Science notes, chapter 3

Nutrition

Textbook Science Form 2 (DLP), pages 44-73

What this chapter covers

Name the seven classes of food, their sources and functions, and the effects of vitamin and mineral deficiency (3.1)
Test for starch, glucose, protein and fat in food (3.1)
Explain a balanced diet, the food pyramid and the factors that influence calorific requirement (3.2)
Estimate the calorific value of a meal and plan a balanced diet (3.2)
Describe the digestive system, the flow of food and the action of digestive enzymes (3.3)
Explain absorption in the villi, assimilation and defecation (3.4)

3.1Classes of food

Food gives us energy and the nutrients that keep the body healthy. There are seven classes of food: carbohydrate, protein, fat, vitamin, mineral, fibre and water.

Class	What it is	Main functions	Sources
Carbohydrate	Contains carbon, hydrogen and oxygen. Starch (stored in plants), glycogen (stored in animals), cellulose (plant cell walls)	Main source of energy; sugar gives energy quickly	Rice, bread, potato, banana, honey, sugar
Protein	Contains carbon, hydrogen, oxygen and nitrogen. Digested to amino acids	Growth, repair of damaged tissue, replace dead cells; makes enzymes, hormones and antibodies	Chicken, meat, egg, seafood, milk, nuts
Fat	Contains carbon, hydrogen and oxygen. Made from glycerol and fatty acids	High-energy source and store; protects the heart and kidneys; carries vitamins A, D, E, K; heat insulator under the skin	Butter, coconut oil, palm oil, groundnuts
Vitamin	Organic compound, needed in small amounts, gives no energy	Keeps the body healthy (see table below)	Fruits, vegetables, milk, meat
Mineral	Non-organic substance, needed in small amounts, gives no energy	Regulates body processes (see table below)	Salt, seafood, milk, liver
Fibre	Cellulose from plant cell walls; cannot be digested	Stimulates peristalsis, so food moves along the tract and constipation is prevented	Vegetables, fruits, grains
Water	Compound of hydrogen and oxygen	Solvent; transports nutrients and oxygen to cells and wastes (urea, salt) out; cools the body by sweat evaporating	Drinks, watermelon, fruit juice

Based on textbook pp. 46-50, Photographs 3.2-3.8.

Each gramme of fat gives about two times the energy of a gramme of carbohydrate. That is why fat is called a high-energy store.

Thinking vitamins and minerals give energy. They are needed in small amounts to keep the body healthy, but they supply no energy.

Vitamins

Vitamins are of two groups: water-soluble (B and C) and fat-soluble (A, D, E and K).

Vitamin	Source	Importance	Deficiency
A	Milk, egg yolk, fish oil	Night vision; healthy skin	Night blindness, skin diseases
B	Yeast, liver, eggs	Nervous system; forms red blood cells	Beri-beri, anaemia
C	Fruits, vegetables	Fights diseases; healthy gums and mouth	Scurvy (bleeding gums)
D	Butter, eggs, fish oil, sunlight on skin	Helps absorb calcium; strengthens tooth enamel; healthy skin	Rickets, toothache, skin diseases
E	Grains, green vegetables	Reproductive system	Sterility, miscarriage
K	Milk, egg yolk, fish oil	Speeds up blood clotting	Prolonged bleeding

Based on textbook Table 3.1, p. 49.

Minerals

Mineral	Source	Importance	Deficiency
Calcium	Milk, anchovies, prawns, green vegetables	Blood clotting; strong bones and teeth	Rickets, osteoporosis
Sodium	Salt, meat, eggs	Nervous system	Muscle cramps
Iron	Liver, meat	Builds haemoglobin in the blood	Anaemia
Iodine	Seafood, fruits	Thyroid gland functions	Goitre
Phosphorus	Cheese, meat, eggs, vegetables	Strong bones and teeth; forms DNA and RNA	Rickets, brittle teeth
Potassium	Plants and animals	Muscle contraction; nervous system	Paralysis, muscle cramps

Based on textbook Table 3.2, p. 50.

Mixing up anaemia causes. Anaemia can come from lack of vitamin B or of the mineral iron. Rickets can come from lack of vitamin D, calcium or phosphorus.

Food tests (Activity 3.1)

Test	Food class	Method	Positive result
Iodine test	Starch	Add 2 drops of iodine solution to 2 mL of the sample	Brown to dark blue (blue-black)
Benedict's test	Reducing sugar, e.g. glucose	Add 2 mL of Benedict's solution and heat in a water bath for 5 minutes	Blue to green, yellow, orange then brick-red precipitate
Millon's test	Protein	Add 2-3 drops of Millon's reagent and heat in a water bath for 5 minutes	Red (pink-red) colour or precipitate
Alcohol-emulsion test	Fat	Shake the sample with ethanol, then pour a few drops into distilled water	Cloudy white emulsion

Methods: textbook Activity 3.1, pp. 51-52.

Heating the Benedict's or Millon's mixture directly over the flame. Use a water bath, so the heat is gentle and even and the liquid does not splash out. Ethanol is flammable: keep it away from fire.

