Photosynthesis Flashcards
1
Q
What organisms have chloroplasts?
A
all plants
unicellular plant-like protists
multicellular plant-like protists (like kelp)
some photosynthetic prokaryotes (cyanobacteria)
2
Q
In what 2 ways do chloroplasts arise?
A
by fission from other mature chloroplasts
from their non-photosynthetic precursors, proplastids
3
Q
in higher plants, where are chloroplasts located?
A
in the mesophyll layer of leaves
4
Q
Approximately how many chloroplasts are there per cell in leaves of higher plants?
A
20-40
5
Q
How many membranes do chloroplasts have?
A
2
6
Q
What separates the double membrane envelope of chloroplasts?
A
an intermembrane space
7
Q
Describe the outer envelope membrane of chloroplasts
A
not very selective, it allows many molecules to pass through its large porins
8
Q
Describe the inner envelope membrane of chloroplasts
A
highly impermeable and substances require a specific transporter to enter
9
Q
Aside from the double membrane, what are the two components of chloroplasts?
A
thylakoids
stroma
10
Q
Describe thylakoids
A
flattened sacs separated from the envelope with their own distinct membrane
11
Q
T or F: thylakoids have their own distinct membrane
A
true
12
Q
How are thylakoids arranged?
A
in stacks called grana (pl.) or granum (s.)
13
Q
What do thylakoid membranes contain?
A
all the protein components for photosynthesis
14
Q
What occurs in the thylakoid lumen?
A
it is where H+ is pumped to create a gradient for ATP synthesis
15
Q
T or F: all thylakoids are stacked as grana
A
false! some are singular and not stacked
16
Q
What are thylakoids that are singular and not stacked called?
A
stroma thylakoids
17
Q
What is the purpose of stroma thylakoids?
A
they usually connect stacks of grana thylakoids
18
Q
Describe the stroma
A
the fluid outside the thylakoid in chloroplasts
19
Q
What 5 things does the stroma of chloroplasts contain?
A
a single, small, circular DNA chromosome
ribosomes
enzymes
lipids
starch granules
20
Q
T or F: the stroma of chloroplasts contains DNA
A
true
21
Q
Describe the structure of the DNA found in the stroma of chloroplasts. What is its purpose?
A
it is a single, circular DNA chromosome
it codes for about 65 synthesizing chloroplast proteins
22
Q
What are the ribosomes found in stroma of chloroplasts associated with?
A
the surface of thylakoids
23
Q
What are the enzymes of the stroma of chloroplasts for?
A
the Calvin Cycle
24
Q
What purpose do the lipids and starch granules in the stroma of chloroplasts have?
A
they are for stored energy
25
What is an example of a protein the stroma DNA chromosome codes for?
RUBisCO
26
Where are most of the chloroplast proteins synthesized and translated?
synthesized in the nucleus
translated in the cytoplasm
27
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
28
What kind of signal do chloroplast-bound proteins synthesized and translated outside of the chloroplast have?
an N-terminal stroma signal
29
What are the 2 large translocation complexes in chloroplasts and where are they located?
Toc = outer envelope membrane
Tic = inner envelope membrane
30
Which of the 2 translocation complexes will proteins encounter first on their journey to the chloroplast stroma?
TOC
31
What happens to proteins when they encounter the TOC complex?
they are unfolded by cytosolic chaperone proteins
32
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
33
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
34
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
35
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
36
What happens to proteins synthesized from genes within the chloroplast DNA?
the chloroplast ribosome will directly translocate the protein through the thylakoid membrane
37
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
38
in the light reactions, where do the high-energy electrons move to once removed from water?
through redox reactions in the ETC
39
What happens in the ETC?
energy is used to pump protons and create an electrochemical gradient
40
What is the electrochemical gradient produced by the ETC? What is its purpose?
a proton gradient that is used to drive ATP synthase (photophosphorylation)
41
What is the final electron acceptor at the end of the ETC?
the carrier NADP+
42
What is the BASIC flow of energy through the light reactions?
light energy --> electrical energy --> chemical energy
43
What are the inputs of the light reactions?
H2O
light
NADP+ (oxidized electron carrier)
ADP + Pi
44
What are the outputs of the light reactions?
