BILD 1: The Cell - Photosynthesis Lecture Notes Fall 2024 PDF

BILD 1: The Cell Photosynthesis Assistant Prof. Gulcin Pekkurnaz Fall 2024 Energy flow and chemical recycling in ecosystems Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O...

BILD 1: The Cell Photosynthesis Assistant Prof. Gulcin Pekkurnaz Fall 2024 Energy flow and chemical recycling in ecosystems Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 molecules Cellular respiration in mitochondria ATP powers ATP most cellular work Heat energy 1 Photosynthesis: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O Respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat) 2 Autotrophs are “self-feeders” that sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules Heterotrophs obtain organic material from other organisms Heterotrophs are the consumers of the biosphere Some eat other living organisms; others, called decomposers, consume dead organic material or feces Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2 3 Photosynthesis occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes 4  Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis in plants Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf Each mesophyll cell contains 30–40 chloroplasts CO2 enters and O2 exits the leaf through microscopic pores called stomata 5  Chloroplasts: The Sites of Photosynthesis in Plants Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 6  Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced Photosynthesis is an endergonic process; the energy boost is provided by light 7  Chloroplasts: The Sites of Photosynthesis in Plants A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma Thylakoids are connected sacs in the chloroplast that compose a third membrane system Thylakoids may be stacked in columns called grana Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes 8  The Two Stages of Photosynthesis Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) : Split H2O, Release O2, Reduce the electron acceptor NADP+ to NADPH, Generate ATP from ADP by photophosphorylation 9  The Two Stages of Photosynthesis Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The Calvin cycle (in the stroma) : forms sugar from CO2, using ATP and NADPH begins with carbon fixation, incorporating CO2 into organic molecules 1 0 The Nature of Sunlight § Light is electromagnetic energy, also called electromagnetic radiation § Electromagnetic energy travels in rhythmic waves § Wavelength is the distance between crests of electromagnetic waves § Wavelength determines the type of electromagnetic energy § Visible light consists of wavelengths (380 nm to 750 nm) that produce colors we can see § Visible light also includes the wavelengths that drive photosynthesis § Light also behaves as though it consists of discrete particles, called photons Photosynthetic Pigments: The Light Receptors § Pigments are substances that absorb visible light § Different pigments absorb different wavelengths § Wavelengths that are not absorbed are reflected or transmitted § Leaves appear green because chlorophyll reflects and transmits green light There are three types of pigments in chloroplasts: § Chlorophyll a, the key light-capturing pigment § Chlorophyll b, an accessory pigment § Carotenoids, a separate group of accessory pigments § The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis § An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process § The action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll § Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis § The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules Structure of chlorophyll molecules in CH3 in chlorophyll a chloroplasts of plants CHO in chlorophyll b CH3 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown § Accessory pigments called carotenoids may broaden the spectrum of colors that drive photosynthesis § Some carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll or react with oxygen Excitation of Chlorophyll by Light § When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable § When excited electrons fall back to the ground state, excess energy is released as heat § In isolation, some pigments also emit light, an afterglow called fluorescence  How a photosystem harvests light Photosystem STROMA Photon Light- Reaction- harvesting center Primary complexes complex electron A photosystem consists of a reaction- acceptor center complex surrounded by light- harvesting complexes Thylakoid membrane The reaction-center complex is an e– association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor Transfer Special pair of chloro- Light-harvesting complexes transfer of energy phyll a molecules Pigment the energy of photons to the chlorophyll a THYLAKOID SPACE molecules molecules in the reaction-center complex (INTERIOR OF THYLAKOID) 18 There are two types of photosystems in the thylakoid membrane Photosystem II (PS II) functions first The reaction-center chlorophyll a of PS II is called P680 because it is best at absorbing a wavelength of 680 nm Photosystem I (PS I) is best at absorbing a wavelength of 700 19 Takes two systems to do the job  H2O CO2 Light NADP+ ADP CALVIN LIGHT CYCLE REACTIONS ATP NADPH O2 [CH2O] (sugar) The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar § The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle § The Calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH § Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phosphate (G3P) § For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2 § The Calvin cycle has three phases: 1. Carbon fixation (catalyzed by rubisco) 2. Reduction 3. Regeneration of the CO2 acceptor (RuBP) Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2 The Calvin cycle has three phases: 1.Carbon fixation (catalyzed by rubisco) incorporates carbon dioxide into an organic molecule. 2. Reduction the organic molecule is reduced. 3. Regeneration of the CO2 acceptor RuBP-ribulose bisphosphate) RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. 24 Life depends on photosynthesis The Importance of Photosynthesis: A Review § The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds § Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells § Plants store excess sugar as starch in chloroplasts and other structures such as roots, tubers, seeds, and fruits  Chloroplast H2O CO2 Light NADP+ ADP 3-Phosphoglycerate LIGHT + REACTIONS: Photosystem II Pi RuBP CALVIN CYCLE Electron transport chain Photosystem I ATP Electron transport chain G3P NADPH Starch (storage) O2 Sucrose (export)  LIGHT REACTIONS CALVIN CYCLE REACTIONS Are carried out by molecules Take place in the stroma in the thylakoid membranes Use ATP and NADPH to convert Convert light energy to the chemical CO2 to the sugar G3P energy of ATP and NADPH Return ADP, inorganic phosphate, Split H2O and release O2 and NADP+ to the light reactions to the atmosphere A Comparison of Chemiosmosis in Chloroplasts and Mitochondria § Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy § Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP § Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities § In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix § In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma A Comparison of Chemiosmosis in Chloroplasts and Mitochondria § ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place § In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH  Comparison of chemiosmosis in mitochondria and chloroplasts Mitochondrion Chloroplast Diffusion of Inter- H+ through membrane ATP synthase Thylakoid H+ space space Electron Inner transport Thylakoid MITOCHONDRION membrane chain membrane CHLOROPLAST STRUCTURE STRUCTURE ATP Pumping synthase of H+ Matrix by ETC Stroma ADP + P i H+ ATP Higher [H+] Lower [H+] H+ More details and extra material (optional) Linear Electron Flow § During the light reactions, there are two possible routes for electron flow: cyclic and linear § Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy § There are eight steps in linear electron flow: 1. A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites P680 2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+) 3. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680 § P680+ is the strongest known biological oxidizing agent § The H+ are released into the thylakoid space § O2 is released as a by-product of this reaction 4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane 5. Potential energy stored in the proton gradient drives production of ATP by chemiosmosis 6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor § P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain 7. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) 8. NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH § The electrons of NADPH are available for the reactions of the Calvin cycle § This process also removes an H+ from the stroma The energy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy e– e– e– Mill makes NADPH e– ATP e– e– n Ph o to e– ATP Ph o to n Photosystem II Photosystem I Cyclic Electron Flow § In cyclic electron flow, electrons cycle back from Fd to the PS I reaction center via a plastocyanin molecule (Pc) § Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH § No oxygen is released § Some organisms such as purple sulfur bacteria have PS I but not PS II § Cyclic electron flow is thought to have evolved before linear electron flow § Cyclic electron flow may protect cells from light-induced damage