Photosynthesis and Cellular Respiration EK2A2: Organisms capture and store free energy for use in biological processes.

Redox Reactions Vocabulary Redox reaction: refers to both reactions together; because an electron transfer requires both a donor and an acceptor, oxidation and reduction go together Oxidation: refers to the loss of electros Reduction: refers to the gain of electrons (e- = negative  reduce charge) Oxidizing agent: causes the oxidation; electron acceptor; must receive e- for a loss to happen Reducing agent: causes the reduction; electron donor; must give e- for a gain to happen OILRIG: oxidation is loss, reduction is gain LEO says GER: loss of electrons oxidation, gain of electrons reduction

Why do redox reactions matter to organisms? The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP EX: Cellular Respiration Glucose is oxidized and O2 is reduced Electrons lose potential eneregy as they move towards O2 e- move towards O2 because it is so electronegative (wants electrons to become more stable) energy is released becomes oxidized becomes reduced

Pair Share 45 seconds: Redox Rxns: Explain the difference between oxidation and reductions What is the difference between oxidizing agent and reducing agent Why are redox rxns important to living things What is the oxidizing agent in cellular respiration? 30 seconds: Overview of Cellular Respiration: What are the three main steps of cellular respiration? Where do these steps take place? What are the reactants and products of cellular respiration? What is the purpose of cellular respiration?

Overview of Cellular Respiration Glycolysis Citric acid cycle (aka Krebs cycle) Oxidative phosphorylation: Electron transport chain and chemiosmosis

Purpose of Cellular Respiration Break down organic molecules (usually glucose) to form ATP (ADPATP) Two main ways to form ATP Oxidative phosphorylation: 90% of ATP made this way; powered by redox reactions of the electron transport chain; inorganic phosphate is added to ADP by ATP synthase (driven by the chemical gradient created by the ETC) Substrate-level phosphorylation: enzyme transfers a phosphate group from a substrate to ADP; happens during glycolsis and citric acid cycle Enzyme ADP P Substrate ATP + Product

An overview of Cellular Respiration Basics Mitochondrion Substrate-level phosphorylation ATP Cytosol Glucose Pyruvate Glycolysis Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative Krebs Cycle phosphorylation: electron transport and chemiosmosis

NAD+ (Nicotineamide adenine dinuclotide) Coenzyme that acts as an electron acceptor Functions as an oxidizing agent during respiration Traps electrons from glucose and other organic molecules with the help of the enzyme dehydrogenase Enzyme removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate  delivers 2 electrons and 1 proton to its coenzyme (NAD+); other H+ is released into the surrounding solution (this H+ will become important later) Each NADH represents stored energy that is used to synthesize ATP Dehydrogenase

Details of Glycolysis Energy investment phase Glucose 2 ADP + 2 P 2 ATP used formed 4 ATP Energy payoff phase 4 ADP + 4 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net 4 ATP formed – 2 ATP used 2 NADH + 2 H+ Glycolysis means “sugar splitting;” glucose is broken into two molecules of pyruvate Occurs in the cytoplasm and has two major phases: Energy investing phase – activation energy is needed to get the exergonic reaction going (2ATP molecules) Energy payoff phase – ATP and NADH is created Glucose + 2 ATP (activation energy) pyruvate (Krebs cycle) + ATP (substrate-level) + 2NADH (carries electrons to ETC) + 2 H+ +2H2O

Energy investment phase The energy input and output of glycolysis Energy investment phase Glucose 2 ADP + 2 P 2 ATP used formed 4 ATP Energy payoff phase 4 ADP + 4 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net 4 ATP formed – 2 ATP used 2 NADH + 2 H+ Figure 9.8

A closer look at glycolysis Glucose-6-phosphate 2 Phosphogluco- ATP 1 Hexokinase ADP Glucose-6-phosphate Glucose-6-phosphate 2 Phosphoglucoisomerase 2 Phosphogluco- isomerase Fructose-6-phosphate Figure 9.9 Fructose-6-phosphate

