Nutrition, Digestion and Respiration

What to remember

• Nutrition is how an organism gets and uses food. Plants make food by photosynthesis (autotrophic); animals depend on ready-made food (heterotrophic).
• Human digestion happens in the alimentary canal with help from the liver, pancreas and salivary glands. Starch digestion begins in the mouth, protein in the stomach, and most digestion and absorption finish in the small intestine.
• Respiration releases energy from food as ATP. Breathing is the exchange of gases; cellular respiration is the release of energy in cells, which may be aerobic or anaerobic.

Modes of nutrition

Autotrophic nutrition. Green plants make food from carbon dioxide and water using sunlight and chlorophyll.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in sunlight, with chlorophyll)

Photosynthesis happens mainly in leaves; chloroplasts hold chlorophyll. Carbon dioxide enters through stomata. Water comes from the roots. Oxygen is released. The glucose is stored as starch. Sachs' iodine test shows starch in a leaf.

Conditions needed: light, chlorophyll, carbon dioxide, water. Light reactions need light; the dark reactions use the carbon dioxide to make sugar.

Heterotrophic nutrition.

Amoeba takes food by engulfing it with pseudopodia (phagocytosis) and digests it in a food vacuole.

Nutrients and balanced diet

Nutrients are carbohydrates, proteins, fats, vitamins, minerals, water and roughage (fibre).

A balanced diet has all nutrients in the right amounts. Energy is measured in calories or joules: 1 calorie = 4.18 joules. Food tests: iodine turns blue-black with starch; Benedict's solution turns brick red on heating with glucose; copper sulphate and sodium hydroxide (biuret) turns violet for protein; a paper spot that turns translucent shows fat.

Human digestive system

The alimentary canal is: mouth → pharynx → oesophagus → stomach → small intestine → large intestine → rectum → anus. The digestive glands are salivary glands, liver and pancreas.

Mouth. Teeth chew the food. Adult humans have 32 teeth: 8 incisors, 4 canines, 8 premolars, 12 molars. Children have 20 milk teeth. Saliva has the enzyme salivary amylase (ptyalin), which turns starch into maltose. The tongue mixes food with saliva. Swallowing pushes food to the oesophagus; peristalsis (waves of contraction) moves it down.

Stomach. Walls secrete gastric juice with hydrochloric acid, pepsin and mucus. Acid kills germs and makes the food acidic for pepsin, which digests proteins. Mucus protects the stomach wall from acid. Food becomes a thick paste called chyme.

Liver. The largest gland. It makes bile, stored in the gall bladder. Bile is not an enzyme; it turns fats into small droplets (emulsification) and makes the food alkaline.

Pancreas. Pancreatic juice has trypsin (proteins), lipase (fats) and amylase (starch).

Small intestine. The longest part, about 6 to 7 metres. Intestinal juice completes digestion: carbohydrates to glucose, proteins to amino acids, and fats to fatty acids and glycerol. Villi, the finger-like folds in the inner wall, increase the surface for absorption.

Large intestine. Absorbs water and some salts. The undigested waste forms faeces stored in the rectum and passes out by the anus (egestion).

Ruminants such as cow and buffalo swallow grass quickly and later chew it again (rumination or chewing the cud). Their stomach has four chambers, and bacteria in the rumen digest cellulose. In horses and rabbits a large caecum does this job.

Respiration

Breathing and respiration. Breathing is the physical intake of oxygen and removal of carbon dioxide. Cellular respiration is the chemical process in which food (glucose) is broken down to release energy.

Aerobic respiration (with oxygen): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (38 ATP). It starts in the cytoplasm (glycolysis) and ends in mitochondria.

Anaerobic respiration (without oxygen): in yeast, glucose → alcohol + carbon dioxide + a small amount of energy (2 ATP). In muscles during hard exercise, glucose → lactic acid, which causes cramps. Anaerobic respiration releases less energy.

Human respiratory system. Air passes through nostrils → nasal cavity → pharynx → larynx (voice box) → trachea (windpipe, kept open by cartilage rings) → bronchi → bronchioles → alveoli in the lungs. Alveoli are tiny, thin-walled air sacs surrounded by capillaries where oxygen enters the blood and carbon dioxide leaves. The diaphragm and rib muscles control breathing.

