TOPIC 2: DIGESTIVE SYSTEM IN HUMAN - BIOLOGY FORM TWO NOTES

TOPIC 2: DIGESTIVE SYSTEM IN HUMAN – BIOLOGY FORM TWO NOTES

The digestive system of human refers to the human alimentary canal and the organs and glands associated with it. Some common terms concerned with the movement of food along the alimentary canal.

Ingestion. This is taking in 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.

PARTS OF THE HUMAN DIGESTIVE SYSTEM AND THEIR ADAPTIVE FEATURES

The main parts are:-

Mouth
Oesophagus
Stomach
Small intestine, (duodenum and ileum),
Large intestine (colon) and rectum.

DIAGRAM OF ALIMENTARY CANAL

THE DIGESTION PROCESS IN HUMAN BEING

Digestion is the process by which food is broken down into a form that can be absorbed and used by the body.

Types of Digestion

Mechanical digestion/physical digestion – It involves breaking down large pieces of food into smaller ones using the teeth.
Chemical digestion – Digestive enzymes achieve chemical break down of food. The digested food is absorbed and assimilated in the body.

DIGESTION IN THE MOUTH

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

Saliva is secreted by salivary glands and it 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.

Components of saliva

Mucin – lubricates the food for easy swallowing.
Water – Acts as a solvent for dissolving food substances ( 95%)
Salts e.g.,bicarbonates, chloride, calcium, potassium, sodium and magnesium
Enzymes e.g. salivary amylase and lingual lipase
Antibacterial compounds eg thiocyanate, hydrogen peroxide
Antimicrobial enzymes i.e. enzymes that kill bacteria eg lysozyme

Functions of saliva

To provide medium for enzyme reactions
Have mucus to lubricate food for easy swallowing
Have salivary amylase enzyme that converts starch into maltose
To moisten the food
Saliva consists of Sodium and Calcium salt to ensure alkaline medium for amylase enzyme

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 are 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 PERISTALYSIS, 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 enzyme

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)

DIAGRAM SHOWING THE STOMACH AND OTHER PARTS

Functions of hydrochloric acid

Provides suitable acidic medium foe enzymes to work best
Kills bacteria present in food
Hydrolyses or breaks down food to simple particles
Activates pepsinogen to pepsin.

Pepsin.

Breaks down proteins into peptides. It is produced in an inactive form called pepsinogen. Hydrochloric acid (HCl) activates pepsinogen into pepsin. Pepsin is produced in inactive form to prevent it from digesting the cells that produce it in the gastric glands, and the wall of alimentary canal.

Rennin

It coagulates soluble milk protein (casein) into an insoluble curd which is then acted by the enzyme pepsin. This enzyme is mostly found in young mammals during the suckling period.

Functions of the stomach

It acts as a temporary storage of food
Digestion of proteins starts in the stomach
Helps in mixing food during churning, also absorbs water, 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 sphincter to the duodenum

Adaptations of stomach

Stomach has sphincter muscles to prevent food from flowing back into the oesophagus
Stomach has gastric glands, which produce gastric juice for the digestive process.
Also in the stomach of a young baby there is renin which coagulates milk
Stomachs have mucus which protects it from corrosion by digestive enzymes
There is hydrochloric acid for the emulsification of fats and killing of bacteria

DIGESTION IN THE DUODENUM

It is the first part of the small intestine.

The liver

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

Functions of bile

Provides an alkaline medium for enzymes to work best
It neutralises the acidic chyme from the stomach
It emulsifies fats (lipids) i.e break down fats into tiny droplets (emulsification) to increase the surface area for enzymatic activities.
Also kills germs present in the food
It helps in excretion of bilirubin (by product of RBC) in the body

The pancreas

The pancreas secrete digestive juices known as pancreatic juice, the juice contains the followings.

Pancreatic amylase- breaks down starch into maltose.
Pancreatic lipase- digest fat droplets into fatty acid and glycerol
Trypsin –breaks down the remaining proteins into peptides
Sodium hydrogen carbonate –provides basic medium for pancreatic enzymes to work best i.e neutralize the acidic chyme from the stomach

DIGESTION IN THE ILEUM (SMALL INTESTINE)

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

Erepsin / peptidase –digest peptides to amino acids
Maltase – breaks down maltose to glucose
Lipase – breaks down fat droplets to fatty acids and glycerol
Sucrase – breaks down sucrose (cane sugar) to glucose

Functions of the ileum

Digest all types of food
Absorption of end products of digestion into the blood stream

The wall of the alimentary canal secrete mucus which perform two major function

Allow smooth movement of food materials along the alimentary, absorption of the end products of digestion into the blood stream
Protect the wall of the alimentary canal against corrosion (digestion) into the blood stream

The end products are:

Amino acids – simple form of proteins
Glucose – simple form of carbohydrates
Fatty acid and glycerol – simplest form of lipids

Absorption

Absorption is the process by which the soluble end products eg glucose diffuses into the blood stream

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

The process of absorption

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 liver and then joins the general body circulation.
Fatty acid and glycerol: They are absorbed into the location of the villi, they can drain into the lymphatic vessel and finally join the body circulation at the venacava.

