Photosynthesis: Capturing Energy I. Chloroplasts A. Facts: 1. double membrane 2. not part of endomembrane system 3. semi-autonomous organelles, grow and reproduce 4. found in plants, algae, cyanobacteria, purple sulfur bacteria 5. plastid B. Structure-3 functional compartments 1. intermembrane space -between two membranes 2. thylakoid space -flattened membrane sacs within chloroplast -contain chlorophyll and other pigments-embedded in membrane -function during photosynthesis to convert light energy to chemical energy -stacked into grana -space-inside membrane 3. stroma-viscous fluid between thylakoids and membrane -reactions that use chemical energy to convert CO 2 into sugar C. All green parts of plants contain chloroplasts -therefore, all can carry out photosynthesis 1. most photosynthesis in leaves -large surface area facing the sun 2. in leaf-mesophyll cells contain largest amount of chloroplasts II. Light is composed of particles that travel as waves A. Facts: 1. travels in waves 2. measured in lambda range-electromagnetic spectrum 380-750 nm-visible light a. red-longest wavelength, lowest frequency b. violet-shortest wavelength, highest frequency B. Behaves as photons -little packets of energy 1. not objects, but each behaves as an object, because has fixed quantities of energy 2. shorter wavelength, greater energy C. Visible light drives photosynthesis 1. blue and red-most effective, least reflected 2. green-most reflected, therefore, see green -absorption spectrum: shows absorption of light at different wavelengths, therefore, main photosynthetic pigment must be green 1
3. action spectrum-shows effectiveness of some process based on absorption of light 2
-1883, T.W. Engelmann -Spirogyra -Spirogyra would carry on photosynthesis -more photosynthesis-more O 2 emitted -aerobic bacteria would crowd in area with most O 2 -red and blue III. Pigments found in thylakoids A. Chlorophyll a (blue-green) -best-red and blue -most important pigment in light-dependent reactions and only pigment that participates in photosynthesis -varies from chlorophyll b by CH 3 B. Chlorophyll b (yellow-green) -differs because it has carbonyl C. Others 1. Carotenoids-yellow, orange, red -ex: carotenes and xanthophylls 2. Phycobilins- -ex: phycoerythrin and phycocyanin D. Why so many? 1. each pigment absorbs a different wavelength of light 2. broadens wavelengths of light that can be absorbed for photosynthesis 3. molecules electrons in pigments actually absorb the energy from light-this begins photo. a. that electron becomes energized (high energy electron) by absorbing photon b. energized electron may return to the ground state and dissipate energy as heat 3
-what occurs when pigments other than chlorophyll a absorb light -heat (energy) is then absorbed by chlorophyll a c. high energy electron may leave atom (of pigment where it was energized) and be accepted by an electron acceptor -what happens to high energy electrons of chlorophyll IV. Photosynthesis -conversion of light energy to chemical bond energy 6 CO 2 + 12 H 2 O C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Net: 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 -reverse of respiration -scientists used 18 O tracers to determine pathways of atoms used in reactions -from this, determined: Reactants: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 Two reactions in photosynthesis 1. Light dependent/photochemical reactions a. light is necessary b. occurs on thylakoid membranes and inside thylakoid space c. solar energy chemical energy d. products ATP, NADPH 2. Light independent/calvin Cycle/Carbon Fixation/Dark reactions (synthesis) a. light not necessary-but products from light rxn are; therefore, usually occurs in light alongside light reactions b. occurs in stroma c. chemical energy glucose I. Light Reaction solar chemical A. Absorption of Light 1. absorbed by pigments (all on thylakoid membranes) 2. electrons within atoms of pigments become excited -absorb photons (energy) -very unstable 3. electrons drop down to ground state releasing energy as heat 4. heat is reabsorbed by other pigments molecules and their electrons become excited, then fall, releasing energy 5. happens over and over until energy is absorbed by one of two specialized chlorophyll molecules- P680 or P700 -named for the wavelength of light that they absorb best 4
6. P680 and P700 are found on thylakoid membrane and are surrounded by other pigments (to absorb energy from more wavelengths of light) -groups of P and pigments a. P700 s pigment cluster makes up PSI b. P680 s- PSII both named from discovery time Light Reaction 1. Begins with exciting of electrons from PSII (chlorophyll a) a. once become excited, they are taken from PSII and captured by primary electron acceptor b. leaves + charge in PSII that must be neutralized c. 2 electrons are supplied by photolysis -splitting of water 1. catalyzed by a Mg-containing compound 2. H 2 O 2 e -, 2 H +, ½ O 2 3. splitting occurs because after light excites e - and e - leave PSII -makes P680 molecule very highly electronegative -therefore, P680 molecule has ability and does oxidize water and removes its e - -causes splitting of water 4. therefore, light does not directly cause splitting of waterthe oxidation by P680 does 5. O 2 released to atmosphere 2 e - absorbed by P680 2 H + absorbed into thylakoid space where they will be used later by NADP+ or will become part of proton pump 2. excited e - s are captured by the primary e - acceptor 3. excited e - s pass from primary e - acceptor to PSII to PSI via ETC a. found in thylakoid membrane b. series of carrier proteins that pass e - from one carrier to the next -each with a higher electronegativity 5
