Especially glucose, play major roles in cell metabolism
Function of dietary carbohydrates
To serve as a source of energy
In a typical diet, 2/3 of daily energy needs are furnished by carbohydrates
Carbohydrate digestion
Disaccharides and polysaccharides are hydrolyzed to form monosaccharides, primarily glucose, fructose, and galactose
Absorption of monosaccharides
Glucose, fructose, and galactose are absorbed into the bloodstream through the lining of the small intestine and transported to the liver
Conversion of fructose and galactose in the liver
Fructose and galactose are rapidly converted to glucose or to compounds that are metabolized by the same pathway as glucose
Blood sugar
Glucose is the most plentiful monosaccharide in blood
Normal blood sugar level in adults after a fast of 8-12 hours is 70-110 mg/100 mL
Blood sugar level reaches a maximum of about 140-160 mg/100 mL about 1 hour after a carbohydrate-containing meal, and returns to normal after 2-2.5 hours
Hypoglycemia
Blood sugar levels are below the normal fasting level, leading to dizziness, fainting, convulsions, and shock
Hyperglycemia
Blood sugar levels are above the normal fasting level, causing glucose to be excreted in the urine
Renal threshold
The blood glucose level at which glucose starts to be excreted in the urine
Liver
The key organ involved in regulating blood glucose levels
Removes glucose from the bloodstream and converts it to glycogens or triglycerides for storage when blood glucose levels are high
Converts stored glycogen to glucose and synthesizes new glucose from noncarbohydrate sources when blood glucose levels are low
Glycolysis
A series of ten reactions, with a net result of converting a glucose molecule into two molecules of pyruvate
Glycolysis
All of the enzymes are found in cellular cytoplasm
There is a net gain of 2 moles of ATP for every mole of glucose that is converted to pyruvate
Regulation of glycolysis
Hexokinase, phosphofructokinase, and pyruvate kinase are the three key regulatory enzymes
Hexokinase
Catalyzes the conversion of glucose to glucose-6-phosphate and initiates the glycolysis pathway, inhibited by high concentration of glucose-6-phosphate
Phosphofructokinase
Catalyzes the irreversible conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, inhibited by high concentrations of ATP and citrate, and activated by high concentrations of ADP and AMP
Pyruvate kinase
Catalyzes the conversion of 3-phosphoenolpyruvate to pyruvate, inhibited by high concentrations of ATP
When ATP use decreases
ATP concentration increases, binding to phosphofructokinase and pyruvate kinase, slowing down their activity and thus slowing the glycolysis pathway
When ATP concentrations are low
ADP and AMP concentrations are high, activating phosphofructokinase and accelerating the glycolysis pathway
Fates of pyruvate
1. Oxidation to acetyl CoA under aerobic conditions
2. Reduction to lactate under anaerobic conditions
3. Reduction to ethanol under anaerobic conditions for some prokaryotic organisms
Oxidation of pyruvate to acetyl CoA
Under aerobic conditions, pyruvate is oxidized in the mitochondria to form acetyl CoA, which can enter the citric acid cycle or be used for fatty acid biosynthesis
Reduction of pyruvate to lactate
Under anaerobic conditions, pyruvate is reduced to lactate to regenerate NAD+ for glycolysis, producing 2 ATP
Reduction of pyruvate to ethanol
Some organisms, including yeast, regenerate NAD+ under anaerobic conditions by alcoholic fermentation, decarboxylating pyruvate to acetaldehyde and then reducing acetaldehyde to ethanol
Complete aerobic oxidation of glucose is 16 times more efficient than lactate fermentation or alcoholic fermentation, producing 32-36 ATP compared to 2 ATP
The total energy available in glucose is 686 kcal/mol, and the efficiency of energy storage in ATP is 34%
Alcoholic fermentation
1. Decarboxylation (removing CO2) of pyruvate to produce acetaldehyde
2. Acetaldehyde is then reduced by NADH to form ethanol (also regenerating NAD+ for glycolysis)
The CO2 thus produced causes beer to foam and wine and champagnes to bubble
Only 2 mol of ATP is produced per mole of glucose by lactate fermentation and alcoholic fermentation
Complete aerobic oxidation of glucose is 16 times more efficient than either of these processes
The total energy available in glucose is 686 kcal/mol
The synthesis of 32 mol of ATP stores 234 kcal/mol
Living cells can capture 34% of the released free energy and make it available to do biochemical work
Automobile engines make available 20-30% of the energy actually released by burning gasoline
Glycogenesis
1. Excess glucose is converted into glycogen
2. Glycogen is stored primarily in the liver and muscle tissue
3. The energy is provided by the hydrolysis of uridine triphosphate (UTP; uracil + ribose + three phosphates)
Glycogenolysis
1. Breakdown of glycogen back into glucose
2. Glycogen phosphorylase cleaves the α(1-4) linkages, releasing glucose 1-phosphate
3. A debranching enzyme hydrolyzes the α(1-6) linkages
4. Phosphoglucomutase isomerizes glucose 1-phosphate to glucose 6-phosphate
5. Glucose 6-phosphatase hydrolyzes glucose 6-phosphate to free glucose (only in liver, kidney, and intestinal cells)
Muscle cells lack glucose 6-phosphatase and cannot form free glucose from glycogen
Liver glycogen is broken down all the way to form free glucose, which is released into the blood during muscular activity and between meals
Gluconeogenesis
1. Synthesizing glucose from noncarbohydrate materials
2. When carbohydrate intake is low and glycogen stores are depleted, the carbon skeletons of lactate, glycerol (derived from the hydrolysis of fats), and certain amino acids are used to synthesize pyruvate, which is then converted to glucose