Honey stays brown with iodine solution but forms a brick-red precipitate with Benedict's solution after heating. What food class does it contain, and what does it not contain?

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It contains reducing sugar (glucose). It does not contain starch, because the iodine test is negative.

3.2Importance of a balanced diet

A balanced diet is a diet that contains all the food classes in the right quantities needed by the body.

Food pyramid, from the base: rice, noodles, bread, grains and potatoes (4 to 8 servings daily); vegetables (3) and fruits (2); milk and dairy products (1 to 3) with fish, chicken, meat and nuts; fat, oil, sugar and salt at the top (eat sparingly) — The food pyramid guides us to choose a balanced diet. Eat most from the base, and sparingly from the top. (Textbook Figure 3.7, p. 53)

Food pyramid: Ministry of Health Malaysia, as printed in the textbook.

Factors that influence calorific requirement

Factor	Effect
Body size	A bigger body frame needs a bigger portion of food
Age	Children and teenagers need more carbohydrate (energy) and protein (growth) because they are growing and active
Work	Farmers, labourers and fishermen do heavy work and need more energy than office workers
Gender	Men are generally more muscular and do heavier work, so they need bigger portions than women
State of health	Sick people need food suited to their condition
Climate	People in cold countries lose heat quickly and need more food to keep their body temperature

Based on textbook Figure 3.8, p. 54.

Calorific value of food

The calorific value (energy value) is the total energy released when 1 g of food is burned completely in the body. It is measured in calorie (cal) or joule (J).

Food class	kJ per g	kcal per g
Fat	37	9
Protein	17	4
Carbohydrate	17	4

Textbook Table 3.3, p. 55.

Conversion: 1 cal = 4.2 J and 1 kcal = 4.2 kJ.

Worked example: adding up a meal (Activity 3.2, p. 55)

Fried rice 640 kcal + 2 bananas (2 × 60 = 120 kcal) + a glass of milk 130 kcal
Total = 640 + 120 + 130 = 890 kcal. In kilojoules: 890 × 4.2 = 3738 kJ.

Worked example: from grams to energy

A snack has 10 g protein, 5 g fat and 30 g carbohydrate.
Protein 10 × 4 = 40 kcal; fat 5 × 9 = 45 kcal; carbohydrate 30 × 4 = 120 kcal.
Total = 40 + 45 + 120 = 205 kcal.

A packet of nuts has 20 g of fat only. Calculate its energy in kcal and in kJ.

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20 × 9 = 180 kcal. In kJ: 20 × 37 = 740 kJ (or 180 × 4.2 = 756 kJ, which is about the same).

Planning a balanced diet and keeping healthy

A balanced meal plan uses the food pyramid and takes into account the person's size, age, work, gender, health and climate. For example, a construction worker needs more energy than a bank officer, and a pregnant woman needs extra protein and minerals.

Many Malaysians have diet-related problems such as diabetes, high blood pressure, high cholesterol and obesity. To lower the risk, eat food with less sugar, salt, oil and fat, eat enough fruits and vegetables, exercise and do not smoke.

Body Mass Index (BMI) = mass (kg) ÷ [height (m) × height (m)].

Worked example: BMI

A student has a mass of 60 kg and a height of 1.5 m.
BMI = 60 ÷ (1.5 × 1.5) = 60 ÷ 2.25 = 26.7 (compare with the BMI chart for his or her age and gender).

Always change height from cm to m before using the BMI formula: 150 cm = 1.5 m.

Why does a man who moved from a cold country to Malaysia usually need less food?

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In the hot climate of Malaysia the body loses less heat to the surroundings, so less energy is needed to keep the body temperature.

3.3Human digestive system

Digestion is the breaking down of large food molecules into small, soluble molecules that can be absorbed. An enzyme is a protein in the body that speeds up chemical digestion.

	Physical digestion	Chemical digestion
What happens	Food is broken into smaller pieces	Food molecules are broken down into simpler molecules
Enzymes	Not involved	Involved
Where	Mouth (chewing)	Mouth, stomach, duodenum and small intestine

Based on textbook Figure 3.10, p. 60.

The digestive system

The digestive system is the digestive tract (a long tube from the mouth to the anus) plus the salivary glands, liver, gall bladder and pancreas. Food only passes through the tract; the other organs make juices that pour into it.

Human digestive system with the mouth, salivary glands, oesophagus, liver, gall bladder, stomach, duodenum, pancreas, small intestine, large intestine, rectum and anus labelled — The digestive tract and its associated organs. (Textbook Figure 3.11, p. 61)

The flow of food

Organ	What happens
Mouth	Teeth chew the food; saliva softens it. Salivary amylase digests starch into maltose
Oesophagus	The food, now called a bolus, is pushed to the stomach by peristalsis
Stomach	Walls secrete protease and hydrochloric acid (activates protease and kills bacteria). Protein is digested into polypeptides. The semi-liquid food is called chyme
Duodenum	First part of the small intestine. Bile (made by the liver, stored in the gall bladder) emulsifies fat into small droplets and neutralises the acid in the chyme. The pancreas adds pancreatic juice with amylase, protease and lipase
Small intestine	Secretes maltase and protease to finish digestion. Digested food is absorbed
Large intestine	Undigested food enters; water and minerals are reabsorbed
Rectum	Faeces are stored
Anus	Faeces leave the body

Based on textbook Figure 3.13, pp. 62-63.

