16- DIGESTION AND ABSORPTION

CHAPTER NO.16 DIGESTION AND ABSORPTION

A161

INTRODUCTION:ABSORPTION is the transfer of end products of digestion from the intestine to the circulatory fluids (blood &lymph) by Physio-Chemical processes.

Absorption of only certain drugs occur in buccal cavity and very little absorption occurs in stomach e.g. alcohol, some water and salts, certain drugs like aspirin, etc. Practically all absorption takes place through small intestine which is adapted to ensure complete absorption:

- Enormous length of small intestine.

- Permanent circular folds on the mucosa called valves of kaeckring or plicae circularis.Yintestinal mucosa raised into number of projections called Jj

- Each intestinal cell has numerous, electron microscopic invaginations

called microvilli.

Absorption depends upon two types of processes:

Passive absorption

Active absorption

PASSIVE ABSORPTION -- It depends upon simple processes like J GL of small and water soluble molecules like some electrolytes like Cl-,small amounts of glucose, some amino acids), (of water and alcohol)and i movement of soluble molecules)

ACTIVE ABSORPTION: It occurs by two processes:

1. ACTIVE TRANSPORT: -It is that active absorption which involves the carrier molecules called permeases or translocases which are generally proteinaceous in nature.(Glucose, galactose, amino acids, Na+ etc. are absorbed by active transport).

2. ENDOCYTOSIS: -By which large sized liquid or solid nutrients are taken in some vessels through the plasma membrane.

ABSORPTION IN DIFFERENT PARTS OF ALIMENTARY CANAL:

Mouth: Certain Drugs

Stomach: Water, simple sugars, some drugs and alcohol

Small intestine: Almost all nutrients including minerals, vitamins etc

Large intestine: Water, some minerals and drugs.ileum is the chief area of absorption due to presence of microvilli (which

increases the surface area of absorption) present in small intestine.Each villus is with a lymph capillary called [ij in the centre which is surrounded by a network of blood capillaries. The molecules of water, minerals, hexose sugar,amino acids and products of nucleic acids are absorbed by blood capillaries of the villi while the fatty acids and cholesterol molecules are absorbed into the lacteals.Main seat of water absorption is small intestine so villus is the unit of absorption.

MECHANISM OF ABSORPTION OF FATS:

PANstS1@]n¢eaMLO)NRClmmm aus). - The long chain fatty acids and monoglycerides are

insoluble in water. In intestinal lumen, fatty acids, monoglycerides, cholesterol and bile salts aggregate to form small, spherical and water soluble molecules called micelles.Each micelle is about 700 times smaller than the emulsion droplet which has bile salts in its outer part, whereas the hydrophobic fatty acids and cholesterol molecules form the core of the micelle.Micelles are small enough to fall between microvilli. These micelles are taken up by intestinal mucosal cells by pinocytosis where these fatty acids and

monoglycerides react to form triglycerides.

The neural fat so formed is surrounded by a coat of lipoprotein to form water

soluble fine globules called J being released in lacteals from where

into the thoracic duct and then into systemic circulation.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A— VERY SHORT ANSWER TYPE QUESTIONS:

A. MCQs:

1. The small intestine has three parts. The first part is called

1. Duodenum

2. Oesophagus

3. Larynx

4. None of the above

2. is a characteristic feature of epithelial cells of the intestine

1. Glottis

2. Pilus

3. Bolus

4. Microvilli

3. A dental condition that is characterized by hyper mineralization of teeth enamel due to excessive intake of . The teeth often appear mottled.

1. Sodium

2. Calcium

3. Fluoride

4. Mercury

4. Spicy food, coupled with anxiety, may lead to

1. Indigestion

2. Hypotension

3. Seizures

4. None of the above

5. Fructose is absorbed into the blood through mucosa cells of intestine by the

process called

1. Active transport

2. Facilitated transport

3. Simple diffusion

4. Co transport mechanism

B. Fill in the blanks:

1... —————___—siS the chief area of absorption.

2. Active transport require the expenditure of

C. True/False:

1. All type of drugs absorbed in mouth

2. Serosa is the outermost layer of alimentary canal

ANSWER KEY: PART-A

a) Multiple choice Questions:

1.1; Duodenum

2. 4; microvilli

3. 3; flouride

4. 1; indigestion

5. 2; facilitated diffusion

b) Fill in the blanks:

1. lleum

2. Energy

c) True or False:

1. False

2. True

PART: B SHORT ANSWER QUESTIONS:

1. Why are villi present in the intestine and not in stomach?

2. Name the part of alimentary canal where major absorption of digested

food takes place. What are the absorbed forms of different kinds of food

materials?

