Photosynthesis Flashcards
(140 cards)
1
Q
What are the two main stages of photosynthesis
A
Light-dependent reactions and carbon reactions (Calvin Cycle)
2
Q
Where do light-dependent reactions occur
A
Thylakoid membranes
3
Q
What are the products of the light-dependent reactions
A
ATP and NADPH
4
Q
What is the role of ATP and NADPH
A
To fuel the Calvin Cycle
5
Q
Where does PSII localise
A
Appressed regions of thylakoid membrane
6
Q
Where does PSI and ATP synthase localise
A
Non-appressed regions of thylakoid membrane
7
Q
What is the structure of LHCII
A
Tetramer
8
Q
What pigments are in LHCII
A
Chlorophyll and carotenoids
9
Q
What does LHCII associate with
A
PSI or PSII
10
Q
What does LHCII kinase do
A
Causes LHCII to dissociate from PSII and bind PSI
11
Q
What does LHCII phosphatase do
A
Causes LHCII to dissociate from PSI and bind PSII
12
Q
What triggers LHCII kinase
A
Build-up of reduced plastoquinol (QH2)
13
Q
How does ATP synthase generate ATP
A
Protons diffuse and rotate c-subunits
14
Q
Where is ATP and NADPH produced
A
Stroma
15
Q
Where are protons produced
A
Thylakoid lumen
16
Q
What is the purpose of ATP and NADPH
A
To drive the Calvin Cycle
17
Q
What are the three stages of the Calvin Cycle
A
- Carboxylation
- Reduction
- Regeneration
18
Q
What enzyme catalyses CO2 fixation
A
RuBISCO
19
Q
What is the product of RuBP + CO2
A
Two molecules of 3-phosphoglycerate (3PGA)
20
Q
Where is RuBISCO large subunit synthesised
A
Chloroplast
21
Q
Where is RuBISCO small subunit synthesised
A
Cytoplasm (imported into chloroplast)
22
Q
What imports the small subunit of RuBISCO
A
TOC and TIC translocons
23
Q
What are the products of the reduction phase
A
G3P and DHAP (triose phosphates)
24
Q
What enzyme interconverts DHAP and G3P
A
Triose phosphate isomerase
25
What happens when plant needs carbs
Sucrose is synthesised in cytoplasm
26
What happens when plant has enough carbs
Starch is synthesised in chloroplast
27
How many triose phosphates are used to make carbs
One
28
What are the other five triose phosphates used for
Regeneration of RuBP
29
Which enzymes are light-regulated
RuBISCO
30
What are the two functions of RuBISCO
Carboxylation (favoured at low temps) and Oxygenation (favoured at high temps) (photorespiration)
31
What does RuBISCO produce when using O2
Phosphoglycolate and 3-phosphoglycerate
32
What are the three photosynthetic pathways
1. C3 photosynthesis
- Rubisco is linked to gas exchange and fixes CO2 (has a 3 carbon product)
2. C4 Photosynthesis
- 4 carbon product that dissociated from gas exchange
3. CAM Photosynthesis
- happens only at night
33
Where is RuBISCO active in C3 plants
Mesophyll cells
34
Where is RuBISCO active in C4 plants
Bundle sheath cells
35
What fixes CO2 in C4 mesophyll cells
PEP carboxylase
36
What is the 4-carbon compound formed in C4 plants
Malate
37
Where is malate transported in C4 plants
Bundle sheath chloroplasts
38
What happens to malate in bundle sheath cells
CO2 is released for RuBISCO
39
What is the waste product of malate decarboxylation
Pyruvate
40
Where does pyruvate go
Back to mesophyll to regenerate PEP
41
What is the main aim of CAM photosynthesis
Reduce water loss by closing stomata during day, instead opening them at night
42
When do CAM plants fix CO2
At night
43
What enzyme fixes CO2 in CAM plants
PEP carboxylase
44
What is CO2 stored as in CAM plants
Oxaloacetate/malic acid
45
When is CO2 released in CAM plants
During the day
46
What are the two main products of the light-dependent reactions?
ATP and NADPH
47
Where do the light-dependent reactions occur?
In the thylakoid membranes of the chloroplast
48
What is the stroma?
The fluid-filled space inside the inner membrane but outside the thylakoids
49
What are thylakoids?
Flattened membrane sacs where light-dependent reactions take place
50
What are grana?
Stacks of thylakoids
51
What is the thylakoid lumen?
The space inside a thylakoid where protons accumulate during electron transport
52
What are the main photosynthetic pigments?
Chlorophyll a, chlorophyll b, carotenoids, phycobilins
53
What pigment is found in all oxygenic photosynthesizers?
Chlorophyll a
54
What wavelengths do chlorophyll a and b absorb best?
Chlorophyll a: ~430 nm & ~660 nm; Chlorophyll b: ~450 nm & ~640 nm
55
What are carotenoids?
