BSCI 105 – SESSION 3● CLICKER 1: energy available to a cell to do work = free energyENZYMES AND METABOLISM● Metabolism: the sum of all biochemical reactions going on in every cell○ Anabolic reactions – biosynthetic (building molecules; using energy)○ Catabolism reactions – degradative (breaks things down)● Metabolic pathways: reactions that occur one after another in orderly fashion● Metabolism is all about ENERGY● Energy: capacity to do work○ Types: Kinetic (being currently used) + potential (stored) energy○ Units: kilocalorie (kcal) or joules● 1st Law of Thermodynamics○ Energy can be converted from one form to another, but can’t be created or destroyed Conservation of Energy● 2nd Law of Thermodynamics○ Entropy in the universe always increases○ Entropy – a measure of disorder○ Energy used to increase order in the system (to decrease entropy)○ Energy made available to do work as order decreases in the system > entropy increases● Energy = “Free Energy”○ Energy AVAILABLE to do work○ G (Gibbs free energy)■ T and P constant in a system (system = cell)○ G (free energy) = H (enthalpy) – T(temp)S(entropy) G = H – TS○ Enthalpy (H) = energy stored in the system■ Contained in chemical bonds○ Delta () = “change”■ G = G(products) – G(reactants)○ Positive G■ Products of the reaction contain MORE free energy than the reactants■ Endergonic (endothermic) – energy needs to go in■ Input of energy is required for reaction to occur○ Negative G■ Products of reaction contain LESS free energy than the reactants■ Exergonic (exothermic)■ Energy is released that is available to do work■ Spontaneous● Chemical Equilibrium○ Chemical reactions are reversible ○ If you know Keq, you can calculate G● Activation Energy (free energy of activation)○ Energy required before a chemical reaction will start○ In biological systems, enzymes lower the energy of activation○ Enzymes DO NOT affect Keq or G○ Enzymes speed up reaction■ Do not change thermodynamics● Enzymes○ Biological catalysts○ Usually proteins○ Reactants of enzyme – substrates○ Area of interaction with substrates – active site○ Enzyme itself is unchanged after reaction and can be used again○ Name tells us what reaction it catalyzes (-ase)● Properties of enzymes○ Specificity■ Due to shape■ Active site and induced fit● Activity of enzymes○ Influenced by:■ Temperature■ pH■ Co-factors (metal ions, coenzymes)■ Inhibitors (competitive, non-competitive)● Control of enzymes○ Allosteric control■ Allosteric site is where “effector” molecules bind■ Binding of effector molecule changes the conformation of the enzyme■ Effector can be activator or inhibitor11/10/10●CLICKER 1: preferred starting point for cellular respiration = GLUCOSE●Metabolic pathways○ How chemical energy is obtained■ Autotrophs – make their own food■ Heterotrophs● Harvest energy from electronsGlucose Metabolism – Cellular respiration●Cellular energy currency = ATP (adenosine triphosphate)●Need energy to do work●ATP carries energy○ In cells, energy released from hydrolysis of ATP is coupled with a second, endergonic reaction●ATP important in coupled reactions●ATP comes from ADP/ATP cycle; 3 ways to make ATP from ADP:○ Substrate level phosphorylation (no oxygen required)○ Oxidative phosphorylation (requires oxygen)○ Photophosphorylation (only in plants)●Substrate-level phosphorylation○ ANAEROBIC○ Energy released when certain covalent bonds are broken and is coupled with the remaking of ATP from ADP and Pi●Cellular Respiration○ Harvest energy to make ATP from complete aerobic breakdown of glucose○ 3 main stages:■ Glycolysis: glucose pyruvate■ Kreb’s cycle (citric acid cycle): pyruvate carbon dioxide■ Electron transport chain■ (Also extra transition step – links glycolysis to kreb’s cycle)●Pathways:○ What steps require energy?○ What step is controlled (allosteric)?○ What are the redox reactions?○ What are the starting points (substrate) and final products?●Glycolysis○ Starting point – GLUCOSE■ Other sugars can enter at other points○ End point – 2 molecules of PYRUVATE○ 10 biochemical reactions■ First 5 are endergonic○ Takes place in cytosol○ DOES NOT REQUIRE OXYGEN!○ All enzymes free in cytoplasm○ Not associated with any organelles■ Any cells can do it!○ ATP made using SUBSTRATE LEVEL PHOSPHORYLATION○ Downside: at end, <4% of potential energy has been harvested from glucose○ Overall reaction:■ Glucose + 2 ADP + 2 Pi + 2 NAD + ■ 2 pyruvate + 2 ATP + 2 NADH○ Net ATP = 2 ATP (plus 2 pyruvate and 2 NADH)●Gluconeogenesis Reverse Pathway○ Making glucose from pyruvate■ Fasting, prolonged exercise○ Requires energy (takes 6 ATP molecules)●When pyruvate enterst he mitochondria it is converted into Acetyl CoA for use by the Kreb’s cycleTHE KREBS CYCLE● Fate of pyruvate○ Major crossroad in metabolism○ If cell needs energy – goes into Krebs cycle○ If cell doesn’t need energy – converted into glucose, then glycogen○ If cell doesn’t have oxygen – goes into fermentation● Transport of pyruvate○ Pyruvate must be actively transported from cytosol into mitochondrial matrix● Conversion of pyruvate○ Pyruvate Acetyl Coenzyme A (Acetyl CoA)○ 3 step process using a single multi-enzyme complex■ **Pyruvate dehydrogenase complex○ Occurs in mitochondrial matrix○ Step 1: oxidation of pyruvate■ Yields 1 CO2○ Step 2: oxidation of 2C fragment acetate■ Yields 1 NADH○ Step 3: conversion of pyruvate● Acetyl CoA○ Produced by oxidation of pyruvate○ Product of metabolism of: proteins, fats, lipids○ Can be used:■ For fat synthesis■ For ATP production● Cycle○ Yield per turn: 1 ATP, 3 NADH, 1 FADH2, 2 CO2○ Recycling● Glycolysis + Krebs cycle○ Glucose + 4 ADP + 4 Pi + 10 NAD+ + 2 FAD ○ 6 CO2 + 4ATP + 10 NADH + 2 FADH2● Enzymes○ Many classes of enzymes used in cellular respiration○ Kinase – transfers phosphate group between molecules○ Dehydrogenase – oxidizes a molecule (removes hydrogen atom)○ Decarboxylase – removes carboxyl group from molecule (makes CO2)○ Isomerase – catalyses structural rearrangement of a molecule11/15/10● CLICKER: chemical energy carried by electrons is “harvested” through a series of ______________
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