Digestives system of human


This  blog post provides readers with the following objectives. The reader will be able to:
  • Explain the concept Nutrition.  
  • Classify nutrients found in food.
  • Demonstrate the presence of various nutrients found in food.
  • Explain the importance of dental care in humans.
  • Explain the concept of balanced diet and malnutrition. 
  • Explain the concept enzymes.
  • Describe the process of digestion and absorption
  • Describe the structure and functions of the liver.



Nutrition is the process by which organisms make or obtain food and utilizing it for growth maintenance. There are two types of nutrition. These are heterotrophic and autotrophic (holophytic) nutrition.


Food is any substance consumed to provide nutritional support for the body. Food contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, water or minerals. The substance is ingested by an organism and assimilated by the body cells to produce energy, maintain life, or stimulate growth.

Why Living Organisms Require Energy

The energy in the form of ATP released from oxidation of food substance is used for
1.      Maintaining a constant body temperature
2.     Electrical transmission of nerve impulses
3.      Synthesis of substances, such as proteins
4.   For transport of molecules against concentration gradients
5.    Vital functions of organisms such as growth, excretion, movement, reproduction etc. 


A carbohydrate is an organic compound that consists of only carbon, hydrogen, and oxygen. The hydrogen and oxygen atoms are present in the ratio 2:1 (as in water). The general formula for carbohydrates is Cm(H2O)n, where m and n are whole numbers

Source of carbohydrates: fruits, sweets, plantain, cassava, potatoes, yam, rice, maize, bread, wheat etc.

Class of Carbohydrates

Three main groups of carbohydrates are monosaccharide, disaccharides, and polysaccharides.


Monosaccharides mono, means "one", and saccharide, means "sugar". The general chemical formula is CnH2nOn; where n is three or more carbon atoms. 

Monosaccharides with three carbon atoms (C3H6O3) are called trioses, those with four (C4H8O4) are called tetroses, five are called pentoses, six are hexoses, and so on.

Hexose sugar:
there are three types of hexose sugar; glucose (dextrose), fructose, and galactose. 

They have the same general formula C6H12O6, but different arrangement of atoms. They are the simplest form of sugar which the body absorb and assimilate. They are the building blocks (monomers) of disaccharides and polysaccharides.

Monosaccharides are usually whitish, crystalline solids, water-soluble, sweet taste and reducing sugar. They are reducing sugars because they reduce blue copper (II) ions to brick red copper (I) ions. i.e. cupric oxide to cuprous oxide.


Disaccharides contain two sugar units i.e. two monosaccharide units bound together by a condensation reaction or dehydration reaction.

Ø     The reaction leads to the loss of water molecule and formation of a glycosidic bond.
Ø     They have a general formula C12H22O11.
Ø     Common disaccharides are maltose, sucrose, and lactose.
Ø     Sucrose is composed of one glucose and one fructose molecule. It occurs in sugarcane, sweet fruits, and certain storage roots e.g. carrots.
Ø     Lactose is composed of one galactose and one glucose molecule, occurs in the milk of all mammals.
Ø     Maltose is made up of two glucose molecules. Maltose occurs in grains.
Ø     Disaccharides can be classified into two types; reducing and non-reducing.
Ø     Reducing disaccharides are maltose, lactose and non-reducing disaccharides: sucrose.
Ø    Disaccharides are white, crystalline, sweeter and more water-soluble than monosaccharides


 Polysaccharides are polymers formed by condensation of several monosaccharide units.  

       Polysaccharides are not sweet, not crystalline and insoluble in water.
       They have a general formula of (C6H10O5)n where n is a large number.
      Examples include storage polysaccharides such as starch and glycogen, and structural polysaccharides such as cellulose and chitin.
       Plants store carbohydrates in the form of starch.
       Cellulose is found in the cell walls of plants.
       Chitin is found in arthropod exoskeletons and in the cell walls of some fungi.

Disaccharides and polysaccharides are broken down by hydrolysis into monosaccharides.

Functions of Carbohydrates

      Provide energy for cell activities

     Form structural components in plants and arthropods (cellulose, lignin, pectin).

     Form components of coenzyme and genetic molecule

    Take part in biological transport, cell-cell recognition, activation of growth factors.

□  Required in biosynthesis of proteins and lipids.   

Test for Carbohydrates

Test for Starch

Iodine test: is used to test for the presence of starch. Iodine solution reacts with the starch producing a blue-black color.

Test for Reducing Sugars

Benedict's or Fehling’s test: The food sample is dissolved in water, and a small amount of Benedict's reagent or Fehling’s reagent is added. The mixture is heated in a water bath for few minutes. A brick red precipitate signifies the presence of reducing sugars.

Test for Non-Reducing Sugar

The food sample is dissolved in water and heated with dilute hydrochloric acid to hydrolysis it into glucose and fructose. Dilute sodium hydroxide is added to neutralize the mixture until it stops fizzing. A positive results of the product with Benedict's or Fehling’s test indicates the presence of disaccharide non-reducing sugar (sucrose).

Test for Carbohydrates


Proteins are complex organic compounds that contain carbon, oxygen, hydrogen, nitrogen and sometimes Sulphur and phosphorus.

