Digestion and Absorption - Details

Mechanical Digestion

• Physically breaking down food into smaller fragments

Special types of contractions:

1. Chewing: In mouth - food turns into bolus

2. Churning: In stomach - bolus turns into chyme via unique peristalsis

3. Segmentation: In small intestines contractions move chyme in both directions to allow greater mixing of chyme with digestive juices

Chemical Digestion

Chemical enzymes help break down matter:

Stomach acids:

• G cells: release Gastrin (hormone) that stimulates Parietal cells to release HCl (gastric acid)

• Chief cells: Pepsinogen

• Parietal cells : Hydrochloric Acid (HCl)

Small Intestine:

• Bile

• Produced in the Liver and stored in Gall bladder – Digestive enzymes

• Pancreatic enzymes

• Brush border enzymes

Carbohydrate Digestion

1. Begins in oral cavity (salivary amylase breakd down carbohydrates)

2. Stomach is too acidic, inhibits amylase activity

3. Continues in small intestines (pancreatic amylase, brush border enzymes) (secretin neutralises pH)

Digestion:

Polymer → Monomers

Complex sugards into simple sugars

Complex sugars: Polysaccharides

Simple sugars: Monosaccharides & disaccharides

Carbohydrate Absorption

Happens in small intestine - enterocytes

Lumen of small intestine → bloodstream in blood vessels

Glucose/Galactose transport:

1. Na⁺ exits enterocytes into blood vessels via Na-K pump.

2. Na⁺ concentration gradient forms: High concentration in lumen, low concentration in enterocytes

3. Na⁺ enters enterocytes via co-transport with glucose and galactose.

4. Other transport proteins facilitate movement of glucose/galactose out of enterocytes, into bloodstream

Fructose transport:

1. Fructose transported via facilitated diffusion

Protein Digestion 1 (Stomach)

Begins in stomach

Key fact: Pepsin breaks down protein. Pepsin has optimal enzyme activity in highly acidic conditions.

1. Bolus enters stomach, distension, pH levels increase (more alkaline)

2. Parasymphathetic activity triggered.

3. G cells release gastrin (hormone).

4. Gastrin triggers chief cells release pepsinogen (inactive pepsin) & gastric lipase

5. Gastrin triggers release of gastric acid (HCL acid) by parietal cells. This increases stomach acidity, lowering pH.

6. pH lowers to optimal levels, activating pepsinogen into pepsin.

7. Pepsin breaks down proteins into polypeptide chains (primary structure)

Proteins: Polymers called polypeptides. Chains of amino acids.

Digestion:

Polymer → Monomers

Polypeptides turn into amino acids.

Protein Digestion 2 (Small Intestine)

Continues in small intestine

• Pancreatic enzymes

  • Inactive: Trypsinogen & Chymotrypsinogen

  • Active: Trypsin & Chymotrypsin

    • Breaks down polypeptide chains into simpler dipeptides/tripeptides

• Brush border enzymes

  • Enteropeptidase

    • Activates trypsinogen into trypsin

  • Aminopeptidase

    • Final step that breaks dipeptides/tripeptides into amino acids

Protein Absorption

Occurs in small intestines via enterocytes

Amino acid transport:

1. Na⁺ exits enterocytes into blood vessels via Na-K pump.

2. Na⁺ concentration gradient forms: High concentration in lumen, low concentration in enterocytes

3. Na⁺ & H⁺ ions enter enterocytes via co-transporters with amino acids or di/tripeptides.

4. Other transport proteins facilitate movement of amino acids out of enterocytes, into bloodstream

Lipid Digestion

Key fact: Lipids are not water soluble, thus tend to cluster as large globules which must be broken down to absorb.

Begins in stomach (minor digestion)

1. Bolus enters stomach, distension, pH levels increase (more alkaline)

2. Parasymphathetic activity triggered.

3. Chief cells release pepsinogen (inactive pepsin) & gastric lipase

4. Fatty acids become partially emulsified.

Continues in small intestine (major digestion)

• Gall bladder releases bile into small intestines (duodenum)

• Pancreas releases lipases

• Bile emulsifies fat globules

• Lipases emulsify smaller fat globules

Digestion:

Polymer → Monomers

Triglycerides broken down into fatty acids & monoglycerides.

Lipid Absorption

Occurs in small intestines via enterocytes

• Short- and medium- chain fatty acids absorbed via simple diffusion

  • Bile salts cluster around these fatty acids/monoglycerides, pushing them close enough to microvilli for absorption

• Long- chain fatty acids diffuse into enterocytes, where they are combined with proteins to create chylomicrons

Water Absorption

Key fact: Movement of sodium out of GI track directs movement of water out of digestive track into the body.

Daily water/fluid intake

• Fluid intake via drinks is roughly 1-2 litres

• Gastric secretions add 6-7 litres

90% of this water is absorbed in the small intestine:

• Tight coupling between water and sodium absorption

• As Na-K pumps push sodium/solutes out of enterocytes into bloodstream, water follows.

Most of remaining water is absorbed within large intestines.