During inhalation the diaphragm moves down and the ribs move up and out, so the chest expands and air rushes in. In exhalation the diaphragm moves up, ribs move down and in, and air goes out. Inhaled air has about 21% oxygen; exhaled air has about 16% oxygen and 4% carbon dioxide. Lime water turns milky with carbon dioxide.

Respiration in other organisms. Amoeba exchanges gases by diffusion through the cell surface. Earthworms breathe through the moist skin. Insects use tubes called tracheae with openings called spiracles. Fishes use gills. Frogs use skin and lungs. Plants exchange gases through stomata (leaves), lenticels (stems) and root hairs. Plants respire day and night but photosynthesise only in light.

Teaching angle

The lesson fits simple experiments: testing food for starch, testing carbon dioxide in exhaled air with lime water, and showing that a leaf needs light by covering part of it. A model of the lungs with a bottle, balloons and a rubber sheet helps learners understand the breathing movements. A teacher should ask learners to predict, observe and then explain.

Exam traps

• Breathing is not the same as cellular respiration.
• Bile is made in the liver and stored in the gall bladder; it has no enzyme.
• Starch digestion starts in the mouth; protein digestion starts in the stomach; fat digestion mainly occurs in the small intestine.
• Aerobic respiration gives 38 ATP; anaerobic gives 2 ATP.
• Lactic acid is formed in muscle; alcohol in yeast.
• Photosynthesis releases oxygen from water, not from carbon dioxide.
• Vitamin C deficiency causes scurvy; vitamin D deficiency causes rickets; vitamin B₁ deficiency causes beri-beri.
• Villi are in the small intestine; alveoli are in the lungs.

One-liners

• 1. The liver is the largest gland in the body.
• 2. Salivary amylase digests starch.
• 3. Pepsin acts in the acid medium of the stomach.
• 4. Villi increase the area for absorption.
• 5. The small intestine is the main site of absorption.
• 6. The large intestine absorbs water.
• 7. An adult human has 32 teeth.
• 8. Alveoli are the sites of gas exchange.
• 9. Insects breathe through tracheae.
• 10. Glycolysis happens in the cytoplasm.
• 11. Iodine gives a blue-black colour with starch.
• 12. Goitre is due to iodine deficiency.

Practice questions

1. The enzyme present in saliva is

   1. trypsin
   2. lipase
   3. pepsin
   4. salivary amylase
   Answer

   D. salivary amylase

   Salivary amylase starts starch digestion.

2. The largest gland in the human body is the

   1. thyroid
   2. salivary gland
   3. liver
   4. pancreas
   Answer

   C. liver

   The liver is the largest gland.

3. Digestion of protein begins in the

   1. large intestine
   2. stomach
   3. mouth
   4. small intestine
   Answer

   B. stomach

   Pepsin acts in the stomach.

4. Bile is stored in the

   1. liver
   2. stomach
   3. pancreas
   4. gall bladder
   Answer

   D. gall bladder

   Bile is made in the liver and stored in the gall bladder.

5. Most absorption of digested food occurs in the

   1. large intestine
   2. oesophagus
   3. small intestine
   4. stomach
   Answer

   C. small intestine

   Villi of the small intestine absorb food.

6. Finger-like projections in the small intestine are called

   1. cilia
   2. villi
   3. bronchioles
   4. alveoli
   Answer

   B. villi

   Villi increase absorption surface.

7. The deficiency of vitamin C causes

   1. night blindness
   2. beri-beri
   3. rickets
   4. scurvy
   Answer

   D. scurvy

   Vitamin C prevents scurvy.

8. Goitre is caused by the deficiency of

   1. calcium
   2. iron
   3. iodine
   4. vitamin A
   Answer

   C. iodine

   Iodine is needed for thyroxine.

9. Gas exchange in human lungs takes place in the

   1. alveoli
   2. larynx
   3. bronchi
   4. trachea
   Answer

   A. alveoli

   Alveoli are thin-walled sacs surrounded by capillaries.

10. In yeast, anaerobic respiration produces carbon dioxide and

    1. lactic acid
    2. alcohol
    3. water only
    4. oxygen
    Answer

    B. alcohol

    Yeast ferments glucose to alcohol.