Chemical structure for Amino acids

□    Proteins are long-chain molecules built from small units known as amino acids.
     Amino acids are joined together with peptide bonds.
    Amino acid has two functional groups; a free carboxyl group (-COOH) known as carboxyl terminus and free amino group (-NH2) known as the amino terminus.

Condensation reaction of Amino acids to form protein

  Essential amino acids are amino acids that the body cannot synthesize on its own. Essential amino acids must therefore be obtained from food.

  Non-essential amino acids are amino acids that the body can manufacture on its own.

  Animal proteins such as meat, fish, eggs, milk contain all the essential amino acids and they are known as first class protein.

 Plant protein contains low amount of essential amino acids and they are described as second-class protein.

Sources of Proteins: meats, milk, fish, eggs, beans, nut butters, cowpea, crayfish and wheat.

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Functions of Proteins

      Provision of energy.
□    Production of enzymes and hormones.
      Formation structural protein e.g. keratin and collagen
□   Formation of tendons and cartilage
□    Production of transport proteins such as hemoglobin and lipoproteins
      Production of antibodies
     Proteins are involved in muscle contraction and movement e.g. actin and myosin.
      Repair body cells and make new ones. 
      For growth

Test for Proteins

     Biuret test: An aqueous food sample is treated with an equal volume of dilute sodium or potassium hydroxide, followed by a few drops of 1% copper (II) sulphate. If the solution turns purple, protein is present.
    Millon's test: A few drops of the reagent are added to the test solution, which is then heated gently. A reddish-brown color indicates the presence of proteins


Biuret test for proteins


Lipids are large organic compounds containing carbon, hydrogen and oxygen. The proportion of oxygen is far smaller. Lipids includes: fats and oils, phospholipids and steroids.

   Fats and oils are a subgroup of lipids called triglycerides or triacylglycerols.
  Triglycerides are esters made up of 3 molecules of fatty acids and 1 molecule of glycerol.
    Fatty acids are long chains of hydrocarbon making fats non-polar (insoluble in water).
 Triglycerides that are solid at room temperature are classified as fats, and occur in animals. They contain saturated hydrocarbons. .
 Triglycerides that are liquid at room temperature are called oils and originate in plants. Oils contain unsaturated hydrocarbon.
    Unsaturated fats are derived from fatty containing double bonds within carbon chain. Unsaturated fats can be converted to saturated ones by the process of hydrogenation.
     Hydrolysis of lipids yields fatty acids and glycerol.

Source of lipids: egg yolk, margarine, butter, soya bean, groundnut, coconut oil, palm oil. 

combination of fatty acids and glycerol to form tryglyceride (fat and oil)

Functions of Lipids

   Fats play a vital role in maintaining healthy skin and hair,
      Insulate body against heat loss and maintaining body temperature.
     Fats serve as a source of energy, containing about 37.8 KJ per gram of fat.
   Protect vital organs such as heart, lungs and intestines.
     Vitamins A, D, E, and K are fat-soluble, (meaning they can only be digested, absorbed, and transported in conjunction with fats).
     Lipids also form the basis of steroid hormones.
   Phospholipids are the main constituents of cell membranes, regulating transport of substance in and out of the cell.

Test for Lipids

     Grease spot test/Brown paper test: Use a cotton swab to put samples of lipids on brown paper bag. Wait for few minutes to dry. Hold the paper up to a light source. The spot becomes translucent (allows light to pass through).

  Sudan III (or Red) test: Add 2ml of any oil and 2ml of water to a test tube. Then add 2-5 drops of Sudan III to the mix. Shake. Sudan III will stain the fat molecules red.


Water is a chemical compound with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds.

     It is tasteless and odorless.
     It provides no calories (energy) or organic nutrients.
     Water is a good polar solvent and is often referred to as the universal solvent.
 Substances that dissolve in water are known as hydrophilic e.g., salts, sugars, acids, alkalis and oxygen while those that are immiscible with water e.g., fats and oils are known as hydrophobic.

Sources of water: water in drinks pineapple, coconut, water melon, water in plants

Functions of Water

    It gives a medium for chemical reactions to take place.
□   It regulates the body temperature.
□   Water forms the major component of the blood and lymph
    Water also helps the blood carry oxygen and nutrients to all parts of the body.
      It forms the major component of digestive juice.
    Water helps formation of urine.
□   It helps removal of waste products.
□  For maintenance of osmotic balance.



Vitamin is an organic compound required by an organism as a vital nutrient in limited amounts. The name is from vital and amine, meaning amine of life. Vitamin cannot be synthesized by an organism, and must be obtained from the diet.

Vitamins are classified as either water-soluble or fat-soluble.

Ø    Fat-soluble vitamins: include A, D, E, and K. They are absorbed through the intestinal tract with the help of lipids (fats).

Ø    Water-soluble vitamins: are vitamin B complex and vitamin C. They dissolve easily in water, they are not stored and readily excreted from the body by the kidney. Many types of water-soluble vitamins are synthesized by bacteria. 


