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CLASSES OF FOOD
1. Food is a basic necessity for all living things.
2. Food is important in order to :
(a) Provide energy to carry out physical activities.
(b) Provide energy for the physiological activities in life such as
reproduction, digestion and
(c) Build new cells and ti repair tissues in the body.
(d) Protect the body from deseases.
(e) Maintain the body temperature.
(f) Promote growth.
3. Food can be divided into seven classes which are :
(e) Mineral salts
1. Carbohydrates are formed from the elements of carbon, hydrogen and
2. Carbohydrates belong to a class of food that provide energy to the body.
3. Carbohydrates are oxidised in the body during respiration to release
4. Carbohydrates consist of four main forms :
(a) Simple sugar is used directly by living cells to produce energy.
Examples of simple sugars are
glucose, sucrose and lactose.
(b) Starch, which is a form of carbohydrate, is stored in plants.
Examples of plants that store starch
are tapioca, banana and wheat. Extra glucose that is produced by
photosynthesis is kept in the
form of starch.
(c) Glycogen, which is a form of carbohydrate is stored in animals.
Glycogen is usually stored in
the liver and muscles. Glycogen is only used when the body lacks of
(d) Cellulose is the main constituent of the cell wall. Therefore,
cellulose is found in abundance in
vagetables and fruits cannot be digested by the human digestive
5. Strach, glycogen and cellulose are made up of glucose molecules joined
together in a different
6. Extra carbohydrates in our body are kept in the form of fat.
7. The presence of starch in food can be tasted by using iodine solution.
The colour of iodine would
change to dark blue starch is present in a food sample.
8. The presence of simple sugar in food can be teated by using Benedict's
solution or the Fehling's
solution. A brick-red precipitate will be obtained when simple sugar is
boiled with the reagents.
1. All types of proteins have carbon, hidrogen, oxygen and nitrogen. Some
proteins also contain
sulphur and phosphorus.
2. Protein molecules are formed from basic units called amino acids. There
are about 20 types of
amino acids. Some of these amino acids can be produced by the body
and some must be
consumed from the diet.
3. Protein is a class of food needed for the growth of the body, especially
in the building of cell
4. Important functions of protein include :
(a) Building new cells.
(b) Repairing damaged tissue.
(c) Building enzyme, antibodies, hormones and haemoglobin.
(d) Supplies energy, especially if carbohydrate and fats are insufficient
in the body.
5. Proteins are divided into two categories :
(a) Plant proteins that are found in beans, corn and wheat.
(b) Animal proteins that are found in meat, fish, egg white and cheese.
6. Children need more protein compared to adults to support their growth.
7. Protein deficiency will cause kwashiorkor. The symptoms of a
kwashiorkor patient are enlargement
of the stomach, no appetite for food and diarrhoea.
8. Extra protein is excreted from the body as urea in the urine.
9. The presence of protein in food can be tested by heating a food sample
with Millon's reagent. A
brick-red precipitate will be obtained if protein is presents in the food
1. Like carbohydrates, fats also consist of carbon, hydrogen and oxygen
elements, but the ratio of
hidrogen to oxygen is higher than 2 : 1.
2. Fat is a food class that provides energy to the body. Fats supply twice as
much energy as
carbohydrates. 1 g of fats produce 38 kJ of energy.
3. Other functions of fats include :
(a) As a heat insulator under the skin to reduce heat loss from the body.
(b) To protect organs such as the heart and the kidneys from the injury.
(c) To dissolve vitamins A, D, E and K in the body.
(d) To prevent the loss of water from the surface of the skin by the
secretion of oil.
4. Fat that exists in the form of liquid is called oil.
5. Fats are divided into :
(a) Plant fats - palm oil, margarine, groundnut oil and coconut oil.
(b) Animal fats - butter, milk, egg yolk and cheese.
6. Animal fats have a higher cholesterol level compared to plant fats.
7. Fat molecules are formed from glycerol and fatty acids.
8. Extra fat is kept by the body under the skin.
9. The presence of fat in food can be tested by rubbing a food sample on a
filter paper. A translucent
spot on the filter paper shows that the food has fat.
10. Fats can also be tested by the alcohol-emulsion test :
(a) A few drops of enthanol are mixed with a fat simple. Then, shake the
(b) Then, cold water is added.
(c) The mixture turns into a cloudy white emulsion if fat is present.
1. Vitamins belong to a food class that maintain our health and protect our
body from disease.
