Cell Respiration & Metabolism Physiology Ch. 5

Carbohydrate Metabolism Most dietary carbohydrate is burned as fuel within a few hours of absorption Three monosaccharides are absorbed from digested food - glucose, galactose, and fructose, but the last two are quickly converted to glucose All oxidative carbohydrate consumption is essentially a matter of glucose catabolism

C 6 H 12 O O 2 6 CO H 2 O

Combustion -vs- Glucose Catabolism Combustion carries out the preceding reaction in a single, uncontrolled step, releasing energy as heat Cellular respiration occurs in many small, enzymatically-catalyzed steps, storing much of the energy as ATP

Three Major Pathways of Glucose Catabolism Glycolysis - splits a glucose molecule into two molecules of pyruvic acid Anaerobic respiration - occurs in the absence of oxygen; reduces pyruvic acid to lactic acid Aerobic respiration - occurs in the presence of oxygen and oxidizes pyruvic acid to carbon dioxide and water

Glucose + 2 NAD + 2 ADP + 2 P i 2 pyruvic acid + 2 NADH + 2 ATP Fig 5.1 P. 105

Fig 5.2 P. 106

Sugar activation The enzyme hexokinase transfers a P i from ATP to glucose, producing glucose 6-phosphate (G6P).  Keeps intracellular glucose concentration low, favoring continued diffusion of glucose.  Traps the glucose within the cell, as phosphorylated molecules cannot pass through the plasma membrane. Phase 1. Step 1. - Phosphorylation

Sugar activation G6P is isomerized to form fructose 6-phosphate (F6P). It is phosphorylated again to form fructose 1,6- diphosphate by the action of phosphofructokinase.  Primes the process by providing activation energy Phase 1. Steps 2 & 3. - Priming  At this point, two molecules of ATP have been consumed

Sugar cleavage F6P is split into glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP).  GAP is on the direct pathway of glycolysis; DHAP is not. The two are isomers and readily interconverted. Phase 2. Step 4. - Cleavage

Sugar oxidation and ATP formation Each GAP molecule is oxidized by removing a pair of hydrogen atoms. A P i is added (from the cell’s pool of free phosphate ions) to form 1,3-bisphosphoglycerate (1,3-BPG)  NAD is reduced by the hydrogens to NADH + + H +. Phase 3. Step 5. - Oxidation

Sugar oxidation and ATP formation A phosphate group is removed from 1,3-BPG and transferred to ADP, phosphorylating it to ATP. 2 ATPs are made. 3-Phosphoglycerate is formed. Phase 3. Step 6. - ATP formation from 1,3-BPG

Sugar oxidation and ATP formation The position of the phosphate group is shifted in the conversion of 3- phosphoglycerate to 2- phosphoglycerate. Phosphoenolpyruvic acid is created by the dehydration of 2-phosphoglycerate. This enol phosphate has a high phosphate transfer potential. Phase 3. Steps 7 & 8. - Isomerizations

Sugar oxidation and ATP formation A phosphate group is removed from phosphoenolpyruvate and transferred to ADP, phosphorylating it to ATP. 2 more ATPs are made. Two pyruvic acid molecules are formed from the single original glucose. Phase 3. Step 9. - ATP formation from phosphoenolpyruvate

The tally from glycolysis…

Lactic Acid Pathway Metabolic pathway by which glucose is converted to lactic acid (anaerobic respiration): Oxygen is not used in the process. NADH + H + + pyruvic acid lactic acid + NAD. Produce 2 ATP/glucose molecule. Fig 5.3 P. 106

Lactic Acid Pathway Some tissues better adapt to anaerobic conditions: RBCs do not contain mitochondria and only use the lactic acid pathway. Occurs in skeletal muscles and heart when ratio of oxygen supply to oxygen need falls below critical level. Skeletal muscle: Normal daily occurrence. Does not harm muscle tissue. Cardiac muscle normally respires aerobically: Myocardial ischemia occurs under anaerobic conditions.

Also study Kreb’s, ETC, lipid and amino acid metabolism in Chapter 5