Food Sources



Vitamin A (Retinol)

orange, leafy vegetables, carrots, egg-yolk, milk, fish

healthy teeth, bones, soft tissue, and skin


Vitamin B1 (Thiamine)

pork, oatmeal, brown rice, vegetables, potatoes, liver, eggs, palm wine

Carbohydrate metabolism, aid in cellular respiration, aid in synthesis of ribose


Vitamin B2 (Riboflavin)

popcorn, milk, beans, green vegetables, mushroom, egg

for growth; production of red blood cells

Skin and corneal lesions

Vitamin B3 (Niacin)

Same as B2

maintain healthy skin and nerves; has cholesterol-lowering effects

Pellagra, Liver damage

Vitamin C (Ascorbic acid)

fruits and vegetables, liver

maintain healthy tissue; promotes wound healing, 


Vitamin D (Calciferol)

fish, eggs, liver, mushrooms, fortified cereals, fortified milk, cheese, yogurt, butter

helps the body to absorb calcium; maintenance of healthy teeth and bones


Vitamin E (Tocopherols)

fruits and vegetables, nuts, seeds, papaya, mango, oil

formation of red blood cells, spermatogenesis, antioxidant

Sterility in males, anaemia, miscarriages

Vitamin K (phylloquinone)

leafy vegetables, egg yolks, liver, cereals, fish, beef

important in blood clotting

Bleeding diathesis

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Source of vitamins and minerals


Mineral nutrients are the chemical elements required by living organisms, other than carbon, hydrogen, and oxygen that are present in organic molecules.

1.       Major Minerals present in human body include; calcium, phosphorus, potassium, sulfur, sodium, chlorine, and magnesium.
2.     Minor minerals necessary for human life; include iron, cobalt, copper, zinc, molybdenum, iodine, and selenium.






Legumes, potato skin, tomatoes, bananas, yams, soybeans

heart and nerve health, is a systemic electrolyte

muscle cramps


Table salt (sodium chloride)

production of HCl in the stomach and maintenance of tissue fluid

Mental apthy, muscle cramps


Table salt, vegetables, milk

is essential in co-regulating ATP with potassium; transmission of nerve impulses

Mental apthy, muscle cramps


Bones, eggs, fish, green leafy vegetables, nuts

is needed for muscle, heart and digestive system health, builds bone, supports synthesis and function of blood cells

Rickets, stunted growth and osteoporosis.


Red meat, fish, poultry, bread, rice, oats

required component of bones, essential for energy processing, formation of nucleic acids



Nuts, soy beans, cocoa mass, vegetables, fruit juice, beans,

required for processing ATP and transmission of nerve impulses

Hypertension (high blood pressure)


Red meat, leafy green vegetables, fish (tuna, salmon), eggs, dried fruits, beans

required for many enzymes, and for hemoglobin and some other proteins



Iodized salt, salt fortified with iodine, sea fish

required for the synthesis of thyroxine

Goitre, reduced growth


Eggs, meat, milk, fish,

for synthesis of amino acids and many proteins (skin, hair, nails, liver, and pancreas)

No deficiency disease known


effect of iodine deficiency


Balanced diet is a diet which contains all the essential food nutrients in their correct proportion. Such diet must provide enough energy, enough material for growth and replacement of worn-out tissues and must give resistance against disease. 

The constituents of balanced diet are roughly in the following proportions:


 about 50%





Minerals and Vitamins

a very small proportion but very necessary


large amount


It is the type, number and arrangement of teeth in a given species of organism.

Type of Dentition

1.     Homodont dentition: is when teeth are of the same size and shape. e.g. as in reptiles, amphibians, and fish.

2.      Heterodont dentition: teeth that differ morphologically e.g. teeth in mammals.   

Set of Teeth

o  Deciduous Teeth or Milk Teeth: is a set of teeth found in infants, which fall out and replaced by permanent teeth. They are smaller and fewer in number.

o   Permanent teeth: is a second set of larger teeth found in the adult that succeed the milk teeth. 

Ø   Monophyodont: is a dentition of animals with only one set of teeth throughout life. 

Ø   Diphyodont: is a dentition in animals that have two sets of teeth. E.g. humans. The first set (milk teeth) normally starts to appear at about six months of age. It has the same dental formula as the permanent teeth except the absence molars.

Ø   Polyphyodont: is a dentition of animals in which the teeth continuously discard and replaced.

