to the chlorophyll if the excited electron is transferred to a neighbouring electron acceptor?m
it will need to replenish its lost electron by receiving a new one from an electron donor
Which pigments are in the light harvesting complexes?
many, but these include the antenna pigments (chlorophyll b and carotenoids)
Which pigments are in the reaction center?
a special chlorophyll a dimer
Describe the transfer of energy in the pigments of the light harvesting complexes
pigments transfer energy from the excited electron amongst each other towards the reaction center
How does the reaction center transfer energy from the excited electron?
the special chlorophyll a dimer directly converts the light energy into chemical energy
How does energy move from pigment molecule to pigment molecule in the light harvesting complexes? What does this really mean?
it always transfers to a neighbouring pigment molecule of equal or lower energy
aka the neighbouring pigment molecule must absorb light of equal or longer wavelength
How is the energy gradient that is required to move energy from the light harvesting complexes into the reaction center maintained?
neighbouring pigment molecules are carefully selected and kept at specific distances from one another to promote the energy transfer
What are the two types of photosystems in the light reactions?
PSI and PSII
Where do the photosystems access light energy?
in their respective light harvesting complexes
Which of the two photosystems occurs in the light reactions first?
PSII then PSI
How are PSII and PSI distinguished?
by their chlorophyll a dimers
What is the chlorophyll a dimer found in the PSII?
P680
What is the chlorophyll a dimer found in the PSI?
P700
Why is the chlorophyll a dimer of PSII called P680?
the chlorophyll a dimer absorbs light at 680 nm
Why is the chlorophyll a dimer of PSI called P700?
the chlorophyll a dimer absorbs light at 700 nm
What is the overall direction electrons move?
from H2O –> PSII –> ETC –> PSI –> ETC –> NADP+
What happens to NADP+ once it accepts the electrons?
it’s reduced to NADPH
What do the photosystems use their chlorophyll a molecules for?
to separate charge
How do the photosystems use their chlorophyll a molecules to separate charge?
the pair of chlorophyll a molecules donate their excited electron to an electron carrier =
chlorophyll a is now positively charged
electron carrier is now negatively charged
What is the result of the photosystems using their chlorophyll a molecules to separate charge?
when the excited electrons are donated from the chlorophyll a molecules to an electron carrier:
chlorophyll a becomes positively charged
electron carrier becomes negatively charged
When the electrons are donated from the chlorophyll a pairs in the photosystems to an electron carrier, what happens?
the electron moves from a primary acceptor (the electron carrier). to the ETC
What is the purpose of the ETC?
to help create a proton gradient to synthesize ATP
What happens to the chlorophyll a dimer that has given up its excited electrons?
it is positively charged and needs to replenish its electrons
it will receives electrons from the splitting of water (photolysis)
What is photolysis?
the splitting of water
How do positively charged chlorophyll a dimers replenish their lost electrons?
by receiving electrons from the splitting of water
How many ETC chains are there in the light reactions?
2
What is at the end of the first ETC?
photosystem 1
What is at the end of the second ETC?
the final electron acceptor, NADP+
T or F: there’s only one type of electron carrier in the light reactions of photosynthesis
false! there’s many (ex. Q, PC, FD)
What are some examples of the electron carriers in the light reactions of photosynthesis?
NADP+
Q
FD
PC
What is the Z pathway?
the energy diagram between the 2 photosystems which forms a ‘Z’
Describe the BASIC steps of the energy diagram between the 2 photosystems
PSII boosts electrons from an energy point lower than water to a midway point
the ETC carries the electrons down in energy to the PSI
PSI boosts electron energy from a midway point to an energy level above the NADP+
How does the Z pathway begin?
sunlight excites electrons of antenna pigments in the light harvesting complexes
What happens after sunlight excites electrons of the antenna pigments of the LHCs?
energy moves through the pigments (as pigments absorb light at a successively longer wavelength) to the reaction center with the P680 dimer
What happens to the energy when it reaches the P680 dimer in the reaction center?
from P680, electrons are passed on to an electron acceptor and leave behind an oxidized photosystem II (P680+)
What happens to the PSII after the electrons are passed to an electron acceptor?
it is oxidized to P680+
Where do the electrons go after they have been passed from the P680 in PSII to the electron acceptor?
