🧬 Biology · Topic 4 of 10
Start with the study notes on Nutrition in Animals & Digestion (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Animals are heterotrophs. Their nutrition happens in five steps:
Ingestion → Digestion → Absorption → Assimilation → Egestion
The alimentary canal is a long tube from mouth to anus. Digestive glands (salivary glands, liver and pancreas) pour juices into it.
Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus
Digestion begins in the mouth. Teeth break food into small pieces, and saliva mixes with it.
Saliva contains the enzyme salivary amylase, which turns starch into sugar.
Example: Chew a piece of chapati for a long time and it begins to taste sweet.
An adult has 32 teeth; a child has 20 milk teeth.
Remember: Bacteria turn leftover sugar into acid, which attacks the enamel and causes tooth decay. Brush after meals.
The tongue mixes food with saliva, rolls it into a ball, helps us swallow, and has taste buds.
The oesophagus (food pipe) carries food to the stomach by peristalsis: wave-like squeezing of its muscles.
The stomach produces:
Example: The epiglottis, a small flap, covers the windpipe when you swallow, so food doesn't go down the wrong way.
Most digestion and absorption happen in the small intestine, which is about 7.5 metres long.
Its inner wall has millions of finger-like villi. They give a huge surface area for absorbing digested food into the blood.
Enzymes are the substances that speed up these breakdowns, e.g. amylase (starch), pepsin (protein) and lipase (fat).
The large intestine absorbs water and some salts from undigested food. The waste is stored in the rectum and passed out through the anus (egestion).
Examples:
Amoeba pushes out finger-like pseudopodia around its food and traps it in a food vacuole, where the food is digested.
Ruminants such as cows swallow grass quickly and store it in the rumen. Later they bring it back up and chew it again: chewing the cud.
Bacteria in their stomach digest cellulose, which humans cannot digest.
Example: Horses and rabbits have a large caecum, where bacteria digest cellulose.
| Misconception | Correct understanding |
|---|---|
| Digestion begins in the stomach. | It begins in the mouth, where saliva starts breaking down starch. |
| Egestion and excretion are the same. | Egestion removes undigested food; excretion removes wastes like urea made by the body. |
| Bile is an enzyme. | Bile has no enzymes; it breaks fat into droplets so enzymes can work on it. |
| Most digestion happens in the large intestine. | Most digestion and absorption happen in the small intestine. |
| Humans can digest the cellulose in grass. | We can't; it passes through as roughage. |
Put the five steps of nutrition in order.
Ingestion, digestion, absorption, assimilation, egestion.
Why does chapati taste sweet after long chewing?
Salivary amylase turns its starch into sugar.
How many teeth does an adult have?
32.
What does hydrochloric acid do in the stomach?
It kills germs and helps the protein-digesting enzymes work.
Which is the largest gland, and what does it make?
The liver; it makes bile.
Where does most absorption of food take place?
In the small intestine, through the villi.
What is the main job of the large intestine?
To absorb water and some salts.
How can cows digest grass?
Bacteria in their stomach digest the cellulose.
Key takeaway: Food is taken in, broken down by enzymes, absorbed in the small intestine, used by the cells, and whatever is left is passed out.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The breaking down of complex food into simpler absorbable substances is called:
Answer: A — Digestion
Digestion converts large insoluble food molecules into small soluble ones that can pass into the blood. It involves both mechanical and chemical action.
Q2. easy
The taking in of food into the body is called:
Answer: A — Ingestion
Ingestion is the first step of nutrition - putting food into the mouth. It is followed by digestion, absorption, assimilation and egestion.
Q3. medium
The five steps of nutrition in animals, in order, are:
Answer: A — Ingestion, digestion, absorption, assimilation, egestion
Food is taken in, broken down, absorbed into the blood, used by the cells (assimilation), and the undigested part is finally removed as faeces (egestion).
Q4. medium
The passing of digested food into the blood is called:
Answer: A — Absorption
Absorption takes place mainly in the small intestine, where the digested nutrients pass through the intestinal wall into the blood stream.
Q6. medium
The removal of undigested food from the body is called:
Answer: A — Egestion
Egestion removes the undigested residue as faeces through the anus. It is different from excretion, which removes metabolic wastes such as urea.
Q8. medium
The correct order of parts of the human alimentary canal is:
Answer: A — Mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus
Food travels from the buccal cavity down the food pipe to the stomach, then through the small intestine where most digestion and absorption occur, and finally through the large intestine to the rectum and anus.
