Cellular Respiration: Harvesting Chemical Energy Principles of energy conservation The process of cellular respiration Related metabolic processes 6O 2 +C 6 H 12 O 6  6H 2 O + 6CO 2 + energy

Concept 9.1 Metabolic pathways that release energy are called catabolic pathways - fermentation and cellular respiration Fermentation: partial degradation of sugars that occurs w/out the help of O 2 Cellular respiration: O 2 is consumed as a reactant along w/ the sugar - more efficient

Redox Reactions Reduction: gain of electrons Oxidation: loss of electrons Cellular respiration is a redox process

Redox Reactions Redox reactions release energy when electrons move closer to electronegative atoms - the relocation of electrons releases the energy stored in food molecules, and this energy is used to synthesize ATP

Concept 9.1 There is a transfer of one or more e- from one reactant to another; the electron transfers are called oxidation-reduction reactions or redox rxns. - the loss of e- from one substance is called oxidation - the addition of e- to another substance is called reduction

Concept 9.1

Electrons “fall” from organic molecules to oxygen during cellular respiration C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy - by oxidizing glucose, cellular respiration takes energy out of storage and makes it available for ATP synthesis - carbohydrates and fats are reservoirs of electrons associated w/ hydrogen

Concept 9.1 The “fall” of electrons during respiration is stepwise, via NAD + and an electron transport chain Glucose is broken down over a series of steps that are each catalyzed by a specific enzyme Hydrogen atoms are stripped from the glucose and usually passed to NAD +. - NAD + is reduced in the rxn.

Concept 9.1 NAD + is transformed to NADH - NADH will later be tapped to make ATP as the electrons continue their fall from NADH to oxygen Respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps instead of one explosive rxn.

Concept 9.1

Electrons removed from food are shuttled by NADH to the “top” end of the chain. At the “bottom”, oxygen captures the electrons along with H + ions to form water Food  NADH  ETC  oxygen

Concept 9.1 Respiration consists of three stages: - glycolysis, the Krebs cycle, electron transport chain (ETC) Glycolysis breaks down 1 glucose into 2 molecules of pyruvate - occurs in the cytosol Krebs cycle breaks down pyruvate into CO 2 - occurs in the mitochondrial matrix

Concept 9.1 ETC accepts electrons from the breakdown products of the first 2 stages - the energy released at each step of the chain is used to make ATP (oxidative phosphorylation); through redox rxns. oxidative phosphorylation accounts for 90% of generated ATP

Concept 9.1

Substrate-level phosphorylation: direct transfer of a phosphate to ADP by an enzyme Each molecule of glucose is degraded into carbon dioxide, water and 38 molecules of ATP

Coenzymes are intermediate energy carriers Found in all cells Assist enzymes in energy transfer NAD and FAD Join with H+ and e- Many coenzymes are vitamins

Concept 9.1 Substrate-level phosphorylation: direct transfer of a phosphate to ADP by an enzyme Each molecule of glucose is degraded into carbon dioxide, water and 38 molecules of ATP

Concept 9.1

Cellular Respiration Glycolysis Krebs cycle Electron transport chain

Concept 9.1 Cellular respiration occurs in the mitochondria Organic + O 2  Carbon + H 2 O + Energy compounds dioxide C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy 1 glucose = -686 kcals

Concept 9.2 Glycolysis means “splitting of sugar” - the 10 steps of glycolysis are broken down into two phases: energy investment and energy payoff - glucose (6C)  2 pyruvate (3C) Energy investment phase: the cell spends 2 ATP to phosphorylate the fuel molecules

Concept 9.2 Energy payoff phase: 4 ATP are produced by substrate-level phosphorylation; 2 NAD + are reduced to 2 NADH by the oxidation of food Net energy yield: 2 ATP and 2 NADH

Glycolysis Catabolic pathway “To split sugar” Glucose + 2ATP  2 pyruvates + 4ATP + 2NADH Occurs in cytosol Anaerobic process 10 step pathway

Concept 9.3 If O 2 is present, energy stored in NADH can be converted to ATP Upon entering the mitochondrion, each pyruvate is first converted to a molecule of acetyl CoA (2C) - another NAD + is reduced to NADH

Concept 9.3

Intermediate Reaction (Oxidative Decarboxylation) Occurs if O 2 is present 2 Pyruvate  2 CO NADH + 2 Acetate Attaches acetate to CoA  Acetyl CoA