Sequence of carrier proteins: Pq-Plastoquinone Cytochrome b 6 - Cytochrome f- both cytochrome complexes (contain Fe) Pc-Plastocyanin 4. e - s fall down ETC and release energy -used in photophosphorylation -same basic process as chemiosmosis and oxidative phosphorylation a. energy from e - is used to pump H + ions into the thylakoid space (proton pump) -ph of space = 5 -ph of stroma = 8 b. creation of proton concentration gradient and electrochemical gradient results c. H + s travel through F0-F1 complex and through ATP synthase -creating ATP from ADP + Pi d. each pair of e - s can phosphorylate about 1.5 ATP molecules 5. ETC ends with PSI (P700) a. here, e - s are again energized by sunlight b. once energized, e - s are accepted by primary e - acceptor of P700 complex c. one of 2 things can happen here: 1. cyclic phosphorylation a. e - s. after becoming excited, drop back down to P700 releasing energy b. when this happens, reaction does not progress to the Calvin cycle c. energy is used to make ATP d. necessary in every reaction because not enough energy is produced from non-cyclic phosphorylation (non-cyclic equal amounts of ATP and NADPH are made; need more ATP) 6
2. noncyclic phosphorylation -continues e - on to ETC 6. Proteins: Fd-Ferredoxin (Fe) FNR-ferredoxin-NADP reductase Here, enzyme, NADP + reductase transfers e - s to NADP + a. stores high energy e - s b. joins with one H + from H 2 O to make NADPH Results of Light Reaction 1. Production of ATP for Calvin Cycle -will provide energy to drive reaction 2. Production of NADPH for Calvin Cycle -will provide H + s and high energy e - s that will be harvested for energy during respiration 3. Therefore, in production of ATP and NADPH, light energy (from sun) is converted to chemical energy -cyclic-primitive form of photosynthesis; still utilized Structure of a typical C 3 plant leaf: See leaf handout Photosynthesis: primarily in palisade layer; also: spongy layer/guard cells For a plant to take in CO 2 for dark reaction, CO 2 must enter leaves of plants through the stoma (stomata, pl.) 7
1. guard cells in C 3 plants typically keep stomates open during the day to absorb CO 2 for photosynthesis 2. close stomates at night to prevent water loss 3. problem open stomata, lose water, but O 2 can enter for photorespiration 4. for plants in dry, arid regions this poses a major problem a. keep stomates open lose water b. stomates open take in CO 2 / O 2 ; increased light, increased photosynthesis, increased O 2 production by plant this increases production of O 2 and fixing of O 2 by the enzyme, rubisco c. close stomates lose CO 2 input Therefore, plants in dry, hot regions must find another way to fix CO 2 without O 2 and losing H 2 O A. C 4 Plants -plants in dry, hot regions -ex: sugar cane (Columbia), corn (Midwest) 1. specialized anatomy Kranz anatomy see handout -region of tightly packed mesophyll cells packed around other photosynthetic cells called bundle sheath cells -tightly line veins in leaves 2. C 4 photosynthesis (Hatch-Slack) a. CO 2 is absorbed into mesophyll cells in usual way b. instead of immediately entering Calvin Cycle, (fixing by rubisco), CO 2 is fixed to PEP (phosphoenol pyruvate) to create OOA (oxaloacetate) a 4-C compound, therefore, C 4 photosynthesis c. enzyme- PEP carboxylase -carboxylation of PEP -has much higher affinity for CO 2, not O 2 (will not bond) d. OOA is converted to compound malate requires 1 NADPH/ CO 2 e. malate is shuttled to the bundle sheath cells through plasmodesmata 8
-spaces in cell walls that link cytoplasm of 2 adjacent plant cells -mesophyll to bundle sheath f. in bundle sheath, malate is converted to pyruvate (3C compound) -decarboxylation reaction -occurs initially because of oxidation by NADP+ molecule g. CO 2 is removed and now enters Calvin Cycle in bundle sheath cells -no O 2 present in bundle sheath cells; none can penetrate mesophyll h. CO 2 begins Calvin cycle by being fixed by rubisco -same light and dark reactions occur-this just with added step to secure CO 2 i. regeneration of PEP -pyruvate is taken back to mesophyll cells where ATP AMP (5 ATPs/ CO 2 ) -convert pyruvate to PEP 3. Advantages of C4 photosynthesis a. little O 2 reaches bundle sheath cells; therefore, little photorespiration b. under these conditions, larger % of sugar produced for energy expended -even with extra added energy, under normal conditions, less light, lower temps- C 3 is optimal c. higher rate of photosynthesis can allow C 4 plants to keep stomata closed longer; therefore, reducing H 2 O loss 9
B. CAM plants (crassulacean acid metabolism) -named after plant family: Crassulaceae -in arid conditions -evolved in succulent plants (water storing plants) -ice plants, cacti, pineapples 1. difficult to keep stomata open during the day-too much H 2 O loss -but CO 2 is needed to carry on Calvin Cycle and light is needed for light reaction 2. plants undergo CAM to combat this 3. Process of CAM Photosynthesis a. at night, stomates are open, CO 2 is absorbed by mesophyll cells, PEP carboxylase fixes CO 2 to OOA, like C 4 b. OOA is converted to malic acid (ionized form of malate) c. malic acid (stores CO 2 ) is shuttled to vacuole of the cell and it is stored overnight for use in day d. day stomata are closed 1. light reaction occurs 2. ATP, NADPH are produced 3. Malic acid is taken out of vacuole 4. malic acid OOA PEP and CO 2 -requires 1 ATP 1 ADP 5. CO 2 is fixed by rubisco and begins Calvin Cycle Advantages: 1. photosynthesis can occur when stomata are closed preventing H 2 O loss 2. reduction of photorespiration -see handout Night stomata Day stomata Open Closed 10
Factors that affect the rate of photosynthesis 1. light intensity number of photons of light/unit time 2. CO 2 concentration 3. Temperature 4. availability of nutrients -ex: NO 3 -, Mg +, Fe 2+ 11