Carbohydrate Digestion Process

Carbohydrate Digestion Process

• Digestion involves hydrolysis, breaking down food molecules into simpler chemical units for metabolic use.
• Carbohydrate digestion starts in the mouth, although it's minimal due to quick swallowing.
• Hydrolysis splits chemical bonds and incorporates water into resulting fragments.
• Salivary α-amylase, an enzyme in saliva, initiates the breakdown of starches into smaller polysaccharides and maltose.
• Stomach digestion of carbohydrates is limited; salivary α-amylase becomes inactive under acidic conditions, and no other enzymes are present.

Small Intestine and Enzymatic Breakdown

• The small intestine is the primary site for carbohydrate digestion.
• Pancreatic α-amylase continues the breakdown of polysaccharides into disaccharides like maltose.
• Disaccharidases, a group of glycoside hydrolases, convert disaccharides into monosaccharides:
  • Maltase: maltose to glucose.
  • Sucrase: sucrose to glucose and fructose.
  • Lactase: lactose to glucose and galactose.
• The resulting monosaccharides (glucose, galactose, and fructose) are absorbed into the bloodstream through intestinal villi, rich in blood capillaries.
• Monosaccharide transport across cell membranes is facilitated by protein carriers.
• Galactose and fructose are converted to glucose metabolic products in the liver.

Glycolysis Overview

• Glycolysis, occurring in the cytosol, is the metabolic pathway converting glucose into two pyruvate molecules, producing energy (ATP and NADH).
• It involves two stages: the six-carbon stage and the three-carbon stage.

Six-Carbon Stage of Glycolysis

• Initial steps (1-3) involve converting ATP to ADP to transform monosaccharides into their phosphorylated forms.
• Step 1: Glucose 6-phosphate is formed via phosphorylation by hexokinase; energy from ATP hydrolysis is required.
• Step 2: Glucose 6-phosphate is isomerized to fructose 6-phosphate by phospho-gluco-isomerase.
• Step 3: Fructose 1,6-bisphosphate is formed through another phosphorylation reaction, facilitated by phospho-fructo-kinase.

Three-Carbon Stage of Glycolysis

• Steps (4-10) focus on the conversion of C6 biphosphate into two C3 monophosphate species.
• Step 4: Aldolase catalyzes the splitting of C6 biphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
• Step 5: Dihydroxyacetone phosphate is isomerized to glyceraldehyde 3-phosphate by triose-phosphate isomerase.
• Step 6: Formation of 1,3-bisphosphoglycerate involves glyceraldehyde 3-phosphate dehydrogenase; NADH is produced and inorganic phosphate is used.
• Step 7: ATP production occurs as 1,3-diphosphoglycerate donates a phosphate to ADP, catalyzed by phospho-glycero-kinase.
• Step 8: 3-phosphoglycerate is isomerized to 2-phosphoglycerate, moving the phosphate group from carbon 3 to carbon 2, mediated by phospho-glycero-mutase.
• Step 9: Formation of phosphoenolpyruvate involves dehydration reactions leading to energy release for later ATP generation.