Photosynthesis Notes PDF

How do plants capture light? -​ Chlorophyll A -​ Main photosynthetic pigment -​ Found in all autotrophic organisms -​ All organisms that produce its own food -​ Accessory chlorophyll pgiments -​ Absorb different wavelengths of light and passes the en...

How do plants capture light? -​ Chlorophyll A -​ Main photosynthetic pigment -​ Found in all autotrophic organisms -​ All organisms that produce its own food -​ Accessory chlorophyll pgiments -​ Absorb different wavelengths of light and passes the energy to chlorophyll a -​ Some wavelengths cannot be absorbed by chlorophyll A -​ “Assistant” chlorophyll -​ Chlorophyll B -​ In all true plants -​ Chlorophyll C -​ In golden brown/brown algae -​ Chlorophyll D -​ In red algae Pigments -​ CH3 is chlorophyll A -​ Main photosynthetic pigment -​ CHO is chlorophyll B -​ accessory pigment -​ Broaden the spectrum used for photosynthesis -​ The difference in absorption spectrum due to slight structural differences between pigment molecules -​ carotenoids -​ Accessory pigments that absorb excessive light that would damage chlorophyll -​ Not the same as chlorophylls BCD -​ Porphyrin ring -​ Light-absorbing head of molecule; magnesium atom at center -​ Hydrocarbon tail -​ hydrophobic -​ Interacts with hydrophobic regions of proteins inside thylakoid membranes Organisms (Eukaryotes) 1.​ Mosses, ferns, seed plants, green algae, and euglenoids -​ Chlorophyll A + B; carotenoids 2.​ Diatoms, dinoflagellates, brown algae -​ Chlorophyll A + C, carotenoids 3.​ Red algae -​ Chlorophyll A + D, carotenoids, phycobiliproteins Organism (Prokaryotes) 1.​ Cyanobacteria -​ Chlorophyll A + D, carotenoids, phycobiliproteins 2.​ prochlorophytes  -​ Chlorophyll A + B; carotenoids Absorption -​ is a graph plotting a spectrum’s light absorption versus wavelength -​ absorption spectrum of chlorophyll A suggests that violet-blue and red light work best for photosynthesis Action spectrum -​ Profiles the relative effectiveness of different wavelengths of radiation in driving a process Reaction Center/Chlorophyll A -​ Absorbs photons -​ HV hits light-harvesting pigments, ie. antenna molecules -​ Passes photon from one molecule to another until it reaches the reaction center/chlorophyll A molecules -​ Chlorophyll A molecules will transfer the photon to the primary electron acceptor -​ Important in light reactions Where are the photosynthetic pigments contained? -​ Inside thylakoid -​ cyanobacteria -​ In the chloroplasts of true plants and algae -​ Only absorbs red and blue-purple light except for green; why plants are green -​ Inside Chloroplast -​ mesophyll -​ Stroma -​ Fluid inside chloroplast -​ Granum -​ Stack of thylakoids -​ Thylakoid -​ Thylakoid space -​ Inside of thylakoid -​ Inner membrane -​ Intermembrane space -​ Space between inner and outer membrane -​ Outer membrane Photosystems -​ Major reaction center/chlorophyll A -​ Accessory pigments -​ Primary electron receptor 1.​ Photosystem 1/P700 -​ Absorbs 700nm wavelength: red light  -​ Primary acceptor is chlorophyll A 2.​ Photosystem 2/P680 -​ Absorbs 680nm wavelength -​ Primary acceptor is pheophytin -​ H2O; 1/2O2 + 2H+ is suspended in the thylakoid lumen Light and Atoms -​ Photon is absorbed by an excitable electron that moves into a high energy level, ie. farther from the nucleus -​ Can end in 2 ways a.​ Cyclic electron flow -​ Electron will return to the lower energy level/ground level by emitting a less energetic photon b.​ Non-cyclic/linear electron flow -​ The electron will be accepted by an electron acceptor molecule -​ Involves P700 and P680 -​ produces NADPH, ATP, and oxygen -​ Oxygen is released in the atmosphere; just a byproduct -​ ATP provides chemical energy and NADPH reduces power -​ Proceeds to the calvin cylce, a carbon fixation process Electron Transport Chain (ETC) -​ When light hits PSii, it hits PSi, there will be an electron transfer and then it travels to ETC -​ ETC is in between PSi and PSii -​ Electron will pass through PSii, ETC, and then will go to PSi to produce NADPH -​ Going through PSii produces oxygen -​ Going through PSii triggers ATP synthase and will produce ATP Non-cyclic electronic flow 1.​ Photon hits a pigment, and its energy is passed among pigment molecules until it excites PSii/680 2.​ An excited electron is transferred to the primary electron acceptor/PSii+ 3.​ H2O is split by enzymes and the electrons are transferred from the hydrogen atoms to PSii+, thus reducing it to PSii -​ PSii+ is the strongest known biological oxidizing agent -​ O2 is released as a byproduct of this reaction -​ Plants produce O2 gas by splitting H2O -​ O2 is liberated by photosynthesis is made from the oxygen in water (H+ and e-) 4.​ Each electron falls down an ETC from the primary electron acceptor of PSii to PSi -​ Accepted by plastoquinone (Pq) and then by plastocyanin (Pc) 5.​ Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane  -​ Diffusion of H+ (protons) across the membrane drives ATP synthesis 6.​ In PSi, like PSii, transferred light energy excites PSi, which loses an electron to an electron acceptor a.​ PSi+ (PSi that is missing an electron) accepts an electron passed down from PSii via ETC 7.​ Each electron falls down an ETC from the primary electron of PSi to the protein ferredoxin (Fd) 8.