Photosynthesis Flashcards
Photosynthesis is broken up into two stage, what are they?
Light and dark reactions
What are they key features of the chloroplast
- Chloroplast envelope
- Granum
- Thylakiod
- Stroma
- Nucleiod + Ribosomes
The light dependent reactions happens …
At the Thylakoids (stacks of grana)
What is the name of the principel photoreceptor
Chlorophyll
(highly conjugated molecule)
The absorption of photons by chlorophyll cause …
Transition from HOMO to LUMO
This energy is then immediately transmitted back OR photoionisation can occur
Plant photosynthesis is a …… process
non-cyclic
How many photosystems are used in photosynthesis
2
There are two reaction centres consisting of ‘special pairs’ of cholophyll molecules:
They are….
- Photosystem II - oxidises H₂O (absorbs lights ~680nm)
- Photosystem I - reduces NADP⁺ (absorbs light ~700nm)
Electrons move from photosystem II to photosystem I by
The electron transport chain
Operate in electrical series using redox reactions
When the electrons move from photosystem II to photosystem I, what is the name of the complex they travel through
Cytochrome b₆f
When electrons travel through this, it generates a proton gradient
What is the function of photosytem II within the electron transport chain
- Catalyses the transfer of e⁻ from H₂O to plastoquinone
- Photoxidation at PSII will lead to the reduction of plastoquinone (Q) to plastoquinol (QH₂)
- This will then gain 2 protons forming QH₂
The PSII reaction centre once oxidised using plastoquinone, how is is reduced again
- Utilises the oxygen evolving complex (containing Mn) to oxidise water (- 4e⁻)
- Forms oxygen
Why when reducing plastiquinone to form 2QH₂ does it create a proton gradient
The protons to form it will be taken from the stoma
What reaction occurs at Cytochrome b₆f complex
Catalyses the transfer of electrons from plastoquinol (QH₂) to plastocyanin (Pc)
The plastoquinone formed is then reduced again in an addition cycle - further increases proton gradient
What happens at photosystem I
- A photon of light with excite the electrons within the chlorophyl - photoxidation to A0
- Electrons move onto Ferredoxin
- Used to for 2NADPH catalysed by Fd-NADP⁺ reductase
- Also increases proton gradient by uptake of protons from stoma
The result of the light dependent reactions is a transmembrane proton gradient
Where is the [H⁺] concentration greatest?
Greater [H⁺] in the thylakoid lumen
The proton gradient is capitalised upon by
The chloroplast ATP synthase
Couples dissipation of proton gradient to enzymatic synthesis of ATP
Per absorbed photon, how much ATP is produced from non-cyclic photosynthesis
Produces 1 ATP per absorbed photon
At saturating light intensity: there is a proton gradient of ~3.5pH
Why?
- Evolution of oxygen through lysis of H₂O will produce 4 protons
- Transport of e⁻ through Cytochrome b₆f will translocate 8 protons into Lumen
- NADP⁺ being reduced in the stroma
The dark reaction of photosynthesis can be considered to have 2 stages
Production and Recovery
Which two enzyme are part of the control for the dark reactions
- Fructose bisphosphatase
- Sedoheptulose bisphosphatase
What is the name of the enzyme which fixes CO₂
Ribulose Bisphosphate Carboxylase (RuBP carboxylase)
(slow enzyme so is about 50% of leaf proteins)
Rubsico is impacted by pH
Why?
- Works best in alkaine pHs (protons are being removed from the stroma in the light dependent reaction)
- (Mg²⁺ effux will balance membrane charge and stimulate RuBP)
What compound will bind to RuBP carboxylase in the dark, to inhibit its activity
CA1P
(2-carboxyarabionitol-1-phosphate)
How do FBPase & SBPase respond to NADPH
As the light reactions starts, NADPH will be generated
Hence FBPase and & SBPase will be stimulated by NADPH
Thioredoxin contains a reversible reducible disulphide group
How does this affect the regulation of the dark reaction
Reduced thioredoxin activates FBPase and SBPase
What is the primary output of photosynthesis
GAP (glyceraldehyde-3-phosphate)
Used in multiple pathways inside and outside the chloroplast with being a precursor to higher order carbohydrates
How is glucose stored within the cell or the plant
α-amylose as a starch component
Driven by PPi hydrolysis
What can GAP go on to form to aid in the structure and function of the plant cells
Sucrose - primary energy molecule
Cellulose - structural molecule
