Biochemistry TCA Cycle Quiz

Study Notes

This presentation covers carbohydrate metabolism, specifically focusing on digestion and absorption, catabolic processes (glycolysis, pentose-phosphate pathway, glycogenolysis), and anabolic processes (gluconeogenesis, glycogenesis).
Cellular respiration, including the Citric acid cycle/TCA cycle/Krebs cycle, is also discussed.

The metabolic processes of catabolism break down larger molecules into smaller components to generate energy.
These smaller molecules also act as precursors to subsequent metabolic processes.

Digestion begins in the mouth with salivary amylase breaking down starch.
Salivary amylase is inactive in the stomach hence no further carbohydrate digestion occurs.
The majority of starch digestion and disaccharide breakdown occurs in the small intestine through pancreatic amylase.
Starch is further broken down into monosaccharides, disaccharides, and oligosaccharides.
Carbohydrate digestion is completed by enzymes on the brush border of the small intestine, converting disaccharides and oligosaccharides into monosaccharides.
Fiber and indigestible carbohydrates are partially broken down by bacteria in the large intestine, forming short-chain fatty acids and gas.
The remaining fiber is excreted in the feces.

Polysaccharide digestion starts in the mouth.
Starch (amylose and amylopectin) is broken down by α-amylase.
α-amylase, produced by salivary and pancreatic glands, hydrolyzes internal α-1,4 bonds between glucosyl residues in the starch chain.
Amylase is deactivated by the highly acidic environment of the stomach.

Glycosidases are located on the membrane of microvilli on absorptive cells in the intestine.
These enzymes convert disaccharides and oligosaccharides into monosaccharides.
Key types include glucoamylase, sucrose-isomaltase complex, trehalase, and β-glycosidase complex (lactase-glucosylceramidase).

Indigestible carbohydrates travel to the colon and are metabolized by colon bacteria.
This produces gases, short-chain fatty acids, and lactate.
Examples of indigestible carbohydrates are cellulose, hemicelluloses, lignin, and several others including those found in pectin, gums, and mucilages.

The table classifies several types of dietary fibers based on their chemical structures and sources including cellulose, hemicelluloses, lignin, water-soluble fiber, pectic substances, gums, and mucilages.

Monosaccharides are absorbed across intestinal epithelial cells and move into the bloodstream, then to tissues and cells.
Glucose absorption occurs through facilitated diffusion and Na+-dependent active transport.

The SGLT protein family facilitates sodium-dependent glucose transport across the intestinal epithelium.
Individual SGLT subtypes are involved in glucose/galactose transport and are found in various tissues like intestine, kidney, brain, and heart.

Facilitative glucose transporters (GLUTs) move glucose from high to low concentration without energy expenditure.
The GLUT protein family includes various isoform transporters associated with different tissues like blood-brain barrier, liver, and intestine.

Glycolysis is the process where glucose is broken down to produce energy (ATP, NADH).
This process occurs in the cytoplasm and doesn't require oxygen.
Glycolysis occurs in both aerobic and anaerobic organisms.

Pyruvate's fate depends on the organism's oxygen conditions.
In aerobic conditions, pyruvate enters the Citric Acid/Krebs cycle, resulting in complete glucose oxidation.
In anaerobic conditions, pyruvate is converted to lactate.
Insufficient glucose stores trigger pyruvate's conversion into glucose (gluconeogenesis).

Cellular respiration describes the process where cells consume oxygen and produce carbon dioxide. It occurs in three major stages.

The primary function of the TCA/Krebs cycle is to generate NADH and FADH2, critical for ATP production in the next stage of cellular respiration.

The rate-limiting step is Isocitrate dehydrogenase.
ADP activates the cycle, especially when ATP is low.
The TCA cycle is crucial for producing NADH and FADH2, essential for ATP generation.

The Citric Acid cycle produces NADH, FADH2, and ATP or GTP. Oxaloacetate is recycled for future use in the cycle.

The process of cellular respiration, including glycolysis, pyruvate oxidation, TCA cycle and oxidative phosphorylation contributes a substantial amount of ATP.

The citric acid cycle acts as an amphibolic pathway for both catabolic (breakdown of molecules) and anabolic (synthesis of molecules) reactions.
The citric acid cycle is vital for the generation of precursors to build amino acids.
It provides intermediate compounds crucial for the synthesis of porphyrin rings found in heme groups, essential components in oxygen-carrying proteins.

Fermentation is an anaerobic process yielding ATP, occurring in situations with insufficient oxygen. Two common types are lactic acid and alcoholic fermentations.

Occurs in tissues and cell types lacking mitochondria (eg., erythrocytes) when oxygen is insufficient.
The need to re-oxidize NADH to NAD+ drives this process; without that, glycolysis will stop.

Gluconeogenesis is the formation of new glucose molecules.
It mainly occurs in the liver and kidney.
Gluconeogenesis serves as a vital backup mechanism during hypoglycemic conditions, ensuring sufficient glucose for the survival of the brain.
It is the reverse process of glycolysis.