Chapter 3

1. Chapter 3 An introduction to metabolism

2. Metabolism and Energy • The sum of all the reactions that take place in our cells. • Reactions require ENERGY • Energy is the ability to do work (cellular work) • Different types of energy • Chemical, Electrical, Mechanical, Light, Thermal • Each form of energy can be converted to other forms • Energy can exist in two states • Kinetic energy • Occurs as a result of motion (molecules, ions moving in solution) • Potential energy • Stored within an object (chemical bonds, glucose)

3. Energy Obeys Laws • The First Law of Thermodynamics • The total amount of energy in any closed system is constant. • Energy cannot by created or destroyed; it can only be converted from one form to another. • If a physical system gains an amount of energy, another physical system must experience a loss of energy of the same amount • Photosynthesis • Sunlight converts to chemical energy • Cellular Respiration • Convert chemical energy into mechanical energy (muscle contraction)

4. Conversion of Energy • Depends on • Breaking and formation of chemical bonds in a chemical reaction • Energy is absorbed when breaking bonds • Energy is released when forming bonds • Combustion of methane (CH₄) • Mole • 6.022 x 10²³ of atoms of a certain element • Gives each element on the periodic table its certain atomic mass 1 mol of carbon dioxide 1 mol of methane 2 mol of oxygen 2 mol of water

5. Bond Energy • Measure of the strength or stability of a covalent bond. • Measured in (kJ/mol) • Different amounts of energy are required to break different types of bonds • Activation Energy • The minimum amount of energy needed to break bonds and start a chemical reaction (match needed to start combustion reaction)

6. Chemical Reaction • Activation energy required • Reaches transition state • Temporary condition in which bonds in the reactants have reached breaking point and new bonds are ready to form in products. • Two types • Exothermic reaction • Net release of energy • Endothermic reaction • Net absorption of energy BEabsorbed – BEreleased= Net energy released/absorbed

7. Energy Obeys Laws • The Second Law of Thermodynamics • In every energy transfer or conversion, some of the useful energy in the system becomes unusable and increases the entropy of the universe. • Entropy • A measurement of disorder in a system • Always increases whenever there is a chemical reaction • Increases when • Solids react to form liquids or gases • Liquids react to form gaseous products • Total number of product molecules is greater than the total number of reactant molecules

8. Spontaneous vs Non-Spontaneous Change • Predict whether a chemical reaction will occur without the continuous input of additional energy • Spontaneous • Will continue to occur on its own (lit match) • No additional energy required • Three Factors to determine whether a reaction will happen spontaneously (favourable) • Energy changes - exothermic changes – release of energy • Entropy – increase • High temperatures • Non-spontaneous • Will not continue to occur on its own (boiling pot of water) • Additional energy is required • Factors determining whether a reaction will not happen (not favoured) • Energy changes – endothermic – absorption of energy • Entropy decreases – increasing order • Low temperatures

9. Favouredvs Not Favoured

10. Gibbs Free Energy • Energy that is not lost during a reaction • Represented by the symbol G • Applies to both chemical and physical processes • Always a reduction in the amount of free energy after completion of process • Responsible for (living organisms) • Synthesis of molecules • Reproduction • Movement • Represented by • ∆G = Gfinalstate – Gintitialstate

11. ∆G = Gfinalstate – Gintitialstate • Difference in the free energy of the final state of molecules to the free energy of the initial state • Negative values (-∆G) (spontaneous) • Free energy of products is less than free energy of reactants • Gives off free energy • C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O ∆G = -2870 kJ/mol glucose oxidized • Exergonic reaction – releases free energy • Positive values (+∆G) (non-spontaneous) • Free energy of products is more than free energy of reactants • Must gain free energy to occur • This reaction cannot happen on its own • 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ ∆G = +2870 kJ/mol glucose formed • Endergonic reaction – requires energy

