Chapter 6 - Part 1.pdf

CHAPTER 6 Photosynthesis and the Chloroplast Image: https://www.micropia.nl/en/discover/microbiology/chloroplast/ The Origin of Photosynthesis Nutrition Heterotrophs survived on nutrients from the environment Chemoautotrophs use energy from inorganic molecules Sulfur oxidizing, nitrogen-fixing,...

CHAPTER 6 Photosynthesis and the Chloroplast Image: https://www.micropia.nl/en/discover/microbiology/chloroplast/ The Origin of Photosynthesis Nutrition Heterotrophs survived on nutrients from the environment Chemoautotrophs use energy from inorganic molecules Sulfur oxidizing, nitrogen-fixing, ironfixing bacteria Autotroph manufacture organic nutrients from CO2 and H2S. Photoautotrophs use radiant energy to make organic compounds. The Origin of Photosynthesis • Photosynthesis converts energy from sunlight into chemical energy stored in carbohydrates.  Low energy electrons are removed from a donor molecule.  First photoautotrophs used H2S as electron source light CO2 + 2H2S (CH2O) + 2S  About 2.7 billion years ago, cyanobacteria used electrons from water to produce oxygen as a waste product: CO2 + H2O light (CH2O) + O2 Fig:6.1:Photosynthetic green sulfur bacteria in a symbiotic relationship with a single anaerobic, heterotrophic bacterium Based on redox potential, which atom can oxidize easily? Sulfur atom in a molecule of H2S Oxygen atom in a molecule of H2O CO2 + 2H2S light (CH2O) + 2S -0.25V CO2 + H2O light (CH2O) + O2 +0.816V Sulfur atom in a molecule of H2S has much less affinity for its electrons and is easier to oxidize (lose electrons) than the oxygen atoms and hard to pull the electrons from water. How chloroplast evolved? symbiotic cyanobacterium transformed from a separate organism into Cytoplasmic organelle called chloroplast form when cyanobacteria took up residence inside a mitochondria containing nonphotosynthetic primitive eukaryotic cell Do chloroplast have their own DNA and ribosomes? Chloroplast Structure Chloroplast, a cytoplasmic organelle, are predominantly in the mesophyll cells of leaves. • Chloroplasts have a double membrane. • The outer membrane contains porins and is permeable to large molecules. (what type of diffusion is it?) • The inner membrane contains lightabsorbing pigment, electron carriers, and ATP-synthesizing enzymes. • The mesophyll cells play important roles in photosynthesis. They have large space within the leaf that allow CO2 to move freely Fig 6.2:The functional organization of a leaf The internal structure of a chloroplast Fig 6.3 Electron micrograph through a single chloroplast Chloroplast • Organelle where photosynthesis takes place. Stroma Outer Membrane Inner Membrane Thylakoid Granum The internal structure of a chloroplast • The inner membrane of a chloroplast is folded into flattened sacs (thylakoids), arranged in stacks called grana. • Thylakoid membranes contain a large percentage of glycolipids, which make the membrane highly fluid for diffusion of proteins complexes. Electron micrograph of a spinach chloroplast showing stacked grana thylakoids Thylakoid Thylakoid Membrane Granum Thylakoid Space (lumen) * Stack of thylakoid disc is called granum which are the membrane like structures found inside the chloroplast of plant cells The internal structure of a chloroplast • The thylakoid membrane contains the chlorophyll molecules and protein complexes that comprise the energy-transducing machinery of the chloroplast. • The space inside a thylakoid sac is the lumen • The space outside the thylakoid and within the chloroplast envelope is the stroma , which contains the enzymes responsible for carbohydrate synthesis. • Like the matrix of a mitochondrion, the stroma of a chloroplast contains small, double-stranded, circular DNA molecules and prokaryotic-like ribosomes. • Chloroplasts are self-replicating organelles containing their own DNA. • Chloroplasts arise by fission from preexisting chloroplast Question Why are plants green? Where is chlorophyll found? Located within the thylakoid membrane Chlorophyll Molecules • Located in the thylakoid membranes. • Chlorophyll has Mg+ in the center. (Magnesium deficiency causes yellowing of leaves – chlorosis- continuous deficiency may cause leaf death) –POWERHOUSE behind photosynthesis • Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red-660 nm are most important). • Plants are green because the green wavelength is reflected, not absorbed. Chlorophyll Short wave (more energy) Long wave Fig 6.7 (less energy) What colors of the light spectrum are best absorbed by plants? Chlorophyll mainly absorbs violet, blue and red light, reflecting lighter blue, green and yellow light How many types of chlorophyll are there? Four types: a, b, c, d Chlorophyll A is the principal pigment involved in the photosynthesis whereas B is the accessory pigment and collect energy in order to