9.2 Digestive processes and enzymes

Digestion is a complex process involving mechanical and chemical breakdown of food. From chewing to enzymatic action, our bodies transform large food particles into absorbable nutrients. This topic explores the intricate mechanisms of digestion, including the roles of various enzymes and hormones.

Understanding digestive processes is crucial for grasping how our bodies extract energy and nutrients from food. We'll examine the specific enzymes involved at each stage of digestion, their optimal pH conditions, and how hormones regulate this vital process.

Mechanical vs Chemical Digestion

Physical Breakdown of Food

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• Mechanical digestion involves physical breakdown of food into smaller pieces through chewing, churning, and mixing
• Increases the surface area for chemical digestion to occur more efficiently
• Begins in the mouth with mastication (chewing) and continues in the stomach through peristaltic contractions (churning and mixing)

Enzymatic Breakdown of Macronutrients

• Chemical digestion involves the enzymatic breakdown of macronutrients (carbohydrates, proteins, and lipids) into their constituent parts that can be absorbed by the body
• Begins in the mouth with salivary amylase, which starts the digestion of starch into maltose
• Continues in the stomach with pepsin and hydrochloric acid, breaking down proteins into peptides
• Pancreatic enzymes (amylase, trypsin, chymotrypsin, and lipase), bile, and brush border enzymes in the small intestine further digest macronutrients into monosaccharides, amino acids, and fatty acids

Absorption and Elimination

• The end products of digestion (monosaccharides, amino acids, and fatty acids) are absorbed by enterocytes in the small intestine
• Absorbed nutrients are transported to the liver via the hepatic portal vein for processing and distribution throughout the body
• The large intestine absorbs water and electrolytes, concentrates the remaining undigested material, and eliminates it as feces through the rectum and anus

Digestive Enzymes and Substrates

Enzymes in the Mouth and Stomach

• Salivary amylase (found in saliva) begins the digestion of starch into maltose
• Pepsin (secreted by the stomach) breaks down proteins into peptides, functioning optimally in the highly acidic environment (pH 1.5-3.5) of the stomach

Pancreatic Enzymes

• Pancreatic amylase further digests starch and glycogen into maltose
• Trypsin and chymotrypsin break down peptides into amino acids
• Pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids
• These enzymes function best in the slightly alkaline environment (pH 7.0-8.5) of the duodenum

Brush Border Enzymes

• Found on the surface of enterocytes in the small intestine, completing the digestion of carbohydrates and proteins
• Maltase, sucrase, and lactase break down disaccharides (maltose, sucrose, and lactose) into monosaccharides (glucose, fructose, and galactose)
• Peptidases break down peptides into individual amino acids
• Optimal function occurs in a slightly alkaline pH (7.0-9.0)

Hormonal Regulation of Digestion

Stimulating Digestive Secretions

• Gastrin (secreted by the stomach) stimulates the secretion of hydrochloric acid and pepsinogen in the stomach, creating an acidic environment for protein digestion
• Secretin (secreted by the duodenum) stimulates the secretion of bicarbonate-rich fluid from the pancreas to neutralize stomach acid in the duodenum, creating an optimal pH for pancreatic enzymes
• Cholecystokinin (CCK) (secreted by the duodenum) stimulates the release of digestive enzymes from the pancreas and the release of bile from the gallbladder, facilitating fat digestion and absorption

Regulating Gastric Emptying and Satiety

• Peptide YY (PYY) and glucagon-like peptide-1 (GLP-1) (secreted by the ileum) slow gastric emptying, promoting a feeling of fullness (satiety) and preventing overeating
• Insulin (secreted by the pancreas) facilitates the uptake of glucose by cells and promotes the storage of excess glucose as glycogen, regulating blood sugar levels

pH Optimization for Digestion

Optimal pH Ranges for Digestive Enzymes

• Digestive enzymes have specific optimal pH ranges in which they function most effectively
• Salivary amylase functions best in a slightly acidic to neutral pH (6.8-7.0) in the mouth
• Pepsin in the stomach requires a highly acidic environment (pH 1.5-3.5) for optimal activity
• Pancreatic enzymes (amylase, trypsin, chymotrypsin, and lipase) function optimally in a slightly alkaline environment (pH 7.0-8.5) in the duodenum
• Brush border enzymes in the small intestine function best in a slightly alkaline pH (7.0-9.0)

Importance of Maintaining Appropriate pH Levels

• Maintaining the appropriate pH levels throughout the gastrointestinal tract is crucial for the proper functioning of digestive enzymes and the efficient breakdown of nutrients
• The stomach's acidic environment denatures proteins and activates pepsinogen to pepsin, while the duodenum's alkaline environment neutralizes stomach acid and provides optimal conditions for pancreatic enzymes
• Imbalances in pH can lead to digestive disorders, malabsorption, and nutrient deficiencies, such as achlorhydria (low stomach acid) or exocrine pancreatic insufficiency (reduced pancreatic enzyme secretion)