Unit 6 Heterotrophic nutrition
Key unit competence: Evaluate heterotrophic nutrition processes and related disorders.
Key ideas
- Heterotrophs cannot make their own food; they obtain ready-made organic food from other organisms. Forms: holozoic (ingest solid food and digest it inside the body: humans, cows, lions), saprophytic (secrete enzymes onto dead matter and absorb the products: fungi, many bacteria) and parasitic (feed on a living host: tapeworm, ticks, Plasmodium).
- The stages of holozoic nutrition: ingestion (taking in food), digestion (breaking it into small soluble molecules), absorption (into the blood), assimilation (use of absorbed food by cells) and egestion (removal of undigested food as faeces).
- Human teeth: incisors (chisel-shaped, cutting and biting), canines (pointed, tearing), premolars and molars (broad with cusps, grinding and crushing). An adult has 32 permanent teeth; a child has 20 milk teeth. Dental formula of an adult: i 2/2, c 1/1, pm 2/2, m 3/3.
- Tooth structure: enamel (hardest substance in the body, covers the crown), dentine (bone-like, under enamel), pulp cavity (nerves and blood vessels), cement and periodontal fibres fix the root in the jaw bone; the gum covers the jaw.
- Dental caries (tooth decay): bacteria in plaque feed on sugar and produce acid that dissolves enamel and dentine, causing holes and pain. Gum disease (periodontitis): plaque causes bleeding, inflamed gums and loosening teeth. Prevention: brush at least twice a day with fluoride toothpaste, floss, eat less sugar, eat crunchy fruits and vegetables, visit a dentist.
- The alimentary canal: mouth → oesophagus → stomach → duodenum and ileum (small intestine) → colon (large intestine) → rectum → anus. Food is moved by peristalsis (waves of muscle contraction). Associated glands: salivary glands, liver (bile), gall bladder (stores bile), pancreas (pancreatic juice).
- Digestion: mouth: chewing (mechanical) and salivary amylase (starch → maltose). Stomach: pepsin (proteins → polypeptides) in hydrochloric acid (kills bacteria, optimum pH for pepsin). Duodenum: bile emulsifies fats (no enzyme) and neutralises acid; pancreatic amylase, trypsin and lipase. Ileum: enzymes finish digestion to glucose, amino acids, fatty acids and glycerol.
- Absorption in the ileum: long, folded, with millions of villi and microvilli (huge surface area), thin epithelium, dense capillaries (glucose and amino acids go to the liver by the hepatic portal vein) and a lacteal (fatty acids and glycerol go into the lymph). The colon absorbs water and salts; faeces are stored in the rectum and egested through the anus.
- Assimilation: glucose is used in respiration or stored as glycogen in the liver and muscles; amino acids build proteins (excess are deaminated in the liver); fats are used for energy or stored under the skin.
- Disorders: constipation (hard, infrequent faeces; caused by little fibre and water; prevented by vegetables, fruits, whole grains, water, exercise); diarrhoea (frequent watery stools from infections or contaminated food; treated with oral rehydration solution (ORS): clean water, salt and sugar); stomach ulcers (often Helicobacter pylori); food poisoning.
Common misconceptions
- "Bile is an enzyme that digests fat." No: bile has no enzymes; it emulsifies fats into small droplets so lipase can act faster.
- "Most digested food is absorbed in the stomach." No: most absorption happens in the ileum (small intestine).
- "Diarrhoea should be treated by stopping all drinking." No: the patient needs more fluids, especially ORS, to replace lost water and salts.
- "Brushing once a week prevents decay." No: plaque forms within hours; brush at least twice a day.
Worked examples
Example 1. Explain how the ileum is adapted for absorption.
Solution
It is very long and folded, with millions of villi and microvilli that give a huge surface area. The villus wall is one cell thick (short diffusion distance), has a dense capillary network to carry away glucose and amino acids, and a lacteal for fats. Cells have many mitochondria for active transport.
Example 2. Explain how eating sweets between meals causes tooth decay.
Solution
Bacteria in plaque feed on the sugar and release acid. Each time sweets are eaten, the mouth stays acidic for a long time, dissolving enamel and then dentine, forming cavities. Brushing with fluoride toothpaste and reducing sugary snacks prevent this.
