Gastrointestinal Tract Carbohydrate and Protein, Digestion and Absorption
• COMPLEX CARBOHYDRATES: POLYSACCHARIDES - CARBOHYDRATE- MAJORITY:
i. Starch (plant source; major food sources) and Glycogen (animal; source) - Digestible
ii. Fibre; Cellulose - Nondigestible
• DISACCHARIDES: Lactose (milk sugar); Sucrose (table sugar)
• MONOSACCHARIDES: Glucose; Fructose
• STARCH IS COMPOSED OF 2 KINDS OF POLYSACCHARIDES:
1. AMYLOSE = a compact chain D-glucose molecules linked via α 1-4 glycosidic bond (~>20,000
glucose units; 15-20% starch)
2. AMYLOPECTIN (and glycogen) = is a highly branched D-glucose chains linked via α 1-6
glycosidic bond (~80-85% starch, ~>2 million glucose units
STEPS OF DIGESTION OF STARCH:
1. IN THE MOUTH, SALIVARY α-AMYLASE breaks starch into shorter polysaccharides (~2%)
• In the STOMACH, the SALIVARY α-AMYLASE is INACTIVATED by LOW pH. NO
CARBOHYDRATE DIGESTION OCCURS
2. IN THE SMALL INTESTINE: complete digestion of starch products; disaccharides into monosaccharides by:
i. PANCREATIC α-AMYLASE
ii. ENZYMES released from the BRUSH BORDER of SI LUMEN
• In small intestine: MOST CARBOHYDRATE DIGESTION and ABSORPTION OCCUR
• STEP 1: PANCREATIC α-AMYLASE:
i. ABSORPTION of GLUCOSE (amino acids and fatty acids) from DUODENUM
ii. STIMULATE the RELEASE of HORMONES (CCK, secretin) from the SMALL INTESTINE
iii. STIMULATE the PANCREATIC EXOCRINE SECRETION of α-AMYLASE
iv. α-amylase digestion liberates short-chain glucans, oligosaccharides, tri- and di-saccharides
• STEP 2: SI BRUSH BORDER ENZYMES
i. FURTHER ENZYME DIGESTION by enzymes released from the brush border of SI lumen
ii. Most starch digestion and breakdown of disaccharides occur
α
ACTION OF α-AMYLASE:
• α-AMYLASE = is an ENDOENZYME
Integration of Physiological Systems Page 1
• COMPLEX CARBOHYDRATES: POLYSACCHARIDES - CARBOHYDRATE- MAJORITY:
i. Starch (plant source; major food sources) and Glycogen (animal; source) - Digestible
ii. Fibre; Cellulose - Nondigestible
• DISACCHARIDES: Lactose (milk sugar); Sucrose (table sugar)
• MONOSACCHARIDES: Glucose; Fructose
• STARCH IS COMPOSED OF 2 KINDS OF POLYSACCHARIDES:
1. AMYLOSE = a compact chain D-glucose molecules linked via α 1-4 glycosidic bond (~>20,000
glucose units; 15-20% starch)
2. AMYLOPECTIN (and glycogen) = is a highly branched D-glucose chains linked via α 1-6
glycosidic bond (~80-85% starch, ~>2 million glucose units
STEPS OF DIGESTION OF STARCH:
1. IN THE MOUTH, SALIVARY α-AMYLASE breaks starch into shorter polysaccharides (~2%)
• In the STOMACH, the SALIVARY α-AMYLASE is INACTIVATED by LOW pH. NO
CARBOHYDRATE DIGESTION OCCURS
2. IN THE SMALL INTESTINE: complete digestion of starch products; disaccharides into monosaccharides by:
i. PANCREATIC α-AMYLASE
ii. ENZYMES released from the BRUSH BORDER of SI LUMEN
• In small intestine: MOST CARBOHYDRATE DIGESTION and ABSORPTION OCCUR
• STEP 1: PANCREATIC α-AMYLASE:
i. ABSORPTION of GLUCOSE (amino acids and fatty acids) from DUODENUM
ii. STIMULATE the RELEASE of HORMONES (CCK, secretin) from the SMALL INTESTINE
iii. STIMULATE the PANCREATIC EXOCRINE SECRETION of α-AMYLASE
iv. α-amylase digestion liberates short-chain glucans, oligosaccharides, tri- and di-saccharides
• STEP 2: SI BRUSH BORDER ENZYMES
i. FURTHER ENZYME DIGESTION by enzymes released from the brush border of SI lumen
ii. Most starch digestion and breakdown of disaccharides occur
α
ACTION OF α-AMYLASE:
• α-AMYLASE = is an ENDOENZYME
Integration of Physiological Systems Page 1