12. ExergonicvsEndergonic Every type of cell in every organism continuously carries out thousands of these reactions

13. Coupled Reactions Transfer of energy from an exergonic reaction to an endergonic reaction Free energy from exergonic reaction drives endergonic reaction When combined free energy is released and both reactions occur spontaneously A → B + C ∆G = - 5 kJ/mol D + E → F ∆G = +4 kJ/mol

14. Catabolic vs Anabolic • Two types of pathways • Catabolic • Complex molecules are broken down into simpler compounds • Release free energy • -∆G • Anabolic • Build complicated molecules from simpler molecules • Consumes free energy • + ∆G

15. ATP Adenosine triphosphate Supplies energy that powers nearly every cellular function Energy “currency”

16. Where Does Energy From ATP Come From? • Consists of • Nitrogenous base (adenine) • 5 carbon sugar (ribose) • 3 phosphate groups • Free energy comes from the three negatively charges phosphate groups

17. Hydrolysis of ATP • Catabolic reaction • Phosphate group is broken off using water • Two products formed • Adenosine diphosphate (ADP) • Inorganic phosphate (Pi) • H⁺ released into solution • ATP + H₂O → ADP + Pi ∆G = -30.5 kJ/mol

18. ATP and Energy Coupling Release of a phosphate group (ATP) attaches itself to a reactant molecule - phosphorylation Molecule gains free energy becoming more reactive Enzyme brings ATP molecule and reactant molecule together – allow for transfer of phosphate group

19. Regeneration of ATP • ATP molecules must be generated for cells to keep functioning • Cells generate ATP by combining ADP with Pi • Reaction called ATP synthesis • Requires free energy • Where do we obtain energy to create ATP? • Food • Breakdown of carbohydrates, fats proteins • All are sources of free energy

20. ATP Cycle ATP is hydrolyzed and resynthesized at least 10 million times per second in our cell

21. Enzymes and Activation Energy • Metabolism in organisms would be slow if enzymes were not present • Just because a reaction can proceed on its own does not mean that it will proceed. • Speed at which a reaction occurs is increased by enzymes • Almost all enzymes are proteins • RNA molecules can also function as enzymes • Only function of enzyme • Lower the potential energy level of the transition state. • Catalyzed reactions written with enzyme name above reaction arrow • Name of enzyme ends in “-ase” and is usually based on substrate

22. What Provides Activation Energy for A Chemical Reaction? • Thermal Energy – combustion reactions • Combustion of propane • A small spark is enough energy for some reactants to overcome the activation barrier • Increase in temperature is bad in biology • Destroy structural components of some proteins and DNA • Speed up all of the reactions in a cell

23. Enzymes Lower Activation Energy

24. How do Enzymes Reduce Activation Energy? • Important – Substrate molecules need to be in transition state for a reaction to proceed. • Enzymes can achieve this in 3 different ways • Bring molecules together (a) • Expose reactant molecules to altered charge environments (b) • Active site contains ionic groups with (+) or (-) charges • Change the shape of the substrate (c) • Weakens its chemical bonds reducing energy required to break them (induced fit model)

25. Food as Fuel • C –H bonds • Position of electrons to atomic nuclei of carbon and hydrogen • Electrons are approx equidistant from two small nuclei (high energy) • Farther away an electron is from the nucleus the more potential energy it has • Size of nucleus affects potential energy • Electrons are more strongly attracted to a larger nucleus than a smaller one

26. Energy Changes During Oxidation • Oxidation of glucose (exergonic) • Transfer of electrons to O₂ • Controlled oxidation • Cells are able to capture more free energy and produce less waste thermal energy • Occurs through a series of enzyme-catalyzed reactions • Energy released transfers to energy-carrying molecules

27. Energy Carriers • Dehydrogenases (Class) • Facilitate transfer of high-energy electrons from food molecules • NAD⁺ • Nicotinamide adenine dinucleotide • Important in metabolic processes • Remove two hydrogen atoms from a substrate molecule • NAD⁺ becomes reduced to NADH • Other H⁺ is released into cytosol • Facilitate the synthesis of ATP