NADPH (reduced e. carrier)
ATP
O2 (waste)
45
Is NADP+ the reduced or oxidized version?
oxidized
46
Is NADPH the reduced or oxidized version?
reduced
47
What two things are used to drive the Calvin Cycle?
ATP + the final electron carrier NADP+ (NADPH when it has accepted the electron)
48
What is the purpose of the Calvin Cycle?
carbon fixation
49
Describe carbon fixation
the conversion of CO2 into simple sugars (CH2O)
50
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
51
What is the BASIC flow of energy through the dark reactions?
transient chemical energy --> stored chemical energy
52
What are the inputs of the dark reactions?
CO2
NADPH (reduced version)
ATP
53
What are the outputs of the dark reactions?
C3H6O3 (simple sugar, later converted to glucose)
NADP+ (oxidized)
ADP + Pi
54
Where in the chloroplast do the light reactions occur?
in the thylakoid membranes
55
Where does the calvin cycle/dark reactions occur in the chloroplast?
the stroma
56
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
57
What is the reaction for photosynthesis?
light energy + 6 H2O + 6 CO2 --> C6H12O6 + 6 O2
58
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
59
What are photosystems? where are they located?
protein complexes that contain light-absorbing pigments
located in the inner membrane system that surrounds the thylakoids
60
What are the 2 basic parts of photosystems?
light harvesting complexes
reaction centre
61
T or F: only the light harvesting complexes have pigments
false! the LHC and the reaction center have pigments
62
What are pigments?
special chemicals that can absorb certain wavelengths of visible light
63
What light is harnessed to undergo photosynthesis?
only light that is absorbed by pigments
64
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
65
What is the key photosynthetic pigment?
chlorophyll a
66
What are the two antenna pigments?
chlorophyll b
carotenoids
67
Why do plants appear green?
because the pigments are absorbing a specific wavelength of light and reflecting green wavelengths
68
What wavelengths of visible light do pigments absorb? what colours are associated with these?
~450 nm
purple-blue
69
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
70
What are the 3 strategies for an excited electron to return to ground state?
1. release of heat and a photon of a different wavelength (fluorescence)
2. transfer of energy to a neighbouring chlorophyll
3. transfer of the excited electron to a neighbouring electron acceptor
71
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
72
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
73
Which pigments are in the light harvesting complexes?
many, but these include the antenna pigments (chlorophyll b and carotenoids)
74
Which pigments are in the reaction center?
a special chlorophyll a dimer
75
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
76
How does the reaction center transfer energy from the excited electron?
the special chlorophyll a dimer directly converts the light energy into chemical energy
77
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
78
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
79
What are the two types of photosystems in the light reactions?
PSI and PSII
80
Where do the photosystems access light energy?
in their respective light harvesting complexes
81
Which of the two photosystems occurs in the light reactions first?
PSII then PSI
82
How are PSII and PSI distinguished?
by their chlorophyll a dimers
83
What is the chlorophyll a dimer found in the PSII?
P680
84
What is the chlorophyll a dimer found in the PSI?
P700
85
Why is the chlorophyll a dimer of PSII called P680?
the chlorophyll a dimer absorbs light at 680 nm
86
Why is the chlorophyll a dimer of PSI called P700?
the chlorophyll a dimer absorbs light at 700 nm
87
What is the overall direction electrons move?
from H2O --> PSII --> ETC --> PSI --> ETC --> NADP+
88
What happens to NADP+ once it accepts the electrons?
it's reduced to NADPH
89
What do the photosystems use their chlorophyll a molecules for?
to separate charge
90
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
91
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
92
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
93
What is the purpose of the ETC?
to help create a proton gradient to synthesize ATP
94
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)
95
What is photolysis?
the splitting of water
96
How do positively charged chlorophyll a dimers replenish their lost electrons?
by receiving electrons from the splitting of water
97
How many ETC chains are there in the light reactions?
2
98
What is at the end of the first ETC?
photosystem 1
99
What is at the end of the second ETC?
the final electron acceptor, NADP+
100
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)
101
What are some examples of the electron carriers in the light reactions of photosynthesis?
