Lecture 6: Photosynthesis Breda, 8 December 1730 Calne (UK), 7 September 1799 Jan Ingenhousz discovered cellular respiration and photosynthesis Photosynthesis: Key Concepts - Some organisms can absorb the electromagnetic energy visible spectrum (light) and convert it into chemical energy → PHOTOAUTOTROPHS Photosynthesis is the most important of all of the energy transformations: without it, there would be no life (as we know it because we couldn’t eat nor have all necessary 02) Photosynthesis: Chemical Reaction 6 C02 + H20 + Light Energy → C6H12 06 + 6 02 Photosynthesis as a redox reaction movement of electrons from one component to another: oxidoreduction reaction → 1 component becomes reduced and the other oxidized OIL RIG What is photosynthesis ? - A series of light-driven reactions that creates organic molecules from inorganic carbon dioxide (CO2) (= carbon fixation) In photosynthesis, water molecules are split (source of electrons), releasing vast quantities of O2 (O2 is a waste product for plants) Here, we will talk about plant photosynthesis Algae (green, red and brown) use different components everything is connected: photosynthesis in the chloroplast: glucose and oxygen are the output of photosynthesis and the input of cellular respiration while CO2 output respiration input photosynthesis Structure of a leaf epidermis transpined because light must go inside mesophyll means middle of leaf divided into palisade and spongy mesophyll, exchange of gas through spongy mesophyll: C02 needed and 02 leaves the cell bottom layer not sealed: exchange between outside environment and inside // stoma: door from which gas can go inside and outside of the leaf veins: vascular tissue, sugar produced need to go in all places Stoma can open and grow the door 2 cells surrounding opening (= guard cells) they can open and close swell or shrink depending on if they absorb or lose water, through stomata CO2 can enter and 02 can go out Photosynthesis in plants: Where? - Chloroplasts → Organelle with 3 membranes - The thylakoid membrane contains the light-capturing and ATPgenerating systems The solar radiation hits the leaf not all radiation from sunlight is useful for chloroplast, some of it is reflected or transmitted and lost. Only a fraction is absorbed (50%) electromagnetic spectrum composed of different wavelength visible light is divided into different components (from UV to infrared) in principle plants could use all these wavelength but they actually don’t Chlorophyll - Chlorophyll contains a porphyrin ring that binds magnesium - Similar to hemoglobin - Light can excite the porphyrin network to an activated (‘energized’) state molecule that plants use for water, absorb water, make photosynthesis called pigment → chemical compound able to absorb light → chlorophyll is able to absorb light blue ring with Magnesium inside which is the part able to absorb the light, then the tail is used by the chlorophyll to connect to the system (inside membrane) Catch the light: photoreceptors - Chlorophylls (a and b) best absorb light in the blue and red wavelengths - These pigments absorb green light poorly: thus, plants look green to us chlorophyll can’t absorb all the wavelength: leaves green because can’t absorb green wavelength and reflect it to us chlorophyll = main pigment for photosynthesis The 2 stages of photosynthesis 1- light reaction 2 - Calvin Cycle Photo-synthesis: photo=light synthesis:Calvin Cycle Using light, water outcome = energy in the form ATP (energy carrier) , NADPH (electron carrier) Then this energy moves to second part: calvin cycle: energy carrier and electrons used to produce sugars: exit calvin cycle in uncharged molecule everyhting happens within the chloroplast Stage 1 (the light reactions) - But: - largely similar to oxidative phosphorylation Electron-transport chain (ETC) in the thylakoid membrane → energy of the electron transport to pump protons into the thylakoid space → proton gradient → ATP The high-energy electrons donated to the photosynthetic ETC come from chlorophyll that has absorbed energy from sunlight The electrons are donated to NADP+ (FINAL ACCEPTOR) to produce NADPH Stage 2 (the dark reactions) - - Light-independent reactions sunlight is NOT needed here! The ATP and NADPH produced in stage 1 are used to produce sugars from CO2 (carbon-fixation reactions) Begins in the stroma (matrix of the chloroplast) with the generation of glyceraldehyde 3-phosphate (G3P) by the combination of CO2 and 1,5ribulose bisphosphate (enzyme: Rubisco) G3P is exported to the cytosol → glucose a nd other organic molecules in the leaves Stage 1 Chlorophyll in photosystems - Antenna complex, which capture and transfer light