1. The Five Steps of Holozoic Nutrition
- Ingestion: Taking food into the body via the mouth.
- Digestion: Breaking down complex, insoluble food molecules into simple, absorbable chemical nutrients.
- Absorption: Digested nutrients pass through the intestinal wall into blood vessels via finger-like micro-projections called villi.
- Assimilation: Body cells utilize absorbed nutrients for growth, repair, and cellular energy.
- Egestion: Expelling undigested, unabsorbed solid waste matter through the anus.
2. The Human Alimentary Canal & Glands
- Buccal Cavity (Mouth): Teeth mechanically chew food; salivary amylase in saliva breaks complex starch into simple maltose sugar.
- Stomach: Muscular J-shaped sac. Secretes:
• Mucous: Protects stomach inner lining from corrosive acid.
• Hydrochloric Acid ($\text{HCl}$): Kills ingested bacteria and creates an acidic medium.
• Pepsin Enzyme: Breaks down complex proteins into peptides. - Small Intestine: Highly coiled ($7.5\text{ m}$ long). Receives:
• Bile from the Liver (stored in gall bladder): Emulsifies large fat globules into tiny droplets.
• Pancreatic Juice from Pancreas: Digests carbohydrates, proteins, and fats.
• Villi: Millions of tiny finger-like projections that increase surface area for rapid nutrient absorption into capillaries. - Large Intestine: Absorbs excess water and mineral salts from unabsorbed slurry.
3. Digestion in Grass-Eating Animals (Ruminants)
Cows and buffaloes swallow grass quickly and store it in a specialized stomach chamber called the Rumen. In the rumen, symbiotic cellulose-digesting bacteria partially digest the plant fiber into cud. Later, the animal regurgitates the cud back into the mouth in small lumps and chews it leisurely (rumination).
Humans cannot digest grass because our digestive tract lacks the specialized bacteria and large caecum needed to break down tough plant cellulose.