5: Food and Humans

Question 1

What is the difference between autotrophic nutrition and heterotrophic nutrition?

Answer 1

Autotrophic nutrition is the mode of nutrition of plants, which make their own food by photosynthesis. They are called autotrophs. Meanwhile, heterotrophic nutrition is the mode of nutrition of humans and other organisms which have to take in organic matter from their surroundings. They are called heterotrophs, and depend on other organisms for food.

Question 2

What are different modes of heterotrophic nutrition?

Answer 2

1. Holozoic nutrition: animals that carry out this mode of nutrition feed on other organisms.
2. Saprophytic nutrition: many bacteria and fungi known as saprophytes carry out this mode of nutrition. They feed on dead organisms or non-living organic matter such as decaying food.
3. Parasitic nutrition: tapeworms and disease-causing bacteria cary out this mode of nutrition. They are known as parasites. They live on or inside the body of other organisms and obtain organic food from them.

Question 3

Why do we need food?

Answer 3

Food provides us with energy for supporting daily activities and keeping us warm. It provides us with raw materials for growth and repair of body tissues. It also provides us with substances that are important for maintaining health.

Question 4

List all food substances.

Answer 4

Food contains 7 types of food substances that are essential to health: Carbohydrates, lipids, proteins, (primary food substances) minerals, vitamins, dietary fibre, and water.

Question 5

What are the features of carbohydrates?

Answer 5

They are organic substances. They are made up of carbon (C), hydrogen (H), and oxygen (O) atoms, in which the H:O ratio is 2:1. Carbohydrates can be classified into monosaccharides, disaccharides, and polysaccharides.

Question 6

List three examples of monosaccharides and where they can be found.

Answer 6

Glucose, fructose: they are commonly found in fruits and honey.
Galactose: it is found in milk and dairy products like cheese and yoghurt.

Question 7

What are the features of monosaccharides?

Answer 7

Monosaccharides are the simplest forms of carbohydrates. They taste sweet and are soluble in water. All monosaccharides are reducing sugars, which form a brick-red precipitate in Benedict’s test.

Question 8

How can monosaccharides become disaccharides and vice versa?

Answer 8

A disaccharide is formed when two monosaccharide molecules combine in the presence of an enzyme. During the reaction, a water molecule is lost. This kind of reaction is called condensation.
A disaccharide can be broken down into two monosaccharide molecules by adding a water molecule in the presence of a different enzyme. This process is called hydrolysis.

Question 9

List three examples of disaccharides, their constituent monosaccharides, and where they can be found.

Answer 9

Maltose (glucose + glucose): found in germinating barley
Sucrose (glucose + fructose): found in sugar canes and sugar beets
Lactose (glucose + galactose): found in milk and dairy products.

Question 10

What are the features of disaccharides?

Answer 10

Disaccharides taste sweet and are soluble in water. They are also reducing sugars, except for sucrose.

Question 11

List three examples of polysaccharides and their usages.

Answer 11

Starch: the major form of carbohydrates stored in plants, found in cereal products, potatoes, and taro.
Glycogen: the storage form of carbohydrates in animals.
Cellulose: the main component of plant cell walls.

Question 12

What are the features of polysaccharides?

Answer 12

Polysaccharides are long chains of monosaccharides joined together by condensation. They do not taste sweet and are insoluble in water.

Question 13

What are the functions of carbohydrates?

Answer 13

1. Carbohydrates are the main energy source of body activities, providing 17.1kJ/g of energy. In our body, glucose is directly broken down in respiration to release energy.
   Disaccharides and polysaccharides are larger molecules. They have to be broken down into monosaccharides so that these smaller molecules can be absorbed and used for respiration.
2. Glycogen is stored in the liver or muscles as energy reserves. It is formed from other kinds of carbohydrates that are taken in excess.
3. Cellulose is not digested in our body. It is an important source of dietary fibre. It retains water in faeces and stimulates peristalsis, preventing constipation.

Question 14

What are the features of lipids?

Answer 14

Lipids are organic substances made up of carbon, hydrogen, and oxygen atoms, but the H:O ratio is much greater than 2:1. Lipids are insoluble in water but dissolve in organic solvents like ethanol.

Question 15

What are the features of triglycerides?

Answer 15

Triglycerides are the most common type of lipids. They are formed from the condensation of three fatty acid molecules and one glycerol molecule.
Triglycerides that are solid at room temperature are fats. They mainly come from animals, with examples being lard and butter. Meanwhile, those that are liquid at room temperature are oils. They mainly come from plants, with corn oil and peanut oil as examples.

