All living organisms require food to carry out their life processes, i.e., growth, repair and for production of energy. There are two main groups of living organisms on the base of mode of nutrition.

1. Autotrophic organisms
2. Heterotrophic organisms

(Auto à Self, troph à feeder)

These are organisms which can prepare their own food by the process of photosynthesis i.e., all green plants.

These are organisms which can not prepare their food. They take prepared food, digest it and then utilize it after absorption i.e., animals, fungi etc. Heterotrophic organisms are further classified into three groups.

1. Parasites
2. Saprophytes
3. Holozoae

Food fulfills the following requirements of animals.

Provide energy which is released from the food by oxidation in cellular respiration.

Provides constituents for repair and growth of cells, tissues and organs.

Provides essential materials to make enzymes which are biological catalysts.

Provide materials to maintain various processes of life including reproduction.

The food which living organisms need composed of following.

1. Carbohydrates
2. Fats
3. Proteins
4. Vitamins
5. Minerals
6. Water
7. Enzymes

These are organic compounds which mostly contain elements such as carbon, hydrogen and oxygen. As the name indicate carbohydrates are hydrated carbons. Each molecule contain hydrogen and oxygen in the ration of 1:2.

Carbohydrates have been classified into following.

1. Monosaccharides
2. Disaccharides
3. Polysaccharides

Those which consist of only single unit of simple sugar e.g., glucose, fructose, galactose.

Those which consist of two units of simple sugar jointed together by bond between them e.g.,

maltose à glucose + glucose

Those which consist of many simple sugar jointed together by bond e.g., starch, cellulose, glycogen.

Plants prepare their own carbohydrates. In man's diet carbohydrates are found in sweet fruits, jam, honey, potato, rice, bread etc.

Most of fats are made up of unit called triglyceride which is composed of 1 glycerol molecule and 3 fatty acid molecules.

Examples of fatty food are butter, oils, ghee etc. Plants can prepare fats themselves.

Proteins are complex organic compound consist of carbon, hydrogen, oxygen, nitrogen and sulphur. Protein is made up of small units called amino acids.

Proteins are made up of smaller units called amino acids. There are 20 kinds of amino acids.

Proteins are found in egg albumen, meat, beans and milk.

Vitamins are essential organic compounds which are needed by our body in very small amount to maintain the normal living processes.

Some selected vitamins are described below.

Vitamin A is necessary for healthy epithelium and for rhodoposia (pigment of red cells).

It is found in milk, fish liver oil, carrots, sweet potatoes, tomato etc.

It is a complex of 12 different vitamins (B₁ - B₁₂).

These vitamins are found in wheat grains, milk, eggs, meat and fresh vegetables.

Chemical name of vitamin C is ascorbic acid.

It is necessary for healthy skin and proper healing.

It is found in citrus fruits, fresh vegetables and milk.

Acute deficiency of vitamin C cause bleeding of gums. The disease is called scurvy.

Vitamin D is needed for the development of strong bones. It helps in absorption of calcium which makes bones hard.

It is present in fish liver oil and also made in our body by sunlight.

The deficiency of vitamin D in children causes the legs to become bowed.

It is needed for fertility.

It is found in green vegetables and certain seeds.

Deficiency of vitamin E cause sterility in some animals.

It helps in clotting of blood.

It is found in liver and also produced by intestinal bacteria.

Deficiency of vitamin K leads to inability of clotting.

There are naturally occurring inorganic compounds. Few important minerals are listed below.

Milk, cheese, eggs, fruits, green vegetables and almond.

It is found in fish, oysters, eggs, milk and cheese.

They occur in body fluids. They are needed for regulation of functions of nerve and muscles.

It is essential for the formation of heamoglobin.

It help to prevent tooth decay.

It is needed for normal functions of thyroid gland.

It is essential constituent of cytoplasm, about 70% of most tissues consist of water.

Enzymes are biological catalyst which regulate different biochemical reactions. All biochemical reaction need enzymes to take place. Enzymes are responsible for chemical digestion of the food.

Modes of nutrition in plants. Plants have been classified into following groups according to the mode of nutrition.

They can prepare their own food by photosynthesis.

They cannot manufacture their own food. They are further classified into following groups.

1. Parasites
2. Saprophytes
3. Insectivores

It is the process by which green parts of the plant prepare their food from carbon dioxide and water with the help of sunlight.

