Digestion and absorption

Digestion is the process of breaking down large, insoluble biological molecules into smaller, soluble molecules that can be absorbed across cell membranes. It involves both physical breakdown and chemical digestion.

The process begins in the mouth, where the teeth and tongue mechanically break food into smaller pieces, increasing its surface area for enzyme action.

The human digestive system.

Chemical breakdown

Chemical digestion is carried out by enzymes that catalyse the hydrolysis of polymers into monomers. Different enzymes target specific types of molecules:

Carbohydrates:

• Amylase: Hydrolyses glycosidic bonds in starch to release maltose molecules.

• Maltase: Hydrolyses the glycosidic bond in maltose to release glucose molecules.

Proteins:

• Endopeptidases: Hydrolyse internal peptide bonds, breaking proteins into polypeptides.

• Exopeptidases: Remove terminal amino acids from polypeptides, forming dipeptides.

• Dipeptidases: Hydrolyse dipeptides into individual amino acids.

Lipids:

• Bile salts: Emulsify large lipid droplets into smaller micelles, increasing the surface area for enzyme action.

• Lipase: Hydrolyses two ester bonds in triglycerides to release a monoglyceride and two fatty acids.

The human digestive system

The human digestive system consists of the alimentary canal (a long tube running from the mouth to the anus) and several accessory organs, including the salivary glands, pancreas, and liver.

Mouth:

• Saliva contains amylase to begin starch digestion.

• Food is chewed into smaller pieces, mixed with mucus and swallowed.

Oesophagus:

• A muscular tube that moves food from the mouth to the stomach.

Stomach: Secretes gastric juice, containing:

• Hydrochloric acid to kill bacteria.

• Protease enzymes for protein digestion.

• Mucus to protect the stomach lining.

Duodenum (first part of the small intestine):

• Bile from the liver (stored in the gallbladder) neutralises stomach acid and emulsifies lipids.

• Pancreatic juice contains enzymes (proteases, lipases, and amylase) for digestion.

Ileum (second part of the small intestine):

• Specialised for absorption with adaptations:

  • Large surface area: Due to length, circular folds, villi, and microvilli.

  • Short diffusion distance: Thin epithelial layer and proximity to blood capillaries.

  • Steep concentration gradient: Maintained by blood flow and active transport mechanisms.

  • Membrane-bound enzymes: Dipeptidases and disaccharidases (maltases, sucrase, lactase) are bound to the membrane of the cells lining to the ileum so that, upon digestion, the products can move directly into the cells.

Large Intestine:

• • Responsible for water absorption and waste formation.

Absorption of glucose and amino acids

Absorption of glucose and amino acids across the epithelial cells lining the ileum (small intestine) ensures that absorption continues even when the concentration is lower in the lumen than in the cell cytoplasm. It occurs in three stages:

1. A Sodium/Potassium Pump: on the capillary side of the epithelial cell transports sodium ions out of the epithelial cell and potassium ions into the cell. This process requires energy from ATP, as both ions are moving against their concentration gradients. This movement results in a lower concentration of sodium ions in the cell than in the small intestine lumen.

2. A Glucose/Sodium Co-Transporter: protein on the lumen side of the epithelial cell brings a molecule of glucose and a sodium ion into the epithelial cell from the gut lumen. Although glucose is moving against its concentration gradient, the sodium ion moves down the concentration gradient, facilitating the 'coupled' transport of glucose without the need for energy from ATP.

3. A Glucose Carrier: allows the facilitated diffusion of glucose, now at a relatively high concentration inside the epithelial cell, into the blood plasma on the capillary side of the epithelial cell.

The absorption of glucose in the ileum

Absorption of lipids

Lipids are absorbed through the following steps:

1. Micelle Formation: Bile salts form micelles, which deliver monoglycerides and fatty acids to the epithelial cells.

2. Absorption and Reassembly: Monoglycerides and fatty acids diffuse into epithelial cells, where they are reassembled into triglycerides.

3. Package into vesicles: In the Golgi apparatus, triglycerides are packaged into vesicles which migrate towards the cell membrane.

4. Transport via Lacteals: Triglycerides are exported via exocytosis and enter lacteals (lymphatic vessels) within the villi, transporting lipids through the lymphatic system.

Definitions