Ch. 5 Photosynthesis Flashcards
Photosynthesis equation
6CO2 + 6H2O –> C6H12O6 + 6O2
Relationship between respiration and photosynthesis
reverse/ interconnected processes, eqn.’s are same but opposite
energy stored in chemical bonds by photosynthesis is extracted from those bonds by respiration to make ATP which is then used to do cellular work through metabolic processes
Pigment
found in chloroplasts
light-absorbing molecules that absorb energy from a narrow range of wavelengths to begin photosynthesis
main is green chlorophyll a w/ other accessory pigments like red, orange and yellow carotenoids –> maximize absorption
“Excited electron”
light is absorbed by pigment and that energy is transferred to e- which become unstable and almost immediately re-emit the absorbed energy
Photosynthesis processes
- Noncyclic/ cyclic photophosphorylation (use H20 and energy from sun to make ATP, NADPH and O2
- Calvin cycle (use CO2, and energy from ATP and NADPH to make glucose)
Noncyclic photophosphorylation (steps) (light dependent rxn)
- Electrons trapped by P680 is PSII are energized by light
- Two energized e- passed to primary e- acceptor
- E- pass through ETC in thylakoid membrane of chloroplast
- As e- move down ETC they lose energy, used to phosphorylate ~1.5 ATP
- ETC ends in PSI w/ P700 where e- again energized by sunlight and passed to primary e- acceptor (dif. from first)
- The two e- again pass through ETC and at end, combine w/ NADP+ and H+ to form NADPH (coenzyme, e- acceptor and energy rich molecule)
- H20 is split to replenish the 2 e- now in NADPH
Splitting of water in photosynthsis
2 e- that go to NADPH, one H+ provides the H in NADPH and 1/2 O2 contributes to O2 gas released
Cyclic photophosphorylation
electrons energized in PSI are “recycled”
energized e- from PSI join w/ protein carriers and generate ATP as they go down ETC, do not go to NADPH but back to PSI over again
occurs simultaneously w/ noncyclic to generate more ATP
Calvin cycle (steps) (dark rxns)
- Carbon fixation: 6CO2 combine w/ 6RuBP (catalyzed by rubisco) to produce 12 PGA
- Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P; ADP, Pi and NADP+ released to go back to light rxn
- Regeneration: 6 ATP are used to convert 10 G3P to 6RuBP
- Carbohydrate synthesis: 12 G3P created but 10 used to regenerate RuBP, 2 remaining used to build glucose
* still occur in presence of light bc dependent upon ATP and NADPH from light rxns
Where does photosynthesis take place?
in chloroplast
Chloroplast structure
- Outer membrane
- Intermembrane space
- Inner membrane
- Stroma (Calvin cycle)
- Thylakoids in grana (protein complexes like PSI and pSII, cytochromes, and other e- carriers in light rxns)
- Thylakoid lumen (H+ ions accumulate here)
Chemiosomsis in chloroplasts (steps)
- H+ ions (protons) accumulate inside thylakoids by way of cytochrome in ETC
- pH and electrical gradient across thylakoid membrane created: H+ in thylakoid reduce pH to 5 while in stroma pH increases to 8; bc H+ are (+) charged, electric gradient also created
- ATP synthase generates ATP: channel protein ATP synthase allows H+ to flow to the stroma, generating energy to phosphorylate ADP to ATP
- Calvin cycle produces G3P using NADPH, CO2 and ATP
Photorespiration
rubisco fixes oxygen instead of carbon, usually in dry and hot environments when stomata close and internal CO2 levels are low
problems: CO2 fixing efficiency goes down bc plant is also fixing O2; products of O2 fixation not productive and energy rich –> instead just broken down by peroxisomes
Evolution of rubisco
early atmosphere in which primitive plants originated had little oxygen, so evolution of rubisco was not influences by the O2 fixing handicap
Chemosynthesis
some chemoautotrophic prokaryotes can use inorganic substances (chemicals) as a source of energy to generate organic molecules
use: H2S, NH3, NO2-
Ex. symbiotic bacteria fond in giant tube words in deep ocean near deep vents.. use H2S to produce carbs
