Nutrition covers how organisms obtain food. Plants make their own (autotrophic nutrition — photosynthesis) while humans consume food (heterotrophic nutrition — digestion and absorption).
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll). Occurs in chloroplasts. Limiting factors: light intensity, CO₂ concentration, temperature. Increasing any one factor increases rate until another becomes limiting. Leaf adaptations: thin (short diffusion distance), large surface area, stomata for gas exchange, palisade cells near top surface, waxy cuticle prevents water loss, spongy mesophyll has air spaces for gas diffusion.
Ingestion → digestion → absorption → assimilation → egestion. Mechanical digestion: teeth (incisors cut, canines tear, molars grind), peristalsis (wave-like muscle contractions move food). Chemical digestion: enzymes break large molecules into small soluble ones. Mouth: amylase (starch → maltose). Stomach: protease/pepsin (proteins), pH 2 (HCl kills bacteria). Small intestine: pancreatic enzymes, bile (emulsifies fats, alkaline). Absorption: villi in small intestine — large surface area, thin walls, rich blood supply, lacteals for fats.
Nutrients: carbohydrates (energy), proteins (growth/repair), lipids (energy store, insulation), vitamins (e.g., C prevents scurvy, D prevents rickets), minerals (iron for haemoglobin, calcium for bones), fibre (prevents constipation), water. Malnutrition: deficiency or excess. Obesity: excess energy intake → heart disease, diabetes. Starvation: insufficient food → wasting, weakened immune system.
The small intestine is approximately 6-7 metres long in humans to maximise the time food spends in contact with the intestinal wall and to maximise the surface area for absorption. The inner surface is further increased by: folds in the wall, villi (finger-like projections), and microvilli (tiny projections on each villus cell). This gives an enormous surface area (~250 m²). The length ensures that virtually all useful nutrients are absorbed before the remaining material passes to the large intestine.
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