Note: the wall of the small intestine has numerous finger like structures called villi (singular villus) which increase the surface area for digestion and absorption of food to take place.

Diagram of the villi

Adaptations of ileum to its functions

The following are the adaptations of ileum to its mode of functions:-

The ileum is very long to ensure sufficient absorptions of food
The villi have large network of blood capillaries for transporting absorbed food to all parts of the body
It has finger like projections called villi for the absorption of food
It is highly coiled to increase the surface area for digestion and absorption
The inner lining of the ileum is folded to increase the surface area for absorption
It has secretory glands, which produce digestive enzymes
It is connected to the liver by the bile duct which enable the bile to reach ileum

Assimilation

This is the process by which the body cells utilize absorbed nutrients for growth, repair, energy generation and reproduction.. It occurs as follows:

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

Amino acids: Some are used in the synthesis 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 glycerol

Are oxidized to release energy during respiration
They are stored in adipose tissues beneath the skin. This helps in insulating the body.

The caecum and the appendix: They have no function in man; in herbivores the caecum and appendix contain bacteria that secrete an enzyme known as cellulose which digest cellulose.

The large intestine (colon)

The large intestine has the following functions

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

Rectum

This is the last chamber of the alimentary canal where faeces are temporary stored and later removed from the body through the anus.

Egestion

Not everything in the food we eat is digestible for example roughage remains undigested and must be removed. The process is controlled by a sphincter muscle. When the rectum is full the sphincter muscles relax allowing faeces to pass out.

Egestion: is the process of removing undigested food from the body.

SUMMARY OF DIGESTION

Part of alimentary canal	Source of enzymes	Enzymes secreted	Substance digested	Product of digestion
Mouth	Salivary gland	Salivary amylase	carbohydrates	Maltose
Stomach	Gastric juice which are secreted by gastric glands found in stomach walls.	Pepsin	protein
Stomach		Rennin	Soluble milk protein (caseinogens)	Insoluble milk protein
Duodenum	Pancreatic juice which are secreted by pancreas.	Pancreatic amylase	starch	Maltose
		Pancreatic lipase	Fats	Fatty acid and glycerol
		Trypsin	proteins	Peptides
Small intestines (ileum)	Intestinal juice which is secreted by secretory cells found in the intestinal walls.	Maltase	maltose	glucose
		Sucrase	sucrose	Glucose and fructose
		Lactase	lactose	Glucose and galactose
		Erepsin (pepsidase)	peptides	Amino acids

Medium in which enzyme work best.

Enzyme	Location	Medium
Salivary amylase	Mouth	Alkaline/ basic
Pepsin, rennin	Stomach	Acidic
Pancreatic lipase, trypsin, pancreatic amylase	Duodenum	Alkaline/ basic
Sucrase, Erepsin, maltase, lactase and lipase.	Ileum	Alkaline /basic

THE RUMINANT DIGESTIVE SYSTEM

A ruminant is an animal that chews food, swallows it then returns it to the mouth later for further chewing. This is called chewing cud. Examples of ruminants are cows, goats, sheep, antelopes and giraffes.

The digestive system of a ruminant is different from that of a human being. Ruminants have a more elaborate system to enable cellulose digestion.

The stomach of a ruminant has four chambers: rumen, reticulum, omasum and abomasums

When a ruminant first chews and swallows a mouthful of plant matter, the food enters the rumen. Bacteria in the rumen immediately start digesting the cellulose present in the material. Chewing cud softens and helps down plant fibres, making them more access to digestion by the bacteria. When the food in the rumen, It is coarse and very green. The food then regurgitated and chewed again, it passes the reticulum; the reticulum has ‘honeycomb’ appearances. In the reticulum, the food is mixed thoroughly with water. The food coarse, more watery, less green and very small compared to the food in the rumen.

The food is regurgitated, chewed again and passes to the omasum. The abomasums has longitudinal folds like the leaves of a book. The folds help to remove water from the food. The food in the omasum is in form of fine particles and has very little water.