Q10. medium
The enzyme in saliva that digests starch is called:
Answer: A — Salivary amylase (ptyalin)
Salivary amylase converts starch into simpler sugars. This is why a piece of chapati or rice begins to taste slightly sweet if chewed for a while.
Q12. medium
The four types of teeth in humans are:
Answer: A — Incisors, canines, premolars and molars
Incisors cut, canines tear, and premolars and molars crush and grind. An adult has 32 teeth in all.
Q14. medium
Which teeth are pointed and used for tearing and piercing?
Answer: A — Canines
The four pointed canines sit next to the incisors. They are especially well developed in carnivores for tearing flesh.
Q16. medium
How many teeth does an adult human normally have?
Answer: A — 32
An adult has 32 permanent teeth: 8 incisors, 4 canines, 8 premolars and 12 molars (including 4 wisdom teeth). Children have 20 milk teeth.
Q18. medium
Tooth decay is caused when:
Answer: A — Bacteria act on leftover sugar and produce acid that attacks the enamel
Food particles stuck between teeth feed bacteria, which produce acid. The acid dissolves the hard enamel, forming cavities that can reach the sensitive pulp.
Q20. medium
The food pipe that carries food from the mouth to the stomach is the:
Answer: A — Oesophagus
The oesophagus pushes food downwards by wave-like muscular contractions called peristalsis, so swallowing works even if you are lying down or upside down.
Q22. medium
The stomach wall secretes hydrochloric acid, mucus and:
Answer: A — Digestive juices containing enzymes that act on proteins
Gastric juice contains hydrochloric acid, mucus and protein-digesting enzymes such as pepsin. Together they begin the digestion of protein.
Q24. medium
Why does the stomach not digest itself despite containing strong acid?
Answer: A — A layer of mucus protects the stomach wall
Mucus secreted by the stomach lining forms a protective barrier against the acid and enzymes. When this layer breaks down, an ulcer can form.
Q26. medium
Bile is secreted by the liver and stored in the:
Answer: A — Gall bladder
Bile is stored and concentrated in the small sac-like gall bladder, and released into the small intestine when fatty food arrives.
Q28. medium
The pancreas secretes pancreatic juice which digests:
Answer: A — Carbohydrates, proteins and fats
Pancreatic juice contains amylase for starch, trypsin for proteins and lipase for fats, making it the most complete of the digestive juices.
Q30. medium
The inner wall of the small intestine has thousands of finger-like projections called:
Answer: A — Villi
Villi enormously increase the surface area for absorption. Each contains blood capillaries that carry the absorbed nutrients away.
Q32. medium
The main function of the LARGE intestine is to:
Answer: A — Absorb water and some salts from the undigested food
By the time food reaches the large intestine, nutrients have been absorbed. Water and salts are reclaimed here, and the remaining semi-solid waste passes to the rectum.
Q34. medium
Diarrhoea causes dehydration, and the recommended immediate treatment is:
Answer: A — Oral rehydration solution (ORS) - a mixture of salt and sugar in clean water
Diarrhoea drains the body of water and salts. ORS replaces both quickly and can be made at home with a pinch of salt and a spoon of sugar in a glass of boiled, cooled water.
Q36. medium
In Amoeba, food is digested inside a:
Answer: A — Food vacuole
Digestive juices are secreted into the food vacuole, and the digested nutrients then diffuse into the surrounding cytoplasm. Undigested matter is thrown out.
Q38. medium
The part of the ruminant stomach where swallowed grass is first stored is the:
Answer: A — Rumen
In the rumen, bacteria begin breaking down the cellulose while the animal is free to move. The softened food is then brought back up to be chewed properly.
Q40. medium
Humans cannot digest cellulose, so the fibre in our food:
Answer: A — Acts as roughage and helps move food through the intestines
Undigested cellulose adds bulk to the food, stimulating the intestinal muscles and preventing constipation. This is why fibre-rich food is important.
Q42. medium
Which animal feeds by sucking blood using a needle-like mouthpart?
Answer: A — Mosquito
A female mosquito pierces the skin with her sharp proboscis and sucks blood, which she needs to develop eggs. She may transmit disease in the process.
Q44. medium
A snake swallows its prey whole because it:
Answer: A — Cannot chew - its jaws stretch wide to take in large prey
A snake's jaws are loosely joined and can open remarkably wide. The prey is swallowed whole and digested slowly by powerful stomach juices.