2. Vitamins are organic substances found in some types of foods.
3. Different vitamins have different functions.
4. Human beings need vitamins A, B, C, D, E, and K. Vitamins A, D, E and K
cannot be dissolved in
water but vitamins B and C are soluble in water.
5. Our body needs vitamins in small quantities. Vitamins deficiency can
cause serious illnesses.
6. Table 2.1 shows the sources and functions of vitamins, as well as the
effects of vitamin deficiencies
in our body.
(E) Mineral salts
1. Mineral salts are inorganic substances that are needed in small
wuantities to carry out body
processes and to maintain a healthy body.
2. Important mineral salts include calcium, phosphorus, iron, iodine,
sodium and fluorine.
3. Calcium and phosphorus are needed in large quantities compared to
4. Mineral salts do not provide energy, but lack of them can cause varoious
types of disease.
Table 2.2 Summarises the sources of mineral salts, functions and the
effect of their deficiencies.
1. Water is a food class taht is needed the most compared to other food classes.
2. Water constitutes about 70% of the body weight of humans.
3. Water has several functions taht are important to the human body.
(a) As a medium for chemical reactions in our body.
(b) As a solvent for the transfer of substances such as digested food,
oxygen, carbon dioxide, urea
(c) Controls our body temperature by releasing heat through
perspiration, respiration, uniration
(d) Controls the concentration of blood and body fluids.
(e) Gives appearances and shape to the body.
(f) To reduce friction between joints.
4. Main source of water is drinking water, fruits and vegetables.
5. The body loses water through perspiration, respiration, as well as
strough disposal of urine and
6. Excessive water loss (about 20% from the amount of water in the body)
can cause death.
7. Water is required by our body about 2 litres daily or about 6 glasses of
water every day.
Direct heating for the glucose and protein tests is not carried out but a
water bath is used instead. This is because the contents of the test tube will
spill out if it is directly heated.
The food tests for carbohydrate ( starch and glucose ), protein and fat are
carried out using specific reagents or materials.
The presence of fat or oil, and vitamin C can be futher tested as shown
1. Fibre or roughage is a food class that consists of fibrous material
(cellulose) that cannot be
digested by the human digestive system.
2. Fibre helps in the movementof food materials through the small and
large intestines. Fibre helps to
smooth the peristalsis process in our intestine by absorbing water for
the ease of passing motion.
3. Sources of food that are rich in fibre are vegetables, fruits, mushrooms
4. Fibre deficiency can slow down the movement of digested food in the
large intestine. This causes a
lot of water to be reabsorbed from the undigested food and the faeces
become hard. This results in
THE IMPORTANCE OF A BALANCED DIET
1. A balanced diet is consuming food that contains all types of food
classes in the right quantity.
2. A balanced diet consist of :
(a) Carbohydrates ( 55% - 65% ).
(b) Fats ( 20% - 35% ).
(c) Proteins ( 10% - 15% ).
(d) Sufficient amount of vitamins and minerals.
(e) Water and fibres.
3. A balanced diet is important to ensure that the body has :
(a) Sufficient energy supply
(b) Good health
4. Several types of deseases such as akwashiorkor, anaemia, beri-beri,
scurvy, rickets and others can
occur as a result of an unbalanced diet.
5. The content of a balanced diet differs according to individual needs and
depends on the following
(a) Sex - men need more energy compared to women.
(b) Age - growing teenagers need more energy because their growth
rate is higher.
(c) Body size - the bigger the size of the body, the more energy is
(d) Type of occupation or activity being carried out - the longer the
work or activity is carry out, or
the heavier the task, the more energy is needed.
(e) Climate - people living in cold countries need more energy
compared countries need more
energy compared to people living in hot countries to maintain their
(A) Food calorific value
1. The calorie is the amount of energy in food.
2. Calories is defined as the amount of heat energy needed to increase 1 g
of water by 10
C at 1
3. The units for food calorie are calorie or joule. ( 1 calorie = 4.2 joules ).
4. Calorific value or energy value is the amount of energy released from 1
gram of food.
5. The calorific value for seven food classes are :
(a) Carbohydrates : 17.2 kJ/1 gram
(b) Fats : 38.5 kJ/ 1 gram
(c) Proteins : 22.2 kJ/1 gram
(d) Vitamins : 0 kJ/1 gram
(d) Mineral salts : 0 kJ/1 gram
(e) Water : 0 kJ/1 gram
(f) Fibre : 0 kJ/1 gram
6. The calorific value for several types of food are shown in table 2.3.
7. A food intake with a higher calorific value than the total energy needed
by the body can cause
8. The following diagrams show a balanced died for breakfast, lunch and
9. The importance of a balanced diet :
(a) To maintain good health of the body.
