7.1 Mechanical and Chemical Digestion

Mechanical and chemical digestion work together to break down food into absorbable nutrients. Mechanical digestion physically breaks food into smaller pieces, while chemical digestion uses enzymes to break down molecules into simpler forms.

The digestive system's processes transform complex food molecules into nutrients our bodies can use. From the mouth to the small intestine, each organ plays a crucial role in breaking down and absorbing the nutrients we need to survive and thrive.

Mechanical Digestion of Food

Physical Breakdown of Food

• Mechanical digestion is the physical breakdown of food into smaller pieces without changing its chemical composition
• Increases the surface area of food particles, allowing for more efficient chemical digestion
• Occurs primarily in the mouth (mastication), esophagus (peristalsis), and stomach (churning)

Mastication and Peristalsis

• Mastication involves the teeth and tongue breaking down food into smaller pieces mixed with saliva (chewing)
• Peristalsis is the rhythmic contraction of smooth muscles in the esophagus that pushes food towards the stomach (swallowing)
• Churning in the stomach further breaks down food particles and mixes them with gastric secretions (mixing)
• Mechanical digestion prepares food for chemical digestion by increasing surface area and exposing more molecules to digestive enzymes

Chemical Digestion in the GI Tract

Overview of Chemical Digestion

• Chemical digestion involves the breakdown of food molecules into smaller components that can be absorbed by the body
• Occurs through hydrolysis reactions catalyzed by digestive enzymes
• Takes place primarily in the mouth, stomach, and small intestine
• Digestive enzymes are specific to the type of nutrient they break down (carbohydrates, proteins, or fats)

Chemical Digestion in the Mouth and Stomach

• In the mouth, salivary amylase begins the breakdown of carbohydrates into simpler sugars (maltose)
• In the stomach, pepsin initiates protein digestion by breaking proteins into smaller peptides
• Lipase in the stomach starts the digestion of fats into fatty acids and monoglycerides
• The acidic environment of the stomach denatures proteins, making them more accessible to digestive enzymes

Chemical Digestion in the Small Intestine

• The small intestine is the primary site of chemical digestion
• Receives digestive enzymes from the pancreas, liver, and intestinal walls to complete nutrient breakdown
• Pancreatic enzymes (amylase, trypsin, chymotrypsin, and lipase) continue digestion of carbohydrates, proteins, and fats
• Brush border enzymes on intestinal cells finish digestion, producing absorbable nutrients (monosaccharides, amino acids, fatty acids)

Digestive Enzymes and their Functions

Salivary and Pancreatic Amylase

• Salivary amylase: secreted by salivary glands, begins carbohydrate digestion in the mouth by breaking down starch into maltose
• Pancreatic amylase: secreted by the pancreas into the small intestine, continues carbohydrate digestion by breaking down starch and glycogen into maltose

Protein-Digesting Enzymes

• Pepsin: secreted by the stomach, initiates protein digestion by breaking proteins into smaller peptides
• Trypsin and chymotrypsin: pancreatic enzymes that continue protein digestion in the small intestine by breaking down peptides into smaller peptides and amino acids
• Peptidases: brush border enzymes that complete protein digestion by breaking down small peptides into amino acids

Fat-Digesting Enzymes

• Pancreatic lipase: secreted by the pancreas, breaks down fats into fatty acids and monoglycerides in the small intestine
• Bile salts from the liver emulsify fats, increasing their surface area for more efficient digestion by lipase

Carbohydrate-Digesting Brush Border Enzymes

• Maltase, sucrase, and lactase: break down disaccharides (maltose, sucrose, and lactose) into monosaccharides (glucose, fructose, and galactose) for absorption

The Small Intestine in Chemical Digestion

Anatomy of the Small Intestine

• The small intestine is divided into three sections: duodenum, jejunum, and ileum
• The duodenum receives digestive enzymes from the pancreas and bile from the liver, which aid in chemical digestion
• The jejunum and ileum are the primary sites of nutrient absorption

Final Stages of Chemical Digestion

• Brush border enzymes on the surface of small intestine cells complete the final stages of carbohydrate and protein digestion
• These enzymes break down disaccharides and small peptides into absorbable monosaccharides and amino acids
• The products of chemical digestion (monosaccharides, amino acids, and fatty acids) are then absorbed by the small intestine cells

Absorption of Nutrients

• The extensive surface area of the small intestine, due to villi and microvilli, facilitates efficient absorption of nutrients
• Villi are finger-like projections that increase the surface area of the small intestine
• Microvilli are microscopic projections on the surface of each intestinal cell that further increase absorption capacity
• The remaining undigested material passes from the ileum to the large intestine for further processing and elimination (feces)