Digestive enzymes

Enzyme	Made by	Digests	Into
Amylase	Salivary glands, pancreas	Starch	Maltose
Maltase	Small intestine	Maltose	Glucose
Protease	Stomach, pancreas, small intestine	Protein to polypeptide; polypeptide to dipeptide; dipeptide to amino acid	Amino acids (end product)
Lipase	Pancreas	Fat	Fatty acids and glycerol

Based on textbook Figure 3.14, p. 64.

Saying bile contains an enzyme. Bile contains no enzyme. It only emulsifies fat (breaks it into droplets) and neutralises acid, so that lipase can work faster.

Digestion starts in different places: carbohydrate in the mouth, protein in the stomach, fat in the duodenum. All three end in the small intestine.

Activity 3.7: the action of saliva on starch (pp. 64-65)

Two boiling tubes: P has starch only, Q has starch and saliva. Both stand in a 37 °C water bath for 30 minutes, then are tested with iodine and Benedict's solution.
Result: P still turns dark blue with iodine and gives no brick-red precipitate. In Q, iodine stays brown and Benedict's gives a brick-red precipitate, because the amylase in saliva digested starch to maltose.
37 °C is kept because it is the body temperature, at which enzymes work best.

A man has his pancreas removed. Which classes of food will he find hard to digest?

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All three. The pancreas makes amylase, protease and lipase, so digestion of carbohydrate, protein and fat becomes slow and incomplete.

3.4Absorption, transportation and defecation

Absorption in the small intestine

The wall of the small intestine has millions of finger-like villi (singular: villus). Digested food is small enough to pass through the wall of a villus and enter the blood.

A villus with blood capillaries and a lacteal; small nutrients pass through the villus wall into the blood — Structure of a villus. (Textbook Figure 3.16, p. 66)

Feature of the villus	Why it helps absorption
Many villi and folds	Large surface area, so absorption is faster
Wall one cell thick	Short distance, so digested food passes through quickly
Network of blood capillaries	Carries nutrients away to all parts of the body
Lacteal	Absorbs digested fat and passes it to the lymphatic system, then the blood

Writing that the villi "digest" food. Villi absorb food that is already digested. Large undigested particles cannot pass through the wall.

Experiment 3.1: absorption of glucose through a Visking tube (p. 67)

The Visking tube stands for the wall of the small intestine and the distilled water outside stands for the blood. One tube holds glucose solution and the other starch suspension, each standing in distilled water for 30 minutes.
Result: the water outside the glucose tube gives a brick-red precipitate with Benedict's solution, so glucose has diffused through. The water outside the starch tube does not turn dark blue with iodine, so starch is too big to pass.
Manipulated variable: content in the Visking tube. Responding variable: presence of glucose in the water. Constant: type and size of tube, temperature, time.

Assimilation

Assimilation is the process of distributing the end products of digestion for the use of the cells.

End product	Used for
Glucose	Producing energy (respiration)
Amino acids	Building cell components (new cells, growth, repair)
Fatty acids and glycerol	Recombined into fat, used as heat insulator and to protect organs

Based on textbook p. 68.

Three systems work together: the digestive system breaks food into small molecules, the blood circulatory system carries them to the cells, and the cells use them for respiration, new cells and body temperature.

Defecation

Fibre, dead cells, digestive secretions and water that are not absorbed move into the large intestine. Water and minerals are reabsorbed into the blood, so the waste becomes solid faeces. Faeces are stored in the rectum and removed through the anus. This is defecation. A diet low in fibre causes constipation.

Do not mix up the two jobs. The small intestine absorbs digested food; the large intestine reabsorbs water.

Explain why a person who eats little fruit and vegetables may become constipated.

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Fruit and vegetables are high in fibre. Without fibre, peristalsis is weak, so faeces move slowly along the large intestine, lose too much water and become hard.

Summary

Remember	Key fact
Seven classes	Carbohydrate, protein, fat, vitamin, mineral, fibre, water
Food tests	Iodine (starch, dark blue); Benedict's (reducing sugar, brick-red, water bath); Millon's (protein, red); alcohol-emulsion (fat, cloudy white)
Balanced diet	All food classes in the right quantities; guided by the food pyramid
Calorific value	Fat 37 kJ/g (9 kcal/g); protein and carbohydrate 17 kJ/g (4 kcal/g); 1 kcal = 4.2 kJ
Energy needs depend on	Body size, age, work, gender, state of health, climate
Digestion	Amylase: starch to maltose; maltase: maltose to glucose; protease: protein to amino acids; lipase: fat to fatty acids and glycerol
Flow of food	Mouth, oesophagus, stomach, duodenum, small intestine, large intestine, rectum, anus
Absorption	Villi in the small intestine; glucose, amino acids, fatty acids and glycerol enter the blood
Defecation	Large intestine reabsorbs water; faeces stored in rectum and leave through the anus