Types of Teeth

There are four distinct types of teeth in mammals:


Ø    occupy the front part of the upper and lower jaws
Ø     they are flat, chisel-shaped teeth 
Ø     each has single root
Ø      used for cutting or gnawing food


o     occur immediately behind the incisors
o     they are pointed, conical shaped teeth
o     project slightly beyond the incisors
o     used for stabbing or piercing and tearing flesh or meat

Premolars (also known as bicuspids)

o    they are small but have broad crowns
o    the broad crown has a small pointed ridges called cusps
o    have one or two roots. 
o    used for chewing or grinding food and crushing bones


·         they are broad with cusps
·         have two or three roots
·         used for chewing or grinding food and for crushing or cracking bones
·         the last molar teeth in each jaw to appear in humans are known as wisdom teeth
·         the premolar and molar are called cheek teeth


Structure of Tooth 

Vertical section through canine and molar


Ø      It’s the hardest substance made by animals
Ø      It is a non-living substance containing calcium phosphate
Ø      It forms a hard biting surface.
Ø      It protects the dentine and pulp cavity.


o      It is bone in structure and occupies the greatest part of the tooth
o      It is secreted by the odontoblasts of the dental pulp
o      It decays more rapidly and subject to severe cavities if not properly treated
o      It encloses the pulp cavity and also supports the crown

Pulp Cavity

o      It is the central part of the tooth filled with soft connective tissue
o      It contains sensory nerve-endings and blood capillaries.


o      It is a thin layer of bone-like material covering the root of the tooth
o      it fixes the root to the jaw bone
o      it allows the periodontal membrane to attach to the tooth for stability

Periodontal membrane

·         a tough fiber found between the cement and the jaw bone.
·         it allows the tooth to moves slightly in its socket during biting and chewing
·         it acts as ‘sock or pain absorber’ during chewing.


Dentition of Herbivores

Herbivores are animals that feed on plant materials, e.g. sheep, goat, rat, rabbit, cow and donkey.

Adaptive Features of Herbivores 

o      well-developed incisors for cutting plant material
o      presence of diastema which allow the tongue to manipulate the grass
o      presence of upper horny pad which allow the lower incisors to cut through vegetation
o      cheek teeth are cusped for grinding and chewing grass
o      side-to-side movement of the lower jaw enable chewing of food (as the teeth grind sideways, the enamel is worn out, exposing the dentine)
o      have open roots ensure continuous growth of the incisors to replenish the worn-out parts

N/B: Diastema is a large gap between the incisors and premolars as results of absence of canines.

Skull of a sheep

Dentition of Carnivores

Carnivore is an organism that feed on flesh and bones of animals often captured alive e.g. dog, lion, cat, tiger. Most carnivores are cannibals; they eat the flesh of their own kind.

Adaptive Features of Carnivores

o      small, closed fitting incisors for grooming
o      canines are large, long and pointed for piercing and holding prey
o    premolars are sharp and pointed for tearing flesh
o      the last upper premolar and first lower molar has modified to form the carnassial teeth for shearing meat.
o      cusped premolars and molars for cracking bones
o      powerful jaw muscles and up and down jaw action avoid dislocation

Dentition of Omnivores

Omnivores feed on both plants and animals e.g. human beings, apes, monkeys, bush pigs and bear. The teeth of omnivores have very little specialization. They have a combination of carnivore and herbivore teeth characteristics.

Check Out Teeth Models for Teaching Dentition

Dentition of omnivores      Dentition of omnivores

Adaptive features of omnivores

o         large and well-developed incisors for cutting plant material
o         long, bluntly pointed canine for tearing flesh or meat
o         cheek teeth have flat broad surface with cusps for grinding and crushing food.
o         the condyle allows up and down movement with little side to side lower jaw movement

Dentition of herbivore (sheep)

structure of carnivore and omnivore dentition

images credited to D G Mackean

Dental Formula 

Dental formula is an expression of the number, type and the arrangement of teeth in one half of the upper and lower jaws. The different types of teeth, denoted as I, C, Pm, and M (incisors, canines, premolars, and molars respectively). Dental formula is written in the form of a fraction; I.C.Pm.M.  The number of teeth of the upper jaw are listed above those of lower jaw below.                     I.C. Pm.M

primary teeth

Dental Care

o      Practice oral hygiene to keep the mouth clean and to prevent bad breath
o     Regular brushing, at least twice a day to avoid formation of plaque.
o     Use fluoride toothpaste or mouthwash to prevent dental decay.
o     Avoid sugary foods.
o      Visit dentist regularly.
o      Eat food containing calcium, phosphorus, vitamin B, C, D for strong and healthy teeth.
o     Avoid biting objects or nails to prevent cracking.
o     Avoid using toothpicks which may lead to gum disease.
o     Avoid eating too hot or too cold foods to prevent cracking.

Dental Diseases

Dental diseases may affect the teeth, the gums, or other tissues of the mouth. Dental diseases can affect the ability to chew, smile, or speak properly and even lead to loss of tongue or teeth.

o  Plaque: is a biofilm of saliva, mucus, bacteria food residues that form on the teeth. Plaque buildup, lead to tooth decays or periodontal problems.

Tooth Decay (Dental Caries)

Tooth decay is caused by acid-producing bacteria. They cause damage in the presence of sugar by producing lactic acid. The acid dissolves the calcium and phosphorus in the enamel. This process is known as "demineralisation" and it leads to tooth destruction.

Causes of Tooth Decay

·         Eating a lot of sugary food
·         Lack of calcium in the diet
·         Lack of vitamin D
·         Failure to brush teeth after meal
·         Not chewing hard foods.

Symptoms of Tooth Decay

o      Pains
o      Uncomfortable when eating food, dinking hot or cold drink
o      Plaque and hole buildup in the enamel

Treatment of Tooth Decay

o  Filling the hole with dental amalgam.
o  In more serious case tooth may have to be extracted
o  Treat with fluoride rinse and fluoride toothpaste to remove the plaque.