(b) To promote growth by producing new cells and tissues.
(c) To repair and replace old and damage cells.
(d) To supply sufficient energ for physiological processes and daily
10. An unbalanced diet over a long period of time leads to malnutrition that
exposes a person to
THE HUMAN DIGESTIVE SYSTEM
1. Digestion is the process of breaking down complex food substances
into simpler and easily
absorbed food particules by the body.
2. The human digestive system sonsists of a long canal ( 9 - 10 m ) known
as the alimentary canal. This
canal begins in the mouth cavity and ends in the anus.
3. The alimentary canal consists of the mouth, oesophagus, stomach,
small intestine ( duodenum and
ileum ), big intestine and anus
4. Several other organs involved in digestion are salivary glands, liver, gall
bladder and pancreas.
5. The passage of food in the alimentary canal is as follows :
(A) Digestion in the duodenum
1. The duodenum is connected to the liver and the pancreas.
2. Food from the stomach is then, pushed into the duodenum and mixed
with bile and pancreatic
3. Bile is secreted by the liver and stored in the gall bladder.
4. Bile is alkaline and neutralizes the acidity of the food mixture from the
stomach and prepares an
alkaline medium for the action of the pancreatic juice.
5. Bile also functions to emulsify fats into small droplets so that they are
easily digested by the lipase
6. Pancreatic juice is produced by the pancreas.
(a) This juice is an alkaline and has three types of enzyme, which are
lipase, amylase and trypsin.
(b) Lipase digests fat into fatty acids and glycerol.
(c) Amylase digests starch into maltose.
(d) Trypsin breaks down protein into polypeptides.
7. Apart from producing bile, the functions of the liver in the digestion
process includes :
(a) To control the sugar level in the blood. Extra glucose is converted to
glycogen and is stored in
the liver. When it is required by the body, glycogen will be converted
back into glucose to
(b) To convert extra amino acids into urea. Then, the urea is excreted in
(c) To store vitamins such as vitamins A and D, as well as mineral salts
such as iron.
Absorption of Digested Food
1. Digested food such as glucose, amino acids, mineral salts and vitamins
are absorbed into the body
through the wall of the small intestine by diffusion.
2. The digestive products are absorbed into the bloodstrem through the
villi ( singular : villus ).
3. Villi are fine projections on the surface of the small intestine. Villi
increase the surface area of the
small intestine to about 40 m2
4. Villi have thin walls to fasten the food absorption process.
5. Each villus is rich with blood capillaries to promote absorption of the
6. The wall of the villus only allow the digestive products that have small
molecules to pass through it.
7. Simple sugar, like glucose, together with amino acids, mineral salts and
water-soluble vitamins are
absorbed into the blood capillaries. While glycerol, fatty acids and fat-
soluble vitamins ( vitamins A,
D, E and K ) are absorbed into the lacteal.
8. Once absorbed into the blood capillaries, the digestive products are
transported to the liver.
9. The digestive products are then, transported to the heart. The heart
pumps blood containing the
digestive products to other parts of the body.
The transport of Digestive Products
1. Digested food is sent to the liver or the lymphatic system. Then, it will be
sent to the netire body
through the blood sirculatory system.
2. Digested food is used by the body cells in the following processes :
(a) Respiration : The oxidation of glucose to produce energy.
(b) Assimilation : The process of converting digested food into
subtances needed for the growth or
repair of damaged tissue.
(c) Decomposition : Waste materials or extra food that cannot be kept
will be decomposed into
materials that can be expelled through the excretory system.
1. Digestion is the process of breaking down complex food to simple
molecules for absorption into the
blood circulatory system.
2. Digestion takes place in two stages :
(a) Physical digestion - big pieces of food are broken down into smaller
pieces by teeth.
(b) Chemical digestion - enzymes break up complex food molecules.
3. Enzymes are proteins that speed up the process of digestion. Enzymes
break up complex food
molecules to smaller and simpler melecules.
1. The digestive system consists of all the organs in the body that help in
the digestion of food.
2. The alimentary canal is made up of the mouth, oesophagus, stomach,
small intestine, large
intestine and anus ( starts from the mouth and ends at the anus )
3. Food is pushed along the alimentary canal by the muscular walls that
contract and expand
alternately through the process of peristalsis.
