Photosynthesis Flashcards
What are some photosynthetic organisms?
Photosynthetic organisms - bacteria, algae and plants
Cyanobacteria can bloom on a lake
These organisms make a considerable contribution to CO2 fixation
Photosynthesis may be oxygenic or non-oxygenic, depending on the electron donor (mainly in bacteria)
Where does photosynthesis take place in plants?
In the chloroplast - it has a double membrane
Stroma - carbon fixation takes place here
Thylakoid - where the light driven transport chain occurs
What are some features of the cholorplast?
Inner/outer membrane - forming an intermembrane space Thylakoid membrane - forms stacks = grana (10-100) Stroma Stroma lamellae (linking grana)
What is the principal of photosynthesis?
Photons fall on the protein complexes, the energy is used to split water to produce oxygen
The electrons that come from splitting water, are passed down the redox potential gradient
This energy is used to move protons form the stroma to the cytosol so they can be used to form ATP
Here we are reducing NADP+ to NADPH
What is necessary for absorption of photons?
Energy of light is harvested by pigment in the chloroplast e.g. Chlorophyll
Chlorophyll a gives a plant it’s green colour, and this is the most abundant one
Similar structure to haem (cyclic tetrapyrrole) with Mg in the middle
It can absorb light and delocalise electrons around the structure and transfers the energy across to adjacent chlorophyll molecules = Mg not redox
There are other pigments e.g. carotenoids
What is the absorption spectrum of pigments?
Due to different side chains, it leads to different absorption properties
Chlorophyll a and b absorb well in the blue and red ends of the spectrum but not in the green = appear green
Having a range of pigments allows more of the absorption spectrum to be utilised
What is the principal of light energy, within photosynthesis?
Light energy causes excitation of electrons within the conjugated double bond system of chlorophyll
This excitation energy (not electrons) can be transferred to adjacent chlorophyll molecules and eventually it reaches the reaction centre, which is part of a photosystem
Therefore light energy can be harvested from a large surface area (antenna chlorophylls or light-harvesting complex (LHC))
What are all the ways in whihc an electronically excited molecule can dissipate its energy?
Internal conversion - converted to kinetic energy and then heat for motion
Fluorescence
Excitation transfer - transfers the energy to unexcited molecules with similar electronic properties
Photooxidation -
What components does photosynthesis use in plants/cyanobacteria?
A non-cyclical process using light driven oxidation of H2O to produce NADPH
This involved 2 reaction centers (RCs), one in photosystem II and the other in photosystem I
Describe the first stage of photosythesis (LDR)?
Photosystem II
Within the oxygen evolving complex 2 molecules of water are bound
- The energy arrives, after jumping from chlorophyll to chlorophyll, at a ‘special’ pair of chloroplasts called P680
○ Named as 680 nm is the peak wavelength to excite the electrons the most - The energy of excitation causes an electron to hop off and go onto a nearby molecule - pheophytin (similar to chlorophyll but doesn’t contain Mg in the middle)
- The electron then jumps to plastoquinone (Qa)
○ Similar to ubiquinone in mitochondria e.g. Lipophilic and can diffuse within the membrane - The electron is passed to an exchangeable plastoquinone (Qb), and here it picks up to protons from the stroma to reduce the quinone
The electrons from water reduce the reaction centre back again
Describe the structure of photosystem II?
14 subunits of 19 sit in the thylakoid membrane, sticking out on both sides into the thylakoid lumen and the stroma
Core proteins - D1 and D2 (part of the reaction center)
On the thylakoid lumen side it has accessor proteins which turn the oxygen evolving complex, with the aid of the manganese centre
How does photolysis of water reduce the RC?
The oxygen evolving complex (OEC) breaks down water into 1/2 O2 2H+ and 2e-
OEC cycles between 5 different states S0 through S4 (very high reduction potentials) and O2 is released between S4-S0
The electrons produced reduce the reaction center and the H+ ions contributer to the transmembrane proton gradient
What is the second stage of photosynthesis (LDR)?
Cytochrome bf complex - it transfers electrons and pumps protons
- The reduced plastoquinone (QH2) is reoxidised by the cytochrome bf complex - to send the Q back to photosystem II to acquire more electrons
- The electrons being donated by plastoquinone are donated to a small molecule called plastocyanin (Pc) - a copper containing protein, the goes from oxidised to a reduced state
- There is enough energy from the electrons travelling through the cytochrome bf complex to allow 4H+ to moved into the thylakoid membrane space
Due to a high positive redox potential at Pc - more energy is needed to continue the journey
What is the third stage of photosynthesis (LDR)?
- The electrons arrives from Pc at a ‘special’ pair of chloroplasts called P700
○ Named as 700 nm is the peak wavelength to excite the electrons the most - Photons are absorbed by the P700 chloroplasts to provide energy to excite the electrons
- The excited electrons move to chlorophyll (A0) and then onto Quinone (A1)
- The electrons then undergo some redox reactions through a series of Iron-Sulphur clusters (4Fe-4S), before being transferred to the protein Ferredoxin
- Ferredoxin can then reduce NADP+ to NADPH
The oxidised reaction centre is reduced by plastocyanin
Describe photosystem I?
Core proteins with chlorophyll molecules associated with it
Light energy is funnelled into the reaction centre with it’s own ‘special’ pair of chlorophylls
The electrons have travelled from water, through photosystem II and cytochrome bf complex and now is donated from plastocyanin to photosystem I