Accessory pigments that absorb light in the blue-green region and protect chlorophyll
56
What is the role of pigments in photosynthesis?
To absorb light energy and funnel it to reaction centers
57
What is resonance energy transfer?
Non-radiative transfer of energy between pigments, funneling it to the reaction center
58
What is a photosystem?
A protein-pigment complex that absorbs light and starts electron transport
59
What is the reaction center of Photosystem II?
P680
60
What is the reaction center of Photosystem I?
P700
61
Where does Photosystem II get its electrons?
From water (H₂O)
62
What is the Manganese Cluster (Oxygen-Evolving Complex)?
A group of atoms in PSII that splits water to replace electrons lost by P680
63
What is the equation for water splitting?
2H₂O → 4H⁺ + 4e⁻ + O₂
64
What happens to the oxygen produced in water splitting?
It is released as a byproduct
65
What is the fate of electrons excited in PSII?
They are transferred to pheophytin, then plastoquinone
66
What is plastoquinone (PQ)?
A lipid-soluble electron carrier in the thylakoid membrane
67
What is plastoquinol (PQH₂)?
Reduced form of plastoquinone that carries electrons and protons
68
What is the cytochrome b₆f complex?
A protein complex that accepts electrons and pumps protons into the lumen
69
Why is cytochrome b₆f important?
It increases the proton gradient across the thylakoid membrane
70
What is plastocyanin (PC)?
A small copper-containing protein that transfers electrons to PSI
71
What happens at Photosystem I (P700)?
It re-excites electrons using light energy
72
Where do electrons go after PSI?
To ferredoxin and then to NADP⁺ via NADP⁺ reductase
73
What is the final electron acceptor in the light-dependent reactions?
NADP⁺
74
What is the role of ferredoxin?
Transfers electrons from PSI to NADP⁺ reductase
75
Where do protons accumulate during the light reactions?
In the thylakoid lumen
76
What is chemiosmosis?
The use of a proton gradient to drive ATP production
77
What drives protons into the thylakoid lumen?
Electron flow through PSII and the cytochrome b₆f complex
78
What are the two major products of the light reactions used in the Calvin Cycle?
ATP and NADPH
79
What is the Z-scheme?
A diagram showing the energy changes of electrons as they move through photosystems
80
Why is the Z-scheme important?
It illustrates the energy input and direction of electron flow in light reactions
81
What is the electron lost in P680 replaced by?
Electrons from water, via the Manganese Cluster in PSII
82
What complex accepts electrons and pumps protons into the stroma in the light dependent reactions of photosynthesis?
The Cytochrome b₆f complex
83
What transfers electrons to photosystem I?
Plastocyanin
84
What are the final products of the light dependent reactions and what do they do?
ATP (chemical energy)
NADPH (reducing power)
O₂ (waste product from water)
These products go to the Calvin Cycle in the stroma to fix CO₂ into glucose
85
Where do the carbon reactions (Calvin Cycle) occur?
In the stroma of the chloroplast
86
What are the 3 stages of the Calvin Cycle?
Carboxylation (fixation), Reduction, Regeneration
87
What enzyme fixes CO₂ in the Calvin Cycle?
RuBisCO
88
What are the substrates for RuBisCO?
Ribulose-1,5-bisphosphate (RuBP) and CO₂
89
What is produced in the carboxylation step?
3-phosphoglycerate (3-PGA)
90
How many CO₂ molecules enter per turn of the Calvin Cycle?
Three
91
What happens during the reduction stage?
3-PGA is phosphorylated by ATP and reduced by NADPH to form G3P
92
How many G3P are produced per 3 CO₂ fixed?
Six
93
How many G3P exit the cycle to form sugars?
One
94
What happens to the remaining G3P?
Five are used to regenerate RuBP
95
How many ATP and NADPH are used per 3 CO₂ fixed?
9 ATP and 6 NADPH
96
What is G3P?
Glyceraldehyde-3-phosphate, a 3-carbon sugar used to make glucose, starch, or sucrose
97
What is triose phosphate isomerase?
An enzyme that interconverts G3P and DHAP
98
What is the role of the triose phosphate antiporter?
Exchanges G3P or DHAP for Pi across the chloroplast membrane
99
Where is sucrose synthesized?
In the cytosol, from exported triose phosphate
100
Where is starch synthesized?
In the chloroplast, from retained triose phosphate
101
What happens when sucrose need is low?
Pi concentration drops, triose phosphate stays in chloroplast, starch is made
102
What is photorespiration?
A wasteful process where RuBisCO binds O₂ instead of CO₂
103
What is the product of RuBisCO’s oxygenation reaction?
Phosphoglycolate (toxic, must be recycled)
104
When does photorespiration increase?
At high temperatures and low CO₂
105
What is C4 photosynthesis?