Q46. medium
Substances that speed up chemical reactions in digestion are called:
Answer: A — Enzymes
Enzymes are biological catalysts. Each acts on a particular type of food - amylase on starch, pepsin on proteins, lipase on fats.
Q48. medium
Which enzyme digests FATS?
Answer: A — Lipase
Lipase, secreted by the pancreas and intestine, breaks fats into fatty acids and glycerol after bile has emulsified them.
Q50. medium
Proteins are finally broken down into:
Answer: A — Amino acids
Amino acids are absorbed into the blood and used by cells to build the body's own proteins for growth and repair.
Q52. medium
Why is it important to chew food properly?
Answer: A — It breaks food into smaller pieces and mixes it with saliva, making digestion easier
Thorough chewing increases the surface area for enzymes to act on and starts starch digestion in the mouth, reducing the load on the stomach and intestines.
Q54. medium
The liver performs which additional function besides producing bile?
Answer: A — It stores glucose as glycogen and detoxifies harmful substances
The liver regulates blood sugar by storing and releasing glycogen, breaks down toxins and old red blood cells, and stores several vitamins.
Q56. medium
Which practice helps keep the digestive system healthy?
Answer: A — Eating fresh food at regular times, chewing well and drinking enough water
Regular, moderate meals of fresh food, eaten slowly with plenty of water and fibre, keep digestion smooth and prevent both indigestion and constipation.
Q58. medium
Which overall statement about nutrition in animals is CORRECT?
Answer: A — Food is ingested, digested by enzymes, absorbed into blood, assimilated by cells, and the undigested part is egested
This sums up the topic: all animals are heterotrophs; the human alimentary canal with its glands carries out the five steps of nutrition; and different animals show remarkably varied ways of obtaining and digesting food.
Q59. medium
Why is the small intestine so long (about 7.5 metres) in humans?
Answer: A — The extra length gives more time and surface area for digestion and absorption
Food moves slowly along the coiled small intestine, giving enzymes time to finish digestion and the villi time to absorb nearly all the nutrients before the residue moves on.
Q5. hard
The use of absorbed food by body cells for energy, growth and repair is called:
Answer: A — Assimilation
Assimilation is the final use of nutrients inside the cells - glucose is oxidised for energy, and amino acids are built into new proteins.
Q7. hard
The human digestive system consists of the alimentary canal and:
Answer: A — The associated digestive glands
The alimentary canal runs from mouth to anus. Glands such as the salivary glands, liver and pancreas secrete juices into it that help digest food.
Q9. hard
Digestion of food begins in the:
Answer: A — Mouth (buccal cavity)
In the mouth, teeth grind the food and saliva mixes with it. The enzyme in saliva begins breaking starch down into sugar even before the food is swallowed.
Q11. hard
Chewing a piece of bread for a long time makes it taste sweet because:
Answer: A — Salivary amylase converts starch into sugar
Prolonged chewing gives the enzyme time to act. The starch is broken down into maltose and other sugars, which the taste buds detect as sweetness.
Q13. hard
Which teeth are used for CUTTING and biting food?
Answer: A — Incisors
The eight sharp, chisel-shaped incisors at the front cut and bite food. There are four in each jaw.
Q15. hard
Which teeth have broad flat surfaces for grinding and chewing?
Answer: A — Molars and premolars
Premolars and molars at the back have wide ridged surfaces that crush and grind food into a fine paste before it is swallowed.
Q17. hard
The first set of teeth that fall out during childhood are called:
Answer: A — Milk teeth
Twenty milk (temporary) teeth appear in infancy and are gradually replaced between about six and twelve years of age by the permanent set.
Q19. hard
The muscular organ that mixes food with saliva and helps in swallowing and tasting is the:
Answer: A — Tongue
The tongue rolls the food into a ball (bolus), pushes it towards the throat for swallowing, and carries taste buds that detect flavours. It also helps in speech.
Q21. hard
The wave-like muscular movement that pushes food along the alimentary canal is called:
Answer: A — Peristalsis
Rhythmic contraction and relaxation of the wall squeezes food along, from the oesophagus right through to the large intestine. It works against gravity too.
Q23. hard
Hydrochloric acid in the stomach:
Answer: A — Kills bacteria in food and provides the acidic medium the enzymes need
The strong acid destroys most germs swallowed with food and creates the acidic conditions in which pepsin works best on proteins.