PART: C LONG ANSWER TYPE QUESTIONS:

1. Discuss the mechanism of absorption.

2. How are fats absorbed in small intestine?

A162

INTRODUCTION:Digestion is the breakdown of |. =: * >.» molecules 8 : ee Sep ey $0 that they can be absorbed into the watery blood plasma.In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream.

The dental formula for the adult human beings is 2123/2123. There are

, 1 canine, 2 premolars and 3 molars in one row. There are total 32 teeth in the adult human being.

The stomach is a ‘j’-shaped organ, with two openings- the oesophageal and the duodenal- and four regions- the cardiac, fundus, body and pylorus.

Mucosa. This is the first and innermost layer or lining.Submucosa. This second layer supports the mucosa.Muscularis. The third layer is made of thick muscles.

Subserosa. This layer contains supporting tissues for the serosa.Serosa. This is the last and outermost layer.

Intestinal villi are tiny, finger-like projections made up of cells that line the entire length of your small intestine. Your villi (villus is the singular, villi is the plural) absorb nutrients from the food you eat and then shuttle those nutrients into your bloodstream so they can travel where they're needed.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A— VERY SHORT ANSWER TYPE QUESTIONS:

M.C.Qs:

1. Digestion is:

(a) Absorption of water

(b) Absorption of food

(d) None of these

2. Function of HCI in stomach is to:

(a) Kill micro-organisms of food

(b) Facilitate absorption of food

(d) Activate pepsinogen to pepsin

3. In the wall of alimentary canal which are/ is true sequence from outer

to inner:

(a) Serosa, longitudinal muscle, mucosa, sub mucosa

(b) Mucosa, serosa, long muscle

(d) Serosa, long muscle, sub-mucosa, mucosa

4. Function of tongue is to:

(a) Help in the act of swallowing

(b) Help in mixing saliva with the food

(d) All the above

B. FILL IN THE BLANKS:

1. helps in increasing the surface area of small intestine

2. Dental formula of adult human being is .

C. TRUE/FALSE:

1. Serosa is the innermost lining of alimentary canal.

2. Liver secretes bile juice.

ANSWER KEY: PART-A

a) Multiple choice Questions:

j.c

2.d

3.c

4.d

b) Fill in the blanks:

1. Microvilli

2. 2123/2123

c) True or False:

1. False;Serosa is the outermost lining of alimentary canal.

2. True

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Write the role of HCI in stomach.

2. Describe various types of teeth in adult human being.

3. Describe various layers of alimentary canal.

PART: C LONG ANSWER TYPE QUESTIONS:

1. Draw well labelled diagram of human Digestive System.

A163

INTRODUCTION:Digestive disorders are characterised according to the part of digestive system where it occurs. There are various causes responsible for the

disorders. Various causes may include disorder due to infection bacteria or protozoan etc.

DISORDERS OF MOUTH AND BUCCAL CAVITY:

1. Hare lip — It is characterized by vertical cleft in upper lip.

2. Dental caries - It is characterised by appearance of cavities in the teeth due

to organic acid produced by anaerobic bacteria (Propionibacterium). The

bacteria act on the food left over or stuck in the teeth and convert it into

organic acid thus causing cavities in them.

3. Tonsillitis- It is inflammation of palatine tonsils due to bacterial

infection. The main symptoms are sore throat, swelling of tonsils and fever.

4. Gingivitis- It is most common disease of gums. It causes irritation, redness

and swelling and bleeding of Gingiva. Tne reason is poor oral hygiene.

DISORDERS OF OESOPHAGUS AND STOMACH:

1. Peptic ulcers- It is characterized by destruction of mucosa of stomach due

to hyper secretion of HCI due to bacterial infections of Helicobacter pyion .

2. Gasteritus -It includes inflammation of gastric mucosa due to excessive

intake of alcohol.

3. Vomiting- It is characterized by ejection of stomach content through mouth.

The reasons of vomiting may include food poisoning, gastroenteritis, infection by

bacteria intestinal obstruction and GERD or ulcers.