The abomasums is the ruminant’s stomach. The abomasums has gastric acid facilitates the digestion of proteins. It also has micro-organisms that may have spilled over the rumen.

Outer side of the ruminant stomach

DIFFERENCE BETWEEN THE HUMAN DIGESTIVE SYSTEM WITH THAT OF OTHER MAMMALS

The ruminants’ digestive system differs from human digestive system in the following ways:

Ruminants have more elaborate system to enable cellulose digestion
The stomach of ruminants have four chambers (rumen, reticulum, omasum, abomasum) while human being have only one chamber.
The food is regurgitated, chewed, and again then passed to omasum.
Ruminant’s digestive system has bacteria that help to digest fibre and sysnthesize vitamin B, human being can not digest fibre.

COMMON DISEASES AND DISORDERS OF THE HUMAN DIGESTIVE SYSTEM

These include diseases and disorders that affect teeth, oesophagus, stomach, small intestine and large intestines

Examples are: – dental caries, heart burn, ulcers, constipation and flatulence.

Heart burn Refers to the burning or painful sensation in the oesophagus or in the chest cavity. It is caused by regurgitation of hydrochloric acid in the stomach which leads to the irritation of oesophagus.
Dental caries. This is commonly referred to as tooth decay. It occurs when bacteria destroy the outer part of the tooth.
Ulcers These occur as a result of the action of enzymes and acid on the epithelial membrane lining of the stomach and duodenum walls. The mucus coating can be broken by bacteria called Hericobacter pylori
Constipation Refers to the decrease in frequency of formation of stool. It occurs when the stool becomes dry and hard due to excessive water absorption in the colon.
Flatulence the condition caused by excessive gas in the digestive track.

CAUSES, SYMPTOMS, EFFECTS AND CONTROL MEASURES OF COMMON DISORDERS AND DISEASES OF THE HUMAN DIGESTIVE SYSTEM

Dental caries (Tooth decay)

Dental caries (Tooth decay)-is a damage to a tooth that can happen when decay causing bacteria in the mouth make acids that attack the tooth’s

Cause

Lack of hard food
Too much sweet food. Sweet encourages the growth of bacteria
Lack of calcium in the diet
Lack of vitamin D
Generally ill health

Symptoms of dental caries

Tooth ache
Holes in the teeth

Control measures to caries

Regular brushing of teeth
Damaged teeth can be filled with artificial crown
Minimization of intake of foods rich in sugar
Avoid eating very hot or cold foods
Have regular dental check ups

STOMACH ULCERS

Stomach ulcers – Are painful sores in the lining of the stomach

Includes:

Peptic ulcers eg gastric ulcers—they develop inside the stomach and duodenal ulcers they develop in the upper section of the small intestines-the duodenum.
Oesophageal ulcers– They develop inside the oesophagus.

Causes

Stress which stimulates acid production
Caffeine
Certain medication like aspirin
Also, can be inherited

SIGN AND SYMPTOMS OF STOMACH ULCERS

Burning pain in the stomach
Nausea and vomiting
Tiredness and weakness
Blood in vomit or stool

Control measures to stomach ulcers

Avoid smoking
Have stress reduction programmes
Avoid taking alcohol
Medication
Avoid eating acidic foods

CONSTIPATION

Constipation – Is the passing of hard, dry bowel motions (stools) that may be infrequent or difficult to pass.

Causes

A diet low in fibre such as roughages
Abnormally slow movement of faeces matter through colon, as a result maximum absorption of water takes place leaving very dry hard faeces
Failure of the sensory cells to signal the presence of faecial matter in rectum

Signs and symptoms of constipation

Hard stools
The urge to go for long call even after you have just been to the toilet
Lack of bowel movements for two or three days

Prevention of constipation

Eat enough fibre
Seek medical help
Drink enough water
Exercise regularly

FLATULENCE

Flatulence – Is the buildup of gas in the digestive system that can lead to abdominal discomfort.

Causes

Swallowing more air than usual
Eating food that causes gas such as beans, cabbage, milk and onions
Poor absorptions of carbohydrates.

Symptoms of flatulence

The constant urge to pass wind
Excessive belching
Accumulation of gas in the stomach
Abdominal pain

Prevention of flatulence

Avoid foods that produce gas
Chewing food properly i.e eating slowly
Using anti acids
Avoid milk if you are lactose intolerant.

HEART BURN

Is the burning feeling in the chest and a sour or bitter taste in the mouth. It is caused by flowing back of stomach content into the oesophagus. The flowing back occurs when the cardiac sphincter does not close completely as a result of eating spicy food, acidic food, foods with too much fat, very cold food and carbonated drinks.