Fructose- 1, 6-bisphosphate Glucose ATP 1 1 Hexokinase ADP Fructose-6-phosphate Glucose-6-phosphate 2 2 Phosphoglucoisomerase ATP 3 Phosphofructokinase: allosteric enzyme Fructose-6-phosphate ATP 3 3 ADP Phosphofructokinase ADP Figure 9.9 A closer look at glycolysis Fructose- 1, 6-bisphosphate Fructose- 1, 6-bisphosphate

Aldolase Isomerase Fructose- 1, 6-bisphosphate 4 5 Dihydroxyacetone Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose- 1, 6-bisphosphate 4 Fructose-6-phosphate Aldolase ATP 3 Phosphofructokinase ADP Figure 9.9 A closer look at glycolysis 5 Isomerase Fructose- 1, 6-bisphosphate 4 Aldolase 5 Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

2 2 ADP 2 ATP 2 3-Phosphoglycerate 1, 3-Bisphosphoglycerate 7 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 1, 3-Bisphosphoglycerate 2 ADP 2 3-Phosphoglycerate 7 Phosphoglycero- kinase 2 ATP Figure 9.9 A closer look at glycolysis 2 3-Phosphoglycerate

2 3-Phosphoglycerate 8 Phosphoglycero- mutase 2 2-Phosphoglycerate Fig. 9-9-7 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 2 3-Phosphoglycerate 8 Phosphoglyceromutase 8 Phosphoglycero- mutase 2 2-Phosphoglycerate Figure 9.9 A closer look at glycolysis 2 2-Phosphoglycerate

2 2-Phosphoglycerate Enolase 2 H2O 2 Phosphoenolpyruvate 9 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 2-Phosphoglycerate 2 3-Phosphoglycerate 8 Phosphoglyceromutase 9 Enolase 2 H2O 2 2-Phosphoglycerate 9 Enolase 2 H2O Figure 9.9 A closer look at glycolysis 2 Phosphoenolpyruvate 2 Phosphoenolpyruvate

Pyruvate 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 Phosphoenolpyruvate 2 ADP 2 3-Phosphoglycerate 8 10 Phosphoglyceromutase Pyruvate kinase 2 ATP 2 2-Phosphoglycerate 9 Enolase 2 H2O Figure 9.9 A closer look at glycolysis 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 Pyruvate 2 Pyruvate

Details of the citric acid cycle If O2 is present, pyruvate from glycolysis enters the mitochondria Pyruvate is then converted into Acetyl CoA In this process CO2 is released into the atmosphere and more NADH is formed (this will carry e- to the ETC) CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Pyruvate Transport protein CO2 Coenzyme A Acetyl CoA

Details of the citric acid cycle (continued) Pyruvate NAD+ NADH + H+ Acetyl CoA CO2 CoA Citric acid cycle FADH2 FAD 2 3 3 NAD+ + 3 H+ ADP + P i ATP Citric acid cycle (aka Krebs cycle) take place within the mitochondrial matrix Cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn (two turns per glucose molecule from glycolysis) Has eight steps, each catalyzed by a specific enzyme NADH and FADH2 produced by the Krebs cycle carry electrons extracted from food to the electron transport chain in the mitochondrial cristae membrane 2 additional molecules of CO2 are released (three total; 1 from pyruvate  acetyl Co A)

Citric Acid Cycle Acetyl CoA Oxaloacetate OAA Malate Citrate CoA—SH NADH +H+ 1 H2O NAD+ 8 Oxaloacetate OAA 2 Malate Citrate Isocitrate NAD+ Citric Acid Cycle NADH 3 + H+ 7 H2O CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 4 6 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

Pair Share At the end of glycolysis and the citric acid cycle…where is the energy that was originally in the glucose molecule???