energy - A reaction center, which converts the light energy into chemo electric energy photosystem: different systems of chlorophylls able to catch the light. As soon as light comes, E transferred from one molecule to another until the central part of the photosystem (reaction center): place from where electrons is able to go to the electron transport chain system uses energy of sunlight to produce an electron which the, produces ATP and NADPH group called antenna complex Not one but 2 photosystems - Photosystem II ATP - Photosystem I → NADPH - Photosystem II donates electrons to photosystem I Photosystem II - Electrons are passed to a mobile electron carrier called plastoquinone, which is part of the photosynthetic ETC → ATP production in chloroplast antenna complex with molecules of chlorophyll reaction center 2 red boxes electron released start to travel in thylakoid membrane (similar to electron transport chain in cellular respiration) as soon as the electron moves along the transport chain, protons are sent out → different concentration between inside and outside → protons want to flow back, they can only through ATP synthase → source of energy (ADP → ATP) but as soon as chlorophyll loses electron it needs to be filled by… water Oxygen is formed in Photosystem II! - Electrons lost by the special pair of chlorophyll are replenished by electrons obtained from H2O through an H2O-splitting enzyme splitting a molecule of water into oxygen, electrons are released into the reaction center 02 is the by-product Photosystems II Essentially all the oxygen in the Earth’s atmosphere has been produced by the water-splitting enzyme of photosystem II (21%) Big History of Life The oxygen revolution or the great oxygenation event – GOE; 2.3 Ga) - ~2.5 Ga, Cyanobacteria began producing O2 as by-product of photosynthesis - At first, any free O2 was chemically captured by dissolved iron or organic matter - ~2.3 Ga, these O2 sinks could no longer capture all produced O2 Photosystems I and NADPH production Electrons are passed to a mobile electron carrier called ferredoxin (Fd), which carries its electrons to ferredoxin-NADP reductase (FNR), which generates NADPH reaction center light, electron released transport chain → electrons arrived on the final component to produce NADPH Photosystems are interconnected electron of photosystem I is the one coming from photosystem II light hits the antenna complex full of chlorophyll → e of sunlight concentrated into reaction center → el leaves molecule of chlorophyll to the electron transport chain to form ATP photosystem I light → el lost connextion: el coming from photosystem II used to replace el from photosystem I → close system 1 similarities between respiration and photosynthesis mitochondria and chloroplast have both inner membrane and inner membrane space the components of electron transport chain lie on the membrane then can form ATP because protons are pumped out of the matrix outside way back is how ATP is formed the use of ATP and NADPH: stage 2 Stage 2: the light-independent reactions - ATP and NADPH are used to drive carbon fixation in the dark reactions in the stroma Carbon fixation occurs in a cycle: carbon-fixation cycle or Calvin cycle Rubisco - The enzyme initiating the Calvin cycle - Ribulose-1,5-bisphosphate carboxylase/oxygenase - It is the most abundant protein on Earth - molecule inside chloroplast with 5 carbons Calvin Cycle in chloroplast, ribulose 15 bisphosphate 5 Ca atoms + C02 from outside RUbisco combines the C02 and the molecule → serie of process → 1st important product: G3P 3C : from 1 Rubisco we end up with 2 mole during this process we use ATP and NADPH 6 ATP are used and 6 NADPH (lots of energy needed for sugars, explain why lots of energy in sugars) G3P 6 molecules → 3 go back in the cylce and 1 becomes a molecule of glucose At the end spend all of this energy just to produce half of the molecule of glucose Moreover, ARP is needed at the end 9 ATP and 6 NADPH to create half of sugar molecule in the whole cycle starting point ribulose biphosphate 5 C + CO2 with Rubisco enzyme → process plants spend huge amount of energy to produce G3P → 1 molecule leaves the chloroplast and is used to form sugars while the other molecules of G3P go back into the cycle - CO2 fixation: capture C02 - CO2 reduction - regeneration Take home message Three Questions: - Why do we need to eat? we need energy for chemical bonds out of food - Why do we breath? need 02 for electron transport chain - Why plant do not need to eat? How can they produce their own food (and also oxygen)? (((Note: NADH → respiration NADPH → Photosynthesis molecular structure similar except for the Phosphate))
