Digestion system

Digestion and absorption 

• Mechanical digestion breaks large food particles into smaller ones 
• Chemical digestion is the breaking of colvalent chemical bonds in organic molecule by digestice enzymes
• Carbonhydrates break down into monosaccharides, lipids break down into fatty acids and monoglyceridess, and protein break down into amino acids. 
• Some molecules are not broken down 
• A relatively small amount of digestion begins in the oral cavity and some occurs in the stomach, vast majority of digestion occurs in the proximal end of the small intestine, especially in the duodenum. 

Carbonhydrates 

• Minor amount of carbonhydrate digestion begins in the oral cavity with the partial digestion of starches by salivary amylase. 
• Digestion continues in the stomach until the food is well mixed with acid, which inactibates salivary amylase. 
• Carbonhydtae digestion is resumed in small intestine by pancretic amylase. 
• Pancreatic lipase in the small intestine is responsuble for the majority of carbonhydrate digestion. 
• Digested carbonhydrates at this point are disaccharides, not absorbed. 
• Carbonhydrate digestion is performed by a series of disaccharidases that are bound to the microvilli of the interstial epithelium. 
• Enzymes digest disaccharides into three monoasaccharides 1) glucose, 2) galactose and 3) fructose
• Diffusion of NA down its concentration gradient provides the energy to transport glucose or galactose across the plasma membrane
• Contrast to glucose and galactose, monosaccharide fructose is taken up by faciliated diffusion. 

Lipids 

•  Lipase enzymes digest lipid molecules 
•  Primary products of lipase digestion are free fatty acids and monoglycerides 

Emulsifaction 

•  Increases the surface area of the lipid exposed to lipase and other digestive enzymes 
• Necessary because lipase is water-soluble and can digest lipids only ay the surface of the droplets 
• The bile salts are secreted by the liver and stored in the gallbladder until needed in the duodenum
• Lingial and gastric lipase, work in the acidic environment of the stomach, do not require bile salts
• Once liids are digested in the intestine, bile salts aggregate around the small droplets. 
• Lipid droplets surrounded by bile salts are called micelles
• Hydrophobic ends of the bile…