DISORDERS OF SMALL INTESTINE:

1. INFECTIOUS DISEASES:

a. Typhoid- It is caused by Salmonella typhyii.

b. Choléra- It is characterized by loose motions and is caused by vibrio cholera.

c. Amoebic dysentry- It is caused by Entamoeba histolytica and charactensed by blood and mucosa in the stool.

d. Ascariasis- It is caused by Ascaris and symptoms include vomitting, indigestion and abdominal pain.

2. INDIGESTION- Improper digestion of food. It might be due to decreased secretion of digestive enzymes or food poisoning or anxiety or intake ofspicy food. It's symptoms may include early fullness during a meal.,uncomfortable fullness after meal, discomfort in the upper abdomen and

burning in the upper abdomen

DISORDERS OF LARGE INTESTINE:

1. Appendicitis- It includes elongation of vermiform appendix.

2. Inguinal hernia- The condition occurs due to protrusion of intestine into inguinal canal of scrotal sac.

3. Piles-lIt is the enlargement of rectal veins causing severe pain.

4. Constipation- It is characterized by very few bowel movements, painfully

defecation because faeces become excessive dry. The main cause could be drinking very less water or having food low in fibre content.

5. Diarrhoea- It is a condition in which there is a frequent and loose passage of

stool caused by bacterial Infection of E. Coli, Salmonella etc. Tne best remedy to

cure it taking, ORS (Oral Rehydration Solution) and continuous feeding.

“LET US KNOW WHAT WE HAVE LEARNT!!”

PART A: VERY SHORT TYPE QUESTION ANSWERS:

A. MULTIPLE CHOICE QUESTIONS:

1 .Which among following disorder is caused by Ascaris?

a) Jaundice

b) Typhoid

c) Ascariasis

d) Vomiting

2. The causative agent for diarrhoea is

a) Salmonella

b) Plasmodium

c) Mosquito

d) Fungus

3. Which of the following is disorder of small intestine?

a) Piles

b) Appendicitis

c) Ascariasis

d) Vomiting

B. FILL IN THE BLANKS:

1. Peptic ulcers causes destruction of of stomach.

2. Cavities in teeth are due to produced by anaerobic bacteria.

C. TRUE & FALSE:

1. Piles are characterized by enlargement of rectal veins.

ANSWER KEY: PART -A

A. MCQs:

1. ; Ascariasis

2. a; Salmonella

3. ¢ ; Ascariasis

B. FILL IN THE BLANKS:

1. mucosa

2. acid

C. TRUE OR FALSE:

1. True

PART: B SHORT ANSWERS TYPE QUESTIONS:

1. Write short note on following:-

a) Inguinal hernia

b) Indigestion

PART: C LONG ANSWER TYPE QUESTIONS:

1. Explain the disorders related to stomach.

A164

RECAPITULATION:DIGESTION: The process in the alimentary canal by which the

complex food is converted mechanically and biochemically into simple substances suitable for absorption and assimilation.

ABSORPTION: It is the process of absorbing the digested food molecules into the blood or lymph.The absorption process occurs by active or passive or facilitated

transport mechanism.

Now let us do some questions based on these diagrams:

PART: A_VERY SHORT ANSWER TYPE QUESTIONS:

a) MCQs:

1. Absorption of food is carried out by Absorption.

(a) Active

(b) Passive

(d) None of these

2. The HCL secreted in the stomach:

(a) Kills Micro-organisms

(b) Digests the proteins

(d) All the above

3. Saliva has enzyme:

(a) Pepsin

(b) Ptyalin

(d) Renin

4. The vitamin C or ascorbic acid prevents:

(a) Rickets

(b) Pellagra

(d) Antibody synthesis

5. Glycogen is stored in:

(a) Liver

(b) Muscles

(d) Blood

(c) True/False

(1) Dental formula for human being is 2123/2123.

(2) Lipids and fats are digested by trypsin.

(3) Anti-sterility vitamin is D.

(b) Fill ups

(a) is the conversion of glycogen to glucose.

(b) Starch is digested into by ptyalin.

ANSWERS KEY: PART -A

(a) MCQs:

1. (c) Both active and passive

2. (d) all of the above kills micro-organisms, digests the proteins, the

fibrous content of the food is loosened

3. (b) ptyalin

4. (c) scurvy

5. (c) Both liver and muscle

(b) TRUE/FALSE:

(1) True

(2) False Lipids and fats are digested by Lipase

(3) True

(c) FILLIN THE BLANKS:

(a) Glycogenolysis

(b) Maltese and dextrin

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Name the different types of teeth present in human being?