Let’s look at the energy so far Following glycolysis and the citric acid cycle we have a total of 4 ATP molecules that have been created via substrate- level phosphorylation 2 from glycolysis and 2 from Krebs Most of the energy extracted from the glucose molecule is currently stored in the Coenzyme carries NADH and FADH2 Some of the energy has been lost as heat Energy in the NADH and FADH2 is now going to be used in the third step of cellular respiration (electron transport chain)

What exactly is the “Electron Transport Chain? NADH NAD+ 2 FADH2 FAD Multiprotein complexes Fe•S FMN Q  Cyt b   Cyt c1 Cyt c Cyt a Cyt a3 IV Free energy (G) relative to O2 (kcal/mol) 50 40 30 20 10 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O e– waste Collection of molecules embedded in the inner membrane of mitochondria in eukaryotic cells (prokaryotes = plasma membrane) Most components are proteins that exist in multiprotein complexes numbered I through IV

Electron Transport Chain NADH NAD+ 2 FADH2 FAD Multiprotein complexes Fe•S FMN Q  Cyt b   Cyt c1 Cyt c Cyt a Cyt a3 IV Free energy (G) relative to O2 (kcal/mol) 50 40 30 20 10 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O e– waste Electron carries alternate between reduced and oxidized states; as electrons move down the chain energy is released Electrons removed from glucose by NAD+ are transferred from NADH to the first molecule of the electron transport chain in complex I The last electron acceptor passes its electrons to O2 (which is known as the terminal electron acceptor) O2 picks up a pair of H+ (hydrogen ions) from the aqueous solution to form H2O ETC does not generate ATP; purpose is to break release of energy into smaller manageable amounts

Chemiosmosis Chemiosmosis: energy-coupling mechanisms that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work In cellular respiration, H+ gradient is established by the electron transport chain, at certain steps along the ETC electron transfer causes H+ to be moved from the inside of the mitochondrial matrix to the inner membrane space; creates a proton H+ gradient

ATP Synthase In addition to the proteins that make up the ETC there are also protein complexes called ATP synthases ATP synthase: enzyme that makes ATP from ADP and inorganic phosphate; uses energy from the hydrogen gradient created by the electron transport chain The exergonic flow of H+ ions through ATP synthase drives phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive work (in this case ATP synthesis) The energy stored in the proton gradient couples with the redox rxn of the ETC to make ATP (oxidative phosphorylation) The H+ gradient is a proton-motive force

Chemiosmosis: Energy Coupling - couples the electron transport chain to ATP synthesis Protein complex of electron carriers Cyt c V Q   ATP synthase  2 H+ + 1/2O2 H2O FADH2 FAD NADH NAD+ ADP + P ATP Figure 9.16 Chemiosmosis i (carrying electrons from food) H+ 1 Electron transport chain: redox 2 Chemiosmosis Oxidative phosphorylation

Energy Flow through Cellular Respiration: A Summary Glucose  NADH  electron transport chain  proton-motive force  ATP Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP Oxidative phosphorylation: electron transport chemiosmosis Citric Acid Cycle 2 Acetyl CoA Glycolysis Glucose Pyruvate 2 NADH 6 NADH 2 FADH2 CYTOSOL Electron shuttles span membrane or MITOCHONDRION ATP yield per glucose molecule oxidized is not an exact number; but it is between 36-38 ATP molecules/ glucose molecule depending on efficiency of cellular respiration 4 ATP by substrate- level phosphorylation 32-34 ATP by oxidative phospohyalation

Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Without the electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation stops Two general mechanisms can oxidize organic fuel and generate ATP w/o O2: anaerobic respiration and fermentation

Anaerobic Respiration Other less electronegative substances can be used in place of O2 as the final electron acceptor in an electron transport chain EX: “sulfate-reducing” marine bacteria use sulfate ion (SO42-) at the end of their ETC; rather than H2O being the end product H2S is the by product.

Fermentation A way of harvesting chemical energy without oxygen or any ETC Recall ATP is created in glycolysis Glucose + 2 ATP (activation energy) pyruvate (Krebs cycle) + ATP (substrate-level) + 2NADH (carries electrons to ETC) + 2 H+ +2H2O In the case of fermentation the Krebs cycle and ETC do not exist…for the organisms to continue fermentation there must be a way to recycle NADH back to NAD+ Organisms has developed two ways to do this: alcohol fermentation and lactic acid fermentation

Two Types of Fermentation Alcoholic Fermentation: Pyruvate is converted to ethanol in two steps Releases carbon dioxide from the pyruvate  produces acetaldehyde Acetaldehyde is reduced by NADH to ethanol (this step generates the supply of NAD+ needed for the continuation of glycolysis Lactic Acid Fermentation: Pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2 Human muscle cells make ATP by lactic acid fermentiaotn with oxygen is scarce

