LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- describe a balanced diet
- identify digestive organs
- compare physical and chemical digestion
- explain enzyme and bile action
- relate villus structure to absorption
THE BIG IDEA
How diet supplies materials and the digestive system makes nutrients absorbable.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
SECTION 01
Balanced diet and deficiency
A balanced diet supplies energy and every required nutrient in proportions suited to age, activity and health.
DETAILED EXPLANATION
- Carbohydrates from foods such as rice, bread and potatoes, and fats from oils, nuts and dairy foods, provide energy. Fats also insulate and store energy.
- Proteins from fish, eggs, beans and meat supply amino acids for growth and repair.
- Vitamin C from fresh fruit and vegetables supports healthy connective tissue; deficiency causes scurvy. Vitamin D from oily fish, eggs and sunlight exposure helps calcium absorption; deficiency can cause rickets.
- Calcium from dairy foods and leafy vegetables supports bones and teeth. Iron from red meat, beans and leafy vegetables is required for haemoglobin.
- Fibre from whole grains, fruit and vegetables supports movement through the gut. Water is a solvent and transport medium.
SECTION 02
Digestive system and physical digestion
The alimentary canal runs from mouth to anus. Associated organs add secretions. Ingestion, digestion, absorption, assimilation and egestion describe different stages.
DETAILED EXPLANATION
- Salivary glands add saliva and amylase; the oesophagus moves a bolus by peristalsis; the stomach stores and churns food while adding acid and protease.
- The pancreas releases digestive enzymes into the duodenum. The liver makes bile, which is stored in the gall bladder before entering the small intestine.
- Incisors cut, canines grip and tear, premolars and molars crush and grind.
- Enamel protects; dentine supports; pulp contains nerves and vessels; cement anchors the tooth in bone.
- Chewing and stomach churning increase food surface area without changing its molecules.
- Bile emulsifies fat into droplets, increasing surface area.
SECTION 03
Chemical digestion
Chemical digestion converts large insoluble molecules into small soluble molecules that can cross the intestinal wall.
DETAILED EXPLANATION
- Amylase converts starch to maltose; maltase on the small-intestine lining converts maltose to glucose.
- Proteases convert proteins to amino acids: pepsin acts in acidic stomach conditions and trypsin in the alkaline small intestine.
- Lipase converts fats and oils to fatty acids and glycerol.
- Hydrochloric acid kills many microorganisms and provides low pH for pepsin. Alkaline bile neutralises acidic chyme in the duodenum.
SECTION 04
Absorption and villi
Most nutrients and water are absorbed in the small intestine; some remaining water is absorbed in the colon.
DETAILED EXPLANATION
- Villi and microvilli create a very large surface area.
- A one-cell-thick epithelium gives a short diffusion path.
- Capillaries rapidly carry glucose and amino acids away, maintaining gradients.
- Lacteals absorb products of fat digestion.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Diet must match the body's needs.
- Physical digestion increases surface area.
- Enzymes make small soluble products.
- Bile emulsifies fat and neutralises acid.
- Villi provide efficient absorption.