Lecture 12. Photosynthesis Flashcards
Where does all free energy needed by biological systems come from?
Solar energy trapped by photosynthesis
What is the function of the stomata and how do they function?
The stomata can open and close to allow exchange of air
What reactions are involved in energy capture?
Light dependent reactions
What reactions are involved in energy expenditure?
Light independent reactions
What are the aims of the light dependent reactions?
The capture of photons
The conversion of light energy to ATP and NADPH
What are the aims of the light independent reactions?
Fixing or capturing of CO₂
Making bigger molecules that can be moved round the plant or be stored
What is the energy of a photon dependent on?
Its wavelength
What is chlorophyll?
A Mg²⁺ porphyrin that contains phytol, highly hydrophobic
When illuminated, what do isolated chloroplasts do?
Evolve O₂
Reduce an electron acceptor (e.g ferricyanide)
Why was the Hill reaction crucial?
It showed that the primary event in photosynthesis is the light-driven energy requiring transfer of an electron from one substance to another
What do photons do to electrons?
Cause electrons to go from ground state to excited state
The excited state can be transferred until captured in reaction centre
Where are the reaction centres in plants?
Chlorophylls - where chemical reactions happen
What are the names of the two reaction centres?
Photosystem I and II
What is photosystem II also known as?
P680, water-plastoquinone oxidoreductase
What makes the enzyme in PSII special?
Water-plastoquinone is the only natural enzyme that splits water
How many electrons does water need to donate to make one molecule of oxygen?
4
How many protons are released into the lumen in PSII?
4
Where do the electrons from PSII go next?
Plastoquinone
Where do the electrons from plastoquinone get transferred to?
Cytochrome bf complex
How many protons are pumped into the lumen by the cytochrome bf complex?
2
Where do the electrons from cytochrome bf complex go next?
Plastocyanine
What is photosystem I also known as?
P700, plastocyanin ferredoxin oxidoreductase
Where do electrons traveling into PSI come from?
Plastocyanine
Where do the electrons from PSI go next?
Ferredoxin
What is the final electron acceptor?
NADP⁺
What happens in the Calvin cycle?
CO₂ is fixated into sugars
CO₂ + 4H⁺ + 4e⁻ → CH₂O + H₂O
What is the first product of the Calvin cycle?
3-phosphoglycerate
What catalyses carbon fixation?
RuBisCo
Steps in the Calvin cycle
- Carbon fixation
- Reduction of 3PG to Glyceraldehyde-3-phosphate (G-3-P)
- Regeneration of starting molecule: RuBP (Reverse of PPP)
Calvin cycle balanced equation
6CO₂ + 18ATP + 12NADPH + 12H₂O → C₆H₁₂O₆ + 18ADP + 18Pi + 12NADP⁺ + 6H⁺
Why does RuBisCo suck big dodongo dick?
Slow catalytic activity: about 3 reactions per second
Isn’t good at distinguishing between O₂ and CO₂
Reaction with oxygen wasteful - costs ATP to lose CO₂
When does more photorespiration occur?
At hotter temperatures
What are two ways that some plants have evolved to concentrate CO₂ at the site of the Calvin cycle?
Mesophyll cells capture CO₂
Bundle cells cary out Calvin cycle
What is the mobile energy storage molecule in plants?
Sucrose
What is the main energy storage molecule in plants?
Starch
Where is starch found?
In stroma in chloroplasts
In seeds and tubers like potatoes
What is starch a mixture of?
Amylose and amylopectin