11. During hard exercise muscle cells may produce

    1. alcohol
    2. starch
    3. lactic acid
    4. oxygen
    Answer

    C. lactic acid

    Anaerobic respiration in muscle gives lactic acid.

12. Which organelle is the main site of aerobic respiration?

    1. Mitochondria
    2. Golgi body
    3. Lysosome
    4. Ribosome
    Answer

    A. Mitochondria

    Aerobic steps occur in mitochondria.

13. Insects breathe through

    1. moist skin
    2. tracheae
    3. lungs
    4. gills
    Answer

    B. tracheae

    Spiracles lead into tracheal tubes.

14. Number of molar teeth in an adult human is

    1. 8
    2. 12
    3. 4
    4. 16
    Answer

    B. 12

    Adult teeth: 8 incisors, 4 canines, 8 premolars, 12 molars.

15. Total number of teeth in an adult human is

    1. 20
    2. 28
    3. 36
    4. 32
    Answer

    D. 32

    Adults have 32 permanent teeth.

16. Iodine solution turns blue-black in the presence of

    1. starch
    2. glucose
    3. fat
    4. protein
    Answer

    A. starch

    This is the test for starch.

17. Benedict's solution on heating with glucose gives

    1. brick red colour
    2. blue-black colour
    3. violet colour
    4. green colour only
    Answer

    A. brick red colour

    Reducing sugar gives a brick red precipitate.

18. Which of these is a parasitic plant?

    1. Rhizobium
    2. Pitcher plant
    3. Sundew
    4. Cuscuta
    Answer

    D. Cuscuta

    Cuscuta (dodder) takes food from its host.

19. Lichen is an example of

    1. parasitism
    2. saprophytic nutrition only
    3. symbiosis between an alga and a fungus
    4. holozoic nutrition
    Answer

    C. symbiosis between an alga and a fungus

    Alga makes food; fungus gives support and water.

20. The mode of nutrition in Amoeba is

    1. parasitic
    2. saprophytic
    3. holozoic
    4. autotrophic
    Answer

    C. holozoic

    It engulfs food with pseudopodia.

21. If a person needs 2,090 joules of energy, how many calories is that? (1 calorie = 4.18 joules)

    1. 8,736
    2. 250
    3. 400
    4. 500
    Answer

    D. 500

    2090 / 4.18 = 500 calories.

22. Air in the human trachea is kept open by

    1. muscle rings
    2. cartilage rings
    3. ligaments
    4. bone rings
    Answer

    B. cartilage rings

    C-shaped cartilage rings stop the tube from collapsing.

23. During inhalation, the diaphragm

    1. moves up and ribs move out
    2. moves down and ribs move up and out
    3. stays still
    4. moves up and ribs move in
    Answer

    B. moves down and ribs move up and out

    The chest cavity enlarges so air enters.

24. The conversion of glucose to pyruvate in the cytoplasm is known as

    1. glycolysis
    2. emulsification
    3. photolysis
    4. peristalsis
    Answer

    A. glycolysis

    Glycolysis occurs in the cytoplasm.

25. The role of bile in digestion is to

    1. digest proteins
    2. digest starch
    3. break fat into small droplets
    4. kill germs in stomach
    Answer

    C. break fat into small droplets

    Bile emulsifies fats; it is not an enzyme.

26. In a leaf photosynthesis, the oxygen released comes from

    1. carbon dioxide
    2. starch
    3. glucose
    4. water
    Answer

    D. water

    Splitting of water gives oxygen.

27. In photosynthesis the number of oxygen molecules released for every 6 molecules of carbon dioxide used is

    1. 12
    2. 3
    3. 6
    4. 1
    Answer

    C. 6

    6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

28. The wave-like muscular movement that pushes food down the food pipe is

    1. rumination
    2. peristalsis
    3. diffusion
    4. emulsification
    Answer

    B. peristalsis

    Peristalsis moves food along the canal.

29. Ruminants like cows digest cellulose with the help of

    1. bacteria in the stomach and caecum
    2. bile from the liver
    3. gastric acid alone
    4. saliva enzymes only
    Answer

    A. bacteria in the stomach and caecum

    Bacteria help to digest cellulose.