Question 16

What are the functions of lipids?

Answer 16

1. Lipids are stored in adipose tissues in our body. These tissues are found under the skin as subcutaneous fat, or around the internal organs. They act as energy reserves, providing 38.9kJ/g of energy (more than twice than that of carbohydrates). They also act as an insulator to reduce heat loss, and a shock absorbed to protect the internal organs.
2. Phospholipids are a type of lipid that are the major component of cell membranes.
3. Lipids are involved in transporting and storing lipid-soluble vitamins (like vitamins A and D) in our body.
4. Lipids are involved in making some hormones.

Question 17

What are the features of proteins?

Answer 17

Proteins are organic substances made up of carbon, hydrogen, oxygen, and nitrogen atoms. Some proteins also contain sulphur.
Amino acids are the basic building blocks of proteins.

Question 18

What is the chemical structure of an amino acid molecule?

Answer 18

An amino acid molecule has a carbon atom at the centre which carries an amino group, a carboxyl group, a hydrogen atom, and a side chain. Different amino acids have different side chains.

Question 19

How can amino acids become dipeptides and vice versa?

Answer 19

When two amino acids are joined together by condensation, a dipeptide is formed. A peptide bond is formed between the two amino acid molecules. This process is catalysed by enzymes in our body. Under the action of different enzymes, a dipeptide can be broken down into amino acids by hydrolysis.

Question 20

How are proteins formed from amino acids?

Answer 20

A number of amino acids join together by condensation to form a polypeptide. The polypeptide coils and folds in specific ways due to the formation of chemical bonds between some amino acids on the polypeptide. This gives the characteristic 3-dimensional shapes of proteins (inorganic components may be attached to this 3-dimensional shape). A protein may consist of one or more polypeptides.

Question 21

How many amino acids do we need?

Answer 21

Our body needs 20 types of amino acids to build up various proteins. 12 of these amino acids cane produced by our body, known as non-essential amino acids. The remaining 8 amino acids have to be obtained from the diet, known as essential amino acids.

Question 22

What are the functions of proteins?

Answer 22

1. Proteins are important for growth and repair of body tissues like muscles.
2. Enzymes, antibodies and some hormones are protein in nature.
3. If the carbohydrates and lipids stored in the body are used up, amino acids may be broken down to release 18.2kJ/g of energy. (about the same as that of carbohydrates, but less than half of that of lipids.)

Question 23

What is the deficiency disease of proteins?

Answer 23

Kwashiorkor: children suffering from kwashiorkor grow poorly and have weak muscles. They have a swollen abdomen because of the accumulation of tissue fluid. Kwashiorkor is common in developing countries where milk, meat, and eggs are limited in diets.

Question 24

What are the features of minerals?

Answer 24

Minerals are inorganic food substances. They are needed in small amounts only compared to primary food substances. They have no energy value, but are important in regulating many metabolic reactions and building body tissues like bones.
Our body needs various minerals like calcium, iron, iodine, potassium, and phosphorus.

Question 25

What are the features of calcium and where can it be found in our diet?

Answer 25

Calcium is essential for the formation of bones and teeth. Children need a lot of calcium to harden their bones and develop teeth as they grow up. Calcium is also involved in blood clotting, muscle contraction, and sending messages in the nervous system.
Calcium is found in canned sardines, milk, and dairy products. Some green vegetables like kale, broccoli, and spinach and tofu are also rich in calcium. Calcium is commonly added to milk powder or prepackaged foods/drinks.

Question 26

What is the deficiency disease for calcium?

Answer 26

Rickets in children: The affected children have soft bones and their legs may bend under the weight of their body. If rickets is not corrected while the child is still growing, the bones may remain bent and the child cannot grow to normal height.

Question 27

What are the features of iron and where can it be found in our diet?

Answer 27

Iron is a component of haemoglobin, a pigment in red blood cells that carries oxygen around the body. It is found in beef, liver, beans, cabbage, spinach, and raisins.

Question 28

What is the deficiency disease for iron?

Answer 28

Anaemia: People with anaemia may faint easily due to the lack of enough haemoglobin to carry oxygen to the brain.

Question 29

What are the features of sodium and where can it be found in our diet?

Answer 29

Sodium is important for maintaining water balance within cells, and for the normal functioning of the nervous system.
Table salt (sodium chloride) is the major source of sodium in the diet. We have to limit our sodium intake because taking in too much sodium may lead to high blood pressure.

