Photosynthesis Lecture Notes - Bl1004 2024-25 PDF

Physiology of Plants and Animals BL1004 Photosynthesis 1 Photosynthesis Campbell and Reece; Edition 7, 8, 9 Chapter 10  overview  concept 10.1  concept 10.2 excluding paragraph on “cyclic electron flow”. ...

Physiology of Plants and Animals BL1004 Photosynthesis 1 Photosynthesis Campbell and Reece; Edition 7, 8, 9 Chapter 10  overview  concept 10.1  concept 10.2 excluding paragraph on “cyclic electron flow”.  concept 10.3  Concept 10.4 excluding paragraph on “photorespiration”. Chapter 9,  overview  concept 9.1 only “stages of cellular respiration” 2 Photosynthesis  Photosynthesis is proces that “impacts” the entire planet 3 The pyramid of life 4 Photosynthesis  Autotrophs  Use inorganic forms of carbon, i.e. CO2  Require an external energy source to reduce CO2 into organic carbon, e.g. sugars  Heterotrophs  Require a supply of organic carbon, i.e. sugar  Depend on dead or living biological matter  Obtain energy as organic carbon, which they can oxidise to produce CO2 Organic carbon is oxidised to create CO2, through respiration by both autotrophs and heterotrophs 5 The carbon cycle Numbers in billion tonnes 6 Photosynthesis  Autotrophs  “self-feeders”  Use inorganic carbon plus other nutrients  Primary producers  Include plants, unicellular and multicellular eukaryotic algae (protista) & prokaryotes  Heterotrophs  Live on organic forms of carbon, as well as other nutrients in organic form, i.e. produced by other organisms  Consumers  Herbivores, carnivores, decomposers 7 Biology; every rule there is an exception! 8  Plants are….? Nr Mallow castle Parasite on ivy Broomrape, a parasitic plant ….sometimes heterotrophs! 9  Animals are …..? Elysia chlorotica, a sea slug that photosynthesises with the help of algal chloroplasts …..sometimes partly autotrophic (mixotrophic)!10  Kleptoplasty Kleptoplasty is the behaviour whereby an animal takes chloroplasts from a food source and incorporates these into the tissues of their own digestive tract This can be considered a partial endosymbiosis (not the entire alga, but only its plastids are absorbed) 11 Energy for autotrophs  Photo-autotrophs use light energy  Plants, algae, prokaryotes  Chemo-autotrophs use chemical energy  OxidiseH2S, NH4+, Fe2+ and so on  Archaea (group of early prokaryotes) 12 Photosynthesis  Photosynthesis; conversion light energy into chemical (sugars etc.) energy 13 Leaf structure 14 Photosynthesis  Mesophyll (parenchyma)  Up to 40 chloroplasts per cell  Green colour of chlorophyll 15 Chloroplast 16 Endosymbiosis & origin of the chloroplast Unicellular photosynthesising Heterotropic algal cell host cell Double membrane Unicellular photosynthesising algal cell 17 Chloroplast  Double membrane  ctDNA (Chloroplast DNA)  Thylakoid membranes  Thylakoids stacked in to grana  Internal lumen  Stroma  Chlorophyll located on thylakoid membranes 18  Understanding photosynthesis (& growth)  Jan Baptist Van Helmont in the 1640’s  Aristotle; plants feed on organic matter in soil  Van Helmont questioned whether soil was really the source of biomass (critical thinking!) 19 Van Helmont Testing a hypothesis: plant biomass derived from organic matter in the soil 5 year 20  Understanding photosynthesis  5lb willow twig in pot with 300lb soil  After 5 year, 16lb willow, with minimal loss of soil  Biomass derived from “water”?!?! (why did he think so?) 21  Understanding photosynthesis  Jan Baptist Van Helmont early 1600’s  Testing a hypothesis  Modern science! 22  Understanding photosynthesis - 2  Joseph Priestley (1733-1804)  Good air / bad air  Oxygen; "dephlogisticated air"  “oxygen better than common air for the purpose of respiration" 23 Character air Light Dark 24  Understanding photosynthesis - 2  Joseph Priestley (1733-1804)  Good air / bad air  Oxygen; "dephlogisticated air"  Invention of soda water (carbonated water - Schweppe)  Priestley “father of the soft-drink” 25 Chlorophyll and light Absorption photon (= package of energy) Chlorophyll in excited state Chlorophyll falls back to ground state; energy released as heat or fluorescence. 26 Photosynthesis; capturing the energy of excited chlorophyll Light energy captured and converted to chemical energy (Photosynthesis) 27  Energy release as heat Temperature of crop informs about efficiency photosynthesis 28 Photosynthesis 6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 29 Photosynthesis; a two-step process 30 Photosynthesis; a two-step process Light reactions: conversion of light energy in chemical energy (ATP and NADPH) Dark reactions (Calvin cycle): Incorporation of CO2 into sugars with energy produced in light reactions 31  Light reactions-1 Redox process (reduction-oxidation) Transfer of electron from donor (water) to an acceptor molecule with a lower redox potential Requires energy Process driven by energy from absorbed photons of light 32 Two-pumps drive photosynthesis 33 Two-pumps drive photosynthesis Photosystem II and Photosystem I The two systems function