pass into chlorophyll a - Chlorophyll a and b are found in all higher plants while c and d are in algae The Absorption of Light • Photosynthetic Pigments – molecules that absorb light of particular wavelengths. • Chlorophyll contains a porphyrin ring (rings made from 4 carbons and 1 nitrogen) that absorbs light and a hydrophobic tail embedding it to the photosynthetic membrane. Q) Is chlorophyll polar or non-polar? How do you know? Any experiment? Q) What is the difference between chlorophyll a and b? Chlorophyll a has methyl group Chlorophyll b has CHO group Q) Which chlorophyll molecule is more polar? Chlorophyll b Fig 6.6 The Absorption of Light • Absorption of photons (light “particles”) by a molecule makes them go from ground state to excited state. • Energy in the photon depends on the wavelength of light. • Energy required to shift electrons varies for different molecules. • Molecules absorb specific wavelengths of light. The Absorption of Light • The alternating single and double bonds along the porphyrin ring form a cloud making it a conjugated system. • Conjugated bond systems absorb energy of a range of wavelengths. Fig 6.7 • Besides chlorophyll, there are accessory pigments called carotenoids. • Carotenoids absorb light in the blue-green region of the spectrum. • Various pigments allow for greater absorption of incoming photons. Question • During the fall, what causes the leaves to change colors? Fall Colors • In addition to the chlorophyll pigments, there are other pigments present. • During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments. are pigments that are either red or yellow. An Overview of Photosynthetic Metabolism • Photosynthesis is a redox reaction transferring an electron from water to carbon dioxide: 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2 • Experiments using 18O showed that O2 molecules released from photosynthesis came from two molecules of H2O, not from CO2 (Ruben and Kamen) One sample of algae was exposed to labeled C[ 18 O 2 ] and unlabeled water Another sample was exposed to unlabeled carbon dioxide and labeled H 2 [ 18 O]. Which of these two samples of photosynthetic organisms released labeled 18 O 2 ? The algae given labeled water produced labeled oxygen, demonstrating that O 2 produced during photosynthesis derived from H 2 O . • Photosynthesis oxidizes water to oxygen; respiration reduces oxygen to form water. Respiration removes high energy electrons from reduced organic substrates to form ATP and NADH. Photosynthesis uses low energy electrons to form ATP and NADPH, which are then used to reduce CO2 to carbohydrate. Overview of the energetic of photosynthesis and aerobic respiration Fig 6.5 An Overview of Photosynthetic Metabolism • Photosynthesis occurs in two stages: • Light-dependent reactions (light reactions)in which sunlight is absorbed, converting it into ATP and NADPH. • Light-independent reactions (dark reactions) use the energy stored in ATP and NADPH to produce carbohydrate. Photosynthetic Units and Reaction Centers • Each photosynthetic unit contains several hundred chlorophyll molecules. • One member of the group—the reaction-center chlorophyll—transfers electrons to an electron acceptor. • Pigments that do not participate directly in the conversion of light energy, they are responsible for light absorption, and are called antenna pigments. Fig 6.10: The transfer of excitation energy The operation of Photosystem II and Photosystem I The reaction center of PSII is referred to as P680, and that of PSI as P700 standing for the wavelengths where absorption is stronger. Two large pigment-protein complexes called photosystems (PSII and PSI) act in series to raise electrons from H2O to NADP+. e- H2O PSII PSI Photosystem II (PSII) boosts electrons from below energy level of water to a midpoint. Photosystem I (PSI) boosts electrons to a level above NADP+. The flow of electrons from H2O to NADP+ is referred to as the Z scheme. NADP+ The operation of Photosystem II and Photosystem I In photosynthesis flow of electrons takes place in three steps: 1. From water to PSII (photolysis) 2. From PSII to PSI (PQ, Cyt b6f and PC) 3. From PSI to NADP+ (Z scheme) The operation of Photosystem II and Photosystem I 1. THE FLOW OF ELECTRONS FROM WATER TO PSII:  Electrons come from water and are released when water is split into protons and molecular oxygen.  Water is a very stable and the splitting of water is the most thermodynamically challenging (endergonic) reaction known to occur in living organisms.  The absorption of light by PSII leads to the formation of two charged molecules, P680+ and Pheophytin (Pheo-) that lacks Mg2+ .  PSII is a complex of more than 20 different polypeptides, most of which are embedded in the thylakoid membranes.  