Periodontal Disease

This is the inflammation of the periodontal membrane and the cement as a result of bacterial infection. The bacteria eat the gum tissue, causing it to pull away from the teeth. This space between the tooth and gum, called periodontal pocket, can traps even more bacteria.

Causes of Periodontal Dises

o   Buildup of plaque on the teeth.
o   Lack of vitamin E in diet
o   Improper cleaning of teeth

Symptoms of Periodontal Disease

o      Bleeding          
o Inflammation 
o      Swelling          
o Deep red color of the gums 

Treatment of Periodontal Disease

If the disease is not too severe it is possible to be treated with fluoride rinse and fluoride toothpaste to remove the plaque, but once the infection has progressed antibiotics will be needed to kill the bacteria.

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Treatment of Tooth Decay



Enzymes are biological catalysts which are protein in nature that speed up the rate of metabolic reaction in biological systems without themselves being altered in the process.

o    Enzymes contain non-protein components known as coenzymes, or cofactors which are essential for their function. E.g. metal ions or water soluble vitamins.

o   Enzyme (inactive) without its cofactor is known as the apo-enzyme and the active form i.e. enzyme plus cofactor is called the holo-enzyme.

Types of Enzymes

1. Exoenzyme: is an enzyme which is secreted by a cell and works outside the cell that produced it. It is usually used for breaking up large molecules that would not be able to enter the cell. E.g. Digestive enzymes and some clotting factors.

2. Endoenzyme: is an enzyme that functions within the cell that produced it was. E.g. dehydrogenases and decarboxylases. 

Characteristics of Enzymes

o  Enzymes are proteins in nature.

o  They are substrates specific (e.g. lipase is for lipids).

Enzymatic reactions are reversible.

o Enzyme activity may be decrease by inhibitors.

o  Remain chemically unchanged at the end of a reaction.

o  Required in small quantities.

o  The rate of enzyme reactions is influenced by enzyme concentration.

o  Some are inactive and requires co-enzymes to activate them.

o Activity is affected by temperature, pressure, pH and the concentration of substrate.

Factors Affecting Enzymes

How pH Affect Enzyme Reaction

Enzymes are affected by changes in the pH. The most favorable pH, at which an enzyme exhibits its maximum activity, is known as the optimum pH. Extremely high or low pH values generally result in complete loss of enzymatic activity. The optimum pH value varies greatly from enzyme to enzyme.

effect of pH on enzymatic reaction

How Temperature Affect Enzyme Reaction

The rate of an enzyme reaction increases as the temperature is increased. A 10°C rise in the temperature will increase the activity of most enzymes. Most enzymes are denatured (lose catalytic activities) at temperature above 40°C. Enzymes lose their activity at extremely low temperatures as well.

effect of temperature on enzymatic reaction

Effect of Enzyme Concentration on Enzyme Reaction

The rate of an enzyme reaction depends upon its own concentration. The rate of reaction is directly proportional to the enzyme concentration if the substrate concentration is fixed. As the amount of enzyme is increased, the rate of reaction increases. If there are more enzyme molecules than the amount needed, reaction rate increases as enzyme concentration increases but it levels off.

relation between enzyme concentration and enzyme reaction

Effect of Certain Molecules on Enzyme Reaction

Inhibitors and activators: Many molecules affect the rates of enzyme reactions. Some molecules bind to the enzyme or the substrate or the enzyme-substrate complex and lower the reaction rate. These are known as inhibitors. Similarly, some bind to the enzyme molecule and increase the reaction rate. These are known as activators.

"Lock and Key" Model

Enzymes are substrates specific. This is because both the enzyme and the substrate possess specific geometric shapes that fit exactly into one another. This is often referred to as the "lock and key" model.  An enzyme, as the key have multi-dimensional shape, with pockets lined with specific amino acids. The particular pockets that are active in catalyzing a reaction is called the active sites of the enzyme. The active site is continuously reshaped by interactions with the substrate. The active site is molded into the precise position that enables the enzyme to perform its catalytic function. 

diagrams illustrating Lock and key model of enzymes

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It is a complex process that breaks down large organic molecules into smaller particles that the body can utilize as fuel. 

It consists of alimentary canal and its accessory glands.

Digestive system is separated into the following steps:
1.   Ingestion: placing food into the mouth (entry of food in the digestive system).
2.  Mastication: the chewing of food into powder.
3.  Deglutition: the swallowing of food.
4.  Peristalsis: the rhythmic contraction and relaxation of the gust muscles which move food through the gastrointestinal tract or gut.
5.   Digestion: is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones.
6.   Absorption: the process whereby soluble end products of digestion pass through the intestinal walls into blood and lymph by osmosis, active transport and diffusion.
7.   Assimilation: the process by which the absorbed food is used by the body cells and tissues.
8.   Egestion: Removal of undigested materials from the digestive tract through defecation.