2. Name the different parts of human stomach?

3. Define the Absorption? Name the processes by which Absorption

Occurs?

PART: C LONG ANSWER TYPES QUESTIONS:

1. Draw a well labelled diagram of human digestive system.

A165

RECAPITULATION:In this chapter we have learnt about digestion being a process involving both mechanical and chemical actions. A brief introduction was provided about the human digestive system to have a better and clear idea of the digestion of various kinds of food stuffs we eat in different parts of digestive system.

In most ME, digestion is a multistage process in the digestive system,

starting from ingestion of raw materials usually involves some type of mechanical

and chemical processing. Digestion is separated into four steps:-

1. INGESTION: It is the placing of food into the mouth.

2. MECHANICAL AND CHEMICAL BREAKDOWN: It involves mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures.

3. ABSORPTION: Nutrients are absorbed from the digestive system into the circulatory and lymphatic capillaries through |

4. EGESTION: It is the removal of undigested materials from the digestive tract through defecation.

“LET US KNOW WHAT WE HAVE LEARNT”

PRACTICE QUESTIONS WITH SOLUTIONS (N.C.E.R.T)

Answer. 1. (a): (i) Pepsin, lipase, and rennin Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted in an inactive form as pepsinogen, which is activated by HCI. Pepsin digests proteins into peptones. Lipase breaks down fats into fatty acids. Rennin is a photolytic enzyme

present in the gastric juice. It helps in the coagulation of milk.

Answer.3. (b): (ii) Intestinal juice

Succus entericus is another name for intestinal juice. It is secreted by the intestinal gland. Intestinal juice contains a variety of enzymes such as maltase, lipases,nucleosidases, dipeptidases, etc.

(a) The mucosal wall of the small intestine forms millions of tiny finger-like

projections known as villi. These villi increase the surface area for more efficient food absorption.Within these villi, there are numerous blood vessels that absorb the digested products of proteins and carbohydrates, carrying them to the blood stream. The villi

also contain lymph vessels for absorbing the products of fat-digestion. From the

blood stream, the absorbed food is finally delivered to each and every cell of the

body.The mucosal walls of the stomach form irregular folds known as rugae. These help

increase the surface area to volume ratio of the expanding stomach.

(a) Pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted into pepsin by hydrochloric acid. Pepsin is the activated in the form of pepsinogen.Pepsinogen —“*> Pepsin + Inactive peptide (Inactive) (Active)

(b) The walls of the alimentary canal are made up of four layers. These are as

follows:

(i) Serosa is the outermost layer of the human alimentary canal. It is made up of a

thin layer of secretory epithelial cells, with some connective tissues underneath.

(ii) Muscularis is a thin layer of smooth muscles arranged into an outer

longitudinal layer and an inner circular layer.

(iii) Sub-mucosa is a layer of loose connective tissues, containing nerves, blood,and lymph vessels. It supports the mucosa.

(iv) Mucosa is the innermost lining of the lumen of the alimentary canal. It is

mainly involved in absorption and secretion.

(a) Bile is a digestive juice secreted by the liver and stored in the gall bladder.

Bile juice has bile salts such as bilirubin and biliverdin. These break down large

fat globules into smaller globules so that the pancreatic enzymes can easily act

on them. This process is known as emulsification of fats. Bile juice also makes

the medium alkaline and activates lipase.

Q.4 State the role of pancreatic juice in digestion of proteins.

Answer.4.Pancreatic juice contains a variety of inactive enzymes such as

trypsinogen, chymotrypsinogen, and carboxypeptidases. These enzymes play an

important role in the digestion of proteins.

PHYSIOLOGY OF PROTEIN-DIGESTION:

The enzyme enterokinase is secreted by the intestinal mucosa. It activates

trypsinogen into trypsin.Trypsinogen ee Trypsin + Inactive peptide Trypsin then activates the other enzymes of pancreatic juice such as chymotrypsinogen and carboxypeptidase.Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into peptides.Chymotrypsinogen Et, Chymotrypsin (Inactive) (Active)

Proteins —“™85*.. Peptides

Carboxypeptidase acts on the carboxyl end of the peptide chain and helps release

the last amino acids. Hence, it helps in the digestion of proteins.Peptides —““2**. Smaller peptide chain + Amino acids

Thus, in short, we can say that the partially-hydrolysed proteins present in the chyme are acted upon by various proteolytic enzymes of the pancreatic juice for their complete digestion.. Trypsin‘ Chymety sin .Proteins, peptones Cibo pene Dipeptides and proteases

Q5 : Describe the process of digestion of protein in stomach.

