Biochemistry - Photosynthesis Flashcards
1
Q
Overall photosynthesis input
A
6 CO2 + 6 H2O + Light
2
Q
Overall photosynthesis output
A
Glucose + 6 O2
3
Q
Overall light reaction input
A
2 H2O + 2 NADP + Light
4
Q
Overall light reaction output
A
2 H+ + O2 + 2 NADPH
5
Q
Structure of a Chloroplast
A
- Stacks of thylakoids forming the Granum
- Thylakoid membrane is impermeable to most ions and molecules except Mg2+ and Cl- (therefore cannot have an electrical gradient like mitochondria have - only chemical/PROTON)
- Contains Chromophores which can absorb viable light (Chlorophyll a & b)
- Stroma is liquid surrounding this
- Inter-membrane space
- Proton gradient generated across most membranes
6
Q
Light Reactions
A
- Are carried out by molecules in the thylakoid membranes
- Convert light energy to the chemical energy of ATP and NADPH
- Split H2O and release O2 to the atmosphere
7
Q
Chlorophyll
A
- A and B
- B had an aldehyde group
- Fluoresce
- Perforin ring with Mg which makes them reflect green
- 540nm is green light
- Neither a or b have very much absorption at this wavelength because they are reflecting the green light
- Lots of absorbance at the other visible light wavelengths
8
Q
Antennae
A
Use a primary chlorophyll molecule surrounded by antennae chlorophyll molecules so that they are able to acquire e- at 100 photons/s instead of 1/s without the antennae
- Via Quantum tunnelling
9
Q
Photosystem 2 (P680) (Light Harvesting Complex)
A
- A photon hits a chlorophyll antenna molecule
- Energy jumps between chlorophyll molecules until it reaches a special Chlorophyll a molecule in the core and the chlorophyll will release an e-
- e- jumps into the reaction centre where it is accepted by the primary acceptor
(Change in energy state drives photosynthesis) - e- is then accepted by Pq and is reduced to Plastoquinol
- e- is then accepted to Cytochrome Complex which transfers 2 protons from the stroma into the thylakoid space, and 2 protons are moved from Plastoquinol as it is oxidised back to Plastoquinone
- e- is passed to Plastocyanin
- e- is passed to PS1
10
Q
Pq
A
Plastoquinone
- Similar to ubiquinone
11
Q
Pc
A
Plastocyanin
- Similar to Cytochrome C
12
Q
Cytochrome Complex
A
- Very similar to Complex 3 in mitochondria
- Has a Q cycle
- Transfers 4 protons into the thylakoid space in total
- Creates membrane potential which is used to make ATP as in the mitochondria
13
Q
Photosytem 1 (P700) (Light Harvesting Complex)
A
- Light hits an antenna chlorophyll and the energy moves through the molecules until it reaches the special chlorophyll a
- e- from PS2 now is energised and e- is released from Chlorophyll a and jumps up the the primary acceptor
- e- is passed down a chain to NADP+ Reductase
- NADP+ Reductase reduces NADP+ and 2H+ to NADPH and 1H+
14
Q
Hills Z Scheme
A
- Movement between PS 2 to Cytochrome Complex to PS 1
15
Q
Missing e- in PS 2
A
There is an “electron hole” where it has got excited and moved down the ETC
- H2O binds to a Mn cluster in PS 2 which rips it apart
- O2 is released into the stroma and moves out of the cell
- 2H+ are moved into the thylakoid space adding to the gradient
- 2e- are now available for more use through PS 2
16
Q
Missing e- in PS 1
A
There is an “electron hole” where it has got excited and moved down the ETC
- Plastocyanin comes from the Cytochrome Complex with its e- and donates it to PS 1
17
Q
Proton Gradient
A
Is developed by
- Splitting of water and H+ is moved into thylakoid space
- Cytochrome complex transfers 4H+ into the thylakoid space
- While NADP + H formation removes H from the stroma this is used by the Calvin cycle and returned as NADP so does not count
18
Q
Chloroplast ATPase
A
- Very similar but there are 14 c-ring subunits
- More protons are being put into the c-ring at once however makes less ATP
19
Q
Differences between Chloroplast and mitochondria
A
- Chloroplasts are more reliant on pH (H+ gradient)
- Chloroplast inner membranes are permeable to Cl- and Mg2+
- So mitochondria use net charge potential and H+ gradient
- Larger pH difference required in chloroplasts as only H+ gradient is used.