the electrons are transferred to an ETC and they move down the chain while releasing energy and creating a proton gradient
While the electrons are being moved down the ETC, what happens to the oxidized P680+ photosystem?
the lost electrons are replaced by electrons from water when it is split
How does energy move through PSI (P700)?
light hits PSI (P700) which excites its electrons
energy moves through PSI to the reaction center with the P700 chlorophyll a dimer
What happens after energy from excited electrons reach the P700 chlorophyll a dimer?
the electrons are transferred to the PSI primary e acceptor and leave behind an oxidized P700+
What reduces the oxidized P700+?
incoming electrons from the ETC1 reduce it back to P700
What transfers electrons from the PSI (P700) to and through the ETC?
primary electron acceptors
What is the last step of the Z pathway?
the electrons reduce NADP+, the final electron acceptor, to NADPH
Describe the composition of the PSII reaction center
it is a complex of 20+ polypeptides, most of which are integral membrane proteins
this complex contains the special chlorophyll a pigment dimer
What type of proteins are most of the polypeptides that make up the PSII reaction center?
integral membrane proteins
Why is the special chlorophyll a dimer of the PSII called P680?
because the pigments most effectively absorb light at the 680 nm wavelength
What molecule is closely associated with the reaction center of PSII?
pheophytin
What is pheophytin?
a molecule similar to chlorophyll but lacks an Mg2+
it is the primary electron acceptor for PSII
What is the function of pheophytin? What is the result of its function?
in the PSII reaction center, electrons move from P680 to pheophytin to generate the separation of charge:
P680+ and Pheo-
Where are the electrons transferred from pheophytin?
to plastoquinone (PQ)
Where does the electron transfer between pheophytin and plastoquinone move the electron toward?
toward the stromal side of the membrane
What becomes the oxidizing agent as a result of the electron transfer between pheophytin and plastoquinone?
P680+
Describe plastoquinone
(PQ)
a lipid-soluble molecule similar to ubiquinone in the mitochondria
What is the function of PQ?
it accepts 2 electrons from pheophytin and 2 H+ (from stroma) to become plastoquinol (PQH2)
What is the reduced form of plastoquinone (PQ)? How does it become this?
plastoquinol (PQH2)
when PQ accepts 2 e- and 2 H+
How many plastoquinones are there? What are they and where are they located?
2
PQA and PQB
located near the stromal side of the PSII reaction center
What is the function of PQA?
It is the type of plastoquinone that accepts electrons from Pheo- and passes them on to PQB
What is the function of PQB?
it accepts the electrons passed from PQA
What is the result of the electron transfer from Pheo- –> PQA –> PQB?
PQBH2
What does PQBH2 do?
it dissociates from PSII and moves laterally through the membrane
T or F: P680+ is a relatively weak oxidizing agent
FALSE! it is the strongest oxidizing agent ever found in a biological system
Why is it important than P680+ is a strong oxidizing agent?
so it can pull electrons from very stable water molecules
What is the photolysis equation?
2 H2O –4 photons–> 4 H+ (lumen) + O2 + 4 e-
In order to produce one oxygen molecule, how many water molecules are required and how many electrons are lost from the water?
2 water molecules required
4 electrons lost
What is the discrepancy between the amount of electrons produced by photolysis and the amount of electrons P680+ can accept?
P680+ only donated 1 electron to Pheo- and therefore can only accept one electron from water to regenerate P680
What is the oxygen evolving complex?
an enzyme that splits water and is associated with the PSII on its luminal surface
What does the oxygen evolving complex contain?
an Mn-Ca cluster that can donate 4 of its electrons
What is the purpose of the Mn-Ca cluster? When does it do its function?
it donates 4 of its electrons, one at a time, to the chlorophyll a dimer every time a photon hits and P680+ is produced
When will the oxygen evolving complex split water molecules? How many molecules will it split
once the Mn-Ca cluster has lost 4 electrons, the OEC can split 2 water molecules
What is produced by the OEC splitting 2 water molecules? what happens to these products?
the 4 electrons from water are used to regenerate the Mn-Ca cluster
the 4 protons accumulate in the thylakoid lumen and contribute to the proton gradient for ATP synthesis
O2 is released as a waste product
What happens to the 4 electrons produced by the OEC splitting water?
they are used to regenerate the Mn-Ca cluster
What happens to the 4 protons produced when the OEC splits 2 water molecules?