Answer.5.The digestion of proteins begins in the stomach and is completed in the small intestine. The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The food that enters the stomach becomes acidic on mixing with this gastric juice.

The main components of gastric juice are hydrochloric acid, pepsinogen, mucus,

and rennin. Hydrochloric acid dissolves the bits of food and creates an acidic

medium so that pepsinogen is converted into pepsin. Pepsin is a protein- digesting

enzyme. It is secreted in its inactive form called pepsinogen, which then gets

activated by hydrochloric acid. The activated pepsin then converts proteins into proteases and peptides.Proteins —="+ Proteases + Peptides Rennin is a proteolytic enzyme, released in an inactive form called prorennin.Rennin plays an important role in the coagulation of milk.

Q6 : Give the dental formula of human beings.

Answer.6. The dental formula expresses the arrangement of teeth in each half of

the upper jaw and the lower jaw. The entire formula is multiplied by two to express

the total number of teeth. The dental formula for milk teeth in humans 2102 5-20 2102 Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine and 2 molar.Premolars are absent in milk teeth.

The dental formula for permanent teeth in humans is:2123—— x 2=32 2123

Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars,

and 3 molars. An adult human has 32 permanent teeth.

Q. 7 : Bile juice contains no digestive enzymes, yet it is important for digestion. Why?

Answer.7. Bile is a digestive juice secreted by the liver. Although it does not

contain any digestive enzymes, it plays an important role in the digestion of fats.

Bile juice contains bile salts, bile pigments like bilirubin, biliverdin and

phospholipids. Bile salts break down large fat globules into smaller globules so that

the pancreatic enzymes can easily act on them. This process is known as

emulsification of fats. Bile juice also makes the medium alkaline and activates

lipase.

Q8 : Describe the digestive role of chymotrypsin. What two other digestive enzymes of the same category are secreted by its source gland?

Answer.8 The enzyme trypsin (present in the pancreatic juice) activates the

inactive enzyme chymotrypsinogen into chymotrypsin.Chymotrypsinogen —““-+ Chymotrypsin (Inactive) (Active)

The activated chymotrypsin plays an important role in the further breakdown of the partially-hydrolysed proteins.

Proteins —“““"2".. Peptides The other digestive enzymes of the same category are trypsinogen and carboxypeptidase. These are secreted by the same source-gland, pancreas.Trypsinogen is present in an inactive form in the pancreatic juice. The enzyme enterokinase is secreted by the intestinal mucosa “activates trypsinogen into trypsin.Trypsinogen ia Trypsin + Inactive peptide The activated trypsin then further hydrolyses the remaining trypsinogen and activates other pancreatic enzymes such as chymotrypsinogen and

carboxypeptidase. Trypsin also helps in breaking down proteins into peptides.

Proteins —“&* Peptides Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids.Peptides —““222** 4 small peptide chain + Amino acids

Q9 : How are polysaccharides and disaccharides digested?

Answer.9. The digestion of carbohydrates takes place in the mouth and the small

intestine region of the alimentary canal. The enzymes that act on carbohydrates

are collectively known as carbohydrases.

Digestion in mouth:As food enters the mouth, it gets mixed with saliva. Saliva a€" secreted by the salivary glands a€" contains a digestive enzyme called salivary amylase. This enzyme breaks down starch into sugar at pH 6.8.Salivary amylase

Starch pies Maltose + lsomaltose + Limit destrins Salivary amylase continues to act in the oesophagus, but its action stops in the stomach as the contents become acidic. Hence, carbohydrate-digestion stops in

the stomach.Digestion in the small intestine: Carbohydrate-digestion is resumed in the small intestine. Here, the food gets mixed with the pancreatic juice and the intestinal juice. Pancreatic juice contains the pancreatic amylase that hydrolyses the polysaccharides into disaccharides.Starch —“4 Disaccharides

(Polysaccharides)Similarly, the intestinal juice contains a variety of enzymes (disaccharidases such as maltase, lactase, sucrase, etc.). These disaccharidases help in the digestion of disaccharides. The digestion of carbohydrates is completed in the small intestine.Maltose —“=+ 2Glucose

Lactose —“““> Glucose + Galactose

Sucrose —““> Glucose + Fructose

Q10 : What would happen if HCl were not secreted in the stomach?