30. Consider the statements. 1. Bile contains digestive enzymes. 2. Bile is made in the liver. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Bile has no enzymes but emulsifies fat.

31. Consider the statements. 1. Starch digestion begins in the mouth. 2. Protein digestion begins in the small intestine. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Protein digestion begins in the stomach.

32. Consider the statements. 1. Breathing and cellular respiration are the same process. 2. Cellular respiration releases energy from food. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Breathing is physical gas exchange.

33. Consider the statements. 1. Anaerobic respiration releases less energy than aerobic. 2. Aerobic respiration needs oxygen. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

34. Consider the statements. 1. Plants respire only at night. 2. Plants photosynthesise only in the presence of light. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Plants respire all the time.

35. Consider the statements. 1. Villi are found in the stomach. 2. Alveoli are found in the lungs. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Villi are in the small intestine.

36. Consider the statements. 1. Vitamin D helps in calcium absorption. 2. Lack of vitamin D causes rickets. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

37. Consider the statements. 1. Earthworm breathes through moist skin. 2. Fish breathes through gills. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

38. Match the nutrient deficiency with the disease. (a) Vitamin A (b) Iron (c) Vitamin D (d) Protein (p) Rickets (q) Anaemia (r) Night blindness (s) Kwashiorkor

    1. a-q, b-r, c-p, d-s
    2. a-p, b-q, c-r, d-s
    3. a-r, b-q, c-s, d-p
    4. a-r, b-q, c-p, d-s
    Answer

    D. a-r, b-q, c-p, d-s

    Standard deficiency diseases.

39. Match the enzyme with the substrate. (a) Pepsin (b) Lipase (c) Salivary amylase (d) Trypsin (p) Starch (q) Proteins in stomach (r) Fats (s) Proteins in intestine

    1. a-s, b-r, c-p, d-q
    2. a-q, b-p, c-r, d-s
    3. a-r, b-q, c-p, d-s
    4. a-q, b-r, c-p, d-s
    Answer

    D. a-q, b-r, c-p, d-s

    Pepsin works in the stomach; trypsin from the pancreas in the intestine.

40. Arrange the path of food in the correct order.

    1. Mouth, oesophagus, stomach, large intestine, small intestine
    2. Mouth, stomach, oesophagus, small intestine, large intestine
    3. Mouth, oesophagus, small intestine, stomach, large intestine
    4. Mouth, oesophagus, stomach, small intestine, large intestine
    Answer

    D. Mouth, oesophagus, stomach, small intestine, large intestine

    Food passes in this order.

41. Arrange the path of air in the correct order.

    1. Nostril, pharynx, larynx, trachea, bronchi, alveoli
    2. Nostril, larynx, pharynx, trachea, bronchi, alveoli
    3. Nostril, pharynx, trachea, larynx, bronchi, alveoli
    4. Nostril, pharynx, larynx, bronchi, trachea, alveoli
    Answer

    A. Nostril, pharynx, larynx, trachea, bronchi, alveoli

    Air passes through the windpipe before the bronchi.

42. A teacher wants Class VII learners to show that exhaled air has carbon dioxide. The best activity is to

    1. blow air on iodine solution
    2. blow air on a burning candle only
    3. blow air through lime water and watch it turn milky
    4. soak a leaf in water
    Answer

    C. blow air through lime water and watch it turn milky

    Lime water turns milky with CO₂.

43. To explain the breathing movement in class, a model with a bottle, balloons and a rubber sheet shows the role of the

    1. alveoli sacs
    2. trachea cartilage
    3. epiglottis
    4. diaphragm
    Answer

    D. diaphragm

    The rubber sheet represents the diaphragm.

44. Which of the following is an insectivorous plant?

    1. Pitcher plant
    2. Cuscuta
    3. Rice plant
    4. Mushroom
    Answer

    A. Pitcher plant

    Pitcher plant traps insects for nitrogen.

45. Approximate percentage of oxygen in the air we inhale is

    1. 21%
    2. 78%
    3. 4%
    4. 16%
    Answer

    A. 21%

    Inhaled air has about 21% oxygen.