Aid Teaching with this Digestive Model 
Stomach Large Intestine Cecum Rectal Anatomy Model Human Digestive System Teaching model

Gastrointestinal Tract (Alimentary Canal) 

The alimentary canal is a muscular tube that runs from the mouth to the anus. It consists of the following parts: 

Mouth (Oral Cavity)

Mouth is an organ for receiving food and breaking up large organic compounds. In the mouth, food is digested mechanically by biting and chewing. 
It has a tongue on its floor.

Function of the Tongue

o      manipulates food for chewing and assists in swallowing
o      it has a touch sense for tasting food
o       it allows the mixing of food with saliva
o      it rolls food particles into a bolus before swallowed


The oesophagus is commonly known as the gullet.
It is a narrow muscular tube which connects the mouth to the stomach.


It is a small, 'J'-shaped pouch, which stores food temporally It is located on the left upper part of the abdominal cavity. It has two sphincters; the oesophageal or cardiac sphincter, found in the cardiac region and the pyloric sphincter dividing the stomach from the small intestine.

The Role of Stomach in Digestion

o      It helps in temporal storage of undigested food.
o      Production of HCl.
o      Production of gastric juice which contains pepsin for protein digestion.
o      The juice contains rennin for coagulation of milk protein.


The intestine or bowel extends from the pyloric sphincter of the stomach to the anus.  It consists of two segments, the small intestine and the large intestine.

The small intestine is further subdivided into the duodenum, jejunum and ileum.

The small intestine possesses numerous projections called villi (singular: villus).

Individual villus cells have microvilli which greatly increase absorptive surface area. Each villus contains blood vessels and a lacteal (lymph vessel).

Large intestine is subdivided into the cecum, colon and rectum.

The transverse section of the alimentary canal reveals;

o  Mucosa: is the innermost layer of the gut surrounding the lumen. It can be divided into the epithelium, lamina propria, and the muscularis mucosae. The epithelium and lamina are filled with gastric glands that secrete mucus, hydrochloric acid, pepsin and rennin. Muscularis mucosa is a thin layer of smooth muscle that contracts and relaxes to move the food forward.

o  Submucosa: consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves.

o   Adventitia or Serosa: consists of an inner circular muscle layer and a longitudinal outer muscle layer. The circular muscle layer prevents food from traveling backward and the longitudinal layer shortens the tract.

The coordinated contraction of these layers is called peristalsis.

Accessory Digestive Gland

An accessory digestive gland (or organ) is a gland that is not a part of the digestive tract. Accessory organs include the salivary glands, liver, gallbladder, and pancreas.

Salivary Glands

The three salivary glands (parotid, submandibular, and sublingual) secrete saliva, which contains enzymes that initiate breakdown of carbohydrates. 

The Role of Saliva in Digestion

o       For lubrication of food.
o      Aid easy movement and swallowing of food.
o      Contains ptyalin (salivary amylase) which convert cooked starch into maltose 
o   It is slight alkaline, providing correct pH for effective functioning of ptyalin.  


The pancreas secretes digestive enzymes into the duodenum through the pancreatic duct.  It contains groups of cells called Islets of Langerhans that secrete insulin and glucagon. The islets are endocrine glands because they are ductless.


o  It converts excess glucose to glycogen for storage in the muscle or liver.
o  It promotes the buildup of fats and proteins.
o  It also inhibits the use of fats and proteins as energy source.
o  Diabetes Mellitus is a disease in which there is insufficient or no insulin at all to convert excess glucose to glycogen for storage.
o  This results in high glucose levels in blood and glucose in the urine.


o   It raises the level of glucose in the blood by converting glycogen back to glucose.
o   It inhibits the uptake of glucose by muscle and other cells.
o   It promotes gluconeogenesis, the synthesis of glucose from amino acids or fatty acids.


The liver is a reddish-brown organ with four lobes of unequal size and shape. It is the largest internal organ (the skin being the largest organ) and the largest gland in the human body. It is located in the right upper quadrant of the abdominal cavity, resting just below the diaphragm. The liver lies to the right of the stomach and overlies the gallbladder. It is connected to two large blood vessels; one called the hepatic artery and other called the portal vein. The hepatic artery carries blood from the aorta, whereas the portal vein carries blood containing digested nutrients from the entire gastrointestinal tract, spleen and pancreas to the liver.

labelled drawing of the liver, Gall bladder and Pancreas

Function of the Liver

The liver has a wide range of functions, including;

o      Formation of red blood cells in a fetus.
o      Production of bile
o      The liver breaks down or modifies toxic substances in a process called detoxification.
o      The liver converts excess amino acids into urea in a process known as deamination.
o      The liver produces heat for body temperature regulation.
o      Decomposition of red blood cells.
o      Storage of a glycogen, vitamin and iron.
o      Storage of blood.
o      The liver produces albumin, the major component of blood serum.
o      It breaks down insulin and other hormones
o      The liver regulates blood sugar level by converting excess glucose into glycogen for storage and converts glycogen back to glucose when the level of blood sugar falls.
o      It synthesized organic compound such as glucose, cholesterol and lipoproteins.
o      It produces coagulation factors such as fibrinogen, prothrombin, and antithrombin.
o      Metabolism of carbohydrates, proteins and lipids.

