L5 2025 Cellular Energetics PDF

Cellular Energetics Dr Denise Hough Lecturer in Physiology, Ageing and Welfare School of Biodiversity, One Health and Veterinary Medicine, MVLS From Course Information Document ILO’s: – Explain the concepts of free energy and entropy – Use thermodynamics to explain why enzymes make met...

Cellular Energetics Dr Denise Hough Lecturer in Physiology, Ageing and Welfare School of Biodiversity, One Health and Veterinary Medicine, MVLS From Course Information Document ILO’s: – Explain the concepts of free energy and entropy – Use thermodynamics to explain why enzymes make metabolic pathways so effective – Describe the role of ATP in coupling processes that release energy with those that require energy – Explain the importance of oxidation-reduction reactions in energy generation – Define the purpose of glycolysis, krebs cycle, electron transport chain, chemiosmosis, oxidative phosphorylation and cellular respiration. – Name the main pathways and their location in the cell by which ATP is generated under aerobic and anaerobic conditions  What is bioenergetics? Resource: Biology: A Global Approach Campbell text book, 12th Ed. Chapter 6: Energy and Life, p.141-153 Chapter 10: Cellular Respiration, p.236-241 How cells produce and use energy Is based on the laws of thermodynamics Relates to energy …  capture (respiration, photosynthesis)  storage (ATP, NADH, proton gradient)  conversion (light, chemical, kinetic)  Energy conservation and conversion 1. First law of thermodynamics: – No energy can be lost within the Universe – Energy can be converted from one form into another 2. Second law of thermodynamics: – Every energy transfer increases the entropy (chaos) of the Universe – Takes the environment into account Waste/chaos: Fuel: Heat & sound Chemical energy energy Work: Kinetic energy  Energy conservation and conversion Not all forms of energy can be converted into biologically relevant work (e.g. movement/growth) An organism loses some of its energy to the environment (typically heat) An organism fixes some of its energy irreversibly (e.g. secondary metabolites in plants for taste/odour) Living organisms require constant energy input Irreversible fixing: Food: e.g. hair & secondary metabolites Chemical Waste/chaos: energy Heat Work: Fat storage: Kinetic Chemical energy energy  Energy balance Ecosystem & Plants Individual – Energy conversion – Energy loss/heat Plants & Animals Cell’s energy currency Cellular respiration: metabolic process with which an organism obtains energy by oxidizing nutrients and releasing waste products Metabolism & ATP energy currency ATP acts as energy reservoir within the cell Energy used for cellular work (chemical, transport, movement ) Metabolism = totality of organism’s chemical reactions – Anabolism: use energy to build complex molecules – e.g. protein synthesis from amino acids – Catabolism: release energy through molecule breakdown – e.g. breakdown of glucose in glycolysis y erg en s es u m is gy bol r e Net Energy a An en es as le re m o lis tab Ca Progress through metabolic pathway  Gibbs free energy Universe = system + surroundings J. W. Gibbs 1878: Free energy of the system, G = the portion of the system’s energy that can perform work Change of free energy when the system changes (e.g. metabolic reaction, transport) D G = H - T S Change in the total energy Change in the disorder Temperature of the system (enthalpy) of the system (entropy) Rule: A reaction or transport process only occurs spontaneously if it decreases G (if G is negative).  Chemical reactions require or release energy A A: Exergonic reaction: Amount of energy released Gibbs free energy free energy released G0 Reactants Products  Enzymes regulate metabolism Metabolic pathways are made up of a group of enzyme- catalysed reactions Enzyme lowers activation energy (EA) required to kick- start reaction Enzymes are often pH- dependent Enzymes are regulated by gene expression and protein modification (e.g. phosphorylation ) Enzymes are often inhibited by the end product in pathway (negative feedback) ATP is used as an energy currency Adenosine-triphosphate (ATP) releases energy when the outermost inorganic phosphate (Pi) is cleaved off and yields adenine-diphosphate (ADP) The (chemical) energy is stored in the outermost Pi because the negative charges repel each other H 2O Energy Pi + ADP  G= -7.3 kcal/mol (-30.5 kJ/mol)  How is ATP formed? A: Substrate-level phosphorylation B: Oxidative phosphorylation (transfer of phosphate group) (Chemiosmosis)  e.g. glycolysis: phosphoglycerokinase  e.g. ATP synthase in inner mitochondrial membrane Chemiosmosis: movement of ions down their electrochemical gradient across a semipermeable membrane  Another energy source: Redox potential & electron transport chain 2 H (e.g. in sugar) Another form of energy is stored in the redox potential Reflects the different affinity of 2 H+ + 2 e– atoms to incorporate or release Free energy, G Elec chain electrons into/from their outer Electronegativity tron shell Energy = electronegativity. tran spor Relocating electrons from 2 e– sugars (weakly electronegative) t to oxygen (strongly /2 O2 1 2H + electronegative) releases energy. H2O  NADH is used as an electron currency Nicotinamide adenine dinucleotide Redox reaction: becomes oxidized Xe- + Y X + Ye- becomes reduced Similar: NADP+  NADPH, FAD  FADH2 Many molecules of NADH are produced during the breakdown of food. How can we transform electron currency into energy currency? Energy harvest: From electrons to ATP CHEMIOSMOSIS – proposed by Peter Mitchell around 1960 Energy can be