Photolysis results in two charged species, P680+ and Pheo- . P680+ is electron deficient and accepts electrons from water while Pheo- has an extra electron and will readily lose its electron making it an oxidizing agent. Pheo- P680+ P680+ e- e- Oxidizing agent e- e- Photolysis +0.82V The operation of Photosystem II and Photosystem I 2. THE FLOW OF ELECTRONS FROM PSII TO PSI  Photosystem II is a huge protein complex that catalyzes the light driven oxidation of water and reduced plastoquinone and it begins with the absorption of light by antenna pigment in the outer light harvesting complex (LHCII)  Absorption of light by P680 generates Pheo- that will transfer its electrons to plastoquinone A (PQA)and then to plastoquinone B (PQB ) and form a negatively charged free radical PQB Absorption of a second photon send a second electron along the same path and will form PQB2 Two photons will enter from the stroma and generate plastoquinol (PQH2)  The reduced PQH2 is an mobile electron that diffuses from the lipid bilayer of the thylakoid membranes and binds to a large protein complex called cytochrome b6f and donates electrons.  Now cytochrome b6f will pass electrons to the another moblile electron carrier (a water soluble, copper containing protein) Plastocyanin that carrier electrons to the PSI and will form P700+ Pheoe- PQB2- Stroma 2H+ PQB- PQA PSII e PQH2 e PSI cytochrome B6f e PQH2 e FeS xytochrome f e PC e Lumen 2H+ The operation of Photosystem II and Photosystem I THE FLOW OF ELECTRONS FROM PSI TO NADP (NADPH Production) H+ NADP+ Ferredoxin reductase NADPH P700-  The PSI of higher plants is composed of polypeptide units and a complex of proteinbound pigments called Light-Harvesting Complex I (LHCI)  Light is absorbed by the antenna pigments of LHCI and passed to LHCI reaction centre pigment (P700*) . -1.0V PSI P700  Absorption of light leads to the production of two charged species (P700- and)  Chlorophyll a (A0- ) is a very strong reducing agent with a redox potential of approximately -1.0 V, which is well suited to reduce NADP+ (Redox potential of -0.32 V)  The reduction of NADP+ is catalyzed by a large enzyme called Ferreodoxin-NADP+ reductase, contains an FAD prosthetic group and accept 2 electrons. Fig 6.16 How does the positive charge of P700 pigment is neutralized? LHCI Plastocyanin carries electrons to the luminal side of PSI and transferred to P700+ Question 1.What are the mobile electron carriers in plants? Can you name them? Plastoquinone (PQH2) Plastocyanin (PC) 2. Why do plants need these mobile electron carriers? Two Photosystem are not in close proximity of each other that is why need mobile carrier to transfer the electrons 3. Any special features of these mobile electron carriers? Plastoquinol (PQH2) accepts two electrons from PSII and two protons from the stroma and soluble in lipid bilayer Plastocyanin (PC) a Cu containing blue protein that transfer electrons from cytb6fcomplex to PSI Photosynthetic Units and Reaction Centers (Summary) 1. PSII Operations: Obtaining Electrons by Splitting Water Fig 6.12 Fig 6.11 Fig 6.16 3.From PSI to NADP+ Fig 6.15 2. From PSII to PSI © 2013 John Wiley & Sons, Inc. All rights reserved. Photosynthetic Reaction The splitting of water during photosynthesis is called photolysis. The redox potential of P680+ pulls electrons from water The formation of one molecule of oxygen during photolysis is requires the simultaneous loss of four electrons from two molecules of water. 2 H2O 4 H+ + O2 + 4 e– • Protons produced in photolysis are retained in the thylakoid lumen. • Oxygen produced is a released as a waste product into the environment. • For every 8 photons absorbed: 2 H2O + 2 NADP+ O2 + 2 NADPH • Electron transport also produces a proton gradient across the thylakoid membrane. © 2013 John Wiley & Sons, Inc. All rights reserved. Question How many protons are required for the synthesis of each molecule of ATP in the light dependent reaction? four Photophosphorylation • The machinery for ATP synthesis in a chloroplast is similar to that of mitochondrial enzymes. • The ATP synthase consists of a head (CF1), and a base (CF0). • The CF1 heads project outward into the stroma, keeping with the orientation of the proton gradient. • Protons move into the lumen through the CF0 base of the synthase. https://www.youtube.com/watch?v=OC677e1DpUc Fig 6.17:Summary of the light-dependent reactions Summary Slide Do you know how herbicides work? • Killing Weeds by Inhibiting Electron Transport • Common herbicides bind to a core protein of PSII. • Light reactions serve as targets of herbicides. • Some herbicide produce oxygen radicals, which are toxic to the human tissue. Question/Recall Which photosystem generates a strong oxidizing agent ? Which photosystem generates a strong reducing agent ? Question/Recall Which photosystem is capable of producing oxygen from water? Which photosystem is capable of producing NADPH from NADP+ ?