Signs and symptoms

A burning feeling in the chest and throat
Difficult swallowing
Sour or bitter taste in the mouth

Effects of heartburn

It causes discomfort
It can lead to diseases like ulcers and cancer of the oesophagus.

Treatment and prevention

Chronic heartburn can be treated with medications that prevent the production of acid in the stomach.
Stop smoking and alcohol consumption and use of foods which acerates heartburn
Take meals a few hours before going bed.

PROPERTIES OF FOOD SUBSTANCE

Basic food substances:

Carbohydrates
Protein
Lipids

CARBOHYDRATES (ALSO KNOWN AS SACCHARIDES)

The word carbohydrates comes from two Greek words i.e: Carbo + hydrates where Carbo means carbon and hydrates means water. It is the food substance made up of carbon, hydrogen, and oxygen.

Sources of carbohydrates

The following are some of the food that contains nutrients of carbohydrates:

Maize
Rice
Cassava
Potatoes
Bananas e.t.c

Types of carbohydrates

Monosaccharide eg glucose, galactose and fructose (fruit sugar)
Disaccharide eg maltose, sucrose (cane sugar) and lactose (milk sugar)
Polysaccharide eg starch, glycogen, chitin and cellulose

Monosaccharide:

Monosaccharide is also known as simple sugar. They include glucose, fructose (fruits sugar) and galactose. These sugars dissolve in water and form sweet solutions. Monosaccharides are also called reducing sugars.

The general formulae for monosaccharide is (CH₂O)n

Where “n” represents a number of carbon atoms and have a value between 3& 6.Example the formulae of glucose is (C₆H₁₂O₆). Other examples of reducing sugars are fructose(fruit sugar), galactose, and etc.

Characteristics of monosaccharides

They are sweet.
They dissolve in water to form sweet solution.
They are crystalline.
They are all reducing sugar.

Disaccharides:

This is the kind of carbohydrate which is formed when two monosaccharide molecules condense and release molecules of water. Their general formula is C₁₁ H₂₂ O₁₁

i. Maltose is formed when two molecules of glucose condense.

Glucose + glucose = maltose + water

ii. Sucrose is formed by condensation of glucose and that of fructose.

Glucose + fructose = sucrose + water

iii. Lactose is found by condensation of galactose molecule and glucose molecule

Glucose + galactose = lactose + water

Characteristics of disaccharides

They are sweet.
They are crystalline.
They dissolves in water to form sweet solution.
Some are reducing sugar (maltose and lactose), and some are non-reducing sugar (sucrose)
They can be hydrolysed by an enzyme or dilute hydrochloric acid.

Polysaccharides:

These are formed when several units of monosaccharides are linked together.

Examples starch, glycogen, chitin, and cellulose.

Example of polysaccharides

1) Starch

Starch is stored in plant cell, in the muscles and liver of vertebrates as (glycogen) in exoskeleton of arthropods and fungal cell as chitin

2) Cellulose

It forms the building material of the plant cell walls.

Source of Carbohydrates

Cereals – e.g. maize, rice, wheat

Sugar – e.g. honey, sugar cane, glucose, sweet fruits.

Characteristics of polysaccharides:

They are not soluble in water
They are non-reducing sugar
They form suspensions with water
They are not sweet
They are non-crystalline.

Functions of carbohydrates (starch, reducing, and non-reducing sugar).

Used as a source of energy in the body.
Regulation of blood glucose in the body.
Dietary fiber.
Biological recognition processes.
Provides flavor and sweeteners. eg sucrose, fructose, and glucose.
They form important component of cell membrane.

PROTEINS

Sources of protein

Egg, Irish potatoes, Beans, Meat, Milk, Fish e.t.c.

Functions of proteins

– Used as a source of energy to the body.
– Used for tissue growth.
– Used for repair the damaged tissue.
– Used in the formation of some tissues. eg hair, nail e.t.c
– They form part of the cell structure.
Lack of proteins in the body can cause disease called Kwashiorkor.

The digestion of proteins in the alimentary canal takes place at the Stomach and duodenum.

At the duodenum, the enzyme involved in the digestion of protein is called Trypsin.

At the stomach, the enzyme involved in the digestion of protein is called Pepsin.

The end- product of digestion of protein is Peptides.

LIPIDS

Lipids includes Fats and Oils.

