Form 2 science

Chapter 2 (part 2)

Nutrition

2.2 Balanced diet

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Balanced diet

• A balances diet is a diet which consists of all

classes of food in a correct proportions.

• A balances diet is important to maintain a • A balances diet is important to maintain a

healthy body and to prevent various

deficiency diseases.

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The factors that determine a person’s balances

diet are:

1. Age

2. Body sizes

3. Gender / sex3. Gender / sex

4. Job

5. Climate

6. State of health

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a) A growing child need more protein

than adult.

b) A big sized person normally needs

more food than a small sizes person.

c) Boys generally need more food than

girls because boys are more active

than girls.

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d) A person doing heavy work needs

to eat more than a person doing

light work.

e) People living in cold countries

need more energy to keep them need more energy to keep them

warm.

f) People with health problems

should be careful with their diets.

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Read the food label

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Calories

The unit of energy in food can be measured

either in joules or calories.joules or calories.

In a complete combustion,

1 calorie = 4.2 joules

In a complete combustion,

• 1 g of carbohydrate gives out 17 joules of

energy

• 1 g of protein gives out 17 joules of energy

• 1 g of fats gives out 39 joules of energy

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• The calorific value

of canned or

packaged foodstuff

are stated on their are stated on their

labels.

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How to calculate the calorific value of food?

A student ate the following for lunch yesterday. Estimate the caloric

value of his meals by referring to the diagram below.

From food label:

Burger = 253 kcal

Orange juice = 157 kcal

French fries = 224 kcal

juice + French fries

The calorific values of food.

Burger + Orange juice + French fries

= 253 + 157 + 224

= 634 kcal s

= 634 x 4.2

= 2662.80 kJ10yschow@smkbpj(a)

2.3 Human Digestive System

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Digestive System

•• Digestion Digestion is the breaking down of food into simpler form so that they can be absorbed into the bloodstream.bloodstream.

• Organs in the digestive system are mouth, stomach, pancreas, small intestine and large intestine.

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In the mouth

• In the mouth the salivary glands produce the

enzyme amylase.

• Salivary amylase acts on starch and changes it • Salivary amylase acts on starch and changes it

into maltose

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Oesophagus

• Digested food is shaped into a bolus by the

tongue and swallowed.

• The bolus moves down the oesophagus is a • The bolus moves down the oesophagus is a

wave-like action called peristalsis .peristalsis .

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Stomach

The stomach secretes gastric juice which contain

hydrochloric acid and proteases.proteases.

Functions of hydrochloric acid:Functions of hydrochloric acid:

( a) Stops the action of amylase

(b) Provides an acidic medium which is

suitable for the action of protease

(c) Kills the germs and microorganisms in

food15yschow@smkbpj(a)

• In the stomach, protease digests proteins into

peptones.

• Mucus protects the stomach walls from being

digested by protease.

Stomach

digested by protease.

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Duodenum

• Duodenum is the first part of the small

intestine.

• It receive bile from the gall bladder and • It receive bile from the gall bladder and

pancreatic juice from pancreas.

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• Bile emulsifier fats into oil droplets for lipase

to act upon.

• Bile also neutralise the acidic chyme.

Duodenum – bile

• Bile also neutralise the acidic chyme.

Chyme is partially digested food.18yschow@smkbpj(a)

Duodenum –pancreatic juice

• Pancreatic juice contains amylase, protease

and lipase.

1. Pancreatic amylase breaks down

starch into maltose.starch into maltose.

2. Protease breaks down proteins

into polypeptides

3. Lipase catalyses the breakdown of

fats into fatty acid and glycerol.19yschow@smkbpj(a)

Lower part of small intestine- ileum

• The digestion of food is completed in the small

intestine.

• Lower part of small intestine secretes

intestinal juice which contains maltose,

protease and lipase.protease and lipase.

(a) Maltase breaks down maltose into

glucose.

(b) Protease breaks down polypeptides into

amino acids.

(c) Lipase breaks down fats into fatty acids

and glycerol.

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The end products of digestion

Foods Enzymes involved End products

(a) Carbohydrates Amylase , maltase Glucose

(b) Proteins Protease Amino acids

(c) Fats Lipase Glycerol and fatty

acids

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2.4 Absorption of Digested Food

– Absorption of digested food takes place in the

small intestine

– The end products of digestion, vitamins and

minerals are absorbed by the villi into the

bloodstream.

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• The small intestine has several characteristics that

enable the effective absorption of end products of

digestion.

I. It has a lot of villi on its surface.

This increases the surface area for

absorption.

I. Each villus has a thin epithelial wall to

enable the diffusion of digested food.

II. There is a rich network of blood

capillaries in each, villus.

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To show the absorption of glucose through a Visking tube.

1. At the beginning of experiment, the water in the beaker is tested with iodine solution and Benedict‘s solution.

2. Both the glucose and starch are not present in the water.

3. After 30 minutes; the two tests above are repeated.

4. It is found that the water in the beaker be contains glucose.

Conclusion :

• This shows that glucose has

diffused through the wall of

Visking tube.

• The starch molecules are too

big and hence cannot diffuse

through the wall of Visking tube.24yschow@smkbpj(a)

2.5 Reabsorption of Water and

Defecation

1. The undigested food, minerals and excess water

pass into the large intestine.

2. The large intestine does not secrete any enzyme

and hence, no digestion occurs in the large and hence, no digestion occurs in the large

intestine.

3. The process of reabsorption takes place in the

large intestine. Water and minerals are

reabsorbed.

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4. After the water is reabsorbed, the

undigested food turns into faeces.

5. The faeces is stored in the rectum.

6. Defecation is the process in which faeces is

expelled from the body through the anus.

7. Insufficient fibre and water in our diet will 7. Insufficient fibre and water in our diet will

cause constipation.constipation.

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2.6 Practice the Habits of Healthy

eating

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1. It is important for us to eat nutritious food to keep

us healthy.

2. Unhealthy eating habits can cause diet- related

diseases such as diabetes, obesity, high blood

pressure and cancer.

3. We should always be sensitive towards the cultural

practices of other races.practices of other races.

4. We must not offend the religious beliefs of others.

We must always respect their religious beliefs.

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