NADP+
Q
FD
PC
102
What is the Z pathway?
the energy diagram between the 2 photosystems which forms a 'Z'
103
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+
104
How does the Z pathway begin?
sunlight excites electrons of antenna pigments in the light harvesting complexes
105
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
106
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+)
107
What happens to the PSII after the electrons are passed to an electron acceptor?
it is oxidized to P680+
108
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
109
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
110
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
111
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+
112
What reduces the oxidized P700+?
incoming electrons from the ETC1 reduce it back to P700
113
What transfers electrons from the PSI (P700) to and through the ETC?
primary electron acceptors
114
What is the last step of the Z pathway?
the electrons reduce NADP+, the final electron acceptor, to NADPH
115
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
116
What type of proteins are most of the polypeptides that make up the PSII reaction center?
integral membrane proteins
117
Why is the special chlorophyll a dimer of the PSII called P680?
because the pigments most effectively absorb light at the 680 nm wavelength
118
What molecule is closely associated with the reaction center of PSII?
pheophytin
119
What is pheophytin?
a molecule similar to chlorophyll but lacks an Mg2+
it is the primary electron acceptor for PSII
120
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-
121
Where are the electrons transferred from pheophytin?
to plastoquinone (PQ)
122
Where does the electron transfer between pheophytin and plastoquinone move the electron toward?
toward the stromal side of the membrane
123
What becomes the oxidizing agent as a result of the electron transfer between pheophytin and plastoquinone?
P680+
124
Describe plastoquinone
(PQ)
a lipid-soluble molecule similar to ubiquinone in the mitochondria
125
What is the function of PQ?
it accepts 2 electrons from pheophytin and 2 H+ (from stroma) to become plastoquinol (PQH2)
126
What is the reduced form of plastoquinone (PQ)? How does it become this?
plastoquinol (PQH2)
when PQ accepts 2 e- and 2 H+
127
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
128
What is the function of PQA?
It is the type of plastoquinone that accepts electrons from Pheo- and passes them on to PQB
129
What is the function of PQB?
it accepts the electrons passed from PQA
130
What is the result of the electron transfer from Pheo- --> PQA --> PQB?
PQBH2
131
What does PQBH2 do?
it dissociates from PSII and moves laterally through the membrane
132
T or F: P680+ is a relatively weak oxidizing agent
FALSE! it is the strongest oxidizing agent ever found in a biological system
133
Why is it important than P680+ is a strong oxidizing agent?
so it can pull electrons from very stable water molecules
134
What is the photolysis equation?
2 H2O --4 photons--> 4 H+ (lumen) + O2 + 4 e-
135
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
136
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
137
What is the oxygen evolving complex?
an enzyme that splits water and is associated with the PSII on its luminal surface
138
What does the oxygen evolving complex contain?
an Mn-Ca cluster that can donate 4 of its electrons
139
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
140
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
141
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
142
What happens to the 4 electrons produced by the OEC splitting water?
they are used to regenerate the Mn-Ca cluster
143
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
144
What happens to the O2 produced when the OEC splits 2 water molecules?
it's released as a waste product
145
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
146
Where does plastoquinol carry electrons to?
the cytochrome b6-f complex
147
What cycle is involved in the transfer of 2 electrons from plastoquinol to the cytochrome b6-f complex?
the Q cycle
148
What is the Q cycle?
the cycle involved in the transfer of 2 electrons from plastoquinol to the cytochrome b6-f complex
149
What happens in the Q cycle?
4H+ are pumped into thylakoid lumen from the stroma which creates the proton gradient
150
What is produced from the Q cycle?
the proton gradient
151
What does the cytochrome b6-f complex transfer the 2 electrons to?
plastocyanin
152
What is plastocyanin?
a small peripheral membrane protein on the thylakoid interior side that accepts electrons from the cytochrome b6-f complex
153
What is plastocyanin analogous to in the mitochondria?
cytochrome c
154
Where does plastocyanin carry electrons to?
the luminal side of PSI where they are transferred to the chlorophyll a dimer P700
155
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
156
where do electrons move from the P700 dimer?
to a primary acceptor called Ao
157
What is Ao?
a chlorophyll molecule that acts as a primary acceptor of the electrons from P700 of PSI
158
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-
159
Is Ao- a strong oxidizing or reducing agent?
strong reducing agent
160
What does Ao- transfer electrons to?
to other molecules in PSI and eventually to ferredoxin
161
How is P700+ reduced back to P700?
by electrons incoming from plastocyanin from the ETC1
162
What is ferredoxin? Where is it located?
a small, water-soluble polypeptide on the stromal side of the thylakoid membrane that accepts electrons
163
What does ferredoxin transfer electrons to?