Photosynthesis as a Redox Process During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. Electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harness that energy to synthesize ATP Photosynthesis reverses the direction of electron flow: water is split, and electrons are transferred along with hydrogen ions from the water to carbon dioxide, reducing it to sugar Because the electrons increase in potential energy as they move from water to sugar, this process requires energy (endergonic)…the energy is provided by light

Photosynthesis Preview The equation of photosynthesis is relatively simply…but the reality is the actual process is not simple…actually two main steps with multiple steps within each Light Reaction Photo part of the reaction; takes place in the thylakoid Light energy is absorbed and converted into a chemical form (ATP and NADPH) Dark Reaction Synthesis part of the reaction; takes place in the stroma Chemical energy from the light reaction is used to to make sugar

More details of the Light Reaction Water is split  provides electrons and protons  O2 is given off as a by-product Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions to an electron acceptor (in photosythesis the electron acceptor is NADP+ (nicotinamide adenine dinucleotide phosphate) Light reaction reduces NADP+ to NADPH by adding a pair of electrons along with an H+ Light reaction also generates ATP using chemiosmosis to power the addition of a phosphate group to ADP  this process is called photophosphorylation

More details of the Calvin Cycle Cycle begins with incorporating CO2 from the air into organic molecules already present in the chloroplast; this process is called carbon fixation Calvin cycle then reduces the fixed carbon to carbohydrates by adding electrons; the reducing power is provided by NADPH (from the light reaction) To convert CO2 to C6H12O6 the Calvin cycle requires chemical energy in the form of ATP (from the light reaction)

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH Fig. 10-5-4 H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast [CH2O] (sugar) O2

Light Reaction EVEN MORE DETAILS (a) Excitation of isolated chlorophyll molecule Heat Excited state Photon Ground (fluorescence) Energy of electron e– Chlorophyll molecule Chlorophyll is a pigment located inside photosystems Photosystems are proteins embedded in the thylakoid’s membrane There are two photosystems associated with photosynthesis: Photosystem I and Photosystem II When chlorophyll inside Photosystem II absorbs light it is excited due to one of its electrons being elevated to an orbital where it has more potential energy

(INTERIOR OF THYLAKOID) Photosystem II THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes Photosystem consists of a reaction-center complex and light- harvesting complexes Light-harvesting complexes funnel the energy of light to the reaction center A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll Enzyme splits water into two electrons, two hydrogen ions and an oxygen molecules (O2) is released and electrons are dumped into the electron transport chain between PSII and PSI

+ H+ Electron transport chain CO2 NADP+ reductase Photosystem II H2O O2 ATP Pc Cytochrome complex Primary acceptor Photosystem I + H+ Fd NADPH Electron transport chain Pq

Photosystem I The exergonic fall of electrons to a lower energy level provides energy for the synthesis of ATP As the electrons pass through certain parts of the electron transport chain, protons are pumpped across the membrane The proton gradient created is used in chemiosmosis to generate ATP via ATP synthase At the same time PSI absorbs light energy in the light-harvesting complex and transfers it to the PSI reaction-center complex, the energy travels down a second electron transport chain and is stored in NADPH ATP and NADPH can not be used in the Calvin cycle to reduce CO2 into glucose

Summary of the complex light reaction Mill makes ATP e– NADPH Photon Photosystem II Photosystem I Light is absorbed by chylorphyll, with the help of photosystems I and II, the light energy travels down two electron transport chains, the first electron transport chain generates ATP, the second electron transport chain generates NADPH Both NADPH and ATP are ready to help the Calvin cycle

H+ Diffusion Electron transport chain ADP + P Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

Dark Reaction: EVEN MORE DETAILS Calvin cycle is anabolic, building carbohydrate from smaller molecules and consuming energy Carbon enters the Calvin cycle in the for of CO2 and leaves as sugar The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar Calvin cycle is broken into three phases: Phase 1: Carbon fixation Phase 2: Reduction Phase 3: Regeneration of the Co2 acceptor (RuBP)

Figure 10.18 The Calvin cycle Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 3 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i Figure 10.18 The Calvin cycle 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

Electron transport chain Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure 10.21 A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)