Heterotrophic Nutrition and Human Digestion

Heterotrophic Nutrition and Human Digestion

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Heterotrophic nutrition is the mode of feeding in which organisms depend on complex organic food made by other living things, because they cannot photosynthesise. Humans are holozoic heterotrophs: they ingest solid food, digest it, absorb the products, assimilate what the body needs, and egest the residue.

The human alimentary canal runs mouth → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon, rectum) → anus, supported by the salivary glands, pancreas, liver, and intestinal glands.

• Carbohydrate digestion starts in the mouth via salivary amylase (ptyalin) converting starch to maltose.
• Protein digestion starts in the stomach via pepsin in HCl (pH ~2), forming polypeptides.
• Fat digestion occurs in the duodenum: bile emulsifies fats, then pancreatic lipase splits them to fatty acids and glycerol.
• Absorption occurs in the ileum through villi; glucose/amino acids enter blood capillaries, fats enter lacteals.

High-yield pointers: know the seven-step process (ingestion → digestion → absorption → assimilation → egestion), distinguish egestion from excretion, and remember pepsin’s optimum pH is acidic (≈2), while amylase and trypsin work in alkaline conditions (≈7–8).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Types of Heterotrophic Nutrition

All heterotrophs rely on autotrophs (green plants) directly or indirectly. Four modes exist:

• Holozoic — ingestion of solid food (humans, Amoeba, goat).
• Saprophytic — feeding on dead/decaying organic matter using external enzymes (Rhizopus, Mucor).
• Parasitic — living on/in a host, harming it (Tapeworm, Plasmodium).
• Symbiotic (mutualistic) — both partners benefit (root-nodule bacteria and legumes).

Humans are obligate holozoic feeders.

Stages in Holozoic Nutrition

Ingestion → Digestion → Absorption → Assimilation → Egestion. Digestion itself has two components: mechanical (teeth-mastication, tongue-mixing, peristalsis, stomach churning) and chemical (hydrolytic enzymes acting on carbohydrates, proteins, lipids).

Chemistry of Digestion

Nutrient	Where digestion begins	Main enzyme(s)	Product(s)
Starch (carbohydrate)	Mouth	Salivary amylase (ptyalin)	Maltose
Maltose	Duodenum	Maltase	Glucose
Protein	Stomach	Pepsin (in HCl)	Polypeptides
Polypeptides	Duodenum	Trypsin, peptidases	Amino acids
Lipids (fats & oils)	Duodenum	Bile (emulsifier) + pancreatic lipase	Fatty acids + glycerol

Enzyme properties to memorise: they are specific (lock-and-key), protein in nature, denatured by heat above ~40 °C, and require an optimum pH (pepsin ≈ 2; amylase/trypsin/lipase ≈ 7–8). Bile from the liver contains no enzyme; it only emulsifies fats into tiny droplets, increasing surface area for lipase.

Peristalsis and Movement

Peristalsis is the wave-like contraction of circular and longitudinal muscles in the gut wall that pushes food along the oesophagus and intestines. The cardiac sphincter closes the stomach entrance to prevent backflow, and the pyloric sphincter controls release of chyme into the duodenum.

Exam Question Patterns in NECO

Expect (1) matching enzymes to substrates and products, (2) naming digestive organs in order, (3) explaining why pepsin needs HCl while trypsin needs an alkaline medium, (4) functions of bile, and (5) adaptations of the ileum for absorption — long villi, thin epithelium, rich capillary and lacteal networks, and presence of microvilli increasing surface area to roughly 250 m².

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Anatomical Adaptations for Absorption

The ileum is the principal site of absorption. Its villi and microvilli amplify surface area dramatically. Each villus contains a network of blood capillaries (for glucose, amino acids, water-soluble vitamins) and a central lacteal (lymphatic vessel) that absorbs fatty acids and glycerol as fine droplets. Absorbed nutrients drain into the hepatic portal vein, which carries them to the liver for detoxification, regulation of blood glucose, and storage of excess as glycogen (assimilation).

Common Mistakes and Examiner Traps

• Egestion ≠ Excretion. Egestion removes undigested material (faeces) via the anus. Excretion removes metabolic wastes (urea, CO₂, sweat) produced by cellular metabolism. The large intestine’s role is to reabsorb water and minerals and compact waste — not to digest food.
• Mouth or stomach for starch? Many candidates say stomach, but salivary amylase works in the mouth; it is inactivated once it meets gastric acid.
• Bile is not an enzyme. It is an alkaline fluid from the liver, stored in the gall bladder, and physically breaks fat into small globules (emulsification).
• Fatty acids enter the lacteals, not the bloodstream directly. They later join the blood via the thoracic duct.
• Pepsin is secreted as inactive pepsinogen, activated by HCl — this prevents the stomach from digesting itself.

Linked Topics Worth Revising

• Teeth and mastication (structure, types, dental formula in mammals).
• Balanced diet (carbohydrates, proteins, lipids, vitamins, minerals, water, roughage).
• Dental caries and constipation as digestive-system disorders.
• Plant nutrition contrast: autotrophic vs. heterotrophic.

Worked Example

A 60 g portion of boiled yam contains roughly 15 g starch. Salivary amylase in the mouth begins hydrolysing starch to maltose; in the duodenum, pancreatic amylase and maltase complete hydrolysis to glucose. The glucose is absorbed across the ileal epithelium by active transport into the capillaries, then transported via the hepatic portal vein to the liver, where excess is converted to glycogen for storage. Energy released from glucose oxidation = ~16 kJ/g (4 kcal/g).

Practice Prompts

1. State three structural adaptations of the ileum that aid absorption and explain how each increases efficiency.
2. A patient has the ileum surgically shortened. Predict the two most likely nutritional consequences and justify each.

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