sequentially 34 A photosystem 35 Photosystems – 3 components Chlorophyllorganised as light harvesting (antennae) complexes that capture photons Reaction centre (special pair of chlorophylls) that catalyses a charge separation Primary electron acceptor 36 Non-cyclic electron flow 37  Non-cyclic electron flow PSII charge separation Transferelectron to primary electron acceptor and onwards to PSI Re-reduction PSII by electron from water PSI charge separation Transferelectron to primary electron acceptor and onwards to NADP+ Nicotinamide adenine dinucleotide phosphate Re-reduction PSI by electron from PSII 38  Non-cyclic electron flow Product-1 NADPH; a form of reducing energy (i.e. energy plus electrons) 39 Water splitting  2 H2O  4H+ + O2 + 4 e- 40 Acidification of the lumen 41  ATP-synthesis Energy stored as H+ gradient Energy H+ gradient utilised to drive ATP-synthesis by ATP-synthase Chemiosmosis; movement of protons across a selectively permeable membrane & against an electrochemical gradient Chemiosmosis drives photo-phosphorylation 42 End-products non-cyclic electron flow Product-1 NADP+ + 2e- + H+  NADPH Product-2 ADP + P  ATP Both NADPH and ATP are forms of chemical energy that will be used to produce (amongst others) sugars 43 Speed Calvin cycle depends on light reactions 44 Calvin cycle Dark reactions (Calvin cycle) Incorporation of CO2 into sugars with energy from light reactions ATP & NADPH 45 The Calvin cycle 46 Calvin cycle phase 1 Carbon fixation  CO linked to ribulose bisphosphate 2  Ribulose bisphosphate carboxylase  C5 + C  unstable C6  2 C3 47 The Calvin cycle 48  Calvin cycle phase 2 Phosphorylation and reduction  C3 are phosphorylated using ATP  C3 reduced by NADPH  Production C3 sugar, can converted to:  Glucose (C6H12O6) and other sugars 49 The Calvin cycle 50  Calvin cycle phase 3 Regeneration acceptor molecule ribulose bisphosphate  5 C3s are combined to give 3 C5s  Result; re-cycling of carbon skeleton that can incorporate CO2 51 52 The Calvin cycle The Calvin cycle; a great example of how all living organisms produce new chemical molecules, without being able to resort to extreme conditions such as high pressure and/or high temperature 53  CO2 in the atmosphere; plentiful? 54 https://www.co2.earth/daily-co2 CO2 in the atmosphere; plentiful? 55  C4 metabolism Adaptations that increase leaf CO2- concentrations without increasing transpiration Adaptations that gather all available CO2 in a small group of cells Calvin-cycle prefaced with C4-cycle 56 C4-photosynthesis C4-cycle (before Calvin cycle) Calvin-cycle (C3) 57 Function linked to structure (C4 leaf anatomy) 58 Function linked to structure In C4 plants: Bundle sheath cells fixate CO2 through Calvin-cycle Rest of mesophyll collects CO2 and transports it as C4 to bundle sheath 59 C4 metabolism  CO2 bound to phosphoenolpyruvate by phosphoenolpyruvate-carboxylase  PEP and PEPC  C3 + CO2  C4  C4 transported to bundle sheath  CO2 released in bundle sheath cells  C3 returned to mesophyll (cycle) 60 Why does it work?  PEPC can operate at very low CO2 (high affinity for CO2)  Large catchment area for CO2  Thus works as concentrating mechanism 61 C4-metabolism comes at a cost 62 C4-metabolism comes at a cost  Most C4-plants originate in hot, dry areas  Maize, sugarcane, millet and sorghum 63 64 CAM-metabolism  Crassulaceae-family acid metabolism  C4 acids accumulate during night in vacuoles  CO2-released during day  Decreases water loss during hot/dry day  Temporal rather than spatial separation C3 and C4 cycles 65 C4 plants, climate change and the rise in CO2 ? 66 C4 plants, climate change and the rise in CO2 ? C4 plants are adapted to low CO2-levels C4 plants are heat/drought adapted 67 Photosynthesis in reverse = mitochondrial respiration Cellular respiration converts the chemical energy contained in complex molecules (e.g sugars) into ATP 68 Mitochondrial respiration Unlike photosynthesis, respiration is shared amongst plants and animals 69  Mitochondrial respiration Aerobic respiration in mitochondria requires O2 Fermentation = a form of anaerobic respiration (not in mitochondria & less efficient) 70  Mitochondrial respiration Aerobic respiration in mitochondria: Sugar + O2  CO2 + water +ATP In the case of glucose C6H12O6+ O2  CO2 + water +ATP 71  Three stages in respiration Glycolysis = sugar splitting Breakdown of C6-sugar into 2 C3 pyruvate molecules Produces net 2 ATP and 2 NADH per glucose 72  Three stages in respiration Citric acid cycle = Krebs cycle (releases CO2) Breakdown of pyruvate into CO2 Produces net 2 ATP, 6 NADH and 2 FADH2 per glucose 73  Three stages in respiration Oxidative phosphorylation & electron transport Use of electron transport chain to oxidise NADH and FADH2 to form H2O from O2 Produces net 26-28 ATP per glucose 74  Aerobic respiration Takes place in mitochondria (except for glycolysis) Present in most eukaryotes, as well as many prokaryotes Conserved mechanism across widely different groups of organisms Plant Human 75