6 CO₂ + 6 H₂O à C₆H₁₂O₆ + 6 O₂

Light plays following important effects.

Provides heat energy.

By the addition of photon water molecule is splitted into H⁺ and OH- which results in release of electrons. This process is called photolysis.

It is green colour matter which is responsible for trapping the sunlight. When light is absorbed by chlorophyll it break water molecule into H⁺ and OH- and release electrons.

It is raw material for the process of photosynthesis. The hydrogen of water molecule becomes a part of glucose.

Photosynthesis is a biochemical reaction, many enzymes are involved in this process. For the normal activities of the enzymes suitable temperature is needed. The temperature which is ideal for the process of the photosynthesis is optimum temperature. The least temperature below which photosynthesis does not take place is minimum temperature and the highest temperature above which photosynthesis does not take place is maximum temperature.

The process of photosynthesis can be divided into following stages.

Light is necessary for this stage of photosynthesis. The light which is absorbed by chlorophyll is used to break water molecule into H⁺ and OH- and emission of electrons. Energy is produced by the movement of electrons which is used in the formation of ATP. The product of light reactions is ATP and NADPH₂.

Light reaction is followed by dark reaction. The products of light reaction are used as a reactant in the dark reaction of the photosynthesis. The first stable product of the dark reaction is glucose, which is then converted into starch and other compounds.

The first stable product of photosynthesis is glucose which is then converted into starch. Presence of starch shows that the process of photosynthesis has taken place.

This test is performed to check the presence of starch. It consist of following steps.

1. Take a green leaf, kill the cells of the leaf by boiling it in water for very small period of time.
2. Boil the leaf in alcohol using water bath to remove chlorophyll from the leaf.
3. Wash the leaf with water and place it in a dish containing iodine reagent.

The whole leaf becomes blue.

Blue colour shows that starch is present in the leaf because iodine reagent gives dark blue colour with starch.

To prove that chlorophyll is necessary for the process of photosynthesis.

Take a potted plant having varigated leaves. Destarch the plant by keeping the potted plant in dark. Wrap it in aluminium foil and expose it to the sunlight for few hours. Detach the leaf from the plant. Draw a rough sketch of the leaf. Apply iodine test on leaf, for the presence of the starch.

The test shows that the parts of the leaf which were previously green turn blue after iodine test while the white parts turn brown.

This result indicated that starch is formed only in those parts of the leaf where chlorophyll exist. This shows that chlorophyll is necessary for the process of the photosynthesis.

To prove that light is necessary for the process of the photosynthesis.

Destarch a potted plant by keeping it in a dark room for two days. It is then transferred to light. Select two of its leaves wrap one leaf completely with black paper. Wrap another leaf with black paper but L shaped part of the paper is cut out so that light can reach the leaf through it. Place the plant in sunlight for 4 to 6 hours. Detach both leaves from the plant and perform iodine test.

It will be observed that the leaf which is covered by paper is free from the starch (remain brown with iodine). In the second leaf the area which is receiving light will turn blue while the area which is covered will remain brown.

This shows that light plays vital role in preparation of starch, which is prepared by photosynthesis. So it is concluded from this experiment that light is necessary for the process of photosynthesis.

To prove that CO₂ is necessary for the process of the photosynthesis.

Destarch two potted plants by keeping them in a dark room. Water them properly during this period. Each pot is enclosed in a transparent polyethene bag. A petri dish containing soda lime is placed in one of the pots to absorb CO₂. In another pot a petri dish is placed containing sodium bicarbonate solution to produce CO₂. Place the plants in sunlight for few hours. A leaf from each plant is detached and tested for starch.

The leaf from the pot containing soda lime does not turn blue. The leaf from another pot will turn blue.

This experiment shows that CO₂ is necessary for the process of photosynthesis.

To prove that oxygen is produced during the process of photosynthesis.

Place the short stem funnel over the aquatic plant hydrilla in a beaker of water. Place the water filled test tube over the stem of funnel. Place the apparatus in sunlight.

Bubbles of gas will soon appear from the plant and will be collected in the test tube. Remove the test tube, bring a burning match stick near the mouth of test tube. It will burn with flame vigorously.

This shows that oxygen is produced during the process of photosynthesis as the gas in test tube is supporting the combustion.

