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photosynthesis
biol 1210
| Question | Answer |
|---|---|
| classify organisms that make their own energy by capturing sun's energy | photoautotrophs - plants, algae, cyanobacteria, some protists & prokaryotes |
| describe structure of a chloroplast (draw it) | site of conversion of light -> chemical energy in eukaryotes. Organelle w double membrane incl. inner & outer membrane & intermembrane space, stroma filled with thylakoids stacked in grana, thylakoid is folded w thylakoid membrane & thylakoid space |
| site of photosynthesis in plants + stomata | chloroplasts in the mesophyll of leaves where chlorophyll absorbs light energy during photosynthesis. Stomata are the microscopic pores in the leaf where CO2 enters and O2 exits |
| equation of photosynthesis. is this similar to cellular resspiration? | 6CO2 + 12H2O + light energy -> C6H12O6 + 6O2 + 6H2O. This is the opposite of cellular respiration! |
| what type of reaction is photosynthesis? is it anabolic or catabolic, exergonic or endergonic? | redox reaction. H2O is oxidized to form O2 (electrons are pulled further from O) and CO2 is reduced to form glucose (electrons are pulled closer to C). Anabolic & endergonic |
| the 2 parts of photosynthesis, where they occur, and the reactants & products of each | light reactions (photo part) in the thylakoids, reactant H2O, O2 product, generate ATP & increase potential energy of e- and the Calvin cycle (synthesis part) in the stroma, reactant CO2, product glucose, generates glucose |
| what type of electromagnetic radiation is important for life & wavelength range? | visible light in the narrow 380 - 750 nm range |
| pigments | substances that absorb visible light. Dif pigments absorb dif wavelengths & wavelengths unabsorbed are reflected or transmitted |
| why are leaves green? | chlorophyll in the chloroplasts transmits (reflects) green light, hence leaves absorb very little green light |
| absorption spectrum of chlorophyll a & action spectrum of photosynthesis | absorption spectrum of chlorophyll a peaks around violet-blue and red light ranges, with the action spectrum overlapping quite nicely, proving that photosynthetic reactions rely on light absorption |
| describe the first evidence for the action spectrum by Theodor Engelmann | filament of green algae was illuminated thru a prism & aerobic bacteria used as indicator of high O2 concentrations. Result was that the aerobic bacteria clustered near blue & red regions of scattered light thru prism |
| describe name & functions of the main photosynthetic pigment v accessory pigments | main: chlorophyll a, which initiates light reactions by transferring electrons to primary electron acceptor. Accessory: chlorophyll b, carotenoids, xanophylls, which widen action spectrum of photosynthesis and/or provide photoprotection from UV radiation |
| what happens when a pigment absorbs light? | when a photon strikes a pigment, its electrons go from a ground state to an excited, unstable state (jumping to a higher energy shell) and different things can happen next |
| describe 4 possible fates of an excited electron in a plant cell | 1. it drops back to ground state & is emitted via fluorescence or 2. it is given off as heat, 3. it excites electrons in nearby pigments, transferring energy (resonance), 4. it jumps from pigment to another molecule - electron acceptor (redox) |
| describe structure of a photosystem | a complex w hundreds of chlorophyll a molecules & accessory pigments embedded in the thylakoid membrane. Made of a reaction-centre complex surrounded by light-harvesting complexes |
| describe light-harvesting complex & reaction-centre complex | light-harvesting: pigment molecules bound to proteins. Reaction-centre: association of proteins that hold a "special pair" of chlorophyll a molecules and a primary electron acceptor |
| describe the onset of a light reaction | photon strikes pigment and resonance is triggered -> energy passed inward from pigment to pigment until reaching special pair in reaction-centre -> electrons are excited & go to primary electron acceptor -> light reaction |
| what makes the pair of chlorophyll a molecules in reaction-centre special? | they are able to use the energy from light to boost an electron to a higher energy level AND transfer that electron to the primary electron acceptor (Pe-A) |
| describe 2 types of photosystems & which acts first | photosystem II (PSII) acts first, best at absorbing wavelength ~680 nm = its reaction-centre is called P680. Photosystem I (PSI) acts 2nd, best at absorbing wavelength of 700 nm = its reaction-centre is called P700. |
| briefly contrast the 2 possible routes for electron flow | linear electron flow: primary pathway, involves both photosystems & produces ATP & NADPH. Cyclic electron flow: uses only PSI & produces ATP but not NADPH |
| describe steps of linear electron flow in PSII | 1. photon strikes pigment -> resonance until exciting P680, 2. P680 electrons transferred to Pe-A (P680+) 3. H2O is split into 2e-, 2H+ & 1O, e- transferred to P680, H+ released to thylakoid space, O immediately combines to form O2, released thru stomata |