Q25. hard
The LARGEST gland in the human body is the:
Answer: A — Liver
The liver is a large reddish-brown gland in the upper right of the abdomen. It secretes bile and performs hundreds of other functions.
Q27. hard
Bile helps in the digestion of:
Answer: A — Fats - it breaks large fat droplets into small ones (emulsification)
Bile contains no enzymes, but by emulsifying fat it greatly increases the surface area available to the fat-digesting enzyme lipase.
Q29. hard
Most of the digestion and absorption of food takes place in the:
Answer: A — Small intestine
The small intestine receives bile and pancreatic juice and secretes intestinal juice of its own. Digestion is completed here and the nutrients are absorbed.
Q31. hard
Villi increase the efficiency of absorption because they:
Answer: A — Greatly increase the surface area in contact with digested food
Millions of villi, each covered with even smaller microvilli, turn the intestinal lining into a vast absorbing surface - roughly the area of a tennis court.
Q33. hard
The undigested waste is stored in the rectum and passed out through the anus. This is called:
Answer: A — Egestion
Faeces are stored briefly in the rectum and expelled through the anus. Adequate roughage and water keep this process regular and comfortable.
Q35. hard
Amoeba takes in food by:
Answer: A — Extending pseudopodia around the food particle and engulfing it
The single-celled Amoeba pushes out finger-like pseudopodia that surround the food and fuse, trapping it in a food vacuole where it is digested.
Q37. hard
Grass-eating animals like cows are called RUMINANTS because they:
Answer: A — Swallow grass quickly, store it in the rumen, and later chew the cud
Ruminants have a four-chambered stomach. Partly digested food is returned to the mouth for thorough chewing - a process called rumination or chewing the cud.
Q39. hard
Ruminants can digest CELLULOSE in grass because:
Answer: A — Bacteria living in their stomach produce enzymes that break down cellulose
No mammal makes cellulase. Ruminants rely on symbiotic bacteria in the rumen to digest cellulose, gaining nutrients that other animals cannot obtain from grass.
Q41. hard
A sac-like structure in some grass-eating animals (like horses) where cellulose is digested is the:
Answer: A — Caecum
Non-ruminant herbivores such as horses and rabbits have a large caecum between the small and large intestines, home to cellulose-digesting bacteria.
Q43. hard
A butterfly feeds on nectar using its:
Answer: A — Long tube-like proboscis that can be coiled up
The butterfly uncoils its proboscis like a straw to reach nectar deep inside flowers, and rolls it up neatly when not feeding.
Q45. hard
A starfish feeds by:
Answer: A — Pushing its stomach out of its mouth onto the prey and digesting it outside the body
The starfish everts its stomach over the soft body of a shellfish, digests it externally, then draws the stomach and the liquefied food back in.
Q47. hard
Which enzyme digests PROTEINS in the stomach?
Answer: A — Pepsin
Pepsin works in the acidic environment of the stomach, breaking proteins into smaller peptides. Trypsin continues the job in the small intestine.
Q49. hard
Carbohydrates are finally broken down into:
Answer: A — Glucose
Starch and sugars end up as glucose, which is absorbed into the blood and oxidised in cells to release energy.
Q51. hard
Fats are finally broken down into:
Answer: A — Fatty acids and glycerol
Lipase splits fat molecules into fatty acids and glycerol, which are absorbed by the villi and later reassembled and stored or used for energy.
Q53. hard
Which of these prevents food from entering the windpipe while swallowing?
Answer: A — The epiglottis, a flap that covers the windpipe
As you swallow, the epiglottis folds down over the opening of the windpipe. If food slips past it, we choke and cough to expel it.
Q55. hard
Which condition is caused by eating too little roughage and drinking too little water?
Answer: A — Constipation
Without enough fibre and water, the waste in the large intestine becomes dry and hard and moves slowly, making it painful to pass.
Q57. hard
Why should we wash hands before eating?
Answer: A — To remove germs that would otherwise enter the digestive system with food
Many digestive illnesses - diarrhoea, cholera, typhoid and worm infections - spread through unwashed hands. Washing with soap is a simple, powerful preventive measure.
Q60. hard
The small intestine of a herbivore like a cow is much longer than that of a tiger because:
Answer: A — Plant food is harder to digest and needs a longer time in the gut
Cellulose-rich grass breaks down slowly, so herbivores need a long gut and helpful bacteria. Meat is digested quickly, so carnivores manage with a much shorter intestine.