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Digestion in the Mouth (Oral Cavity)

Digestion begins in the Mouth, where food is chewed or masticated with the teeth. Saliva is secreted by three pairs of salivary glands in the oral cavity, and is mixed with the chewed food by the tongue. Saliva moistens the food.  It contains digestive enzymes, salivary amylase, which breakdown starch into maltose. It also contains mucus that soften the food and form it into a bolus. An additional enzyme, lingual lipase, hydrolyzes fats/oil into partial glycerides and fatty acids. The tongue manipulates the food and roll it into a bolus which is swallowed into the stomach. The bolus moves by peristalsis through the oesophagus. 


Digestion in the Stomach

The bolus enters the stomach through the esophageal sphincter. The presence of food in the stomach stimulates the stomach walls to secrete a hormone called gastrin. Gastrin stimulates the gastric glands in the stomach to secrete gastric juice. The gastric juices contain hydrochloric acid, mucus and proteases (pepsinogen and prorennin).

o      HCl is produced by special cells of the gastric glands called parietal cells, pepsinogen by chief cells and mucus by mucus neck cells.
o     Rennin digests milk protein, caesinogen into caesin (curds) i.e. curdling the milk.
o    Pepsin breaks down proteins into peptides.
o    Mucus lubricates the food and also prevents HCl from acting on the stomach walls.

Functions of Hydrochloric Acid

o      kills or inhibits bacteria
o      provides the acidic pH for the proteases activity
o      converts the inactive pepsinogen into active form pepsin
o      stops the action of salivary amylase
o      maintains a pH in the stomach of approximately 2.0
o      dissolves bone accidentally swallowed

The food is partially digested and semi-liquid, referred to as chyme. Chyme slowly passes through the pyloric sphincter and into the duodenum of the small intestine.

Digestion in the Small Intestine

In the duodenum, acidic chyme from stomach triggers the cells of the duodenum to produce secretin. Secretin stimulates the pancreas to produce pancreatic juice. It also stimulates the liver to secrete bile. The presence of food in the duodenum also stimulates the release of another hormone called cholecystokinin, which function the same way as the secretin.

Pancreatic Juice

Pancreatic juice contains
o   Pancreatic amylase digests starch to maltose
o   Trypsin and Chymotrypsin digest proteins to peptides.
o   Pancreatic lipase digests fats to glycerol and fatty acid.
o   Sodium bicarbonate which neutralizes the acidic chyme from the stomach.


Bile is a greenish-yellow fluid that contains water, salts and bile pigment (bilirubin, biliverdin), but has no enzyme. It is produced by the liver when worn out red blood cells are broken down. It is then stored in the gallbladder where it released via the bile duct.

Functions of Bile 

o    Bile emulsifies fats (separates it into small droplets which then acted upon by lipase).
o     It neutralizes the acidic chyme 
o    Eliminates waste products such as bilin and bile acids from the blood.
o    Bile pigment gives faeces their brown colour

Intestinal juice

Intestinal juice (succus entericus) is secreted by the intestinal glands in the small intestine. It contains large number of enzymes such as enteropeptidase, erepsin, trypsin, lipase, maltase, lactase and sucrase etc. which complete the process of digestion.

The Table Below Shows Digestive Enzymes Grouped by Source of the Enzyme



Food (Substrates)


Mouth (salivary glands)

Salivary amylase







Milk protein (caseinogens)




Pancreatic amylase

Trypsin & chemotrypsin









Fatty acid and glycerol

Small Intestine


















Amino acids

Glucose and fructose

Glucose and galactose

Fatty acid and glycerol

Amino acids


Sugar, phosphorus and nitrogen base

Large Intestine

The large intestine absorbs water from the chyme. Food products that cannot go through the villi form feces. Faeces contain cellulose (dietary fiber), water, mucus, dead cells, bile pigments and bacteria. Bacteria of large intestine metabolize bilirubin to urobilinogen, which is responsible for the brown color of feces.


Most absorption in the small intestine occurs in the duodenum.

Adaptation of Small Intestine for Absorption of Nutrients

       Long tube to provide large surface area for absorption
       Presence of small, finger-like structures called villi, with numerous microvilli increase surface area for absorption.
       Thin epithelial cells of the villi (one cell thick), allow easy diffusion of nutrients.
       Epithelial cells secrete mucus to prevent friction
       Epithelial cells contain numerous mitochondria which provides energy/ATP for active transport
       Lacteal (lymphatic vessels) to transport fatty acids and glycerol.
       Rich network capillaries supply blood to keep a concentration gradient.

Structure of small intestine

Structure of villus and microvillus
Diagram of a villus

Absorption of fatty acids and glycerol

Fatty acid and glycerol enter the villi by diffusion. In the villi, they reassembled into fats (triglycerides) and package into chylomicrons or liposomes. They move into the lacteals for transport to the rest of the body through the lymphatic system.

Absorption of Glucose and Amino acids

Diffusion and active transport is responsible for the movement of glucose and amino acids. Villus capillaries collect amino acids and glucose taken up by the villi into the blood stream. Blood containing the nutrients is transported via the hepatic portal vein to the liver for filtering, removal of toxins, and nutrient processing. 


