Why we need food, heterotrophic nutrition and digestion
Food gives energy and builds the body. Heterotrophs depend on others for food. In humans, enzymes digest food along the alimentary canal, and villi absorb it.
Why living things need food
- Food gives energy for life processes such as respiration, digestion and excretion.
- Food helps in growth and in the repair of worn-out and damaged cells and tissues.
- Food is needed to make enzymes and hormones in the body.
- Without food for a few days, we become weak and exhausted; without food for long, we cannot survive.
Modes of nutrition
| Mode | How the organism gets food | Examples |
|---|---|---|
| Autotrophic (auto = self) | Makes its own food by photosynthesis | Green plants, algae, some bacteria |
| Holozoic (a kind of heterotrophic; heteros = other) | Takes in whole food, then digests and absorbs it | Amoeba, frog, humans |
| Parasitic (heterotrophic) | Lives on or inside another living organism, the host, and takes food from it | Head louse, bed bug, leech, worms in the gut, Cuscuta (amar bel) |
| Saprotrophic (heterotrophic) | Feeds on dead and decaying matter, and so cleans the surroundings | Mushroom, bread mould, yeast, the white fungus on wet leather |
Digestion along the alimentary canal
**Digestion** breaks complex food into small, simple substances that cells can absorb. The **alimentary canal** is one long tube: mouth, pharynx, oesophagus, stomach, small intestine, large intestine and rectum. With the **salivary glands, liver and pancreas**, it forms the digestive system.
- 1Mouth: teeth chew the food. Saliva wets it for swallowing, and its enzyme salivary amylase breaks starch into sugar.Chew a piece of chapati for long and it tastes sweet
- 2Oesophagus (food pipe): there is no digestion here. Waves of muscle contraction, called peristalsis, push the food down to the stomach.
- 3Stomach: gastric juice has hydrochloric acid (HCl) and pepsinogen. HCl turns pepsinogen into pepsin and kills germs. Pepsin breaks proteins into peptones.
- 4Duodenum (first part of the small intestine): bile, made by the liver and stored in the gall bladder, breaks fat into tiny droplets (emulsification). It also makes the food alkaline.Bile has no enzyme
- 5Pancreatic juice also enters the duodenum. Its trypsin digests proteins, amylase turns starch into maltose, and lipase turns fats into fatty acids and glycerol.
- 6★Digestion is completed in the small intestine: proteins become amino acids, carbohydrates glucose, and fats fatty acids and glycerol.
- 7Absorption: finger-like villi on the inner wall give a large surface. Blood capillaries in them take the digested food to all cells.
- 8Large intestine: water is absorbed from the undigested food. The solid waste (faeces) passes to the rectum and leaves through the anus.
★ key step
The five steps of nutrition
**Ingestion**: taking food in through the mouth. **Assimilation**: cells use the absorbed food to release energy and build cell parts. **Egestion**: removing undigested waste through the anus.
Digestive juices and their enzymes
| Juice (where it is made) | Enzyme or substance | What it does |
|---|---|---|
| Saliva (salivary glands) | Salivary amylase | Starch → sugar |
| Gastric juice (stomach) | Hydrochloric acid | Makes the medium acidic, turns pepsinogen into pepsin, kills bacteria |
| Gastric juice (stomach) | Pepsin | Proteins → peptones |
| Bile (liver; stored in the gall bladder) | No enzyme | Emulsifies fat; makes the medium alkaline for the pancreatic enzymes |
| Pancreatic juice (pancreas) | Trypsin | Peptones → smaller peptides |
| Pancreatic juice (pancreas) | Amylase | Starch → maltose |
| Pancreatic juice (pancreas) | Lipase | Fats → fatty acids and glycerol |
How to draw the human alimentary canal
- Outline the head and trunk. Mark the mouth and draw the salivary glands near the jaw.
- Draw the pharynx (throat) behind the mouth, then a long, narrow tube down the chest: the oesophagus (food pipe).
- Below it, draw a J-shaped bag: the stomach.
- Draw the large liver beside and partly over the stomach, with the small gall bladder under it.
- Draw the leaf-shaped pancreas just below the stomach.
- Fill the middle of the abdomen with the long, coiled small intestine; mark its first part, the duodenum, just after the stomach.
- Draw the wider large intestine around it, ending in the rectum and anus. Write all labels on one side with straight lines.
When you eat idli and sambar, salivary amylase starts on the rice starch at once, and the dal protein waits for pepsin in the stomach. Jaundice, a liver infection by the hepatitis virus that often spreads through dirty water, turns the skin and urine yellow, so drink clean water.
2 marks: “Pancreatic juice acts on all three main kinds of food. Name its three enzymes and say what each one does.” Trypsin digests proteins (peptones into smaller peptides); amylase digests starch into maltose; lipase digests fats into fatty acids and glycerol.
2 marks: “Bile has no enzyme. How does it still help digestion?” It emulsifies fats, breaking them into tiny droplets, so lipase can act faster. It also makes the medium alkaline, which the pancreatic enzymes need.
5 marks: “Describe the digestion of food in humans from the mouth to the large intestine. Draw a labelled diagram of the alimentary canal.” Mouth: salivary amylase, starch to sugar; oesophagus: peristalsis; stomach: HCl and pepsin, proteins to peptones; small intestine: bile emulsifies fat, trypsin, amylase and lipase, digestion completed, villi absorb; large intestine: water absorbed, faeces leave through the anus. Labelled diagram.
Pancreatic juice is TAL: Trypsin for proteins, Amylase for starch, Lipase for fats.