stored in a transmembrane proton (H+) gradient This energy can be used to make ATP Establishment of H+ gradient is a separate process from ATP synthesis A directional movement of H+ ions across the membrane is accompanied by a chain of reactions involved with electron transport Intact membranes are an essential requirement for chemiosmosis. Energy harvest: From electrons to ATP NADH used in oxidation-reduction (REDOX) reactions to transfer H+ with electrons in electron transport chain H+ gradient used to produce ATP  What is cellular respiration? Cellular respiration: metabolic process with which an organism obtains energy by oxidizing nutrients and releasing waste products Resource: Biology: A Global Approach Campbell text book,12th Ed. Chapter 10: Cellular Respiration, p.236- 258 Involve pathways concerned with energy metabolism Organised across different subcellular locations What is the energy balance of respiration? Metabolism Metabolic pathways interact in a network Catabolic route: energy release Anabolic route: energy required The yellow highlighted area shows the pathways that we will discuss for cellular respiration: The breakdown of glucose to capture energy as ATP Cellular respiration can be divided into 3 stages Cellular respiration can be divided into 3 stages Cellular respiration can be divided into 3 stages Glycolysis Glycolysis Starts with glucose, a 6-carbon molecule – C6H12O6 1st step commits molecule to pathway and needs energy in the form of ATP Incorporates phosphate group * Enzyme name tip: – Kinase: adds phosphate Glycolysis Step 2 shifts the chemical bonds to form an isomer molecule * Enzyme name tip: – Isomerase: converts molecule from one isomer to another Glycolysis Step 3 incorporates another phosphate group and needs energy in the form of ATP * Enzyme name tip: – Kinase: adds phosphate  Glycolysis Step 4 splits the 6-carbon molecule into two smaller 3-carbon molecules (enzyme type: lyase) Step 5 converts the one molecule, yielding 2 identical 3-carbon molecules  Glycolysis Step 6 is the first redox reaction, where electrons are captured and carried by NADH * Enzyme tip: dehydrogenase enzymes oxidizes a substrate by reducing an electron acceptor  Glycolysis Step 7 is the first payoff reaction A phosphate group comes from the 3-carbon molecule and gets added to ADP to make energy rich ATP * Enzyme = kinase  Glycolysis Step 8 moves the chemical bonds around again in preparation for another substrate phosphorylation  Glycolysis Step 9 cleaves off water * Enzyme tip: Enolase is a hydro-lyase type enzyme  Glycolysis Final step: phosphate group taken to produce ATP, makes pyruvate  Preparing pyruvate for citric acid cycle Also known as the “link reaction” Does not proceed when there is no oxygen Citric acid cycle Also known as: Tricarboxylic acid (TCA) cycle Or Krebs cycle  Citric acid cycle One cycle with 1 Acetyl CoA: 3 NADH 1 FADH2 1 ATP Two cycles with 2 Acetyl CoA: 6 NADH 2 FADH2 2 ATP Recall that the link step which is the conversion of Pyruvate to Acetyl CoA generates 1NADH (happens twice) Aerobic respiration through glycolysis & citric acid cycle = 8 NADH and 2 FADH2 and 2 ATP Electron harvest and ATP production during respiration Electron transport chain and ATPsynthase  Electron transport chain and ATPsynthase Note: the actual number of ATP produced through chemiosmosis is variable, see textbook page 178 for details or see if you can find out why (not examinable). (Nice video resource: www.khanacademy.org) Depending on whether oxygen is present or not, glycolysis feeds into respiration or fermentation  Energy balance of fermentation (no or low oxygen) Fermentation produces 2 ATP per glucose (Compared to 32 ATP produced by oxidative phosphorylation!!!) O2 and oxidative phosphorylation led to an ‘explosion’ in evolution  Summary Living organisms require constant energy input Energy can be converted from one form into another e.g. light energy  chemical energy  kinetic energy Exergonic reactions release energy that can be used for chemical work (endergonic reactions), transport and movement ATP is formed by substrate-level and oxidative phosphorylation of ADP and acts as an energy currency NAD+ captures electrons from redox reactions and in the form of NADH delivers them to the electron transport chain The electron transport chain establishes a proton gradient across the inner membranes of mitochondria and chloroplasts H+ flow back through membrane-bound ATP synthases which produce ATP The trans-membrane H+ gradient acts as a ‘high-energy’ intermediate between redox potential and ATP synthesis  Summary Metabolism = totality of an organism’s chemical reactions, can be anabolic or catabolic Biochemical reactions are organised in metabolic pathways, which form complex metabolic networks Cellular respiration can be divided into three stages: glycolysis, citric acid cycle and oxidative phosphorylation Overall reaction for cellular respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O ∆G= -2870kJ/mol Cellular respiration produces approximately 30-32 ATP per glucose Under anaerobic conditions, glucose is fermented and only produce 2 ATP per glucose Metabolism is regulated by and through enzymes To do: Interactive Revision Use this Revision QuizBot to develop and test your knowledge on cellular energetics: Access via link/QR code: https://mizou.com/login-thread?ID=Gca_oOaKiF1WTjnWTrdwlVC9iww1mBNDnC6TObnWpzs-50720 It is in a chatbot style, which means that you can also ask for explanations, focus on particular topic or adjust difficulty level. 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