Contents
1 · Essential Nutrients
| Nutrient | Chemical nature | Functions | Sources |
|---|---|---|---|
| Carbohydrates | Sugar, starch, glycogen | Energy source; structural (cellulose in plants) | Rice, bread, pasta, fruits |
| Proteins | Amino acids | Growth and repair; enzymes, antibodies, haemoglobin | Meat, fish, eggs, legumes |
| Fats/Lipids | Glycerol + fatty acids | Energy store; insulation; cell membranes; fat-soluble vitamins | Oils, butter, nuts, meat |
| Vitamins | Organic molecules (various) | Coenzymes, antioxidants, bone health (see deficiencies) | Vegetables, fruits, dairy |
| Minerals | Inorganic ions | Bone/teeth (Ca²⁺), haemoglobin (Fe²⁺), nerve impulses (Na⁺, K⁺) | Dairy, meat, leafy greens |
| Water | H₂O | Solvent; reactant in reactions; transport medium; temperature regulation | Drinks, fruits, vegetables |
| Dietary fibre | Cellulose (indigestible) | Adds bulk to faeces; stimulates peristalsis; prevents constipation | Whole grains, vegetables |
2 · The Alimentary Canal
The alimentary canal is a muscular tube running from the mouth to the anus. Food passes through it by peristalsis — waves of muscular contraction and relaxation that push food along.
Rhythmic waves of muscular contraction (circular muscles contract, longitudinal muscles relax, then vice versa) that propel food along the alimentary canal. It occurs throughout the oesophagus, stomach and intestines.
Sequence of Organs
Mouth → Oesophagus → Stomach → Duodenum → Ileum → Colon → Rectum → Anus
Accessory organs (not part of the canal but contribute secretions): salivary glands, liver, pancreas, gall bladder.
3 · Digestive Enzymes
| Enzyme | Produced by | Substrate | Product | Optimal pH |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose | ~7 (neutral) |
| Pepsin | Stomach (gastric glands) | Protein | Polypeptides | ~2 (acidic) |
| Pancreatic amylase | Pancreas | Starch | Maltose | ~7–8 |
| Lipase | Pancreas | Lipids | Fatty acids + glycerol | ~7–8 |
| Trypsin | Pancreas | Protein / polypeptides | Shorter polypeptides | ~8 (alkaline) |
| Maltase | Small intestine (ileum) | Maltose | Glucose + glucose | ~7 |
| Peptidases | Small intestine (ileum) | Polypeptides | Amino acids | ~7 |
The role of bile is commonly tested. Bile is produced by the liver, stored in the gall bladder, and released into the duodenum. It is NOT an enzyme — it emulsifies fats, breaking large fat globules into smaller droplets to increase the surface area for lipase to act on.
4 · The Stomach
The stomach has three main functions:
- Mechanical digestion: Churning of food by muscular walls creates a liquid called chyme.
- Chemical digestion: Gastric glands secrete hydrochloric acid (pH 2) and the enzyme pepsin. HCl kills bacteria, denatures salivary amylase, and provides the acidic pH needed by pepsin.
- Storage: Holds food while it is partially digested before releasing it into the duodenum.
Students say the stomach "produces acid to digest food." The acid itself does not digest food — it creates the correct pH for pepsin and kills microbes. Only enzymes digest food chemically.
5 · The Small Intestine
The small intestine consists of the duodenum and the ileum. Most chemical digestion is completed here, and virtually all nutrient absorption occurs in the ileum.
In the duodenum, secretions from the pancreas (pancreatic juice: amylase, lipase, trypsin, sodium bicarbonate) and liver (bile) are added. Sodium bicarbonate neutralises the acidic chyme from the stomach, raising pH to ~8 for optimal pancreatic enzyme activity.
6 · Villus Structure & Absorption
A finger-like projection of the inner lining of the ileum that massively increases the surface area for absorption. The surface of each villus is further covered in microvilli (the "brush border"), increasing surface area even more.
Adaptations of the Ileum for Absorption
- Large surface area: Villi and microvilli greatly increase the absorptive surface.
- Thin epithelium: Only one cell thick, reducing the diffusion distance.
- Rich blood supply: Each villus contains a capillary network to maintain a steep concentration gradient and carry absorbed nutrients to the liver via the hepatic portal vein.
- Lacteal: A lymph vessel in the centre of each villus that absorbs fatty acids and glycerol (as chylomicrons), which enter the lymphatic system before reaching the bloodstream.
1. The villus has a large surface area (due to microvilli) which increases the rate of absorption by diffusion.
2. The epithelial layer is only one cell thick, reducing the diffusion distance for glucose.
3. A dense network of blood capillaries in the villus constantly removes absorbed glucose, maintaining a steep concentration gradient.
4. Glucose is absorbed by active transport (as well as diffusion) using carrier proteins, allowing absorption even against a concentration gradient.
7 · The Large Intestine
By the time food reaches the large intestine (colon), most digestion and absorption are complete. The main function of the colon is to absorb water from the remaining indigestible material, compacting it into faeces. The rectum stores faeces until they are expelled through the anus.
The colon also contains large populations of bacteria that produce certain vitamins (e.g. vitamin K) and break down some remaining organic matter.
8 · Nutritional Deficiency Diseases
| Deficiency | Nutrient lacking | Symptoms |
|---|---|---|
| Anaemia | Iron (Fe²⁺) | Fewer/smaller red blood cells; fatigue; pale skin; breathlessness |
| Scurvy | Vitamin C (ascorbic acid) | Bleeding gums; poor wound healing; bruising; joint pain |
| Rickets (children) / Osteomalacia (adults) | Vitamin D and/or Calcium | Soft, weakened bones; bowed legs in children |
| Night blindness | Vitamin A | Inability to see in dim light; dry eyes |
| Kwashiorkor | Protein | Muscle wasting; swollen abdomen (oedema); poor growth; hair discolouration |
| Marasmus | Overall energy (protein + calories) | Severe weight loss; muscle and fat wasting; stunted growth |
Kwashiorkor vs Marasmus is commonly confused. Kwashiorkor = adequate calories but protein-deficient (hence swollen abdomen from oedema). Marasmus = overall starvation (both protein and energy severely deficient, no swelling — just wasting).
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