they accumulate in the thylakoid lumen to help produce the proton gradient for ATP synthesis
What happens to the O2 produced when the OEC splits 2 water molecules?
it’s released as a waste product
After water is split, what happens to the reduced plastoquinol (PQH2)?
it moves through the membrane bilayer while carrying the electrons to the cytochrome b6-f complex
Where does plastoquinol carry electrons to?
the cytochrome b6-f complex
What cycle is involved in the transfer of 2 electrons from plastoquinol to the cytochrome b6-f complex?
the Q cycle
What is the Q cycle?
the cycle involved in the transfer of 2 electrons from plastoquinol to the cytochrome b6-f complex
What happens in the Q cycle?
4H+ are pumped into thylakoid lumen from the stroma which creates the proton gradient
What is produced from the Q cycle?
the proton gradient
What does the cytochrome b6-f complex transfer the 2 electrons to?
plastocyanin
What is plastocyanin?
a small peripheral membrane protein on the thylakoid interior side that accepts electrons from the cytochrome b6-f complex
What is plastocyanin analogous to in the mitochondria?
cytochrome c
Where does plastocyanin carry electrons to?
the luminal side of PSI where they are transferred to the chlorophyll a dimer P700
How does light energy move from the LHC of PSI towards the reaction center at the chlorophyll a dimer (P700)?
antenna pigments move light energy in the same way as in PSII LHC
where do electrons move from the P700 dimer?
to a primary acceptor called Ao
What is Ao?
a chlorophyll molecule that acts as a primary acceptor of the electrons from P700 of PSI
What results from the transfer of the electrons from P700 to the primary acceptor Ao?
a separation of charge:
P700 oxidized to P700+
Ao reduced to Ao-
Is Ao- a strong oxidizing or reducing agent?
strong reducing agent
What does Ao- transfer electrons to?
to other molecules in PSI and eventually to ferredoxin
How is P700+ reduced back to P700?
by electrons incoming from plastocyanin from the ETC1
What is ferredoxin? Where is it located?
a small, water-soluble polypeptide on the stromal side of the thylakoid membrane that accepts electrons
What does ferredoxin transfer electrons to?
Ferredoxin-NADP+ Reductase (FNR)
What does ferredoxin-NADP+ Reductase pass electrons to?
NADP+
What are the electron carriers involved in the transfer of electrons from H2O to PSII and from PSII to PSI?
chlorophyll a P680 to pheophytin
Pheo- to plastoquinone (PQA)
WATER (separate): water to Mn-Ca of OEC to P680+
PQA to PQB
PQBH2 (plastoquinol) to cytochrome b6-f complex
cytochrome b6-f complex to plastocyanin
plastocyanin to P700
What are the electron carriers involved in the transfer of electrons from PSI to NADP+?
P700 to Ao
Ao- to others and then ferredoxin
plastocyanin (from ETC1) to P700+
ferredoxin to ferredoxin-NADP+ Reductase
FNR to NADP+
What are the 3 sources of the H+ gradient?
photolysis
Q cycle
Reduction of NADP+
How is photolysis a source of H+ gradient?
it generates H+ in the thylakoid lumen
How is the Q cycle a source of H+ gradient?
it transfers the H+ from the stroma to the thylakoid lumen
How is the reduction of NADP+ a source of H+ gradient?
H+ concentration is reduced on the stromal side of the thylakoid
What are the major products of the light reactions?
NADPH (reduced NADP+)
ATP
O2 (waste)
What is the major process of the dark reactions?
the Calvin Cycle
Where does the Calvin Cycle occur?
the stroma
What sugar does the Calvin Cycle produce?
glyceraldehyde 3-phosphate (G3P)
What does the Calvin Cycle require that is produced in the light reactions per G3P made?
9 ATP
6 NADPH
Why is the Calvin Cycle referred to as a cycle?
the starting material (RuBP) is regenerated
What are the 3 stages of the Calvin Cycle?
- fixing CO2 into an organic molecule
- reducing the organic molecule with NADPH and ATP to form a sugar
- regenerating the CO2 acceptor
How many carbons of G3P does each cycle of the Calvin Cycle fix?
ONE
How many times does the Calvin Cycle have to occur in order to net produce one G3P molecule? why?
3 times
because only one carbon is fixed per cycle and G3P has 3 carbons
What molecule incorporates CO2 during carbon fixation?