Assimilation is the process by which the absorbed food is used by the cells and tissues of the body i.e., the metabolism of end-products of digestion.


Glucose metabolism

Glucose (end-product of carbohydrates) is the only sugar used by the body cells to provide energy. Therefore, all monosaccharides must be converted into glucose metabolite by liver. In the liver, insulin produced by pancreatic cells convert excess glucose to glycogen for storage in the muscle or liver. This reduces the level of glucose in the blood.

In contrast, glucagon is secreted into the bloodstream by pancreatic cells. It promotes the breakdown of glycogen in the liver to release glucose into the blood. Glucagon also promotes gluconeogenesis, a process involving the synthesis of glucose from amino acids or fatty acids.


Amino acids metabolism

Ingested proteins are broken down into free amino acids by enzymes called proteases. Amino acids are used for protein biosynthesis. Excess amino acids are converted to glucose through gluconeogenesis if glucose run short. Excess amino acids cannot be stored, and can be poisonous if accumulated in the body. They are converted to a less toxic compound called urea by the liver. This process is referred to as deamination, which involved the removal of the amine group from amino acids. The remaining organic acids or keto-acids are converted into glycogen for storage.


Fatty Acids and Glycerol Metabolism

Fatty acids and glycerol recombine, after absorption to form fats in the body. They are oxidized to release energy. The liver also converts excess carbohydrates and proteins into fats which are stored in the adipose tissues. Excess fats are stored around vital organs such as heart, digestive, reproductive and excretory organs.


Roughage (Dietary fibre or Bulk)

Roughages are the indigestible part of plants consumed as food. Roughage consists of primarily of cellulose, a component of plant cell walls. It found in fruits, vegetables and grains.

Functions of Roughage

o     For easy digestion of food
o     Stimulates bowel movement
o     Prevention of constipation
o     Prevention of colon cancer
o     It increases the body’s sense of fullness or gives bulk to the diet. This helps to prevent overeating and obesity.
o     It slows the body’s absorption of sugar. This help people with type II diabetes as it lowers blood sugar level.



Malnutrition is a diet condition caused by a deficiency or excess of one or more essential nutrients in the diet. The intake of inadequate nutrients results in deficiency diseases. Some deficiency diseases are


Marasmus is a form of severe protein deficiency and is one of the forms of protein-energy malfunction. A child with marasmus looks emaciated. Body weight may be reduced to less than 80% of the average weight.

Signs and symptoms of Marasmus

o    extensive tissue and muscle wasting
o    dry, cracked skin
o    thin legs
o    poor growth
o    loose skin folds hanging over the buttocks, thighs and armpit


It can progress to the point of no return when the body's machinery for protein synthesis degrades.



Kwashiorkor is an acute form of childhood protein-energy malnutrition. Insufficient protein consumption, but with sufficient calorie intake, distinguishes it from marasmus. It can have a long-term impact on a child's physical and mental development, and may lead also to death.

Kwashiokor child

Signs and symptoms of Kwashiorkor

o     a distended abdomen   
o     an enlarged liver with fatty infiltrates
o     thinning hair   
o     loss of teeth
o     skin depigmentation and dermatitis 
o     irritability and anorexia

Treatment of Kwashiorkor 

It can be treated by adding protein to the diet.   


Constipation also known as costiveness refers to the difficulty in emptying the bowel. It is a common cause of painful defecation. Severe constipation includes obstipation (failure to pass stools or gas) and fecal impaction, which can become life-threatening. Constipation is a symptom, not a disease, therefore effective treatment requires first determining the cause.

Causes of Constipation 

o      insufficient dietary fiber intake
o      side effects of medications
o       inadequate fluid intake
o      colon cancer
o      decreased physical activity 
o      voluntary withholding of the stool

Symptoms of Constipation 

o     bloating, distension and abdominal pain,
o     difficult to pass stool
o     headaches,
o     a feeling of fatigue
o     nervous exhaustion, or a sense of incomplete emptying

Prevention of Constipation 

o      maintenance with adequate exercise
o      increased fluid intake
o      high fiber diet is recommended

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Indigestion also known as dyspepsia is a medical condition characterized by recurrent pain in the upper abdomen, upper abdominal fullness and feeling full earlier than expected when eating.

Causes of Indigestion

o     swallowing air while eating
o     improper chewing of food
o     overeating or eating too quickly 
o     eating too fatty foods
o     emotional upset/stress
o     eating too much acidic food e.g. cabbage, onions

Signs and Symptoms pf Indigestion

o      bloating  
o      belching  
o     heartburn    
o     loss of appetite
o     nausea 
o    upper abdominal pain
o    abdominal swelling and persistent vomiting

Ways of preventing indigestion

o     cook food properly 
o     chew food properly
o     avoid overeating
o     avoid going to sleep immediately after eating
o     avoid oily or fatty food

Treatment of indigestion: by medication or drug therapy


Click Here for WAEC/ SSCE/ WASSCE Biology  Past Questions and Answers on Nutrition in Mammals

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