A process where CO₂ is fixed into malate in mesophyll cells and released in bundle sheath cells
106
Why is C4 photosynthesis beneficial?
It concentrates CO₂ around RuBisCO, reducing photorespiration
107
What enzyme is used in C4 and CAM plants for initial CO₂ fixation?
PEP carboxylase
108
What is CAM photosynthesis?
A process where CO₂ is fixed at night into acids and released for the Calvin Cycle during the day
109
What is the key difference between C3, C4, and CAM plants?
C3: CO₂ fixed directly by RuBisCO; C4: spatial separation; CAM: temporal separation
110
Why are the carbon reactions not truly 'dark' reactions?
They are regulated by light, including RuBisCO activity
111
Where are the small subunits of RuBisCO made?
In the cytoplasm (from nuclear genes)
112
Where are the large subunits of RuBisCO made?
In the chloroplast (from the plastid genome)
113
What is the function of RuBisCO?
It catalyzes carbon fixation by attaching CO₂ to RuBP
114
How is RuBisCO activity regulated?
Indirectly by light signals and redox status
115
What is the result of the reduction phase of the Calvin cycle?
Glyceraldehyde-3-phosphate
116
What is the result of the carbon fixation phase of the Calvin cycle?
3-Phosphoglycerate
117
What molecule splits water in photosynthesis?
The manganese cluster
118
Give the order of complexes/proteins in the light dependent reactions
PSII
Pheophytin
Plastoquinone A
Plastoquinone B
Lipid soluble plastoquinone
Cytochrome B6F
Plastocyanin
PSI
119
In the Calvin cycle, how much ATP do you need to make a whole glucose molecule?
18
120
In the Calvin cycle, how much NADPH do you need to make a whole glucose molecule?
12
121
What 2 enzymes are used in the reduction stage of the Calvin cycle?
PGA kinase
G3P dehydrogenase
122
How many carbons are in 3 molecules of RuBP?
15 - 5 each
123
What is the point of C4 photosynthesis?
It prevents photorespiration by separating where CO2 is fixed from where RuBisCO works
124
What kind of cell does C3 photosynthesis usually happen in/
Mesophyll
125
What are the 2 cells containing active chloroplasts in C4 photosynthesis plants?
Mesophyll and bundle sheath
126
What is CAM photosynthesis used for?
Dry environments
127
Describe step 1 of C4 photosynthesis?
CO2 combines with PEP by way of PEP carboxylase to form oxaloacetate, which then creates malate, a stable 4 carbon molecule which can carry carbon across to the bundle sheath cells. This happens in the mesophyll cells
128
What does decarboxylation of malate in C4 photosynthesis create?
CO2 for the calvin cycle
Pyruvate which is sent back to mesophyll cells and converted into PEP to start the cycle again
129
What are the different ATP costs for C3 and C4 photosynthesis? Not including Calvin cycle
C3: ~3 ATP per CO₂ fixed
C4: ~5 ATP per CO₂ (extra cost from regenerating PEP)
130
Which is temporal and which is spatial separation out of C4 and CAM?
C4 is spatial
CAM is temporal
131
Describe what happens at night in CAM plants
Stomata open at night when it’s cooler and more humid.
CO₂ enters and is fixed by PEP carboxylase
Malate is converted to malic acid and stored in vacuoles inside the cell
132
What form is CO2 stored in in CAM plants?
Malic acid - PEP + CO2 is malate, then this is converted into malic acid and stored in vacuoles
133
Describe what happens during the day in CAM plants
Stomata are closed to conserve water.
Malic acid is transported out of vacuoles and decarboxylated:
The released CO₂ enters the Calvin Cycle (same as in C3) in the chloroplast.
Pyruvate is used later to regenerate PEP
134
Why do plants need the glyoxylate cycle?
To convert stored lipids into sugars
To recycle toxic compounds from photorespiration
135
What are the 2 functions of RuBisCO?
Carboxylation
Oxygenation
136
What is synthesised when there is insufficient carbohydrate content for metabolic needs?
Sucrose
137
What is synthesised when there is sufficient carbohydrate content for metabolic needs?
Starch
138
What is the triose phosphate antiporter and what does it do?
The triose phosphate antiporter is a transport protein found in the inner membrane of the chloroplast.
One triose phosphate (G3P or DHAP) out of the chloroplast and one inorganic phosphate (Pi) in
139
Describe the activity of the triose phosphate antiporter in a surplus of sucrose
Sucrose need DECREASES
Less conversion to SUCROSE
[Pi] DROPS
No exchange
TRIOSE PHOSPHATE stays inside chloroplast
STARCH produced instead
140
Where does starch production happen compared to sucrose? highlights the need for the antiporter
Starch in the stroma of the chloroplast, sucrose in the cytosol - this is why the antiporter is active in the absence of sucrose as it needs to bring G3P out into the cytosol by swapping it with Pi