Question 30

What are the features of vitamins?

Answer 30

Vitamins are organic food substances. Like minerals, they are needed in small amounts only. They have no energy value, but they help regulate many metabolic reactions.
There are over 10 types of vitamins. A few of them can be made by our body. Many of them have to be obtained from the diet.

Question 31

What are the features of vitamin A and where can it be found in our diet?

Answer 31

Vitamin A is lipid-soluble. It is used to form a pigment necessary for vision in dim light in the retina of our eyes. Vitamin A is also important for keeping the cornea, the skin, the lining of the alimentary canal, and the breathing system healthy.
Vitamin A is found in fish liver oil, liver, eggs, milk and dairy products. Some vegetables and fruits contain an orange pigment called carotene, which can be converted to vitamin A in our liver.

Question 32

What are the deficiency diseases of vitamin A?

Answer 32

1. night blindness or poor vision in dim light
2. drying up of the cornea and skin
3. easy infection of the lining of the lungs and trachea.

Question 33

What are the features of vitamin C and where can it be found in our diet?

Answer 33

Vitamin C is water-soluble. It is destroyed when exposed to oxygen and high temperatures. It is needed for the growth and repair of connective tissues, which help keep body structures in place. It also promotes the absorption of iron from plant foods and helps the immune system work properly to protect the body from diseases.
Vitamin C can be found in fresh vegetable and fruits, especially bell peppers, broccoli, guavas and kiwi fruits.

Question 34

What is the deficiency disease for vitamin C?

Answer 34

Scurvy: the symptoms of scurvy include weak and bleeding gums, poor healing of wounds, small red spots on the skin, and joint pain.

Question 35

What are the features of vitamin D and where can it be found in our diet?

Answer 35

Vitamin D is lipid-soluble. It promotes the absorption of calcium and phosphate ions, which are essential for keeping bones and teeth strong. Vitamin D is particularly important to actively growing children.
Vitamin D is produced by the skin under sunlight. It can also be obtained from diet from fatty fish, fish liver oils, liver, and egg yolks.

Question 36

What is the deficiency disease of vitamin D?

Answer 36

Rickets in children: The affected children have soft bones and their legs may bend under the weight of their body. If rickets is not corrected while the child is still growing, the bones may remain bent and the child cannot grow to normal height.

Question 37

What are the features of B vitamins and where can it be found in our diet?

Answer 37

B vitamins refer to a group of water-soluble organic molecules including vitamins B1, B2, B6, and B12. Their functions include helping the release of energy from food in respiration, maintaining healthy skin, hair and muscle contraction, and enhancing the function of the nervous system.
B vitamins can be found in meat, fish, eggs, dairy products, unpolished cereals, leafy green vegetables, beans, and peas.

Question 38

What are the deficiency diseases for B vitamins?

Answer 38

A lack of vitamin B1 can cause beriberi, a disease of the nervous system with symptoms including impaired sensation and weakness in limbs. A lack of vitamin B6 or B12 can cause anaemia.

Question 39

What are the features of dietary fibre?

Answer 39

Dietary fibre is an organic food substance. It consists mainly of cellulose from plant cell walls. Only plant foods contain dietary fibre.
Dietary fibre does not provide energy. It passes through the gut undigested because our body does not have the enzyme (cellulase) to digest cellulose. However, it adds bulk to food to stimulate its movement along the gut/peristalsis. It also holds a lot of water to allow faeces to remain soft. This way, faeces can be passed out of the body more easily.

Question 40

What are the deficiency diseases of dietary fibre?

Answer 40

Constipation and colorectal cancer.

Question 41

What are the functions of water?

Answer 41

1. It acts as a solvent that dissolves chemicals in the body.
2. It acts as a medium for chemical reactions to take place.
3. It acts as a transport medium.
4. It acts as a cooling agent to help regulate body temperature.
5. It is a reactant in some metabolic reactions.

Question 42

How can we test for the presence of glucose?

Answer 42

Dip glucose test paper into a drop of the solution, and see if a colour change takes place. (depends on the colour chart on the packing of the test paper.)

Question 43

How can we test for the presence of reducing sugars?

Answer 43

Benedict’s test: Mix equal volumes of the testing solution with Benedict’s solution. After boiling for 5 minutes, see if brick-red precipitate appears. A brick-red precipitate is formed if reducing sugars are present.

Question 44

How can we test for the presence of starch?