Practice questions
Name the three forms of heterotrophic nutrition and give an example of each.
Show answer
Holozoic (humans, cows); saprophytic (mushrooms, bread mould, decomposing bacteria); parasitic (tapeworm, tick, Plasmodium).
Common mistake: Calling fungi holozoic.
Name the four types of human teeth and give the function of each.
Show answer
Incisors: cutting and biting. Canines: tearing. Premolars: crushing and grinding. Molars: grinding.
Common mistake: Saying canines grind food.
Describe how tooth decay occurs and give three ways to prevent it.
Show answer
Bacteria in plaque feed on sugar and produce acid that dissolves enamel and dentine, forming cavities that may reach the pulp. Prevent by brushing twice daily with fluoride toothpaste, eating less sugar, flossing and regular dental check-ups.
Common mistake: Saying decay is caused by cold water.
State the role of: (a) salivary amylase; (b) bile; (c) hydrochloric acid.
Show answer
(a) Digests starch into maltose. (b) Emulsifies fats and neutralises stomach acid. (c) Kills bacteria and provides acidic pH for pepsin.
Common mistake: Calling bile an enzyme.
Describe what happens to proteins from the mouth to the blood.
Show answer
Chewing breaks food; in the stomach pepsin digests proteins to polypeptides; in the duodenum trypsin and in the ileum peptidases digest them to amino acids, which are absorbed into villus capillaries.
Common mistake: Saying amylase digests proteins.
State four ways the ileum is adapted for absorption.
Show answer
Long; villi and microvilli; thin epithelium; rich capillary network; lacteals; many mitochondria for active transport.
Common mistake: Listing features of the stomach.
Explain how to prepare oral rehydration solution at home.
Show answer
Boil and cool one litre of clean water; dissolve six level teaspoons of sugar and half a level teaspoon of salt (or one ORS packet); give small sips often; make fresh daily.
Common mistake: Using too much salt.
State the causes and prevention of constipation.
Show answer
Causes: little fibre, too little water, lack of exercise, ignoring the urge to defecate. Prevention: eat fruits, vegetables and whole grains; drink enough water; exercise.
Common mistake: Recommending more sugar.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The teeth used for grinding food are: a. molars and premolars 2. The hardest substance in the human body is: b. enamel 3. Taking in solid food and digesting it inside the body is: c. kills bacteria and gives pepsin its pH 4. Hydrochloric acid in the stomach: d. holozoic nutrition Show answer
1 → a (molars and premolars) · 2 → b (enamel) · 3 → d (holozoic nutrition) · 4 → c (kills bacteria and gives pepsin its pH)
- They have broad surfaces with cusps.
- It covers the crown.
- Humans are holozoic.
- The stomach pH is about 2.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
clean water, salt and sugar · producers · heterotrophs · the pH there is alkaline- Pepsin stops working when it reaches the duodenum because .
- Oral rehydration solution contains .
- Organisms that feed on ready-made organic food are .
Show answer
1: the pH there is alkaline · 2: clean water, salt and sugar · 3: heterotrophs
- Pepsin's optimum pH is about 2.
- It replaces lost water and salts.
- Animals and fungi are heterotrophs.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Tooth decay is caused by acid produced by: a. ileum 2. Most absorption of digested food takes place in the: b. water and salts 3. The main function of the colon is to absorb: c. fibre-rich fruits and vegetables 4. Constipation can be prevented by eating: d. bacteria feeding on sugar Show answer
1 → d (bacteria feeding on sugar) · 2 → a (ileum) · 3 → b (water and salts) · 4 → c (fibre-rich fruits and vegetables)
- Bacteria live in plaque.
- It has villi.
- Faeces become firmer.
- Fibre adds bulk and helps peristalsis.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
premolars · incisors · proteins in the stomach · pulp cavity- Pepsin digests .
- The teeth used for cutting and biting are .
- The part of the tooth with nerves and blood vessels is the .
Show answer
1: proteins in the stomach · 2: incisors · 3: pulp cavity
- It works in acid.
- They are chisel-shaped.
- Decay reaching it causes pain.