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Respiration • Organisms can be classified based on how they obtain energy: • Autotrophs – Able to produce their own organic molecules through photosynthesis • Heterotrophs – Live on organic compounds produced by other organisms • All organisms use cellular respiration to extract energy from organic molecules 1 Cellular respiration • Cellular respiration is a series of reactions • Aerobic respiration (in presence of oxygen) – Final electron receptor is oxygen (O2) • Anaerobic respiration (without oxygen) – Final electron acceptor is an inorganic molecule (not O2) • Fermentation (without oxygen – non humans) – Final electron acceptor is an organic molecule 2 • Aerobic respiration (in presence of oxygen) – Final electron receptor is oxygen (O2) • Anaerobic respiration (without oxygen) – Final electron acceptor is an inorganic molecule (not O2) • Fermentation (without oxygen – non humans) – Final electron acceptor is an organic molecule 3 Cellular Energy •In order to sustain life (steady state), cells constantly expend energy in the form of ATP hydrolysis –the hydrolysis of ATP yields a molecule of ADP (adenosine diphosphate) and a Phosphate group •ATP → ADP + P + E •Following ATP hydrolysis, the cell synthesizes new molecules of ATP using ADP and P –ADP + P + E → ATP –“recycling energy” –cells use the energy stored in the covalent bonds of monosaccharides, fatty acids and amino acids as an energy source (fuel) to resynthesize ATP Cellular Respiration -Most cells of the body use glucose in the presence of O2 that we inhale, as fuel source to synthesize molecules of ATP in a process called cellular respiration •The products include molecules of carbon dioxide that we exhale, and water -C6H12O6 + 6O2 + 38 ADP +38 P 6H2O + 6CO2 + 38 ATP –note that the number of atoms in the reactants equal the number of atoms in the products –That is the reaction is balanced Cellular Respiration The process of cellular respiration is divided into 3 sequential phases 1. Glycolysis occurs in the cytoplasm 2. Krebs cycle occurs in the mitochondrial matrix 3. The electron transport chain and oxidative phosphorylation occurs across the inner mitochondrial membrane - between the matrix and the intermembrane space •During the first 2 phases of glycolysis and the Krebs cycle, a molecule of glucose is gradually broken apart in 20 sequential biochemical reactions which transforms the glucose into different carbohydrate intermediates as bonds are broken or rearranged -4 molecules of ATP are synthesized as a result of energy released from the breaking of bonds of the carbohydrate intermediates –the 12 hydrogen atoms of the original glucose molecule are removed and used to create a large H+ gradient across the inner mitochondrial membrane -the H+ gradient is used as an energy source by the electron transport chain and oxidative phosphorylation to synthesize the remaining 34 molecules of ATP Summary of ATP Production Glycolysis -Glycolysis = sugar breaking •The first 7 biochemical reactions of cellular respiration are catalyzed by enzymes in the cytoplasm •Begins with 1 molecule of glucose (6 carbons) and ends with 2 molecules of pyruvic acid (3 carbons x 2 = 6 carbons) •During glycolysis: –2 molecules of ATP are hydrolyzed –one 6 carbon carbohydrate intermediate is split into two 3 carbon carbohydrate intermediates (PGAL) –in one reaction, each of the 3 carbon carbohydrate intermediates molecules lose a H (oxidized) which is transferred to a molecule of NAD+ (reduced to NADH) –4 molecules of ATP are synthesized Glycolysis Glycolysis -The products of glycolysis are: –2 molecules of ATP •4 are synthesized, but 2 are hydrolyzed at the beginning of glycolysis -2 molecules of pyruvic acid (3 carbon molecule) –2 molecules of NADH •move from the cytosol to the mitochondrial matrix •Only 2 of 38 molecules of ATP are synthesized during