Answer 44

Iodine test: drop iodine solution to the testing solution. A colour change of the iodine solution from brown to blue-black indicates that starch is present.

Question 45

How can we test for the presence of lipids?

Answer 45

Grease spot test: Drop the testing solution on a piece of filter paper. If lipids are present, a translucent spot is left, which disappears after immersing it in an organic solvent.

Question 46

How can we test for the presence of proteins?

Answer 46

Dip protein test paper into a drop of the solution and see if a colour change takes place. (depends on the colour chart on the packing of the test paper.)

Question 47

How can we test for the presence of vitamin C?

Answer 47

Add DCPIP (dichlorophenol indophenol) solution drop by drop to the testing solution. If vitamin C is present, the DCPIP solution will be decolourised.

Question 48

What is a balanced diet?

Answer 48

A balanced diet consists of all the food substances in the right amounts and proportions.

Question 49

What is the food pyramid?

Answer 49

The food pyramid is a guideline for us to plan a balanced diet. In the food pyramid, food are classified into 6 groups. The amounts of the 6 food groups in a balanced diet are represented by their relative sizes in the food pyramid. In addition to the amounts and proportions, we should also choose a variety of food from each group.

Question 50

What are the food groups and their suggested proportions in the food pyramid?

Answer 50

1. Fat, oil, salt, and sugar: eat less. Eating too much fatty and sugary food leads to being overweight and heart disease. Too much salt may lead to high blood pressure.
2. Dairy products: eat moderately.
3. Meat, eggs, and beans: eat moderately. They are rich in proteins, vitamins and minerals, which are needed for growth and repair of the body and maintaining health.
4. Vegetables: eat more.
5. Fruits: eat more. They are rich in vitamins, minerals, and dietary fibre, which are needed for maintaining health.
6. Cereals and grains: eat the most. They provide us with energy readily. Some of them (eg. wholemeal bread) are rich in dietary fibre.

Question 51

What are some factors affecting our dietary requirement?

Answer 51

Age, sex, body status, level of activity

Question 52

How does age affect our dietary requirement?

Answer 52

Children and teenagers are growing actively. They need a large amount of proteins, calcium, and iron for building body tissues like muscles bones, teeth, and blood.
Children require the greatest amount of energy per unit body mass because they have the highest growth rate and the highest metabolic rate. Children also have the highest rate of heat loss as their surface area to volume ratio is the highest among all age groups.

Question 53

How does sex affect our dietary requirement?

Answer 53

After puberty, males generally need more energy than females. This is because males usually have a higher metabolic rate due to their larger body size and being more muscular. Moreover, males have less fat under the skin. They lose heat more rapidly through the body surface. Hence, a higher metabolic rate is required to generate more heat to keep the body warm.
Males need more proteins than females for the growth and repair of muscles.
Females need more iron than males to replace the loss of iron during menstruation.

Question 54

How does level of activity affect our dietary requirement?

Answer 54

People who are more physically active need more energy for muscular activities (muscle contraction). For example, the diet of construction workers should include more carbohydrate-rich foods than that of an office worker of the same sex and age.

Question 55

How does body status affect our dietary requirement?

Answer 55

Pregnant women need more energy and proteins for the growth of the foetus. They also need more iron for the formation of foetal red blood cells.
Breast-feeding mothers need an extra supply of various types of nutrients for milk production. However, their recommended iron intake is much lower than that of ordinary females. This is because menstruation stops during breast-feeding and the loss of iron is greatly reduced.

Question 56

What are the effects of eating too little?

Answer 56

When we eat too little, our energy intake is smaller than our energy needs/expenditure. Under such conditions, the subcutaneous fat or muscle proteins may be broken down to release energy for body activities. This results in excessive weight loss and the person may become thin and weak. As the body cannot get enough nutrients for maintaining health, various deficiency diseases may develop.

Question 57

Give an example of a disease of under-eating.

Answer 57

Anorexia nervosa: affected patients have a distorted body image. They consider themselves fat even though they are actually very slim. They are afraid of gaining weight, thus refusing to eat. Without proper treatment, they will become dangerously weak or even die.

Question 58

What are the effects of eating too much?

Answer 58

If our energy intake is greater than our energy needs/expenditure, the excess energy will be stored as fats in the body and we will gain weight. When our body weight exceeds a normal level, we are said to be overweight. The condition of being seriously overweight is called obesity.
Being overweight or obese increases the risk of a number of diseases like diabetes, heart disease, stroke, and arthritis.