biochem xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Biochemistry

1. Digestion of Starch / Absorption of Glucose Dr.SOWMIYA D

2. SYNOPSIS Definition Classification of carbohydrates Structure of starch Enzymes in starch digestion Digestion in the mouth Digestion in the stomach Digestion in the small intestine Role of brush border enzymes Final products for digestion Absorption of glucose Regulation of blood glucose Clinical relevance Effect of high starch diet Summary of digestion

3. INTRODUCTION CARBOHYDRATE: Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. FUNCTIONS: 1. They are the most abundant dietary source of energy (4 Cal/g) for all organisms. 2. It is precursors for many organic compounds (fats, amino acids). 3. Carbohydrates (as glycoproteins and glyco -lipids) are involved in cell growth, adhesion and fertilization. 4. They are structural components of many organisms. 5. It act as energy storage

4. Classification of carbohydrates

5. STRUCTURE OF STARCH Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals, including man. High content of starch is found in cereals, roots, tubers, vegetables etc. Starch consists of two polysaccharide components-water soluble amylose (15–20%) and a water insoluble amylopectin (80–85%).

6. Chemically, amylose is a long unbranched chain with 200–1,000 D-glucose units held by α (1 → 4) glycosidic linkages. Amylopectin, on the other hand, is a branched chain with α(1 → 6) glycosidic bonds at the branching points and α (1 → 4) linkages everywhere else.

7. Enzymes in Starch Digestion • Salivary amylase: Initiates breakdown of starch in the mouth. • Pancreatic amylase: Secreted into the small intestine; continues starch digestion. • Brush border enzymes: Maltase, isomaltase , sucrase complete digestion to glucose.

8. Digestion in the Mouth • Salivary glands secrete α-amylase which hydrolyzes α-1,4 glycosidic bonds. • Produces maltose and small oligosaccharides. • The activity is limited due to short retention time in the mouth and inactivation in stomach acid.

9. Digestion in the Stomach • The acidic pH (~2) of the stomach denatures salivary amylase. • No significant starch digestion occurs here. • Why st arch digestion halts in the stomach? Acidic Environment Lack of carbohydrate splitting enzymes Primary function is protein digestion • Mechanical churning helps prepare chyme for enzymatic action in the intestine.

10. Digestion in the Small Intestine • The pancreas secretes pancreatic amylase into the duodenum. • When the acidic stomach contents reaches the small intestine , they are neutralised by bicarbonate secreted by the pancreas and pancreatic alpha amylase continues the process of starch digestion It hydrolyses the dextrins to mixture of maltose, isomaltose, limit dextrin.

11. Role of Brush Border Enzymes These enzymes responsible for final digestion of carbohydrate.

12. Final Products of Digestion • Glucose is the predominant monosaccharide produced. • Small quantities of galactose and fructose may also be absorbed from dietary disaccharides. • Glucose is then ready for absorption by enterocytes.

13. Absorption of Glucose • Occurs in the jejunum of the small intestine. • Two main transport mechanisms: active co-transport with sodium and facilitated diffusion. • Transport proteins ensure efficient absorption even against concentration gradients.

14. Role of Sodium-Potassium Pump • Located in the basolateral membrane of enterocytes. • Pumps sodium out of cells and potassium in, maintaining sodium gradient. • Provides energy for SGLT-1 mediated glucose uptake.

15. SGLT-1 Transporter • Located in the apical membrane of enterocytes. • Couples glucose transport with sodium ions, using energy from the sodium gradient. • Ensures active absorption of glucose even at low luminal concentrations.

16. GLUT-2 Transporter • Located in the basolateral membrane of enterocytes. • Mediates passive facilitated diffusion of glucose into blood capillaries. • Allows rapid transport of glucose into the portal circulation.

17. Transport into Bloodstream • Glucose enters the portal vein after absorption. • Transported to the liver where it is metabolized or stored. • Liver regulates systemic glucose availability by glycogenesis and glycogenolysis.

18. Utilization of Glucose • Glucose undergoes glycolysis to produce ATP. • Excess glucose is stored as glycogen (glycogenesis). • If glycogen stores are full, glucose is converted into fat (lipogenesis).

19. Regulation of Blood Glucose • Insulin promotes glucose uptake and storage in liver and muscle. • Glucagon stimulates glycogen breakdown and gluconeogenesis during fasting. • Maintains homeostasis of blood glucose levels (~70–110 mg/dl).

20. Clinical Relevance • Glucose-galactose malabsorption: rare genetic transporter defect. • Diabetes mellitus: impaired glucose uptake/utilization due to insulin deficiency or resistance. • Lactose intolerance indirectly affects carbohydrate digestion.

21. Effect of High Starch Diet • Leads to rapid rise in blood glucose (high glycemic index foods). • Excess insulin secretion may lead to insulin resistance over time. • Associated with obesity, type 2 diabetes, and metabolic syndrome.

22. Summary of Digestion

23. Thank You REFERENCES: Lippincott illustrated review of biochemistry Textbook of biochemistry – sathyanarayana Textbook of biochemistry – D M Vasudevan