BIOLOGY: FOOD TEST | SUMMARY OF DIGESTION

By: Mwl Ombeni Mkamba

FOOD TEST

Food test is used to determine which nutrients are present in a food specimen. At this level we will learn how to test for carbohydrates, proteins and lipids.

Test for carbohydrates

	Procedure	Observation	Conclusion
	Test for reducing sugar
	Dissolve specimen in water	colour changes from blues to green to yellow then orange	Reducing sugar is present
	Add an equal amount of Benedict’s solution to the solution
	Boil the mixture
	Test for non reducing sugar
1.	Dissolved the specimen in water Put 2cm³of the solution in a test tube. Add 1cm³ of(dilute hydrochloric acid neutralizes disaccharides to monosaccharide’s) Boil the mixture Allow the mixture to cool Add small amounts of sodium hydroxide at a time (sodium addition) Continue until fizzing stops. Add 2cm³ of Benedict’s solution, then boil the mixture	Colour changes from blue, green to yellow to orange.	Non – reducing sugar is present

3.

5.

	Test for starch
	Add a few drops of iodine solution to the specimen	Colour changes to blue - black	Starch is present

Test for Protein

Procedure	observation	Conclusion
Biuret test
The specimen should be in solution form	Color changes to purple	Proteins are present
Pour 2cm³of specimen in a test tube
Add 1cm³ of sodium hydroxide solution then a drop of copper sulphate solution shaking the mixture after each addition

Test for lipids

Procedure	observation	Conclusion
Grease spottiest
Rub the specimen on a piece of dry filter paper	a translucent mark is formed	Specimen contain lipids
Hold the paper against the light	a translucent mark is formed	Specimen contain lipids
Sudan III test
The specimen should be in solution form	Droplets of oil turn red	Specimen contains lipids
Add some drops of Sudan III test	Droplets of oil turn red	Specimen contains lipids
Emulsion test
Ensure the specimen is in solution form	The clear mixture turns cloudy forming a milky suspension	Specimen contains lipids are present.
Put the specimen in a test tube along with an equal amount of acetone, benzene or ethyl alcohol.
Shake the mixture
Add an equal volume of water

DIGESTIVE SYSTEM IN HUMAN

Some common terms concerned with the movement of food along the alimentary canal.

INGESTION – This is taking in a food to the mouth.

DIGESTION – This is the process by which food is broken down to small particles which are absorbed and assimilated in the body.

ABSORPTION – This is the process by which soluble end products of digestion diffuse into the blood stream.

ASSIMILATION – This is the incorporation of products of digestion into the cell metabolism.

EGESTION –This is the process by which indigested food particles are removed from the body through the anus.

The digestive system is made up of alimentary canal (gut) and associated organs such as the pancreas and liver

PARTS OF ALIMENTARY CANAL

The alimentary canal is a long hollow tube that runs from the mouth to the anus. It is also known as digestive tract.

It consists of the mouth, pharynx, gullet, stomach, duodenum, ileum, caecum, colon and rectum

DIGESTION

Digestion is a process by which food is broken down into form that can be absorbed and used by the body. It involves both mechanical and chemical breakdown of food. Mechanical take place in the mouth; the teeth chew the food to reduce it to small piece that can be swallowed easily

The small pieces have a large surface area to facilitate the action of digestive juice called enzymes

Chemical breakdown is achieved by digestive enzymes. The digested food is absorbed and assimilated in the body. The undigested and indigestible materials are egested as faeces.

THE DIGESTIVE SYSTEM AND DIGESTION PROCESS

The major regions where digestion occur in the alimentary canal are mouth, stomach, duodenum and ileum

DIGESTION IN THE MOUTH

In the mouth, food is chewed by teeth and mixed with saliva to form a ball like to break down food into small particles thus increase the surface area for enzymatic activities.

-Saliva is alkaline in nature, so it makes the food alkaline when in the mouth.

- Secretion of saliva is controlled by the nervous system. These smell, taste, sight or thoughts of food cause saliva to flow from the gland.

-Saliva is a mixture of mucus water, a variety of salts and the enzymes known as salivary amylase.

FUNCTION OF SALIVA

1. Water acts as solvent for dissolving food substance

2. Mucus lubricates thus food for easy swallowing.

3. Salivary amylase turns starch to maltose.

- The tongue rolls the food in the round mass known as bolus(plural is boli)

- The boli are pushed down the oesophagus by the tongue

THE PHARYNX

The region which crosses the air passage is known as glottis
There is a structure known as epiglottis which prevent food from entering the wind pipe or trachea.
There is no digestive enzymes

THE OESOPHAGUS (GULLET)

This is the tube which connects the pharynx and the stomach.
The food passes the gullet rapidly by contraction and relaxation of the gullet mode this is known as PERISTALSIS
Peristalsis is the process by which food substances move down the alimentary canal in the form of bolus through muscle valve known as sphincter
There is no digestive enzymes.

DIGESTION IN THE STOMACH

The food is mixed with hydrochloric acid and gastric juice to produce a semi – solid mass known as chyme.
The wall of the stomach contains gastric glands which secrete gastric juices.
The gastric juices contain water, hydrochloric acid, mucus and enzymes(pepsin and rennin).

FUNCTION OF THE HYDROCHLORIC ACID

1) Provides suitable acidic medium for enzymes to work best

2) Hydrolyses or breaks down food to simple particles

3) Kills bacteria present in food

- The function of pepsin is to breakdown proteins into peptides.

