Seminar 11: Photosynthesis - Harvesting energy from light Flashcards
inputs & outputs of photosynthesis
inputs:
- H2O (soil, root uptake)
- CO2 (atmosphere, stomata)
- energy (sunlight)
output:
- O2 (released thru stomata)
- glucose (some used for energy immediately - cellular resp, convert > cellulose, sucrose)
what are the redox reactions in photosynthesis?
Oxygen in H2O reduced to O2
Carbon in CO2 oxidised to C6H12O6
where does photosynthesis occur in the diff photosynthetic organisms
plants & green algae - chloroplasts
cyanobacteria - thylakoid
what are light reactions
phase of photosynthesis that requires light
what does light reactions produce
ATP, NADPH, O2
where does the light reactions occur? what is here that allows it to occur?
- thylakoid membrane
- chlorophyll pigment (a & b are main ones)
what is photosystem
the process by which plants, algae, & some bacteria use the energy of sunlight to drive the synthesis of organic molecules from CO2 & H2O.
desc briefly how ATP is produced in the light reaction
- photon of light strikes a pigment molecule in a light harvesting energy complex
- e- boosted to higher energy lvl (excited)
- causes energy to be passed (e- carry energy)
- when passed to P680 PAIR: e- > primary e- acceptor (redox reaction) & P680 oxidised to P680+
- enzyme catalyses splitting of H2O > 2e- (1/2 O2) & 2H+
- e- prod: transferred to primary e- acceptor (to replace lost e-)
- H+ prod: released > thylakoid membrane (creates proton gradient)
- 1/2 O2 combine to form O2
- e- from primary e- acceptor > photosystem 1 via ETC
- H+ move thru ATP synthase, ATP prod
differentiate b/w C3, C4 & CAM plants
C3 uses RuBP & Rubisco to fixate CO2 whilst C4 & CAM use PEP & PEP carboxylase
(because PEP carboxylase won’t oxygenate)
C4 separates CO2 fixation & Calvin cycle SPATIALLY (mesophyll - fixation, bundle sheath - Calvin)
CAM separates TEMPORALLY (CO2 fixation at night, less heat so less H2O loss & cycle in DAY)
what is photorespiration
when Rubisco fixates O2 instead of CO2
wastes energy (ATP, NADPH) because toxic product must be removed by cell
what is the 1st prod of CO2 fixation in the 3 plant types?
C3: 3PH
C4 & CAM: Oxaloacetate
what are light independent reactions?
2nd stage of photosynthesis, CO2 > glucose using ATP & NADPH from light dependent stage
Inputs & Outputs of Calvin cycle
INPUTS: 6 CO2, 18 ATP (12 - step2: phosphorylation in reduction, 6 - step 3: regeneration in conversion of RuMP to RuBP), 12 NADPH
OUTPUTS: 1 glucose (from 2 G3P), 18 ADP, 12 NADP+
where does light indp reactions occur & what happens?
- in stroma
- CO2 reduced to glucose
desc the 5 steps of Calvin cycle
- CARBON FIXATION:
CO2 fixed to intermediary 6 C molecule (very unstable & immediately breaks down to 2 x 3PG) - REDUCTION:
3PG reduced to G3P (req oxidation of 12 ATP > 12 ADP + 12 Pi & 12 NADPH > 12 NADP+ & 12 H+) - GLUCOSE PRODUCTION:
1/6 of G3P formed is used to make GLUCOSE (exits cycle)
- WHY 6CO2 req for every 1 GLUCOSE molecule (cause only 1/6 is used for glucose prod) - RuMP FORMATION:
other 5/6 of G3P rearranged to RuMP - RuMP > RuBP
req. hydrolysis of 6ATP > 6 ADP + 6 Pi
g3P + phosphate REFORMS RuBP to fixate more CO2 so cycle can continue
What is the advantage of PEP Carboxylase?
- Doesn’t fix O2, no oxygenate activity
- Will fix CO2 to PEP even when CO2 lvls are LOW
What characteristic of Bundle Sheath cells allow for the Calvin cycle to occur efficiently?
They have modified chloroplasts which allow CO2 to be highly concentrated around RubisCo
How are photosynthesis & respiration closely linked to the Calvin Cycle?
- some G3P prod from Calvin cycle will enter glycolysis & be converted to pyruvate (in cytosol) , pyruvate can be used in cellular resp for energy or anabolic reactions to prod lipids, proteins, other carbs
- some G3P can enter gluconeogensis