- In chloroplasts PSII adds H+ to thylakoid space through splitting H2O, mitochondria consume O2
- Mitochondria have actual H+ pumps: Complex I, and IV pump
- In mitos e-s reduce Oxygen
- In chloroplasts e-s reduce NADP
20
Q
Cyclic Electron Flow
A
- PS 2 is in thylakoid membrane in grana
- PS 1 is in thylakoid membrane in stroma (connecting grana) along with ATPase STROMA LAMELLAE
- In PS 1 e- move from Chlorophyll A to the Primary Acceptor, down to Ferredoxin (Fd), and then rather than move to NADP+ Reductase, the e- moves to the Cytochrome complex, then to Pc and then back to Chlorophyll A
- This will transfer 1 H+
21
Q
Linear Electron Flow
A
Production of ATP using the flow of e- from PS 2 to PS 1
22
Q
Why have cyclic electron flow
A
- This produces ATP only (no NADPH)
- Some bacteria only have PS 1
- Mutant plants without cyclic flow still grow, but not in bright light?
- Some plants can operate with less water C4
23
Q
Calvin Cycle Input
A
- Confined to the Stroma, partitioning stops intermediates reacting or being metabolised in mitochondria
- 3 CO2
- 9 ATP
- 6 NADPH
- 5 H2O
24
Q
Calvin Cycle output
A
- 6 G3P sugar (only 1 removed from cycle)
- Glucose requires two of these to form via gluconeogenesis
25
Q
Phase 1: Carbon Fixation
A
- 3 CO2 are combined with 3 Ribulose biphosphate (RuBP) by rubisco to form 3 of a very reactive 6C intermediate
- Intermediate breaks down into 6 3C 3-Phosphoglycerate (3PGA)
- 3-Phosphoglycerate is phosphorylated (6 ATP used) to form 6 1,3-Biphosphoglycerate
26
Q
Phase 2: Reduction
A
- 6 NADPH are oxidised to NADP+ and 6 of 1,3-Biphosphoglycerate is reduced and loses phosphate group to form 6 of the 3C Glyceraldehyde-3-Phosphate (G3P)
- 1 of G3P is released from the cycle and goes toward forming glucose
27
Q
Phase 3: Regeneration
A
- 5 remaining of G3P are phosphorylated by 3 ATP and form 3 molecules of RuBP
28
Q
Rubisco
A
- Ribulose Bisphosphate Carboxylase
- The most abundant protein on earth
- Binds CO2 and RuBP to form the short lived 6C intermediate
29
Q
How is the Calvin Cycle Regulated by light?
A
- The more alkaline the Stroma is the faster the Calvin Cycle works, the stromal pH rises when the light reactions are working as it is pumping protons into the thylakoid space
- An enzyme which is active in light and reduces RuBisCo, also needs NADPH
30
Q
Photorespiration
A
- RuBisCo binds O2 (low affinity) and makes useless products and this wastes water
- SO when O2 levels are high and CO2 levels are low wasteful work occurs and there is no carbon fixation
- Many plant shut down photosynthesis at midday
- Doing this because O2 can be dangerous?
31
Q
C4 Plants
A
- Live in hot climates and can concentrate CO2
- Maize, sugar cane, grasses
- Partitioning of specific cycles among leaf cells
- Draw in CO2 and use 4-carbon oxaloacetate (has 4 Cs hence C4) as a “CO2” shuttle
- Oxaloacetate turns into Malate which moves into cell where the Calvin cycle occurs and splits into CO2 and pyruvate
- (Pyruvate is converted to be able to bind CO2 again which uses an ATP)
- Acts like a chemical pump
- Decreases RuBisCo competition with O2
32
Q
Why are C4 Plants Useful
A
- If C3 plants split water at a high rate, a lot of O2 accumulates. O2 accumulates and results in photorespiration.
- This is wasteful use of H2O and carbon.
- The C4 pathway means more CO2 is concentrated so less O2 is present for photorespiration through RUBISCO binding O2
- But C4 plants uses more ATP!
- This extra ATP comes from cyclic electron flow and sunlight is cheap, especially in the tropics.
- Ultimately C4 plants can use water more efficiently.
