Bio Review Notes

Bio Review Notes #38
NUTRITION IN ANIMALS Performance Objectives:

After completing this lesson, students will be able to:

Describe the differences between inracellular and extracellular digestion, and between mechanical and chemical digestion

Describe the advantages of "assembly line" digestion in higher animals

Unicellular organisms ingest food by phagocytosis.
Most digestion by lower animals is intracellular, but extracellular digestion (both mechanical and chemical) predominates among higher animals.
"Assembly line" (mouth-to-anus) digestion allows regional specialization to evolve, so that digestion happens in stages.

Intracellular digestion predominates in lower animals.
In phagocytosis, folds of the cell membrane engulf food material originally outside the cell, forming a vacuole. Chemical digestion then follows when these vacuoles fuse with lysosomes.

Extracellular digestion predominates in higher animals.

Mechanical digestion: Food is minced or crushed, exposing more surface area.
Chemical digestion: Food is broken down chemically with the help of enzymes.

Mouth: Major site of mechanical digestion (using teeth, etc.). Chemical digestion begins with amylase enzyme in saliva that breaks down starches. Food passes from mouth to stomach via the esophagus.

Stomach: Major site of protein digestion. Pepsin breaks proteins into small peptides. Hydrochloric acid (HCl) acidifies stomach contents; this helps pepsin, which works best at acidic pH (near 2.0). Mechanical digestion also occurs by contraction of 3 muscle layers, kneading food. Some species (like birds) have a large storage crop, followed by a strong muscular gizzard, specialized for mechanical digestion.

Liver: Secretes bile, containing bile pigments (derived from hemoglobin) and bile salts, which emulsify fats. Bile is stored in the gall bladder until needed.

Small intestine: Long and coiled, with large surface area.

Duodenum: Digestion of lipids, carbohydrates, and proteins, using enzymes secreted by the duodenum itself and by the pancreas. Lipids are emulsified by bile salts secreted by the liver.
Jejunum: Chemical digestion of most nutrients completed here.
Ileum: Site of most absorption of digestion end-products.

Large (large-diameter) intestine: Includes:

Caecum (size varies), site of microbial digestion of cellulose in herbivores
colon site of much water resorption

Anus: Undigested wastes are eliminated.

Index Syllabus

rev. Aug. 2011