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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

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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

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ATP

• Adenosine Triphosphate (ATP) is the energy currency of the cell.

– used to drive movement– used to drive endergonic reactions

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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.

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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+.

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NAD+ & NADH

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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

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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

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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.

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Glucose Catabolism

• Cells catabolize organic molecules and produce ATP in two ways:

– substrate-level phosphorylation– aerobic respiration

in most organisms, both are combinedglycolysispyruvate oxidationKrebs cycleelectron transport chain

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Aerobic Respiration

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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

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Priming Reactions

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Cleavage Reactions

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Energy-Harvesting Reactions

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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

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Recycling NADH

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Stage Two - Oxidation of Pyruvate

• Within mitochondria, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2.

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Stage Three - Krebs Cycle

• Acetyl-CoA is oxidized in a series of nine reactions.

– two steps: priming energy extraction

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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

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Krebs Cycle

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Krebs Cycle

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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.

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Electron Transport

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Stage Four: The Electron Transport Chain

• NADH molecules carry electrons to the inner mitochondrial membrane, where they transfer electrons to a series of membrane-associated proteins.

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Electron Transport Chain

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Chemiosmosis

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ATP Generation

• This process begins with pyruvate, the product of glycolysis, and ends with the synthesis of ATP

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Theoretical ATP Yield of Aerobic Respiration

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Regulating Aerobic Respiration

• Control of glucose catabolism occurs at two key points in the catabolic pathway.

– glycolysis - phosphofructokinase– Krebs cycle - citrate synthetase

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Electron Transport System

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Control of Glucose Catabolism

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Catabolism of Proteins and Fats

• Proteins are utilized by deaminating their amino acids, and then metabolizing the product.

• Fats are utilized by beta-oxidation.

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Cellular Extraction of Chemical Energy

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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