glycolysis Pyruvic Acid -The presence or absence of O2 in the cell determines how the cell will use the 2 molecules of pyruvic acid at the end of glycolysis –If O2 is present pyruvic acid will be used in the process of aerobic respiration -36 more molecules of ATP are synthesized as cellular respiration continues through the Krebs cycle and the electron transport chain and oxidative phosphorylation •If O2 is absent the pyruvic acid will be used in the process of anaerobic respiration (fermentation) •produces no more ATP Anaerobic Fermentation (Anaerobic Respiration) •In the absence of O2 (oxygen debt), the 2 molecules of pyruvic acid from the end of glycolysis are converted into 2 molecules of lactic acid •Anaerobic fermentation most commonly occurs in active skeletal muscle cells when they use O2 faster than it can be delivered by the respiratory and circulatory systems •The accumulation of lactic acid causes: –muscle proteins to partially denature •leading to muscle weakening (fatigue) –muscle soreness Aerobic Respiration -In the presence of O2, the 2 molecules of pyruvic acid from the end of glycolysis are transported into the mitochondrial matrix •Each molecule of pyruvic acid is converted into a molecule of Acetyl Coenzyme A (Acetyl CoA) -required step between glycolysis and the Krebs cycle 2 Pyruvic Acid → 2 Acetyl CoA During the conversion of each of the 2 molecules of pyruvic acid to Acetyl CoA the following occurs: •Decarboxylation (CO2 is removed) of pyruvic acid making acetic acid (2 carbons) -Acetic acid is oxidized while NAD is reduced to NADH -Coenzyme A in the mitochondrial matrix is added to the acetic acid to make acetyl CoA During this step the products are: –2 molecules of CO2 –2 molecules of NADH –2 molecules of Acetyl CoA •used in the Krebs Cycle Pyruvic Acid → Acetyl CoA Krebs Cycle (Citric Acid Cycle) •A series of 10 biochemical reactions which begins and ends (cyclic) with a molecule of citric acid -Each acetic acid (2 carbons) is combined with a molecule of oxaloacetic acid (4 carbons) to make citric acid (6 carbons) •Each citric acid is decarboxylated and oxidized as it is converted into carbohydrate intermediates –NAD+ and FAD are reduced to NADH and FADH2 –energy released from the hydrolysis of bonds is used to synthesize ATP •These reactions produce: –3 molecules of NADH –1 molecule of FADH2 –2 molecules of CO2 –1 molecule of ATP –1 molecule of oxaloacetic acid Krebs Cycle •For each molecule of glucose entering glycolysis, two molecules of acetyl CoA enter the Krebs cycle yielding : Products of Krebs Cycle –6 NADH –2 FADH2 –4 CO2 –2 ATP –2 oxaloacetic acid •reenter the Krebs cycle of reactions after combined with another acetic acid Products of Glycolysis → Krebs Cycle •4 ATP –2 from glycolysis –2 from Krebs •10 NADH (10 H from original glucose molecule) –2 from glycolysis –2 from (2) Pyruvic Acid → (2)Acetyl CoA –6 from Krebs •2 FADH2 (2 H from original glucose molecule) –2 from Krebs •6 CO2 (6 C from original glucose molecule) –2 from (2) Pyruvic Acid → (2)Acetyl CoA –4 from Krebs Electron Transport Chain A group of 5 integral and peripheral membrane enzymes associated with the inner mitochondrial membrane called the electron transport chain (ETC) creates a H gradient between the intermembrane space and the matrix of the mitochondria using the H of molecules of NADH and FADH2 that have accumulates in the mitochondrial matrix as products of glycolysis and the Krebs cycle •3 Enzyme complexes of the ETC oxidizes the NADH and FADH2 (remove the H) in the matrix and split it into an electron (e-) and a proton (H+) •The e- is passed from one enzyme of the ETC to the next until it reaches the last enzyme of the ETC •NADH is oxidized by Enzyme complex 1 of the ETC •FADH2 is oxidized by Enzyme complex 2 of the ETC -The electron that is shuttled from enzyme to enzyme in the ETC initially has a high amount of energy -the