8.3 Photosynthesis (HL) Flashcards
Chloroplast: How many membranes?
- Inner and outer membrane
Chloroplast: What is the stroma?
Similar to the cytoplasm, it surrounds the thylakoids
Chloroplast: What is the stroma involved in?
- Involved in light independent reactions of the cell like Calvin’s cycle, carbon fixation
Chloroplast: What do thylakoid membranes and granum look like in microscope?
Shows as darker or thicker green clumps
Chloroplast: Function of thylakoid and grana
Offer large surface area where light-dependent reactions occur like photolysis. Photosystems, ATP synthase are embedded
Chloroplast: Function of thylakoid intermembrane space
Small volume allows fast generation of H+ ion gradient for chemiosmosis
Chloroplast: What does stroma contain?
Rubisco, enzymes, substrates, NADPH, molecules for Calvin cycle
Chloroplast: 70S ribosomes
Synthesize some of the proteins and enzymes needed within the chloroplast
Chloroplast: Naked DNA
Codes for some for the chloroplast proteins
Purpose and mitochondrial equivalent: Chloroplast envelope
- Membranes which compartmentalize organelles in cytoplasm
- Outer mitochondrial membrane
Purpose and mitochondrial equivalent: Thylakoid membrane
- Carries out ETC, has ATP synthase, makes use of chemiosmosis
- Inner mitochondrial membrane
Purpose and mitochondrial equivalent: Grana
- Maximize surface area for reactions
- Cristae
Purpose and mitochondrial equivalent: Low volume intermembrane space
- Rapid generation of H+ conc. gradient
- Low volume intermembrane space
Purpose and mitochondrial equivalent: Stroma
- Fluid medium for diffusion of molecules and enzymes for reactions
- Matrix (But instead for Krebs cycle)
Light-dependent reactions: Steps
- Photoactivation
- Photolysis
- ETC
- Chemiosmosis
- ATP synthesis
- NADP reduction
Photoactivation and photolysis: What is a photosystem?
A collection of chlorophyll molecules and other accessory pigments that combine with a protein
Photoactivation and photolysis: Which photosystem is activated first?
PSII
Photoactivation and photolysis: Difference between PSI and PSII
PSI is sensitive to wavelengths of 700nm while PS2 to wavelengths of 680nm
Photoactivation and photolysis: Function of PSII
Absorbs the light energy, splits water and gathers electrons from photolysis
Photoactivation and photolysis: What is a reaction center?
A collection of proteins and pigments wherein pigments transfer all the energy from light photons to a central chlorophyll a molecule
Photoactivation and photolysis: Summary of process in PSII
- 2 excited electrons are passed to primary electron acceptor which passes it to plastoquinone and then PSI
- PSII will repeat so 4 electrons are lost in one cycle
- Water is split as reaction becomes powerful oxidizing agent
Photoactivation and photolysis: Why can the electrons be used in light-dependent reactions again?
- They constantly replace electrons lost by PSII
- Electrons lost from the reaction center are replaced by electrons derived from water
ETC: Summary of process/photophosphorylation
- Excited electrons from PSII are transferred to thylakoid membrane
- Electrons lose energy passing through chain so H+ ions accumulate in thylakoid
- This creates EC gradient
- Protons return to stroma via ATP synthase in transmembrane
- ATP synthase catalyzes synthesis of ATP
- De-energized electrons are taken to PSI
What receives the electrons from PSI?
Ferredoxin
What does ferredoxin do?
It reduces NADP+ which is carried to light-independent reactions
What does the NADP molecule accept?
2 electrons from PSI and 2 H+ ions from stroma
What happens in cyclic phosphorylation?
- Involves use of one PS and no reduction of NADP+ or splitting of water. - - Excited electron enters ETC and comes back to same PS to restore electron supply
What happens in non-cyclic phosphorylation?
- Involves 2 PS and reduction NADP+
- Light is absorbed by PSII and excited electron enters ETC
- Photoactivation results in release of electrons for NADP+ and photolysis of water replaces those lost
Why is cyclic used?
Can be used to produce a steady supply of ATP in presence of sunlight for cell energy demands
Why is cyclic not ideal?
ATP is highly reactive and so it cannot be readily stored in the cell
Why is non-cyclic the preferred phosphorylation?
- Both NADPH and ATP are required to produce organic molecules via light independent reactions
- Only non-cyclic allows for synthesis of organic molecules and long-term energy storage
Is oxygen evolved in cyclic and non-cyclic?
Oxygen is only evolved in non-cyclic
Light independent: What are the 3 steps?
- Carbon fixation
- Carboxylation of ribulose biphosphate (RuBP)
- Triose phosphate production (Form organic molecules or reform stocks of RuBP)
Light independent: Where does it take place?
In the stroma of the chloroplast
Light independent: Can they occur without absence of light?
Only for a short while as they stop when stock of NADPH and ATP runs out
Light independent: 1st step- What is carbon fixation? Which enzyme is used?
Atmospheric CO2 is fixed by adding RuBP. It is catalyzed by rubisco. RuBP is carboxylated to form a hexose biphosphate compound
- Results in breakdown into molecules of PGA
Light independent: What is RuBP?
5 carbon molecule
Light independent: 2nd step- Reduction of 3-PGA (What is 3PGA)
- 3-PGA consists of 3 C atoms and it stands for glycerate 3-phosphate
- It is reduced to triose phosphate which also has 3 C atoms by ATP and NADPH
Light independent: Release of triose phosphate (3rd step)
Only 1 molecule is released and it is used for carb synthesis
Light independent: Regeneration of RuBP (4th step)
- 6 triose phosphates are produced per cycle but since it’s 3C, one glucose monomer is made through 2 cycles.
- Remaining 5 TP molecules are recombined to regenerate RuBP
- Regeneration requires energy from hydrolysis of ATP
Why are light-independent reactions costly?
- One molecule of triose phosphate requires 3 turns of the Calvin cycle which uses 3 ATP and 2 NADPH per cycle
> 3C + 3C from RuBP = hexose + 2C
> 2C + 4C from RuBP= Hexose + 1C
> 1C + 5C from RuBP = hexose
Calvin’s lollipop experiment: Which element was used and why?
Carbon-14, a radioactive isotope because he argued that if CO2 was starting material, radioactive products would appear from the organism (like Chlorella)
Calvin’s lollipop experiment: What apparatus was used and why?
Used the lollipop apparatus to control when the carbon was added to the green algal liquid
Calvin’s lollipop experiment: Summary of process
- Drained samples at certain intervals and let it drop into boiling ethanol, stopping the reaction
- Samples were used in chromatography
- Chromatograms were exposed to X-ray films to make radiograms that capture position of products during photosynthesis
- Analyzed products to deduce pathway of Calvin cycle
Calvin’s lollipop experiment: What did he find?
- Large amount of 3-PGA was found after 5 seconds
- After 30s, PGA, PEPA, triose phosphate were showing high percentage of radioactivity
