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How Cells Harvest Energy Chapter 9 Outline •Cellular Energy Harvest •Cellular Respiration –Glycolysis –Oxidation of Pyruvate –Krebs Cycle –Electron Transport Chain •Catabolism of Protein and Fat •Fermentation 1 Cellular Respiration • Cells harvest energy by breaking bonds and shifting electrons from one molecule to another. – aerobic respiration - final electron acceptor is oxygen – anaerobic respiration - final electron acceptor is inorganic molecule other than oxygen – fermentation - final electron acceptor is an organic molecule 2 ATP • Adenosine Triphosphate (ATP) is the energy currency of the cell. – used to drive movement – used to drive endergonic reactions 3 ATP • Most of the ATP produced in cells is made by the enzyme ATP synthase. – Enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient. ATP synthesis is achieved by a rotary motor driven by a gradient of protons. 4 NAD+ & NADH • • • • • Nicotinamide adenine dinucleotide, NAD+, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups: with one nucleotide containing an adenosine ring, and the other containing nicotinamide. In metabolism, NAD+ is involved in redox reactions, carrying electrons from one reaction to another. The coenzyme is therefore found in two forms in cells: NAD+ is an oxidizing agent – it accepts electrons from other molecules and becomes reduced, this reaction forms NADH, which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD+. 5 NAD+ & NADH 6 • • • • The Cellular isms Metabolism: is the set of chemical reactions that occur in living organisms in order to maintain life. – These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. – Usually divided into two categories. Catabolism and Anabolism Catabolism – breaking down Anabolism – building up 7 The Cellular isms • Catabolism: the set of metabolic pathways which break down molecules into smaller units and release energy. – Large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino acids, respectively. – These processes produce energy 8 The Cellular isms • Anabolism: the set of metabolic pathways that construct molecules from smaller units. – These reactions require energy. – Anabolism is powered by catabolism. Many anabolic processes are powered by adenosine triphosphate (ATP). – Anabolic processes tend toward "building up" organs and tissues. – These processes produce growth and differentiation of cells and increase in body size, a process that involves synthesis of complex molecules. 9 Glucose Catabolism • Cells catabolize organic molecules and produce ATP in two ways: – substrate-level phosphorylation – aerobic respiration in most organisms, both are combined glycolysis pyruvate oxidation Krebs cycle electron transport chain 10 Aerobic Respiration 11 Stage One - Glycolysis • • • For each molecule of glucose that passes through glycolysis, the cell nets two ATP molecules. Priming – glucose priming – cleavage and rearrangement Substrate-level phosphorylation – oxidation – ATP generation 12 Priming Reactions 13 Cleavage Reactions 14 Energy-Harvesting Reactions 15 Recycling NADH • As long as food molecules are available to be converted into glucose, a cell can produce ATP. – Continual production creates NADH accumulation and NAD+ depletion. + NADH must be recycled into NAD . aerobic respiration fermentation 16 Recycling NADH 17 Stage Two - Oxidation of Pyruvate • Within mitochondria, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2. 18 Stage Three - Krebs Cycle • Acetyl-CoA is oxidized in a series of nine reactions. – two steps: priming energy extraction 19 20 Overview of Krebs Cycle • • • • • • • 1: Condensation 2-3: Isomerization 4: First oxidation 5: Second oxidation 6: Substrate-level phosphorylation 7: Third oxidation 8-9: Regeneration and oxaloacetate 21 Krebs Cycle 22 Krebs Cycle 23 Harvesting Energy by Extracting Electrons • Glucose catabolism involves a series of oxidation-reduction reactions that release energy by repositioning electrons closer to oxygen atoms. – Energy is harvested from glucose molecules in gradual steps, using NAD+ as an electron carrier. 24 Electron Transport 25 Stage Four: The Electron Transport Chain • NADH molecules carry electrons to the inner mitochondrial membrane, where they transfer electrons to a series of membraneassociated proteins. 26 Electron Transport Chain 27 Chemiosmosis 28 ATP Generation • This process begins with pyruvate, the product of glycolysis, and ends with the synthesis of ATP 29 Theoretical ATP Yield of Aerobic Respiration 30 Regulating Aerobic Respiration • Control of glucose catabolism occurs at two key points in the catabolic pathway. – glycolysis - phosphofructokinase – Krebs cycle - citrate synthetase 31 32 Control of Glucose Catabolism 33 Catabolism of Proteins and Fats • • Proteins are utilized by deaminating their amino acids, and then metabolizing the product. Fats are utilized by beta-oxidation. 34 Cellular Extraction of Chemical Energy 35 Fermentation • Electrons that result from the glycolytic breakdown of glucose are donated to an organic molecule. + – regenerates NAD from NADH ethanol fermentation lactic acid fermentation 36