Sources of lipids

Groundnuts
Coconuts
Avocado
Milk
Meat e.t.c

Functions of lipids

Used as a sources of energy in the body.
They protect internal organs like hear, kidney etc.
Insulate the body against heat loss.
They assist mechanism of buoyance in marine organisms.
They are important component of cell membranes.
Excessive amount of lipids in the body can cause Obesity.
Lack of lipids in the body can cause the deficiency of vitamin: A, D, E, K etc

* The digestion of lipids in the alimentary canal takes place at the Duodenum.

* The enzyme involved in the digestion of lipids is called Pancreatic Lipase.

* The end- product of digestion of lipids is Fatty acids and Glycerol.

FOOD TEST

Food test: Is the systematic procedures to find out the food substances present in a given food sample.

The process accomplished by the use chemical reagents.
The absence/presence of food substance tested is indicated by the colour changes in the given sample.

Chemical reagents used during food test

Iodine solution, Benedict’s solution, Hydrochloric acid (HCl), Sudan III solution, Sodium hydroxide (NaOH)/potassiumhydroxide(KOH)/solution or Sodium hydrogen carbonate(NaHCO₃), 1%Copper II sulphate solution.

Food tested	Reagent used	Observations
Starch	Iodine solution	Blue-black colouration
Reducing sugar	Benedict’s solution and heating	Series of colours from green, yellow, orange, brick red
Non reducing sugar	Dilute HCl, NaOH/KOH/NaHCO_3,Benedict’s solution and then heating	Series of colours from green, yellow, orange, brick red.
Proteins	Sodium hydroxide solution (NaOH)/potassium hydroxide solution (KOH), 1% copper II sulphate solution (CuSO₄).	Violet/ purple colour
Lipids	Sudan III solution.	Red droplets. will appear on the surfaces

NOTE:

– The function of hydrochloric acid is to neutralizes disaccharides to monosaccharides.

– The function of Sodium hydrogen carbonate is to hydrolyses the acid.

PRINCIPLES OF WRITTING FOOD TEST PRACTICAL REPORT:

It must have four parts in a tabular form: Food tested ( test for), Procedure, Observation, and Inference.
Passive past tens must be used instead of present tense.
If not guided, the food must be tested in a series of: Starch, Reducing sugar, Non- reducing sugar, Proteins, and the last is Lipids.
Use specific amount of sample solution and reagents; eg 2mls or two drops and not few mls or few drops.
It must have a heading at the top.
If reducing sugar present, no need for testing non- reducing sugar.

NB: During Reporting

Use of past tense
Don’t command eg put, add, take etc
Quantity of the contents such as 3mls, 2mls, 1mls

NB: Assume you are provided with sample solution W

Food tested	Procedure	Observation	Inference
Starch	2drops of iodine solution were added in a test tube containing 2mls of sample solution W.	Colour of the food sample solution W changed to blue black	Starch was present in food sample solution W
Starch		The food sample solution W retained the brown colour of iodine solution	Starch was absent in sample solution W
Reducing sugar	2mls of Benedict’s solution were added in a test tube containing 2mls of food solution W. Then, the mixture was heated to boil in a water bath.	A series of colour change from blue to green, to yellow, to orange and finally to brick red precipitate was observed in sample solution W	Reducing sugar was present in sample solution W
Reducing sugar		The sample solution W retained the blue colour of Benedict’s solution.	Reducing sugar was absent in sample solution W
Non-reducing sugar	1ml of dilute HCl was added in a test tube containing 2mls of sample solution W. Then the mixture was boiled and allowed to cool. Then 1ml of Sodium hydroxide was added to the cooled mixture followed by 2mls of Benedict’s solution. Then the mixture was boiled again.	A series of colour change from blue to green, to yellow, to orange and finally to brick red precipitate was observed in sample solution W	Non-reducing sugar was present in sample solution W
Non-reducing sugar		The food sample solution W retained the blue colour of Benedict’s solution.	Non-reducing sugar was absent in sample solution W
Protein	1mls of sodium hydroxide solution were added in a test tube containing 2mls of tested food sample solution W followed by 1 mls copper (II) suphate solution. Then, the mixture was shaken well/vigorously shaken and allowed to settle.	The purple colour was observed in the food sample solution W	Protein was present in sample solution W
Protein		The food sample solution W retained the light blue colour of 1% copper (II) suphate solution	Protein was absent in sample solution W
Lipids	2drops of Sudan (III) solution were added in a test tube containing 2mls of food sample solution W. The mixture then was shaken and allowed to settle.	Red ring was formed at the top of the food sample solution W	Lipid was present in food sample solution W
Lipids		The food sample solution W retained the colour of Sudan (III) solution.	Lipid was absent in sample solution W

TOPIC 2: DIGESTIVE SYSTEM IN HUMAN – BIOLOGY FORM TWO NOTES

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