5. Photosynthesis- Light-dependent reactions Flashcards
What is the overall process in higher plants?
- Chlorophylls absorb light energy for oxidation of water, releasing oxygen and generating NADPH and ATP (thylakoid reactions).
- NADPH and ATP are used to reduce carbon dioxide to form sugars (carbon fixation reactions
Light reactions result in:
1) The creation of reducing power for the production of NADPH
2) The generation of a transmembrane proton gradient for the formation of ATP
3) The production of O2
How does light behave and how does it affect chlorophylls?
- Light - behaves as both particle and wave, delivering energy as photons - some of which are absorbed and used by plants.
- Chlorophyll energised by light may fluoresce, transfer energy to another molecule, or use its energy to drive chemical reactions.
- All photosynthetic organisms contain a mixture of pigments with distinct structures and light-absorbing properties.
What are pigments?
Pigments: Molecules that absorb wavelengths in the visible spectrum
The specific wavelengths absorbed by a particular molecule are characteristic of that type of molecule
What do antenna complex do?
Light drives the reduction of NADP+ and the formation of ATP. Oxygen-evolving organisms have 2 photosystems (PSI and PSII)
Contained in photosystems, they collect light energy and transfer it to reaction centre complexes
Components contain a variety of compounds:
Plants: Chlorophylls a, b and carotenoids
Where are antenna complex found? What else is found there?
Thylakoid membranes: Contains reaction centres, light harvesting antenna complexes and most of the electron carrier proteins.
PSI and PSII are spatially separated in thylakoids
Excitation energy passes from pigments that absorb short wavelengths to those that absorb longer wavelength, and ends up in the reaction centre pigment
What are some similarities of the production of ATP between photosynthesis and oxidative phosphorylation?
1) Occurs across a membrane
2) Involves an electron transport system
3) IT generates a proton gradient across the membrane which is used to drive ATP synthesis
4) Many of the components in the systems are similar, a few are the same
5) Both processes occur in cellular organelles in eukaryotes
Where does photosynthesis get the energy from to generate redox potentials?
Photosynthesis: Use energy of light photons
Oxidative phosphorylation: Uses chemical reactions
What does cyclic mode produce?
What does linear mode produce?
This is what it says on the powerpoint:
Cyclic mode results in ATP production only
Linear mode results in ATP production, and the production of reduced cofactor (NADH or NADPH)
Linear mode photosynthesis must oxidise something to balance out the production of reduced cofactor
Two photosystems are required in the noncyclic electron transport
What is a reaction centre?
What does the reaction centre convert light energy?
Reaction centre: The site at which the charge separation takes place within each photosystem
Converts light energy into chemical energy
The excited chlorophyll a molecule is a reducing agent (electron donor) (Chl*)
A is an acceptor molecule, oxidising agent
Chl* + A –> Chl+ + A-
What happens during stage 1 of photosynthesis?
1) Light is absorbed by ‘special pair’ chlorophyll a molecules in Photosystem II which responds to wavelengths shorter than 680nm
2) The P680 which absorbs the light, raises electrons in chlorophyll a to a higher energy state which ejects out an electron
3) Electron is immediately accepted by an electron acceptor, Quinone Q. This leads to a positive charge on the chlorophyll a + negatively charged acceptor (Q-)
4) The Q- is a reducing agent which ultimately transfers electrons to NADP+ (stage 2)
5) Chlorophyll a+ is now a strong oxidising agent which attracts an electron from an electron donor to regenerate the original chlorophyll a
6) 4 chlorophyll a+ is used to remove 4 electrons from 2H2O molecules:
2H20 + 4 chlorophyll a+ –> 4H+ + O2 +4 chlorophyll a
STAGE 1: Absorption of light, generation of a high-energy electrons and the formation of O2
What happens during stage 2 of photosynthesis?
1) Electrons are transported from the reduced quinone using carriers in the thylakoid membrane
2) The electrons then reach the ultimate electron acceptor= NADP+
3) This forms NADPH
This takes place in photosystem I
4) The transport of electron in the thylakoid membrane is coupled to the movement of protons from the stroma to thylakoid lumen= FORMS PH GRADIENT across the membrane as photosystem II release protons into the thylakoid lumen and takes them up from the stroma
STAGE 2: Electron transport leading to reduction of NADP+ and generation of a proton-motive force
What is the overall reaction of stages 1 and 2?
2H2O + 2NADP+ —-> 2H+ + 2NADPH + O2
What is stage 3 of photosynthesis?
1) Protons move down their concentration gradient from the thylakoid lumen to the stroma through ATP synthase
Protons are transported into the thylakoid lumen when electrons pass through the cytochrome bf complex
2) Couples proton movement to the synthesis of ATP from ADP and Pi
STAGE 3: Proton gradient drives ATP synthesis
What are the steps in stage 1 in more detail?
1) In chloroplasts, light energy is absorbed by light-harvesting complexes and is transferred to chlorophyll a molecules in reaction centres
2) Absorption of photon by PSII, causing an electron to move from P680 chlorophyll a to an acceptor plastoquinone (Qb) on the stromal surface
3) Result= P680+ (strongly oxidised) able to oxidise H2O to remove an electron from it. Forms intermediate in O2 formation and a proton= contributes to proton-motive force
4) P680 absorbs a second proton and semiquinone Q’- accepts a second electron and binds with 2 protons from the stromal space.
RESULT= QH2
2Q + H2O –> O2 + QH2
5) QH2 diffuses in the membrane, QH2 binds to Cytochrome bf and transfers electrons to it. Q cycle operates= Net protons pumping
6) Cytochrome bf then transfers electrons to plastocyanin which diffuses into thylakoid lumen= Carrying electron to Photosystem I
What happens after photosystem I absorbs a photon?
1) Removal of an electron from the reaction-centre chlorophyll a (P700)= oxidised P700+
2) P700+ is then reduced by an electron passed from plastocyanin
3) Electron is then moved within PSI to the storm surface of the thylakoid membrane, accepted by ferredoxin protein
4) Linear electron flow: Electrons released from P700 by excitation of P700 are transported to NADP+ to generate NADPH+
