Carbohydrate Metabolism Digestion of Carbohydrates In the mouth
Carbohydrate Metabolism
Digestion of Carbohydrates In the mouth: – Salivary amylase hydrolyzes starch into dextrin +maltose In the stomach: – due to drop of p. H salivary amylase acts for a very short time In the small intestines: – Pancreatic and intestinal enzymes hydrolyze the oligo- and polysaccharides follows: as
Glucose Oxidation Extracting Energy from Glucose: There are 3 major biochemical processes that occur in cells to progressively breakdown glucose with therelease of various packets of energy: • Glycolysis (occurs in the cytoplasm and is only moderately efficient). • Krebs' cycle (takes place in the matrix of the mitochondria and results in a great release of energy). • Electron transport chain.
I. Glycolysis (Embden Meyerhof Pathway): A. Definition: 1. Glycolysis means oxidation of glucose to give pyruvate (in the presence of oxygen) or lactate (in the absence of oxygen). B. Site: cytoplasm of all tissue cells, but it is of physiological importance in: 1. Tissues with no mitochondria: mature RBCs, cornea and lens. 2. Tissues with few mitochondria: Testis, leucocytes, medulla of the kidney, retina, skin and gastrointestinal tract. 3. Tissues undergo frequent oxygen lack: skeletal muscles especially during exercise.
C. Steps: Stages of glycolysis 1. Stage one (the energy requiring stage): a) One molecule of glucose is converted into two molecules of glycerosldhyde-3 -phosphate. b) These steps requires 2 molecules of ATP (energy loss) 2. Stage two (the energy producing stage): a) The 2 molecules of glyceroaldehyde-3 -phosphate are converted into pyruvate (aerobic glycolysis) or lactate (anaerobic glycolysis(. b) These steps produce ATP molecules (energy production).
Energy Investment Phase (steps 1 -5) Fig. 9. 9 a
Fig. 9. 9 b Energy-Payoff Phase (Steps 6 -10)
Energy production of glycolysis: ATP produced ATP utilized Net energy In absence of oxygen 4 ATP (anaerobic glycolysis) (Substrate level phosphorylation) 2 ATP from 1, 3 DPG. 2 ATP from phosphoenol pyruvate 2 ATP 2 ATP From glucose to glucose -6 -p. From fructose -6 -p to fructose 1, 6 p. In presence of oxygen 4 ATP (aerobic glycolysis) (substrate level phosphorylation) 2 ATP from 1, 3 BPG. 2 ATP from phosphoenol pyruvate. 2 ATP 6 ATP -From glucose to Or glucose -6 -p. 8 ATP From fructose -6 -p to fructose 1, 6 p. + 4 ATP or 6 ATP (from oxidation of 2 NADH + H in mitochondria).
Differences between aerobic and anaerobic glycolysis: Aerobic Anaerobic 1. End product Pyruvate Lactate 2. energy 6 or 8 ATP 2 ATP 3. Regeneration of NAD+ Through respiration chain in mitochondria Through Lactate formation 4. Availability to TCA in mitochondria Available and 2 Pyruvate Not available as lactate can oxidize to give 30 is cytoplasmic substrate ATP
Importance of lactate production in anerobic glycolysis: 1. In absence of oxygen, lactate is the end product of glycolysis: Glucose Pyruvate Lactate 2. In absence of oxygen, NADH + H+ is not oxidized by the respiratory chain. 3. The conversion of pyruvate to lactate is the mechanism for regeneration of NAD+. 4. This helps continuity of glycolysis, as the generated NAD+ will be used once more for oxidation of another glucose molecule.
Biological importance (functions) of glycolysis: 1. Energy production: a) anaerobic glycolysis gives 2 ATP. b) aerobic glycolysis gives 8 ATP. 2. Provides important intermediates: a) Dihydroxyacetone phosphate: can give glycerol-3 phosphate, which is used for synthesis of triacylglycerols and phospholipids (lipogenesis). b) 3 Phosphoglycerate: which can be used for synthesis of amino acid serine. c) Pyruvate: which can be used in synthesis of amino acid alanine. 3. Aerobic glycolysis provides the mitochondria with pyruvate, which gives acetyl Co. A Krebs' cycle.
Krebs'Cycle (Citric Acid Cycle) (Tricarboxylic Acid Cycle) "TCA" • Site: mitochondria of every cell • Series of biochemical reactions that are • responsible for complete oxidation of CHO, fats and Ptns to form : CO 2 + H 2 O + Energy 12
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Energy yield of Krebs' cycle: • 1 mole of acetyl Co. A through Krebs' cycle produces 12 ATPs: • 1 ATP (substrate level oxidative phosphorylation). • 1 FADH 2 → 2 ATP (respiratory chain oxidative phosphorylation). • 3 NADH+H+→ 9 ATP(respiratory chain oxidative phosphorylation) • oxidative decarboxylation of pyruvate gives 1 NADH+H+ → 3 ATP Thus net ATP gain is: 12 + 3 = 15 ATP • Since 1 glucose molecule by undergoing glycolysis gives 2 pyruvate • Thus 1 glucose molecule yields 15 × 2 = 30 ATP. 14
Importance of Krebs' cycle Energy production: 1 acetyl Co. A yields 12 ATP. 2. It is the final common metabolic pathway for complete oxidation of acetyl Co. A which results from the partial oxidation of CHO, fats and proteins (amino acids). 3. Interconversion of carbohydrates, fats and proteins (gluconeogenesis, lipogenesis, and formation of nonessential amino acids). 15
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