Ferredoxin-NADP+ Reductase (FNR)
164
What does ferredoxin-NADP+ Reductase pass electrons to?
NADP+
165
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
166
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+
167
What are the 3 sources of the H+ gradient?
photolysis
Q cycle
Reduction of NADP+
168
How is photolysis a source of H+ gradient?
it generates H+ in the thylakoid lumen
169
How is the Q cycle a source of H+ gradient?
it transfers the H+ from the stroma to the thylakoid lumen
170
How is the reduction of NADP+ a source of H+ gradient?
H+ concentration is reduced on the stromal side of the thylakoid
171
What are the major products of the light reactions?
NADPH (reduced NADP+)
ATP
O2 (waste)
172
What is the major process of the dark reactions?
the Calvin Cycle
173
Where does the Calvin Cycle occur?
the stroma
174
What sugar does the Calvin Cycle produce?
glyceraldehyde 3-phosphate (G3P)
175
What does the Calvin Cycle require that is produced in the light reactions per G3P made?
9 ATP
| 6 NADPH
176
Why is the Calvin Cycle referred to as a cycle?
the starting material (RuBP) is regenerated
177
What are the 3 stages of the Calvin Cycle?
1. fixing CO2 into an organic molecule
2. reducing the organic molecule with NADPH and ATP to form a sugar
3. regenerating the CO2 acceptor
178
How many carbons of G3P does each cycle of the Calvin Cycle fix?
ONE
179
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
180
What molecule incorporates CO2 during carbon fixation?
Ribulose bisphosphate (RuBP)
181
How many carbons does RuBP have?
5
182
What does RuBP stand for?
Ribulose bisphosphate
183
What is required for the incorporation of CO2 into RuBP?
the enzyme Rubisco
184
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
185
What does the incorporation of CO2 into RuBP produce?
an unstable 6C intermediate
186
What does the unstable 6C intermediate of the CO2 + RuBP quickly turn into?
it splits into two 3C molecules of 3-phosphoglycerate (PGA)
187
How many carbons are in each of the 2 molecules of 3-PGA?
3
188
What does PGA stand for?
3-phosphoglycerate
189
1 CO2 + 1 RuBP --Rubisco--> ?
2 molecules of 3-phosphoglycerate
190
What happens to the 3-PGA?
it is phosphorylated by ATP and converted into 1,3-BPG
191
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
192
What does BPG stand for?
1,3-bisphosphoglycerate
193
What is produced when 1,3-BPG is reduced by the NADPH carrier?
glyceraldehyde 3-phosphate (G3P)
194
What does G3P stand for?
glyceraldehyde 3-phosphate
195
What reduces BPG to G3P?
the electrons stored in the NADPH carrier
196
What is G3P?
a 3 carbon sugar produced by the carbon fixation reactions
197
How many CO2 molecules are fixed in 6 turns of the Calvin Cycle?
6
198
How many ATP and NADPH are used in 6 turns of the Calvin Cycle?
12 ATP + 12 NADPH
2 of each per turn
199
How many GAPs (aka G3Ps) are produced by 6 turns of the Calvin Cycle?
12 (2 per turn)
200
Why do the numbers of the Calvin Cycle matter?
not all the G3Ps produced are net Calvin Cycle products
201
What happens to most of the G3Ps (GAPs) produced by the Calvin Cycle?
they are used up in regenerating the starting product, RuBP
202
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
203
What are the 2 net G3P products produced by the Calvin Cycle used for?
to make sugars
204
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
205
What happens to the remaining 10 GAPs that were not used to make sucrose or starch?
they will regenerate 6 molecules of RuBP again
206
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
207
What is the full name of rubisco?
ribulose bisphosphate carboxylase/oxidase
208
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
209
What is the desired product of RuBP + CO2?
2x 3-PGA
210
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
211
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
212
What process occurs when Rubisco uses O2 as a substrate?
photorespiration
213
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
214
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
215
What is C3 carbon fixation?
the normal Calvin Cycle process which produces G3P from CO2
216
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
