cell respiration & metabolism physiology ch. 5. carbohydrate metabolism most dietary...
TRANSCRIPT
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
C6H12O6 + 6 O2 6 CO2 + 6 H2O
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 Pi 2 pyruvic acid + 2 NADH + 2 ATP
Fig 5.1P. 105
Fig 5.2P. 106
Sugar activation
The enzyme hexokinase transfers a Pi 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 byproviding 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 readilyinterconverted.
Phase 2.
Step 4. - Cleavage
Sugar oxidation and ATP formation
Each GAP molecule is oxidized by removing a pair of hydrogen atoms.
A Pi 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 5.3P. 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