Answer.10. Hydrochloric acid is secreted by the glands present on the stomach

walls. It dissolves bits of food and creates an acidic medium. The acidic medium

allows pepsinogen to be converted into pepsin.Pepsin plays an important role in the digestion of proteins. Therefore, if HCl were not secreted in the stomach, then pepsin would not be activated. This would affect protein digestion. A pH of about 1.8 is necessary for proteins to be digested. This pH is achieved by HCl.

Q11 : How does butter in your food gets digested and absorbed in the body?

Answer.11. DIGESTION OF FATS: Butter is a fat product and gets digested in the

small intestine. The bile juice secreted by the liver contains bile salts that break

down large fat globules into smaller globules, so as to increase their surface area for the action of lipase. This process is referred to as emulsification of fats.

After this, the pancreatic lipase present in the pancreatic juice and the intestinal

lipase present in the intestinal juice hydrolyse the fat molecules into triglycerides,diglycerides, monoglycerides, and ultimately into glycerol.

PancreaticFats ipwxe —” Triglycerides + Diglycerides Diglycerides and monoglycerides te, Fatty acids + Glycerol

Absorption of fats Fat absorption is an active process. During fat digestion, fats are hydrolyzed into fatty acids and glycerol. However, since these are water insoluble, they cannot be directly absorbed by the blood. Hence, they are first incorporated into small droplets called micelles and then transported into the villi of the intestinal mucosa.

They are then reformed into small microscopic particles called chylomicrons, which are small, protein-coated fat globules. These chylomicrons are transported to the lymph vessels in the villi. From the lymph vessels, the absorbed food is finally released into the blood stream and from the blood stream, to each and every cell of the body.

Q12 : Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal

Answer.12 The digestion of proteins begins in the stomach and is completed in

the small intestine. The enzymes that act on proteins are known as proteases.

DIGESTION IN THE STOMACH:The digestive juice secreted in the gastric glands present on the stomach walls is

called gastric juice. The main components of gastric juice are HCl, pepsinogen,

and rennin. The food that enters the stomach becomes acidic on mixing with this

gastric juice.The acidic medium converts inactive pepsinogen into active pepsin. The active pepsin then converts proteins into proteases and peptides.Pepsin

Proteins > Proteases + Peptides

The enzyme rennin plays an important role in the coagulation of milk.

DIGESTION IN THE SMALL INTESTINE:

The food from the stomach is acted upon by three enzymes present in the small

intestine Pancreatic Juice, Intestinal Juice (known as succus entericus), and Bile

Juice.

ACTION OF PANCREATIC JUICE:

Pancreatic juice contains a variety of inactive enzymes such as trypsinogen,

chymotrypsinogen, and carboxypeptidases. The enzymes are present in an

inactivated state. The enzyme enterokinase secreted by the intestinal mucosa

activates trypsinogen into trypsin.

Trypsinogen ee + Trypsin + Inactive peptide

Theactivated trypsin then activates the other enzymes of pancreatic juice.

Chymotrypsinogenis a proteolytic enzyme that breaks down proteins into peptides.

Chymotrypsinogen —E“ + Chymotrypsin

Proteins —"“"""+Peptides Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids.Peptides —“"2"** smaller peptide chain + Amino acids

ACTION OF BILE JUICE:Bile juice has bile salts such as bilirubin and biliverdin which break down large, fat

globules into smaller globules so that pancreatic enzymes can easily act on them.

This process is known as emulsification of fats. Bile juice also makes the medium

alkaline and activates lipase. Lipase then breaks down fats into diglycerides and

monoglycerides.ACTION OF INTESTINAL JUICE:

Intestinal juice contains a variety of enzymes. Pancreatic amylase digests

polysaccharides into disaccharides. Disaccharidases such as maltase, lactase,

sucrase, etc., further digest the disaccharides.The proteases hydrolyse peptides into dipeptides and finally into amino acids.Dipeptides 2" + Amino acids

Pancreatic lipase breaks down fats into diglycerides and monoglycerides.

The nucleases break down nucleic acids into nucleotides and nucleosides.

Q13 : Explain the term thecodont and diphyodont

Answer.13. Thecodont is a type of dentition in which the teeth are embedded in the

deep sockets of the jaw bone. Ankylosis is absent and the roots are cylindrical.