Ribulose bisphosphate (RuBP)
How many carbons does RuBP have?
5
What does RuBP stand for?
Ribulose bisphosphate
What is required for the incorporation of CO2 into RuBP?
the enzyme Rubisco
T or F: rubisco is the most prevalent enzyme on earth. Explain why/why not
true because it is required for the fixation of carbon from CO2
What does the incorporation of CO2 into RuBP produce?
an unstable 6C intermediate
What does the unstable 6C intermediate of the CO2 + RuBP quickly turn into?
it splits into two 3C molecules of 3-phosphoglycerate (PGA)
How many carbons are in each of the 2 molecules of 3-PGA?
3
What does PGA stand for?
3-phosphoglycerate
1 CO2 + 1 RuBP –Rubisco–> ?
2 molecules of 3-phosphoglycerate
What happens to the 3-PGA?
it is phosphorylated by ATP and converted into 1,3-BPG
What does the phosphorylation of 3-PGA produce? How many ATP molecules does this take per turn?
1,3-bisphosphoglycerate (BPG)
takes 2 ATP per turn
What does BPG stand for?
1,3-bisphosphoglycerate
What is produced when 1,3-BPG is reduced by the NADPH carrier?
glyceraldehyde 3-phosphate (G3P)
What does G3P stand for?
glyceraldehyde 3-phosphate
What reduces BPG to G3P?
the electrons stored in the NADPH carrier
What is G3P?
a 3 carbon sugar produced by the carbon fixation reactions
How many CO2 molecules are fixed in 6 turns of the Calvin Cycle?
6
How many ATP and NADPH are used in 6 turns of the Calvin Cycle?
12 ATP + 12 NADPH
2 of each per turn
How many GAPs (aka G3Ps) are produced by 6 turns of the Calvin Cycle?
12 (2 per turn)
Why do the numbers of the Calvin Cycle matter?
not all the G3Ps produced are net Calvin Cycle products
What happens to most of the G3Ps (GAPs) produced by the Calvin Cycle?
they are used up in regenerating the starting product, RuBP
Of the 12 GAPs (G3Ps), how many will be NET products of the Calvin Cycle?
2 of these 12 will be net products of the Calvin Cycle
What are the 2 net G3P products produced by the Calvin Cycle used for?
to make sugars
What are the 2 possibilities for the 2 net G3Ps produced by the Calvin Cycle?
they can be converted into starch granules and be stored in the stroma
OR
they can be exported to the cytosol to make sucrose
What happens to the remaining 10 GAPs that were not used to make sucrose or starch?
they will regenerate 6 molecules of RuBP again
How many molecules of RuBP will the remaining 10 GAPs make? What else is required for this process?
6 molecules of RuBP
also requires 6 ATP total or 1 ATP per turn
What is the full name of rubisco?
ribulose bisphosphate carboxylase/oxidase
Why is the carboxylase/oxidase ending of the rubisco name important?
it means that rubisco can use CO2 or O2 as a substrate and if rubisco is using O2 as a substrate, the sugars produced will not be the same as if it were CO2
What is the desired product of RuBP + CO2?
2x 3-PGA
What is the undesired product from RuBP + O2? How is it produced?
2-phosphoglycolate
produced when rubisco uses O2 as a substrate instead of CO2
Why can rubisco use CO2 or O2 as a substrate?
it first appeared billions of years ago when CO2 was plentiful in the atmosphere and O2 was not
there was no selective pressure to adapt a binding site that distinguished between CO2 and O2
What process occurs when Rubisco uses O2 as a substrate?
photorespiration
Why is it problematic for rubisco to use O2 as a substrate?
it produces an undesirable 2C product which will undergo photorespiration which releases up to half of the unfixed carbon to be released as CO2 in the atmosphere = a huge waste of a plant’s energy
Why is the ability of rubisco to use O2 as a substrate particularly problematic in hot climates?
because plants close their stomata on hot days (prevent water loss) which blocks the influx of CO2 –> O2 produced by photosynthesis can build up in the leaf
What is C3 carbon fixation?
the normal Calvin Cycle process which produces G3P from CO2
What is C4 carbon fixation?
a method of carbon fixation which reduces photorespiration in some plant groups that can fix CO2 into 4-carbon oxaloacetate