electron energizes the 1st, 2nd and 3rd Enzyme complexes which provides enough energy to each complex to pump a free proton from the mitochondrial matrix into the intermembrane space –the energy in the electron decreases as it is passed along the ETC, as each Enzyme complex takes some of the energy away from the electron using it to pump protons –the 3rd Enzyme complex transfers the electron to an atom of oxygen in the matrix ETC and Oxidative Phosphorylation Free Energy of Electrons in the ETC Proton (H+) gradient •The movement of protons from the matrix into the intermembrane space creates a high H+ (pH = 7) concentration in the intermembrane space and a low H+ (pH = 8) concentration in the matrix (Chemiosmosis) –this proton gradient becomes the source of energy used by the mitochondria to synthesize ATP, which is released as H+ diffuse from the intermembrane space back into the matrix •The oxidation of NADH energizes all 3 Enzymes complexes (1st, 2nd and 3rd) moving 3 H+ across the inner membrane -The oxidation of FADH2 energizes only 2 enzymes (2nd and 3rd) moving 2 H+ across the inner membrane •The oxidation of NADH contributes more to the proton gradient than FADH2 ATP Synthase •ATP synthase is an integral membrane protein in the inner mitochondrial membrane that has 2 functions: –it acts as a H+ channel •allows H+ to diffuse down its gradient into the matrix –it acts as an enzyme •uses the energy that is released by the diffusion of H+ to synthesize ATP from ADP and P –this type of ATP synthesis is called oxidative phosphorylation because, this process requires the oxidation of NADH and FADH2 - and attaching Phosphate to ADP to make ATP ATP Synthesis During Oxidative Phosphorylation -Because the oxidation of NADH contributes more to the proton gradient than the oxidation of FADH2, more ATP is synthesized from NADH than FADH2 •On average, for each NADH that is oxidized 3 molecules of ATP are synthesized –10 NADH x 3 ATP = 30 ATP •On average, for each FADH2 that is oxidized 1.5 molecules of ATP are synthesized –2 FADH2 x 2 ATP = 4 ATP -34 of the 38 molecules of ATP are synthesized by oxidative phosphorylation Formation of Water -After the electrons arrive to the last enzyme of the ETC they are transferred to atoms of oxygen in the mitochondrial matrix -this makes oxygen especially negative -In the matrix, these oxygen atom are combined with the H+ that have diffused back into the matrix from the intermembrane space to form water (H2O) –the protons become “reunited” with their electron Oxidation Without O2 1. Anaerobic respiration – Use of inorganic molecules (other than O2) as final electron acceptor – Many prokaryotes use sulfur, nitrate, carbon dioxide or even inorganic metals 2. Fermentation – Use of organic molecules as final electron acceptor (lactic acid or ethyl alcohol) 35 Anaerobic respiration • Methanogens – CO2 is reduced to CH4 (methane) – Found in diverse organisms including cows • Sulfur bacteria – Inorganic sulphate (SO4) is reduced to hydrogen sulfide (H2S) – Early sulfate reducers set the stage for evolution of photosynthesis 36 37 Fermentation • Reduces organic molecules in order to regenerate NAD+ 1.Ethanol fermentation occurs in yeast – CO2, ethanol, and NAD+ are produced 2.Lactic acid fermentation – Occurs in animal cells (especially muscles) – Electrons are transferred from NADH to pyruvate to produce lactic acid 38 39 Catabolism of Fat • Fats are broken down to fatty acids and glycerol – Fatty acids are converted to acetyl groups by b-oxidation – Oxygen-dependent process • The respiration of a 6-carbon fatty acid yields 20% more energy than 6-carbon glucose. 40 Catabolism of Protein • Amino acids undergo deamination to remove the amino group • Remainder of the amino acid is converted to a molecule that enters glycolysis or the Krebs cycle – Alanine is converted to pyruvate – Aspartate is converted to oxaloacetate 41