| describe steps of linear electron flow from PSII to PSI & production of ATP | 4. photoexcited e- from Pe-A move from PSII to PSI via ETC 5. ETC pumps H+ from stroma to thylakoid space & produces proton gradient & chemiosmosis makes ATP, very similar to the cellular respiration ETC but less complexes, e- do not form H2O at the end |
| describe steps of linear electron flow in PSI after ETC | 6. light energy transferred from pigments by resonance to excite P700 -> P700+ -> P700+ reduced by e- from P680 7. photoexcited e- go down 2nd ETC which does NOT create a proton gradient (and therefore no ATP) |
| describe steps of linear electron flow - how NADPH is produced | 8. enzyme NADP+ reductase catalyzes transfer of e- from 2nd ETC to NADP+ -> NADPH (note: 2e- are required for reduction & this also removes an H+ atom from the stroma) |
| what are the products of linear electron flow? where do they go? | O2, ATP, & NADPH. O2 is released to environment by stomata, ATP & NADPH are produced to the stroma side of the thylakoid membrane |
| describe the maintenance of the proton gradient in linear electron flow | maintained by 3 processes: 1. splitting of H2O by PSII, releasing 2H+, 2. cytochrome complex in 1st ETC uses energy of the e- flow to pump H+ across, 3. removal of 1H+ from stroma during reduction of NADP+ |
| compare/contrast oxidative phosphorylation & photophosphorylation | both use ETC & proton pumps to form a proton-motive force (H+ gradient across a membrane) that powers ATP synthase. Oxidative uses energy from breakdown of glucose, photophosphorylation uses energy captured from sunlight |
| conditions for the cyclic flow of electrons | when light levels high = light energy absorbed begins to overwhelm Calvin cycle's use of NADPH. If no NADP+ returned, high energy e- can damage cell. Hence, e- shunted into alternate pathway to increase ATP production & lower NADPH production |
| describe cyclic flow of electrons | same as linear, except instead of e- from PSI going to NADPH, it goes to ferredoxin & back to the 1st ETC (therefore cyclic). PSII is shut down & only PSI is used |
| brief summary of the Calvin cycle | like citric acid cycle, regenerates its starting material after molecules enter/leave, builds glucose from CO2 by using ATP & reducing power of e- in NADPH. Carbon enters as CO2 & leaves as glyceraldehyde-3-phosphate (G3P). Has 3 phases |
| relationship between glyceraldehyde-3-phosphate & glucose & name the phases of Calvin cycle | G3P is "half" a glucose. Phases: fixation, reduction & regeneration |
| fixation phase of Calvin cycle | CO2 added to 5' sugar ribulose 1,5-biphosphate (RuBP), catalyzed by rubisco enzyme, produces 6' that breaks down into two 3' molecules (3-PGA; 3-phosphoglycerate) |
| reduction phase of Calvin cycle | for energy to increase, carbon must be reduced in form of 3-PGA. ATP is used to phosphorylate 3-PGA, then NADPH transfers 2 high energy e- -> resulting 6G3P. Only 1G3P can be transferred out of chloroplast, remaining 5G3P r recycled |
| regeneration phase of Calvin cycle | 5' RuBP is regenerated thru 12/15 steps of cycle. 3 5' RuBP molecules are made from remaining 5 G3P molecules, requiring 3ATP |
| how many CO2, NADPH & ATP to make 1 G3P? to make 1 glucose? | 1 G3P: 3 CO2, 9ATP, 6NADPH. 1 glucose: 6CO2, 18ATP, 12NADPH (cycle spins twice) |
| describe the conditions that would require alternate methods of CO2 fixation | in hot & arid climates, plants may lose a lot of water due to evaporation thru their stomata while they r open during the day = have to close stomata, which reduces CO2 access, sugar production & causes build-up of O2 |
| describe photorespiration & why it is wasteful | as CO2 becomes scarce, rubisco binds O2 to 5' RuBP instead of fixing CO2, causing it to split into 3-PGA and 2-PGA, slowing down Calvin cycle. It is wasteful bc it uses ATP, but sugar production is significantly decreased |
| describe C3 plants & photorespiration | ~85% of plants are C3 - rice, wheat, soybeans, all trees. Called C3 since 1st step of Calvin cycle is fixing CO2 to 3' molecule. Photorespiration occurs in C3 plants when conditions r hot & dry; they cannot fix this problem |
| describe C4 plants & photorespiration | light reactions in mesophyll & Calvin cycle in bundle-sheath cells around leaf veins. Minimize photorespiration cost by incorporating CO2 into oxaloacetate, using enzyme PEP carboxylase w higher affinity for CO2 than rubisco |
| what happens to oxaloacetate in C4 plants? | it is converted into malate which moves into bundle-sheath cells -> pyruvate & CO2, CO2 used in Calvin cycle. Requires ATP to move pyruvate back into mesophyll to pick up more CO2, but maintains high concentration of CO2 > O2 around rubisco |
| describe CAM plants & photorespiration | use crassulacean acid metabolism (CAM) to fix carbon - light reactions & Calvin cycle aren't separate physically (no bundle-sheath cells), but the release of CO2 is controlled, minimizing photorespiration |
| metabolism of CAM plants during night v. day | open stomata at night & uptake CO2, fixing it in oxaloacetate w PEP carboxylase -> convert into malate/organic acids -> stored in vacuoles until day, when stomata close to reduce water loss & CO2 is released from vacuoles to enter Calvin cycle |
Created by:
AntBanana
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