Photosynthesis Flashcards
(18 cards)
1
Q
2 major processes of photosynthesis
A
- oxidation of water is driven by solar energy during the light reactions
- electrons from water oxidation pass through an ETC to NADPH and create a proton gradient
- proton gradient is used to synthesise ATP
2
Q
Dark phase of photosynthesis
A
- light independent and carbon-fixation reactions
- reduction of CO2 to carbohydrate
- requires reducing power and energy which is provided by NADPH and ATP
3
Q
Light dependent reactions
A
- electrons derived from H2O oxidation drive the formation of a proton gradient
- this is used to synthesise ATP
- these electrons eventually reduce NADP+ to NADPH
- oxygen is a byproduct of water oxidation
4
Q
What is the thylakoid
A
- flattened sacks
- contain chlorophyll and other components required for light dependent phase
5
Q
What are photosystems I (PSI) and II (PSII)?
A
Photosynthetic reaction centres
- pigment protein complexes embedded in the thylakoid membrane
- pigments are chlorophyll and carotenoids
- PSII and PSI operate in series, connected by a non-pigmented cytochrome bf complex
- electrons are transferred from H2O (PSII) ultimately to NADP+ (PSI)
6
Q
Describe PSII
A
- enormous transmembrane assembly = more than 20 subunits
- responds to wavelengths shorter than 680 nm
- several antenna pigments capture light and transfer it among themselves
- until they reach a special pair of chlorophyll molecules in the reaction centre
- the special pair acts as an energy trap
7
Q
Photoreceptors in PSI and PSII
A
Pigments are photoreceptors - absorb light
- electrons are excited to higher energy
- return to ground state and releases energy or heat = fluorescence
- returns to ground state and energy released excites an electron from a neighbouring pigment molecule and passes onto another molecule = resonance energy transfer
- moves to a nearby pigment molecule via electron transfer and pigments have a +/- charge = photoinduced charge separation
8
Q
What happens in PSII?
A
- special pair of chlorophyll a molecules at the active centre absorb a photon of 680 nm
- the pair is often named P680 or P680* when excited
- after excitation, P680* transfers electrons to pheophytin then plastaquinone (Q) at site Qa and site Qb
- second electron reduces mobile Q to plastaquinol (QH2)
- ionised P680* extracts electrons from water bound at a manganese centre and forms oxygen
9
Q
Oxygen revolving centre in PSII
A
- one calcium and 4 manganese ions
- manganese changes its oxidation state
- tyrosine residues mediate electron transfer from the centre to chlorophyll
10
Q
Oxygen evolution centre
A
- oxygen manganese calcium cluster
- oxidised one electron at a time
- water molecules bound to Ca and Mn4 are linked to form a molecule of oxygen
- oxygen is released from the centre
11
Q
Electron transfer to Q
A
- 2 electrons are needed to reduce plastaquinone (PQ) to (PQH2)
- mobile PQH2 diffuses through the membrane carrying 2 electrons
- 4 photons are needed to generate one oxygen so 4 electrons are transferred
- 4 protons from water are released into the thylakoid lumen and 4 protons from the stroma are transferred to PQH2
12
Q
Proton translocation
A
- protons from oxidised water molecules (2 for each) are released into the thylakoid lumen
- PQ is reduced into PQH2
- two protons needed to reduced each PQH2 are captured from the stroma
- PQH2 is oxidised by cytochrome bf
- the 2 protons are released into the lumen
13
Q
Electron transfer to cytochrome bf
A
- Similar to complex III in oxidative phosphorylation
- Receives electrons from PQH2
- 2 additional protons transferred from the stroma via the Q cycle
- Cytochrome bf transfers the electrons to plastocyanin
14
Q
What is plastocyanin?
A
- blue copper protein
- soluble in thylakoid lumen
- reduced at the copper atom one electron at a time
- PQH2 oxidise in 2 steps one electron at a time
15
Q
Role of PSI
A
PC (plastocyanin) carries electrons between cytochrome bf and PSI
16
Q
What happens in PSI?
A
- Special pair of chlorophyll a molecules = P700/P700*
- After excitation, P700* transfers electrons to chlorophyll a0 then phyloquinone then 3 iron sulphur clusters
- Electrons are transferred finally to ferredoxin
- Ionised P700* recovers electron from PC
17
Q
Chloroplast ATP synthase
A
Similar structure to complex V
- CF0 = stalk and forms a proton channel
- CF1 = catalytic subunits
18
Q
3 stages of the Calvin cycle
A
- Fixation of atmospheric CO2 by ribulose-1,4-biphosphate to form 2 molecules of 3-phosphoglycerate
- Reduction of 3-phosphoglycerate to form G3P which can be converted into hexoses
- Regeneration of ribulose-1,5-biphosphate from 2x G3P so that more CO2 can be fixed