- The function of rennin is to coagulate (solidifies) soluble milk protein (casein) into an insoluble form which is then acted on by the enzymes pepsin. This enzyme is mostly found in young mammals during sucking period.

The function of mucus is to protect the stomach against corrosion by the hydrochloric acid.

GENERAL FUNCTIONS OF THE STOMACH

It is a temporary storage of food
Digestion of proteins starts in the stomach
Helps in mixing food during churning, also absorb water alcohol and some vitamins.

There is a muscle valve between the stomach and the duodenum known as pyloric sphincter

The chyme (liquid food) passes periodically from the stomach through the pyloric sphincter to the duodenum.

DIGESTION IN THE DUODENUM

Duodenum is the first part of the small intestine
It is associated with the liver and pancreas.

THE LIVER

The liver has cells which secrete bile.
Bile is stored in the gall bladder and is released through the bile duct. It is greenish yellow in colour and contains bile salts.

FUNCTION OF THE BILE

i) It emulsifies fats (lipids) i.e. break down fats into tiny fat droplets to increase the surface area for enzymatic activities.

ii) Provides an alkaline medium for enzymes to work best.

iii) It neutralizes the acidic food from the stomach.

THE PANCREASE

The pancreas secretes digestive juices known as pancreatic juice. The juice contains the following.

1. Pancreatic amylase – it breaks down the starch into maltose.

2. Pancreatic lipase – digest the fat droplets into fatty acids and glycerol.

3. Sodium hydrogen carbonates (NaHCO₃) provides basic medium for pancreatic enzymes to work best i.e. neutralize the acidic chyme from the stomach. The resulting food mixture in the duodenum is known as chyme.

4. Trypsin – break down proteins into peptide.

DIGESTION IN THE ILEUM (SMALL INTESTINE)

The ileum is the largest section in the alimentary canal.
The intestine contains secretory cells which secrete mucus and digestive juice known as intestinal juice or succus entericus.
The juice has 4 enzymes

1. Erepsin or peptidase digests peptides to amino acids.

2. Maltose – breaks down maltose to glucose.

3. Lipase – breaks down fat droplets to fatty acids and glycerol.

4. Sucrose – breaks down sucrose (cane sugar) to glucose.

The ileum has two main functions

a) Digests all types of food.

b) Absorption of end products of digestion into the blood stream

NOTES: The walls of the alimentary canal secrete mucus which performs two major functions.

a) Allows smooth movement of food materials along the alimentary, absorption of the end product of digestion into the blood of stream

b) Protect the wall of the alimentary canal against corrosion (digestion) by digestive enzymes.

-End products are:

Amino acids – simple form of proteins.
Glucose – simple form of carbohydrates.
Fatty acids and glycogen – simplest forms of lipids.

ABSORPTION

Absorption is the process by which the soluble end products e.g. glucose diffuses into the blood stream.

Absorption takes place mainly in the small intestine however; absorption of some water, soluble vitamins B and C, and soluble salts take place into the stomach.

PROCESS OF ABSORPTION

1. Amino acids and glucose. These materials are absorbed into the blood stream through the process of active transport

- These materials diffuse into the blood with the dissolved materials to the HEPATIC PORTAL VEIN

- The hepatic portal vein takes the blood with the dissolved materials to the river and then joins the general body circulation.

2. Fatty acids and glycerol.

- They are absorbed into the location of the villi

- They can drain into lymphatic vessel and finally join the body circulation at the vena cava.

NOTE: The wall of the small intestine has numerous fingers – like structure called villi: (singular villus) which increase the surface area for digestion and absorption of food to take place.

Diagram of villi

Adaption of ileum to its functions

1) It is long to provide large surface area for digestion and absorption.

2) It is highly coiled to increase the surface area for digestion and absorption.

3) It has villi and micro – villi which increase the surface area for absorption.

4) It has dense network of blood capillaries which facilitate easy diffusion of digested materials.

ASSIMILATION

This is the process by which the end products of digestion are incorporated in the cell metabolism. Assimilation occurs as follows:

Glucose: some is oxidized during respiration to produce energy (ATP) some is stored as glycogen in muscle some is converted to fats and stored as tissue beneath the skin.

AMINO ACIDS

Some are used in the synthesis (formation) of proteins, some are used in growth and repair of worn out cells. In absence of glucose and fats they may be oxidized to release energy during respiration.

FATTY ACID AND GRYCELOR

Are oxidized to release energy during respiration.
Stored as a dispose tissue beneath the skin. This helps in insulating the body.

THE CAECUM AND THE APPENDIX

These have no function in man.
In herbivores the caecum and appendix contain bacteria that secrete an enzyme known as cellulose.

THE LARGE INTESTINE (COLON)

The large intestine has four functions.

Absorb water from the undigested materials
Absorb small amount of digested food.
Glandular lining of the colon produces mucus which lubricate the passage of faeces
It is a temporary storage of faeces up to the time of defecation (egestion).

EGESTION: The undigested and indigested materials are known as faeces. The faeces are removed from the body through anus by the process of egestion.

SUMMARY OF DIGESTION

Part of alimentary canal	Enzymes secreted	Substance digested	Product of digestion
Mouth	Salivary amylase	Carbohydrates	Maltose
Stomach	Pepsin	Protein	Peptides
	Rennin	Soluble milk protein (caseinogens)	Insoluble milk protein e
	Trypsin	Protein	Peptides
Duodenum	Pancreatic amylase	Starch	Maltose
Ileum	Pancreatic juice	lipase	Fattys acids and glycerol
		Maltase	Glucose
		Sucrase	Glucose and fructose
		Lactose	Glucose and galactose

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