LEC 6: PHOTOSYNTHESIS 2 Flashcards
What photosystem is most prevalent in the stromal side of the thylakoid membrane?
Photosystem 1
What photosystem is most prevalent in the lumen side of the thylakoid membrane?
Photosystem 2
What are the three types of energy transfer possibly used in plants/plan pigments?
- Fluorescence/heat energy transfer
- Resonance energy transfer
- Electron transport via electron transport chains
What is a photosystem?
A network of pigments that absorb light energy via resonance energy transfer. The energy is funneled to a reaction center which contains 2 chlorophyll A pigments
Explain how resonance energy transfer allows for light energy to reach the reaction center of an antenna complex
- energy is absorbed by a pigment on the photosystem
- that pigment is energized with photons
- it passes its energy on to a nearby pigment thereby reducing its energy level and increasing the nearby pigments’
What are the main components of Chlorophyll A pigment?
- 4 nitrogens attached to carbon rings
- Magnesium center
- Hydrophobic tail
Chlorophyll A is reduced during light absorption. Is this true?
No it is not. Chlorophyll A loses electrons so it becomes oxidized.
Differentiate between a Light Harvesting System and Light Harvesting System 2
Many LH1 systems surround a single LH2
LH1s do not have a chlorophyll A reaction center. They just transfer energy to LH2
LH2 has a reaction center, and this is where electrons are lost in Chlorophyll A.
Differentiate between a light harvesting system and a photosystem
Light harvesting systems are the singular units that make up a photosystem. There are several photosystems in a single chlorophyll, and multiple light harvesting systems in a photosystem
What does the 680 and 700 correspond to in P680 and P700?
The nano-wavelengths each reaction center is activated by. In photosystem 2, the reaction center is activated at 680 nano-wavelengths for example.
Briefly list the components of the light dependent reactions in order
- P680
- Cytochrome B6/f complex
- P700
- ATP Synthase
Where is ATP synthesized?
In the stromal side of the membrane
Where is NADPH produced?
In the stromal side of the membrane
Protons are accumulated to create the electrochemical gradient responsible for ATP production on which side of the membrane?
Lumen side
What are the two goals of Photosystem 2?
- Water photolysis (splitting of water) to produce protons and oxygen
- Release 2 electrons from Chlorophyll A pair.
What redox reaction happens to water?
Water is oxidized to oxygen. It loses electrons to form the oxygen.
Where are the electrons from water directed to after oxidation?
2 electrons are sent to Chlorophyll A to replace the electron hole (electrons lost by P680 during energization)
What is the water-splitting complex on the base of P680 called?
oxygen-evolving complex
What are the components and chemical formula for the oxygen-evolving complex?
Manganese, calcium and oxygen form: Mn4O5Ca at the base of P680
What is the first electron acceptor to take the electrons donated from P680?
Pheophytin
What structure does pheophytin have that allows it to accept electrons from P80?
Pheophytin is a modified chlorphyll A molecule. It does not have the Magnesium center - but instead has 2 protons.
What are the two electron acceptors that follow pheophytin?
Plastoquinone A and Plastoquinone B
What besides pick up electrons does PQB (plastoquinone B) do before electrons are passed to Cytochrome B6/f complex?
It picks up hydrogen protons from the stroma to be donated to Cytochrome B6/f
What molecular characteristic of PQB allows it to pick up electrons and protons?
The benzoquinone portion of PQB allows for hydrogens and electrons to be picked up
What molecular characteristic of PQB allows it to be anchored in the thylakoid membrane?
The isoprenyl is hydrophobic and allows for anchorage.
What is the function of Cytochrome B6/f complex?
To pick up electrons and protons from PQB. The protons are ejected into the lumen of the membrane, and the electrons are passed on to plastocyanin electron acceptor.
What are the 2 sources for hydrogen accumulation in the lumen?
- P680 oxidizing water and releasing hydrogen protons in the lumen.
- Cytochrome B6/f complex ejecting protons into the lumen (gathered by PQB)
What is a characteristic of plastocyanin?
It is the only water soluble protein/electron acceptor
What causes the P700 reaction center to be energized?
The electrons that were donated by plastocyanin
What are the main electron acceptors after P700 loses 2 electrons?
phylloquinone, iron-sulfur proteins, ferrodoxin and flavoprotein
What is the final electron acceptor in the series of reactions? What is its function?
Flavoprotein reduces NADP+ to NADPH creating another energy currency
What is the redox pair in the NADPH/P700 electron transfer?
P700 is oxidized
NADP+ is reduced to NADPH
How are plastoquinone and phylloquinone similar?
They both have a benzoquinone and isoprenyl group that allows acceptance of electrons/protons and anchoring in the membrane
What makes Photosystem 2 - ATP synthase a non-cyclic electron flow?
Electrons move in a single direction and are not recycled.
What is ATP production via the non-cyclic electron flow called?
Non-cyclic photophosphorylation
What are the conditions for cyclic electron flow?
High light intensities
What does high light intensity cause in order to trigger cyclic electron flow?
Photosystem 2 is deactivated and only Photosystem 1 is used.
What is the main goal of cyclic photophosphorylation?
To produce MORE ATP
How is ATP synthesis supplemented by cyclic photophosphorylation?
Photosystem 1 transfers its electrons to Cytochrome B6/f complex, which grabs hydrogens from the stroma into the lumen. The electrons are then returned to PS1 and the cycle continues.
What protein transfers electrons from PS1 to Cytochrome B6/f?
Ferrodoxin
What protein transfers electrons from cytochrome b6/f to P700?
plastocyanin
What is the ratio of ATP to NADPH needed for carbon fixation?
3:2 ATP to NADPH
How does proton build up in the lumen contribute to ATP synthesis?
An electrochemical gradient is created when hydrogen concentration is greater in the lumen than in the stroma, therefore the protons move down the gradient. This movement powers the ATP synthase enzyme which phosphorylates ADP into ATP.
What other organelles/organisms do chemiosmotic coupling?
Mitochondria, Chloroplasts and Bacteria