Krebs Cycle In mitochondrial matrix Oxidation of acetyl CoA to CO 2 Products include: 2 ATP + 6 NADH + 2 FAD + 4 CO 2 8 step pathway

Concept 9.3 Acetyl CoA will enter the Krebs cycle for further oxidation Krebs cycle - 8 steps, each catalyzed by a specific enzyme - Acetyl CoA (2C) enters, 2 CO 2 (1C) leave, 3 NAD +  3 NADH, 1 FAD  1 FADH 2, 1 ADP  1 ATP

Concept 9.4 Cristae: inner membrane folding of the mitochondria - increases surface area for more ETC’s Electrons removed from food during gycolysis/Krebs are transferred by NADH to the first molecule of the ETC

The Electron Transport Chain Embedded in the inner mitochondrial membrane Most ETC molecules are proteins Primary ATP generating pathway Produces ATP via Substrate Level Phosphorylation and Chemiosmosis

Concept 9.4 Most of the electron carriers in the ETC are proteins called cytochromes (cyt). The process goes downhill with oxygen being the final e- acceptor - for every 2 NADH, 1 O 2 molecule is reduced into 2 molecules of water

Concept 9.4 FADH 2 adds its e- at a lower energy level than NADH on the ETC. -NADH = 3 ATP - FADH 2 = 2 ATP ETC makes no ATP directly. It moves e- from food to oxygen breaking the energy drop to manageable amounts.

Concept 9.4 Inside the inner membrane are enzymes called ATP synthase. - makes ATP from ADP and a phosphate ATP synthase uses energy from the ion gradient to synthesize ATP. - proton gradient

Concept 9.4 The ETC is an energy converter that uses the exergonic flow of e- to pump H + ions across the membrane - from the matrix to the inner membrane space. ATP synthases are the only place that are freely permeable to H +

Chemiosmosis: the energy coupling mechanism Electron flow actively transports H+ from the matrix into the intermembrane space Flow of H+ “down the gradient” thru the ATP synthase complex generates ATP

Concept 9.4 H + gradient across a membrane couples the redox rxns. of the ETC to ATP synthesis - chemiosmosis: connection between the chemical rxn. Makes ATP and transport across a membrane

Concept 9.4 Chemiosmosis is also found in the chloroplasts - ATP is generated during photosynthesis - light drives both e- flow down the ETC and H + gradient formation

Concept 9.4 H + ions are pumped by members of the ETC. The resulting gradient is called a proton-motive force: the gradient has the capacity to do work

Concept 9.4

Energy flow during respiration: Glucose  NADH and FADH  ETC  proton-motive force  ATP 38 ATP formed; 4 from substrate phosphorylation, 34 from oxidative phosphorylation

Concept 9.5 Alcohol fermentation: pyruvate  ethanol - CO 2 is released to recycle NAD step process to regenerate NAD +

Concept 9.5

Lactic acid fermentation: pyruvate  lactic acid - human cells make ATP by (LAF) when oxygen is scarce - lactate is carried away by blood to the liver; lactate is converted back to pyruvate by liver cells

Concept 9.5

w/out oxygen, the energy still stored in pyruvate is unavailable to cells Facultative anaerobes: yeasts and bacteria that can make enough ATP to survive using either fermentation or respiration

Concept 9.5 Ancient prokaryotes probably used anaerobic fermentation before oxygen was present in the atmosphere Also, glycolysis does not require mitochondria to occur

Related Metabolic Processes (The versatility of catabolism) Can use proteins, fats and polysaccharides Monomers of fats and proteins enter pathways at various points Can result in generation of energy or biosynthesis of new molecules

Concept 9.6 Proteins must be broken down to amino acids - various amino acids can be converted as intermediates of glycolysis and the Krebs cycle

Concept 9.6 Carbohydrates can be hydrolyzed to form glucose monomers to enter into glycolysis Metabolism works on supply and demand!!!!

Concept 9.6 Carbs and fats can be converted to fats through intermediates of glycolysis and the Krebs cycle We will store fat even if we have a fat free diet

Regulation of Cell Respiration Feedback mechanism Allosteric enzyme- phosphofructo- kinase Citrate and ATP are the inhibitors ADP and AMP are allosteric activators

Concept 9.6 Cellular respiration is controlled by feedback mechanisms Feedback inhibition: end products inhibit the enzymes that catalyze the early steps of the process Phosphofructokinase (enzyme for step 3 of glycolysis) is the pacemaker

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