The food prepared by the plants is used in respiration and assimilation (formation of new protoplasm). The excess amount of food is stored in different storing tissues.

He planted willow tree which weighted 2.3 kg. After 5 years he observed that weight of tree was 2.3 kg weight of soil was 91 kg. After next five years the weight of plant become 76.9 kg and weight of soil become 90.8 kg. Increase in weight of plant was 74.6 kg and decrease in weight of soil was 2 kg.

Van Helmont concluded from this experiment that decrease in weight was due to utilization of minerals from soil and increase in weight of plant was due to absorption of water from soil by the plant.

Joseph Prestly enclosed one mouse in a jar in which potted plant was present and second mouse in a jar without plant. It was noticed that the mouse in jar without plant was died due to suffocation but the mouse in jar having plant remained alive.

It was concluded by Pristly that plant take some substance from the air and replace it with a substance which is essential for the life. Later it was found that plant take CO₂ from the air and give oxygen in the air.

On the basis of mode of nutrition, animals have been classified into following groups.

These are animals which feed on plants. E.g., elephant, cow, sparrow.

These are animals which feed on flesh of animals. E.g., dog, lion.

These are animals which feed on animals as well as plants. E.g., man, crow.

Animals which get their food from other living organisms. Parasites may be internal or external e.g., plasmodians, tape worm, ticks, mites etc.

The relationship in which living organisms which are partners are mutually beneficial is called symbiosis. Such animals are called symbionts.

Conversion of complex form of food into a simpler form by the action of different enzymes.

Starch à Glucose

Protein à Amino acid

Fats à Fatty acid + glycerol

Intake of food is called ingestion

The digested food is utilized in cellular respiration and growth. This process by which the absorbed food become a part of protoplasm is called assimilation.

Elimination of undigested food from the body is called egestion.

Human digestive system has been classified into two main parts.

1. Alimentary canal.
2. Digestive glands associated with alimentary canal.

Alimentary canal consist of following parts.

1. Mouth
2. Buccal cavity
3. Pharynx
4. Oesophagus
5. Stomach
6. Duodenum
7. Ilium
8. Colon
9. Rectum
10. Anus

Salivary glands are found in mouth. They secrete saliva which contain an enzyme called Ptyalin. Ptyalin convert starch into maltose.

Starch à Maltose

Four types of teeth are found in mouth which help to grind the food. The process is called mastication.

From mouth food is passed to the stomach through oesophagus.

Inner walls of the stomach posses many gastric glands which secrete gastric juice. Gastric juice contain dilute HCl, pepsin and rennin.

Dilute HCl kill bacteria, makes food soft and change the nature (pH) of food to acidic. Pepsin converts protein into polypeptide. Rennin converts milk into curd.

Liver secrete bile juice which is stored in gall bladder. Bile juice is brought to the duodenum through bile duct. Bile juice contain bile salts which converts fats into tiny droplets.

Pancreas secrete pancreatic juice which is alkaline in nature and contain three enzymes called trypsin, amylo trypsin and lipase. These three enzymes work along with other enzymes present in the small intestine. Pancreatic and bile juice are alkaline in nature.

1. Trypsin converts peptides to amino acids.
2. Amylotrypsin converts carbohydrates into glucose.
3. Lipase converts fats into fatty acids and glycerol.

Absorption of food takes place in ileum through villi. Villi are finger like folds of intestine lined by epithelium and provides large surface area for absorption of digested food. Absorbed food is carried to liver through hepatic portal vein and lymph vessels where it is brought to the heart.

Undigested food is stored in large intestine for some time and eliminated through anus.

The diet which contain all the nutrients in proper proportion.

Balanced diet comprise of carbohydrates, fats, proteins, vitamins and minerals. Deficiency in any group of diet results in physiological disorders.

Milk is considered to be most balanced diet for babies because it contain protein, carbohydrates, fats, vitamin A, vitamin D and calcium.

Person which doesn't get as much food as required by his body become weak, his immune system also become weak and in case of acute starvation person is starved to death.

If a person taking sufficient amount of diet but missing any one or more groups of balanced diet he will suffer from certain physiological disorders.

They are added to the food for colouring and preservation by industries. These dyes and preservatives should be used with care.

Certain harmful bacterial found in food cause food poisoning by producing poisonous substance toxic in food or by entering into the body.

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