LECTURE 11 Flashcards
In photosynthesis, e- from the H atoms in H2O are
Transferred to CO2, reducing it to carbohydrate
When light hits, 3 things can occur
1) light is reduced
2) light is transmitted (goes through)
3) light is absorbed (disappears)
Pigments
Substances that absorb light
Chlorophyll a
The main photosynthetic pigment
Accessory pigments
1) . Chlorophyll b
2) . Carotenoids
Chlorophyll b
Broadens spectrum used for photosynthesis
Carotenoids
Absorbs excessive light that may damage chlorophyll
dissipates E in the form of heat
What happens when a pigment molecule absorbs a photon ?
Photon absorbed by pigment, e- raised form ground state.
There are two possibilities from here:
1) returns to ground stage and gives off heat
2) returns to ground state and gives off
- photon < E (longer wavelength)
- heat
How does absorbing a photon of a specific wavelength giving off a slightly larger wavelength allow the plant to use the photons as E?
In a plant chloroplast, the chlorophyll a molecules are assembled with other components of the thylakoid membrane.
- this allows the E of the photons to be absorbed for a useful purpose.
What are all the parts of a light reactions
1) light reaction
2) pigment
3) structures involved in photosynthetic electron transfer
Structures involves in photosynthetic electron transfer
Located in the thylakoid. Many small molecules and large enzyme complexes are involved :
- PS ll
- water splitting enzyme
- cytochrome b6f complex
- PS l
- SADP reductase
PS l and PS ll
The photosystems consist of a reaction center complex surrounded by light harvesting complexes
Reaction center
E form absorbing of photons in all other pigment molecules in the photo system is transferred to a pair of chlorophyll a molecules in a reaction center
What do the special pair of chlorophyll a molecules in the rxn Center do?
They absorb a specific wavelength of light
The light harvesting complexes
Transmit E via resonance E transfer
Reasonance E transfer
Involves the transfer of E. Required the molecule to be very close to each other
When the special pair of chlorophyll a molecules receive E, they
Donate an e- to the primary e- acceptor (pheophytin), the special pair is now photooxidized because oxidation was caused by light.
What is the primary e- accceptor
Pheophytin
Where does the reaction on PH ll happen
In the stoma
First three steps to PS ll and l
1) photon strikes a pigment modules in PS ll
2) e- is transferred to primary e- acceptor (pheophytin). Loss of e- leaves holes that must be filled.
3) H2) is plot and its e- are transferred one at a time to P680+ (reducing it to P680). The 2 P+ are released into the thylakoid space.
P680 +
Very strong oxidant agent
How many e- required to split H2O
2
Where are protons stored
Thylakoid
What causes “e- holes”
The ejection of e- (using E form light) leaves holes in reaction center chlorophyll a molecules
What are steps 4 and 5 of the light PS ll and l
4) photo excited e- mov3e to PS l via e- transport chain
5) e- “fall” to a lower E level (exogeronic)
Proton pumping driven by e- moving through cytochrome b6f complex (4H+ enter per H2O lysed)
What is the primary e- acceptor for PS l
It’s unknown
What do light harvesting PS l pigments cause P700 to do
They lose e-, to becomes P700+
What does the proton gradient in steps 4 and 5 form?
The thylakoid space
Summary of linear e- flow down e- transport chain
Primary e- acceptor (pheophytin)
Plastoquinone (Pq)
Cytochrome b6f complex
Plastocyanin (Pc)
- (Potential E decreases with each transfer)
- at PS l, the e- replace those lost form PS l as it harvest light E.
Steps 6,7,8 of PS ll and l
6) light harvesting PS 1 pigments independently cause P700 to lose e- (becoming photooxidized P700+)
7) P700+ accepted e- that arrive via the e- transport chain of PS ll
8) NADP+ reductase catalyzes the transfer of e- form freedom in (fd) to NADP+, 2e- and 1H+ (from stroma) are required to make NADPH.
NADPH caries on to the Calvin cycle.
Non-cyclic photorylation
Formation of ATP via the process of linear electron flow pathway.
(H2O one one end, NADP on the other)
What is the source of electrons in the linear electron flow patheway ?
H2O since its continuously replacing those lost from chlorophyll a reaction center molecules
Within the thylakoid space,
High conc. Of P+ (H+) boules up (high potential E)
Chemiosmotic coupling
Explains how ATP can be made from a P+ conc. Gradient across a membrane
How discovered chemiosmotic coupling
Peter Mitchell
The linear e- flow pathway creates
ATP and NADP simultaneously (required light and the lysis of H2O)
The carbon fixation rxn requires
1.5 X more ATP then NADPH
How can the cell produce ATP with out marketing excess NADPH?
Plants use PS l plus some of the e- transfer system to make only ATP.
- plants sense levels of NADPH, depending on the balance of ATP : NADPH, that can negate in Schlick electron flow pathway.
Cyclic electron flow
- no production of NADPH
- no release of oxygen
- but it can make ATP form P+
(2H+ transferred from stroma into thylakoid space per e-)
Plants have a defective cyclic pathway that can only grow in
Low light levels; might protect plants form high intensity light
What PS system does cyclic electron flow use?
PS l, NOT PS ll
Summary of cyclic e- flow
1) e- end up back to full “holes” in the PS l rxn Center
2) the transfer of e- is from ferredacin to plastoqiunone, instead of the NADP+
3) P+ pumping of cytb6f complex from stroma to thylakoid space creates a H+ gradient.
4) in this manner, ATP is produced via: cyclic photophosphorylation
