Life Processes · Lesson 6 of 13
Nutrition in Human Beings
“Your digestive tract is a carefully organised food-processing journey.”
• Trace food through the human alimentary canal. • Explain mechanical and chemical digestion in the mouth and stomach. • Describe the roles of bile, pancreatic juice and intestinal juice. • Explain digestion of carbohydrates, proteins and fats. • Relate villus structure to efficient absorption. • Explain water absorption, egestion and dental caries.
A bite of food may contain starch, protein and fat mixed together, yet the body must convert each into small absorbable molecules. The alimentary canal solves this through mechanical processing, controlled movement, specialised chemical environments and many enzymes.
The alimentary canal is the continuous muscular tube extending from the mouth to the anus through which food is ingested, digested, absorbed and finally egested.
Digestion In The Mouth
Teeth crush food into smaller pieces, increasing the surface available to enzymes. The tongue mixes food with saliva and helps swallowing. Saliva moistens food and contains salivary amylase, which begins converting starch into simpler sugars.
Observing The Action Of Saliva
Purpose: test whether saliva acts on starch. Setup: place equal starch solution in two tubes and add saliva to only one. After waiting, add dilute iodine. The tube without saliva turns blue-black because starch remains. The saliva-treated tube shows little or no blue-black colour because amylase has broken down much of the starch.
Movement To The Stomach
Rhythmic muscular contractions called peristalsis push food through the oesophagus and along the gut. In the stomach, muscular walls mix food with gastric juice. Gastric glands release hydrochloric acid, pepsin and mucus. Acid creates the medium needed by pepsin and helps destroy many microbes; pepsin digests proteins; mucus protects the stomach lining.
Peristalsis is the wave-like, rhythmic contraction and relaxation of the alimentary canal that moves food forward.
| Secretion or structure | Source | Main function |
|---|---|---|
| Saliva and amylase | Salivary glands | Moistens food and begins starch digestion |
| Hydrochloric acid | Gastric glands | Creates an acidic medium and helps kill microbes |
| Pepsin | Gastric glands | Digests proteins in the stomach |
| Mucus | Stomach lining | Protects the wall from acid and enzymes |
| Bile | Liver | Neutralises acidic food and emulsifies fats |
| Pancreatic juice | Pancreas | Trypsin acts on proteins; lipase acts on fats |
| Intestinal juice | Small-intestine glands | Completes digestion into absorbable molecules |
Complete Digestion In The Small Intestine
A sphincter releases stomach contents into the small intestine in controlled amounts. Bile makes the mixture alkaline and breaks large fat globules into smaller droplets, increasing the area available to lipase. Pancreatic trypsin continues protein digestion, while lipase breaks fats. Intestinal enzymes complete the conversion of proteins to amino acids, complex carbohydrates to glucose, and fats to fatty acids and glycerol.
Cellulose-rich plant food takes longer to process than meat. Herbivores therefore generally have a longer small intestine, providing more time and area for digestion and absorption.
Absorption Through Villi
The inner lining of the small intestine bears numerous finger-like villi. Their large combined surface area and rich blood supply allow rapid absorption. Absorbed nutrients are carried to cells for energy release, growth and tissue repair.
Villi are microscopic finger-like projections of the small-intestine lining that greatly increase the surface area for absorption.
Large Intestine And Egestion
Unabsorbed material enters the large intestine, where additional water is absorbed. The remaining material is stored temporarily and removed through the anus. The anal sphincter regulates its release. This removal of undigested food is egestion, not excretion; excretion removes metabolic wastes produced by cells.
Dental Caries
Bacteria in dental plaque act on sugars and produce acids that soften enamel and dentine. Plaque prevents saliva from reaching the covered surface effectively, so acid is not neutralised. Regular cleaning removes plaque before extensive damage occurs. Untreated decay may reach the pulp and cause inflammation and infection.
Problem
Explain how fat in a meal is prepared for absorption.
- 1.Fat enters the small intestine as relatively large globules.
- 2.Bile salts divide the globules into small droplets; this is emulsification, not chemical digestion.
- 3.The increased surface area allows lipase to act efficiently.
- 4.Lipase breaks fat molecules into fatty acids and glycerol.
- 5.These products are then absorbed through the intestinal lining.
Problem
Predict the effect of severe damage that flattens intestinal villi.
- 1.Flattening greatly reduces absorptive surface area.
- 2.Digestion may still produce small nutrient molecules.
- 3.However, fewer molecules cross into blood efficiently.
- 4.The person may become undernourished despite eating adequate food.
Bile is not an enzyme; it emulsifies fat and helps create alkaline conditions. Digestion breaks molecules down, while absorption moves the products into blood or lymph. Egestion removes undigested material, while excretion removes cellular wastes.
Quiz
Which enzyme begins starch digestion?
What is a major role of hydrochloric acid?
Why does bile improve fat digestion?
Which feature makes villi effective?
Which process removes undigested food?
Practice Problems
- Trace a starch-rich bite from the mouth to absorption, naming the main mechanical and chemical changes.
- Explain the roles of hydrochloric acid, pepsin and mucus in the stomach.
- A duct carrying bile is blocked. Predict two effects on digestion in the small intestine.
- Compare digestion, absorption, assimilation, egestion and excretion.
- Explain why damage to the villi can cause weakness even when food intake remains normal.
Key Takeaways
• Digestion combines mechanical breakdown with enzyme-controlled chemical change. • Salivary amylase begins starch digestion in the mouth. • Peristalsis moves food through the alimentary canal. • The stomach provides acidic conditions for protein digestion. • Bile neutralises acid and emulsifies fats. • The small intestine completes digestion and absorbs products through villi. • The large intestine absorbs water before egestion. • Dental plaque promotes acid damage to teeth.