23.7 Chemical Digestion and Absorption: A Closer Look

Chemical digestion breaks down complex nutrients into simpler forms our bodies can absorb. Enzymes in the mouth, stomach, and small intestine work together to break apart carbs, proteins, fats, and nucleic acids into their basic building blocks.

Nutrient absorption happens mainly in the small intestine. Water-soluble nutrients enter the bloodstream directly, while fats are packaged into chylomicrons and enter the lymphatic system. This process fuels our bodies with essential nutrients.

Chemical Digestion

Chemical digestion of macromolecules

Top images from around the web for Chemical digestion of macromolecules

• 

  Digestive System : Introduction | BIO103: Human Biology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look · Anatomy and Physiology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look | Anatomy and Physiology View original

  Is this image relevant?

• 

  Digestive System : Introduction | BIO103: Human Biology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look · Anatomy and Physiology View original

  Is this image relevant?

1 of 3

Top images from around the web for Chemical digestion of macromolecules

• 

  Digestive System : Introduction | BIO103: Human Biology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look · Anatomy and Physiology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look | Anatomy and Physiology View original

  Is this image relevant?

• 

  Digestive System : Introduction | BIO103: Human Biology View original

  Is this image relevant?

• 

  Chemical Digestion and Absorption: A Closer Look · Anatomy and Physiology View original

  Is this image relevant?

1 of 3

• Carbohydrates
  • Salivary amylase in the mouth initiates carbohydrate digestion by breaking down polysaccharides (starch) into smaller molecules (maltose, maltotriose, and α-dextrins)
  • Pancreatic amylase secreted into the small intestine further digests remaining polysaccharides into disaccharides (maltose, sucrose, lactose)
  • Brush border enzymes (lactase, sucrase, maltase) attached to the microvilli of the small intestine break down disaccharides into monosaccharides (glucose, fructose, galactose) for absorption
• Proteins
  • Pepsin secreted by the stomach initiates protein digestion by breaking down proteins into smaller polypeptides in the acidic environment
  • Pancreatic enzymes (trypsin, chymotrypsin, elastase, carboxypeptidase) secreted into the small intestine further digest polypeptides into smaller peptides and individual amino acids
  • Aminopeptidases and dipeptidases attached to the brush border of the small intestine break down peptides into individual amino acids for absorption
• Lipids
  • Lingual lipase in the mouth and gastric lipase in the stomach initiate lipid digestion by breaking down a small amount of triglycerides into fatty acids and monoglycerides
  • Bile salts produced by the liver and stored in the gallbladder are released into the small intestine to emulsify lipids, increasing their surface area for enzymatic action
  • Pancreatic lipase secreted into the small intestine breaks down the majority of triglycerides into fatty acids and monoglycerides for absorption
• Nucleic acids
  • Ribonucleases and deoxyribonucleases secreted by the pancreas into the small intestine break down RNA and DNA into smaller nucleotides
  • Nucleotidases and phosphatases attached to the brush border of the small intestine further digest nucleotides into individual nitrogenous bases (purines and pyrimidines), pentoses (ribose and deoxyribose), and phosphates for absorption

Enzymatic processes in chemical digestion

• Hydrolysis: The primary mechanism by which enzymes break down macromolecules into smaller units
• Emulsification: The process of breaking down large fat globules into smaller droplets, increasing surface area for enzymatic action
• Enzymes: Biological catalysts that speed up chemical reactions in the digestive process without being consumed

Nutrient Absorption

Nutrient absorption into circulatory systems

• Absorption in the small intestine occurs through the mucosa, which consists of epithelial cells with microvilli (brush border) that increase the surface area for efficient nutrient uptake
• Water-soluble nutrients (monosaccharides, amino acids, nucleotides, minerals, and water-soluble vitamins) are absorbed through the intestinal wall into the bloodstream via:
  1. Passive diffusion: Movement of nutrients down their concentration gradient without energy input (facilitated by aquaporins for water)
  2. Facilitated diffusion: Movement of nutrients down their concentration gradient using carrier proteins (GLUT transporters for monosaccharides)
  3. Active transport: Movement of nutrients against their concentration gradient using energy (ATP) and carrier proteins (sodium-glucose cotransporter for glucose and amino acids)
• Lipid-soluble nutrients (fatty acids, monoglycerides, and fat-soluble vitamins) are absorbed into the lymphatic system via:
  1. Diffusion of fatty acids and monoglycerides into intestinal epithelial cells
  2. Reassembly of triglycerides within epithelial cells using smooth endoplasmic reticulum
  3. Packaging of triglycerides, cholesterol, and fat-soluble vitamins (A, D, E, K) into chylomicrons
  4. Transport of chylomicrons into the lymphatic system through lacteals in the intestinal villi

Water-soluble vs lipid-soluble nutrient absorption

• Water-soluble nutrients (monosaccharides, amino acids, nucleotides, minerals, and water-soluble vitamins)
  • Absorbed directly into the bloodstream through the intestinal wall via passive diffusion, facilitated diffusion, or active transport
  • Enter the hepatic portal system and are carried to the liver for processing, storage, or distribution before entering the systemic circulation
  • Water-soluble vitamins (B-complex and C) are absorbed along with other water-soluble nutrients
• Lipid-soluble nutrients (fatty acids, monoglycerides, and fat-soluble vitamins)
  • Absorbed into intestinal epithelial cells and reassembled into triglycerides using smooth endoplasmic reticulum
  • Packaged into chylomicrons along with cholesterol and fat-soluble vitamins (A, D, E, K)
  • Transported into the lymphatic system through lacteals in the intestinal villi, bypassing the liver
  • Enter the systemic circulation via the thoracic duct, allowing for distribution to peripheral tissues before reaching the liver