Heterotrophic nutrition and its types
Heterotrophs get food made by others. How they take it depends on the food and the organism:
- Holozoic: swallow whole food and digest it inside the body (humans, cow, amoeba).
- Saprophytic: break down dead and decaying matter outside the body and absorb it (bread mould, mushrooms, many bacteria).
- Parasitic: take food from a living host, usually harming it (Cuscuta or amarbel, ticks, lice, tapeworm).
How single-celled organisms get food
Amoeba pushes out finger-like projections (pseudopodia) around a food particle, which fuse to form a food vacuole. Inside it, the food is digested and the useful part diffuses into the cytoplasm; the leftover is thrown out at the cell surface.
Paramecium has a fixed shape; its tiny hair-like cilia sweep food into a particular spot (the cell mouth).
The human digestive system step by step
Mouth
Teeth chew food into small pieces; the tongue mixes it. Salivary amylase in saliva breaks starch into simple sugar.
Oesophagus (food pipe)
Rhythmic muscle contraction, peristalsis, moves food down. Peristalsis runs along the whole gut.
Stomach
Gastric glands release hydrochloric acid (acidic medium, kills germs, helps pepsin work), pepsin (protein digestion) and mucus (protects the lining from acid). A sphincter muscle lets food into the small intestine a little at a time.
Small intestine
The longest part (very long and coiled, to give time and area). Herbivores have a longer small intestine than carnivores because cellulose is hard to digest.
- Liver → bile: makes the food alkaline and breaks large fat globules into small droplets (emulsification). Bile has no enzyme.
- Pancreas → pancreatic juice: trypsin (proteins), lipase (emulsified fats), amylase (starch).
- Intestinal glands: final enzymes turning proteins into amino acids, carbohydrates into glucose, fats into fatty acids and glycerol.
Villi, finger-like folds of the inner wall, hugely increase the absorbing area. Each is rich in blood vessels that carry the food to every cell (assimilation).
Large intestine and anus
More villi absorb extra water. The waste is removed through the anus (egestion), controlled by the anal sphincter.
Enzyme summary and exam focus
| Juice / gland | Enzyme or part | Acts on → gives |
|---|---|---|
| Saliva | Salivary amylase | Starch → sugar (maltose) |
| Gastric juice | Pepsin, HCl, mucus | Protein → smaller peptides |
| Bile (liver) | Bile salts, no enzyme | Fat → droplets (emulsified) |
| Pancreatic juice | Trypsin, lipase, amylase | Protein, fat, starch |
| Intestinal juice | Final enzymes | → amino acids, glucose, fatty acids + glycerol |
Exam tip: role of HCl, bile, villi and the enzyme table are the usual 2–3 mark questions; the labelled diagram of the alimentary canal is a common 5-mark part.
Key formulas and definitions
- Starch → (salivary amylase) → sugar
- Protein → (pepsin, trypsin) → peptides → amino acids
- Fat → (bile: emulsify) → droplets → (lipase) → fatty acids + glycerol
- Peristalsis: wave-like muscle squeezing that moves food
- Villi: finger-like folds that increase absorbing area
- Holozoic, saprophytic, parasitic: three types of heterotrophic nutrition
Worked examples
1. What would happen if the stomach did not make mucus?
HCl would slowly eat into the stomach's own lining, causing painful sores (ulcers). Mucus forms a protective coat between the acid and the wall.
2. Why is the small intestine of a cow longer than that of a tiger?
A cow eats grass, rich in cellulose, which is digested very slowly. A longer intestine gives more time and area. A tiger eats meat, which is easier to digest, so a shorter intestine is enough.
3. Bile has no enzyme. Then how does it help in digestion?
It does two things: it makes the acidic food from the stomach alkaline, so pancreatic enzymes can work, and its bile salts break big fat globules into small droplets. Smaller droplets give lipase much more surface to act on.
4. Name the type of nutrition: (a) mushroom, (b) Cuscuta, (c) amoeba.
(a) Saprophytic: absorbs food from dead matter. (b) Parasitic: takes food from a host plant. (c) Holozoic: engulfs whole food and digests it inside.
5. Trace the digestion of a piece of paneer (mostly protein and fat).
Mouth: chewed, little chemical change. Stomach: pepsin in acid starts breaking protein. Small intestine: bile emulsifies fat; trypsin continues protein digestion; lipase breaks fat; intestinal enzymes give amino acids, fatty acids and glycerol, which villi absorb.
Common mistakes
- Writing that bile contains enzymes. Bile has bile salts, no digestive enzyme.
- Saying digestion ends in the stomach. Most digestion and nearly all absorption happen in the small intestine.
- Confusing egestion (removing undigested food through the anus) with excretion (removing metabolic waste like urea).
- Thinking bile is made in the gall bladder. It is made by the liver and only stored in the gall bladder.