1. Food is chewid and broken up into small pieces by the teeth.
2. Small pieces of food have a wide surface area for the saliva to act upon.
3. The salivary gland secretes saliva that is alkaline and contains salivary
amylase enzymes ( ptailin ).
4. Salivary amylase digests starch into maltose ( sugar ).
1. Food is pushed through the oesophagus into the stomach by the
alternating muscular contraction
and relaxation of the oesophagus.
2. This process of alternating muscular contraction and relaxation is
known as peristalsis.
3. No digestion tales place in the oesophagus because there are no
digestive enzymes secreted.
1. Food is mixed with gatsric juices in the stomach.
2. Gastric juices are secreted from the cells of the stomach wall. Gastric
(a) hydrocloric acid.
(b) enzymes ( rennin and pepsin ).
3. The functions of hydrocloric acis include :
(a) providing an acidic medium for enzymic action.
(b) killing bacteria found in food.
(c) neutralising the alkaline property of saliva and stopping the action
of salivary amylase
4. Pepsin digests proteins into peptones.
5. Rennin coagulates milk in the stomach to help in the ezymic action of
1. The duodenum is the first part of the small intestine.
2. The duodenum receives bile and pancreatic juice.
3. Bile is produced by the liver and is stored in the gall bladder. The bile
flows from the gall bladder
into the duodenum through the bile duct. The functions of bile are :
(a) emulsification of fat, i.e. breaking up large fatty globules into small
droplets for enzymic
(b) preparation of an alkaline medium for enzymic action.
4. Pancreatic juice, which is produced by the pancreas cells, contains three
types of enzymes, i.e.
panceatic amylase , protease and lipase.
5. The pancreatic amylase digests starch into maltose.
6. The protease digests peptones into amino acids.
7. The lipase digets fat into fatty acid and glycerol.
8. The small intestine ( ileum ) produces enzymes which digest maltose
into glucose ( simple sugar ).
9. Digestion is compketed in the small imtestine.
10. The digested food is then ready to be absorbed through the thin walls of
the small intestine into the
1. About 2 ml of albumen solution is poured into test tubes labelled A, B, C
2. Test tubes A to D are filled with the following :
A : 6 ml distilled water
B : 6 ml pepsin solution ( 1% )
C : 6 ml pepsin solution ( 1% ) and 1 ml dilute hydrochloric acid.
D : 6 ml pepsin solution ( 1% ) and 1 ml dilute sodium hydroxide solution
3. At the beginning of the experiment the contents of all the test tubes are
tested with Millon's
4. Then all the four test tubes are heated in a water bath at a temperature of
C for 30 minutes.
5. The contents of all the test tubes are tested with Millon's reagent again
at the end of the
6. The observations are recorded.
1. Pepsin acts best at a temperature of 370
C, i.e. our body temperature.
2. The digestion of proteins by pepisn requires an acidic medium.
3. The acid speeds up the pepsin action on protein.
The enzyme in the stomach digests proteins.
1. Starch molecules are too big to diffuse through the Visking tube. The
smaller glucose molecules
can diffuse through the Visking tube wall that is semipermeable.
2. The Visking tube is used as an anology to represent the human small
3. The water in the boiling tube is used an anlogy to represent the human
bood circulatory system.
1. The hypothesis made can be accepted.
2. The simple digested food molecules diffuse through the small intestine
walls into the bloodstream.
The smaller glucose molecules diffuse through the small intestine walls into
REABSORPTION OF WATER AND DEFECATION
Reabsorption of water
1. The main function of the large intestine is to reabsorb water.
2. The substances that enter the large intestine consist of water and
undigested food substances like
cellulose from the fibre of vegetables and fruit ( roughage ).
3. Water is reabsorbed from these undigested food substances.
1. Undigested food in the large intestine is expelled as faeces through the
process of defecation.
2. The faeces that reach the rectum are made up of undigested food
substances. The faeces
accumulate in the rectum.
3. When the rectum is full of faeces, the rectum undergoes peristalsis and
assested by abdominal
contraction, will push the faeces through the anus to be expelled.
4. If an individual has problems passing motion, he or she is said to be
5. Constipation takes place because of the lack of water and roughage in
HEALTHY EATING HABITS
1. Healthy eating habits will help maintain a healthy body.
2. Unhealthy eating habits cause various health problems.