Digestive System: Study Notes

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Digestive System

HOLOZOIC NUTRITION

Holozoic nutrition involves the ingestion of complex food, its enzymatic breakdown into simple diffusible forms, absorption, and utilization for energy.

1. Ingestion:
   • Protozoans use phagocytosis; metazoans use the mouth.
   • Microphagous feeders (e.g., Daphnia) use limbs; macrophagous feeders use tentacles.
2. Digestion:
   • Extracellular: Outside cells.
   • Intracellular: Inside cells.
   • Includes mechanical digestion (breaking food by churning or mastication) and chemical digestion (enzymatic hydrolysis).

Types of Digestive Systems:

• Sac-like: Single opening (mouth), no anus, incomplete system. The mouth opens into a large sac-like body cavity which also functions as the digestive cavity.
• Tube-like: the digestive cavity is separate from the body cavity. Separate mouth and anus, complete system.

1. Absorption: Diffusion of digested food into blood.
2. Assimilation: Energy production from digested products.
3. Egestion: Expulsion of undigested food

The pathway of the digestive tract is: oral cavity → pharynx → oesophagus → stomach → smallintestine → large intestine → rectum

The accessory organs of digestion are the salivary glands, pancreas, liver, and gallbladder.

The five distinct layers of the gut are: the mucosa, submucosa, muscularexterna and serosa.

• Mucosa is the innermost layer of the gut and has three layers; the epithelium, lamina propria and muscularis mucosa.
• Submucosa is a layer of connective tissue containing nerves, blood and lymph vessels, collagen and elastic fibres.
• Muscularis externa is composed of an inner circular and an outerlongitudinal layer of smooth muscle.
• Subserosa consists of Connective tissue
• Serosa is the outermost coat of the gut wall.

DIGESTIVE ORGANS & THEIR FUNCTIONS

1. Buccal Cavity

• Structure: The chamber just inside the mouth where food is chewed.
• Digestive Gland: Salivary glands (Parotid, Submandibular, Sublingual, Minor salivary glands).
• Secretion/Enzymes: Saliva containing Salivary amylase (ptyalin) and Lipase.
• Digestion/Function:
  • Mechanical digestion: Mastication (chewing) increases the surface area of food for enzymatic digestion.
• Chemical digestion:
  • • Salivary amylase breaks down starch into maltose and dextrins.
  • Lipase catalyzes the hydrolysis of lipids.

2. Oesophagus

• Structure: Tube connecting the mouth to the stomach.
• Digestive Gland: None.
• Secretion/Enzymes: None.
• Digestion/Function: Peristalsis (muscle contractions that move food along the digestive tract).

3. Stomach

• Structure: Composed of four parts: fundus, body, antrum, and pylorus. It has a lesser and greater curvature, and the lining is thrown into folds called rugae. Muscularis externa consists of three layers: inner oblique, middle circular, and outer longitudinal.
• Digestive Gland: Gastric glands (Mucous cells, Chief cells, Parietal cells, G cells).
• Secretion/Enzymes:
  • Mucous cells secrete mucus.
  • Chief cells secrete pepsinogen.
  • Parietal cells secrete hydrochloric acid (gastric acid) and intrinsic factor.
  • G cells secrete gastrin.
• Digestion/Function:
  • Mucus protects the stomach lining.
  • Pepsinogen is activated to pepsin, which breaks down proteins into peptones.
  • Hydrochloric acid activates pepsin and inhibits microbial growth.
  • Intrinsic factor aids in Vitamin B12 absorption.
  • Gastrin increases hydrochloric acid secretion and gastric motility.
  • Digestion results in chyme (an acidic, semifluid mixture).

4. Small Intestine

• Structure: Consists of three parts: Duodenum, Jejunum, and Ileum. Lined with villi (finger-like projections), which increase surface area for absorption. At the base of villi are the crypts of Leiberkuhn, with capillary beds for water-soluble nutrient absorption and lacteals for fat absorption.
• Digestive Gland:
  • Duodenum: Intestinal glands, pancreas, and liver secretions (via the hepatopancreatic duct).
• Secretion/Enzymes:
  • Duodenum: Disaccharidases (maltase, isomaltase, lactase, sucrase), Aminopeptidase, Dipeptidases, Enteropeptidase, Secretin, Cholecystokinin.
  • Liver: Bile.
  • Pancreatic juice: Amylase, Lipase, Trypsinogen (converted to trypsin), Chymotrypsinogen (converted to chymotrypsin), Carboxypeptidases.
• Digestion/Function:
  • Disaccharidases break down sugars (e.g., maltase breaks maltose into glucose).
  • Aminopeptidase removes the N-terminal amino acid from peptides.
  • Dipeptidases break dipeptides into amino acids.
  • Enteropeptidase activates proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidases).
  • Secretin stimulates pancreatic juice release and slows motility.
  • Cholecystokinin stimulates bile release and pancreatic juice secretion.
  • Bile emulsifies fats for easier digestion by lipase.
  • Pancreatic amylase breaks down starch into maltose, lipase breaks lipids into fatty acids and glycerol, trypsin and chymotrypsin break proteins into smaller polypeptides, carboxypeptidases break peptides into amino acids.
  • Jejunum and Ileum: Absorption of nutrients (simple sugars, amino acids, triglycerides) via secondary active transport and facilitated diffusion.

5. Large Intestine

• Structure: Includes the cecum (with the appendix) and the colon (ascending, transverse, descending, and sigmoid parts).
• Digestive Gland: None.
• Secretion/Enzymes: None.
• Digestion/Function: Absorbs water and salts, forming semisolid feces.

6. Rectum

• Structure: End of the large intestine.
• Digestive Gland: None.
• Secretion/Enzymes: None.
• Digestion/Function: Stores feces, which are then excreted through the anus.

7. Anus

• Structure: External opening of the anal canal, surrounded by two sphincters (internal smooth muscle and external striated muscle).
• Digestive Gland: None.
• Secretion/Enzymes: None.
• Digestion/Function: Egestion (elimination of feces).

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