Photosynthesis Flashcards
What is photosynthesis?
A reverse of oxidative carbohydrate metabolism
- light energy drives carbon reduction
- Carbohydrate produced serve the producing organism & consumers
2H20 =(light)= O2 + 4H+
Electrons then reduce CO2
4H+ + CO2 = CH20 + 4H2O
What are the features of the chloroplast?
- Thylakoids (Thylakoid space and membranes)
- Chloroplast envelope (outer and inner membrane plus intermembrane space)
- Stroma
- Granal thylakoids
- Nucleoids
- Ribosome
- Plastoglobules
- Starch granules
What are the sizes of the chloroplast?how many per algal plant cells?
features of the membranes?
- 5 micrometres approx.
- 1-1000 per cell
- ‘impermeable’ inner membrane
- permeable outer membrane
What is chlorophyll? Features
The principal photoreceptor
- act as light-harvesting antennas
- highly conjugated bonding system (networks of alternating single & double bonds) so is an excellent receptor
What happens at the Light Harvesting Complex? (LHC)
Light energy is absorbed and travels along the antenna pigments to the reaction centre. This then receives donor electrons and emits acceptor electrons
How is light transformed to chemical energy?
1) Electromagnetic radiation is propagated as ‘photons’
- Electrons can dissipate its excitation energy by ‘Resonance Energy Transfer’
2) By photooxidation
What are the 2 reaction centres? Where are they located?
- Photosystem II (Oxidises h20, special pair: P680)
- Photosystem I (Reduces NADP+, special pair: P700)
The electrons from PSII to PSI by an ‘electron transport chain’
-Operate in electrical series coupling oxidation with reduction
What light does PSI respond to?
lightwaves <700nm which creates a biosynthetic reducing power in the form of NADPH
-Electrons from 2NADPH come from 2h2o
What is the cytochrome b6f complex?
- Electrons travel from PSII to PSI
- Homologous to Complex III (oxidative phosphorylation)
- Generates a H+ gradient
What light does PSII respond to?
lightwaves <680nm
- generates a molecule of O2
- transfer from h2o to plastoquinone
- Generates a H+ gradient
What are the features of Photosystem II?
A very large protein assembly (20subunits)
- Catalyses the transfer of e- from h2o to plastoquinone
- reassembles ubiquinone
- cycles between oxidised (Q) to reduced (QH2)
-Electrons in QH2 are at a higher redox potential than in H20 (light drives the reaction thermodynamically uphill)
What does P680 do?
Extracts electrons from h2o at: -Oxygen Evolving Complex
- Each time photon kicks electron from P680, e- is extracted from Mn centre
- 4 photons absorbed to ‘extract’ 4e-
- 4e- used to reduce 2Q to 2QH2
How is a proton gradient obtained across the membrane?
- Quinone reduction on stroma side of PSII
- Site of water oxidation on the lumen side
Creates a proton gradient from lumen side to stroma side
What is the ctyochrome bf complex?
- Catalyses the transfer from plastoquinol (QH2) to plastocyanin (Pc)
- Helps the proton gradient due to enzyme positioning as cytochrome bf reduces platoquinol to plastocyanin (takes up 2H+ and reoxidises on the stromal side)
What happens at PSI?
Electrons from P700 travel down through chlorophyll to FeS cluster then transferred to Ferredoxin
What happens at Ferredoxin?
-Transfer electrons to NADP+ which is catalysed by ‘ferredoxin-NADP+ reductase’ on the stromal side (enhances proton gradient)
How does phosphorylation of ATP come from this?
The result of photosynthesis is a transmembrane proton gradient where there is greater protons in the
How does phosphorylation of ATP come from this?
The result of photosynthesis is a transmembrane proton gradient where there is greater protons in the thylakoid lumen
-This is capitalised upon by the chloroplast ATP synthase which couples dissipation of proton gradient to enzymatic synthesis of ATP
How many ATP are produced?
- Non-cyclic transport: 1ATP per photon
- NADPH indirectly produces 2.5ATP
- 1ATP for every 3 protons through Chloroplast ATP Synthase
How do plants receive carbohydrates in the dark?
Calvin Cycle
Stage 1: 3 moluecules of RuBP react with 3CO2 which ultimately creates 6 molecules of Glyceraldehydes-3-phosphate (GAP) and uses 9ATP and 6NADPH
-The GAP can be used for biosynthesis for carbohydrates
Stage 2: Carbon atoms of remaining 5 GAP are rearranged into 3 RuBP
What is the enzyme used to fix CO2 in Calvin Cycle?
Ribulose Bisphosphate Carboxylase (RuBP Carboxylase)
What veriables effect the rate of Calvin Cycle?
1) pH level (stroma increase to pH 8.0 in light near RuBP optimum)
2) Mg2+ influx to balance the charge caused by H+ stimulates RuBP
How is RuBP production affected in the dark?
CAP1 is produced in the dark, inhibiting RuBP Carboxylase
What is the ferredoxin-thioredoxin reductase system?
Responds to the soluble ferredoxin in stroma
-Thioredoxin system deactivates phosphofructokinase (PFK) to glycolysis
What is the overall stoichiometry of Calvin Cycle?
3CO2 + 9ATP + 6NADP -> GAP + 9ADP + 8Pi + 6NADP+
How is GAP used to make larger carbohydrates?
- The primary product of photosynthesis
- Can be converted into fructose-6-phosphate then in G1P which is the precursor to higher order carbohydrates
What are the features of alhpa-amylose?
- starch component
- synthesised in the chloroplast stroma
- storage of glucose units
- starch synthase transfers the glucose residue to alpha-amylose molecule which is driven by PPi hydrolysis
What other components other than starch can be produced?
Cellulose and Sucrose