​ The electrons are then transferred to NADP+ and reduce to NADPH -​ The electrons of NADPH are available for the reactions of the calvin cycle -​ This process also removes H+ from the stroma Cyclic Electron Flow -​ Only happens in PSi -​ Circles form PSi, primary acceptor, ferredoxin, cytochrome complex, and plastocyanin -​ Electrons cycle back from Fd to the PSi reaction center -​ This flow only uses PSi and only produces ATP -​ No oxygen is released because water was not produced -​ Is thought to have evolved before noncycllic electron flow -​ May protect cells from light-induced damage Summary 1.​ light-dependent reaction -​ Increase the potential energy of electrons by moving them from H2O to NADPH -​ ETC/process -​ Needs water and light -​ Produces ATP, NADPH, and O2 (byproduct) 2.​ Carbon-fixation process -​ Calvin process -​ Needs ATP, NADPH, CO2 -​ Produces carbohydrates/glucose Chemiosmosis | 02/08/25 -​ When ions move by diffusion across a semi-permeable membrane, eg. a membrane in mitochondria -​ Ions will move down from the area of high concentration to low concentration -​ Ions move out to balance the electrical charge -​ Ions are molecules with net electric charge -​ Ions move down an electrochemical gradient, a form of potential energy -​ A type of diffusion Comparison of chemiosmosis in chloroplasts and mitochondria -​ Both generate ATP by chemiosmosis but use different sources of energy -​ Mitochondria transfers chemical energy from food to ATP  -​ Chloroplasts transform light energy into chemical energy of ATP -​ Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but has similarities Mitochondria vs chloroplasts -​ Mitochondria -​ Protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix -​ Chloroplasts -​ Protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma Calvin Cycle -​ Also called “carbon fixation” -​ Requires CO2 -​ Fixing carbon from CO2 -​ Also called “dark reaction” -​ Complements “light reaction” -​ Plant groups -​ C3, C4, and CAM -​ Differs in the way they fix carbon -​ Has to adjust in the environment they are planted/environmental stressors -​ Occurs in the stroma, ie. the side facing the stroma -​ 6 calvin cycles needed to produce one glucose molecule -​ Uses 18 ATP and 12 NADPH per calvin cycle Calvin cycle phases 1.​ CO2 uptake -​ Creates 6 molecules of ribulose bisphosphate (RuBP) -​ 6 molecules of CO2 -​ CO2 molecules are captured by RuBP, resulting in unstable intermediate that is immediately broken apart into 2 PGA -​ 12 molecules of phosphoglycerate (PGA) will proceed with the process -​ will be needing 12 molecules of ATP 2.​ Carbon reduction -​ 12 ATP is needed to start the process -​ 12 NADPH is needed for the process to continue -​ PGA is phosphorylated by ATP and reduced by NADPH -​ Removal of phosphate results in G3P formation -​ G3P is rearranged into new RuBP molecule or another sugar via series of reactions -​ 12 G3P is 3.​ RuBP regeneration\ -​ 6 molecules of ribulose phosphate or RP will need 6 ATP  -​ Produces 6 molecules of RuBP -​ Restarts cycle from CO2 uptake phase Most notable features of calvin cycle -​ Their large nitrogen requirement for rubisco and other photosynthetic enzymes -​ Rubisco -​ Enzyme that drives the whole process calvin cycle -​ It accounts for about 25% of the nitrogen in photosynthetic cells -​ Their dependence on the product of the light reaction, ie. ATP and NADPH, which depend on irradiance -​ The light received by the photosynthetic cell -​ Their frequent limitation by CO2 supply to the chloroplast What will happen if there is little CO2/what will happen if calvin cycle will not continue -​ Calvin cycle will not proceed as often -​ More ATP and NADPH will become concentrated in the stroma due to it not being used -​ Lesser glucose will be made -​ Plants will not survive Rubisco: a carboxylase and oxygenase -​ As carboxylase -​ Initiates calvin cycle -​ As oxygenase -​ Catalyzes reaction between rubisco and oxygen under conditions of CO2 limitations -​ Initiates breakdown of CO2 sugars -​ Photorespiration -​ Occurs in the light (photo) and consumes O2 while producing CO2 (respiration); uses ATP but no sugar molecules -​ Reduces photosynthetic efficiency of the calvin cycle as much as 50% -​ Opposite of photosynthesis Alternative mechanisms of carbon fixation -​ Certain plants minimixe cost of photorespiration -​ Via incorporation of CO2 into 4 carbon compouds in mesophyll cells -​ Alternative pathways: -​ Hatch-stack pathway/C4 pathway -​ Crassulacean acid metabolism/CAM C3 and C4 comparision 1.​ C3 -​ Calvin cycle takes place in mesophyll cells -​ Bundle sheath cells are nonphotosynthetic  2.​ C4 -​ Reaction that fix CO2 into 4 carbon compounds take place in mesophyll cells -​ They are then transferred form the mesophyll cells to the photosynthetic sheath cells where calvin cycle happens C4 cycle -​ 2 organelles needed: mesophyll and bundle sheath cells -​ CO2 enters mesophyll cells via stomata -​ CO2 will be fixed by PEP carboxylase -​ Produces into oxaloacetate, a carbon molecule -​ Oxaloacetate turns into malate, a 4 carbon molecule -​ Malate goes into bundle sheath cells -​ 3 carbons from malate will be reduced to pyruvate, a 3 carbon molecule -​ Other molecule will turn into CO2 -​ Calvin cycle will start and sugar travels into vascular tissue -​ Pyruvate goes back to mesophyll and becomes ATP, reduced into ADP, then calvin cycle restarts C4 plants -​ Thrive in hot and moist conditions -​ 15% of plants, eg. grass, corn, and sugarcane -​ Divides photosynthesis spatially -​ Light rxn: mesophyll cells -​ Calvin cycle: bundle sheath cells