Examples include living crocodilians and mammals.Diphyodont is a type of dentition in which two successive sets of teeth are

developed during the lifetime of the organism. The first set of teeth is deciduous and the other set is permanent.

The deciduous set of teeth is replaced by the permanent adult teeth.This type of dentition can be seen in humans.

Q14 : Name different types of teeth and their number in an adult human.

Answer.14,There are four different types of teeth in an adult human. They are as follows:

(i) Incisors The eight teeth in the front are incisors. There are four incisors each in the upper jaw and the lower jaw. They are meant for cutting.

(ii) Canines The pointy teeth on either side of the incisors are canines. They are four in number, two each placed in the upper jaw and the lower jaw. They are meant for tearing.

(iii) Premolars They are present next to the canines. They are eighteen in number, four each placed in the upper jaw and the lower jaw. They are meant for grinding.

(iv) Molars They are present at the end of the jaw, next to the premolars. There are twelve molars, six each placed in the upper jaw and the lower jaw.

This means each half of the upper jaw and the lower jaw has;2 incisors, 1 canine, 2 premolars, and 3 molars.Hence, an adult human has 32 permanent teeth.

Q15 : What are the functions of liver?

Answer.15.Liver is the largest and heaviest internal organ of the body. It is not

directly involved in digestion, but secretes digestive juices. It secretes bile which plays a major role in the emulsification of fats.

A166

RECAPITULATION:Digestion is a catabolic process involving both physical and chemical action over food in various parts of digestive system under the activity of different enzymes.Digestion is either intercellular or intracellular.

Absorption is passing diffusible food from small intestine into blood or lymph.

Main site of absorption is villi and microvilli in small intestine.

LETS MEMORISE THE CHAPTER WITH THE HELP SOME MORE QUESTIONS & ANSWERS:

Q1. What is the dental formula of a human baby?

Ans. The dental formula of a human baby is

2120/2120 = 20

Q2.Name the enzyme that curdles milk?

Ans. Renin, Chymotrypsin.

Q3.What is emulsificatrion?

Ans. It is the breakdown of fat globules into small sized fat droplets by bile salts of bile juice.

Q4.Name the hardest substance in the body?

Ans. Enamel.

Q5.State one difference between chyme and chyle?

Ans. Chyme is semi digested acidic food present in stomach.Chyle is fully digested alkaline food present in small intestine.

Q6.What is diastema?

Ans. It is the gap between incisors and premolars due to absence of canines which

is a characteristic of ruminants on account of herbivorous diet.

Q7.Does gall bladder make bile?

Ans. No, it concentrates bile.

Q8.What is the role of gall bladder?What happens if it stops functioning or is removed?

Ans. Gall bladder stores and concentrates the bile. In the absence of gall bladder,

the availability of bile for emulsification of fat is reduced. Therefore there is reduced digestion of fat.

Q9.How is intestinal mucosa protected from the acidic food present in stomach?

Ans. The secretion including mucous, sodium bicarbonate, other electrolytes,

water and a few enzymes from Brunner’s gland, present in mucosa of duodenum,

protects the intestinal mucosa from acidic food.

Q10.What are the functions of pancreas?

Ans. Pancreas has two types of components:

a) Exocrine:- It secretes trypsin, chymotrypsin, carboxypeptidase, pancreatic

lipase and pancreatic amylase.

b) Endocrine: - it produces glucagon and insulin which is required for sugar

metabolism.

Q11.Enumerate the functions of HCI of gastric juice.

Ans. Functions of HCi of gastric juice are as follow:

|. HCl stops the activity of salivary amylase by changing the pH of food.

Il. It activates the inactive enzymes.

Ill. It makes the food acidic which is essential for activity of rennin and pepsin.

Q12.Describe the enztmatic action on fats in the duodenum?

Ans. In duodenum, pancreatic lipase acts on fats. The fats are not broken down

completely as food stays for short time in duodenum.Fat + water -—-------—------> Diglyceride + Monoglyceride + Fatty acids + Glycerol This step occurs in the presence of enzyme pancreatic lipase.

Q13.What do you mean by incomplete and complete type of digestive tracts?

Ans. The incomplete digestive tract has only one opening for intake of food and

elimination of indigestible matter. e.g. Coelenterates, Planaria, Liver fluke etc.

Complete digestive tracts have a separate opening for intake of food and

elimination of indigestible matter e.g. in man, frog, rabbit, etc.

Q14.What are microvilli?State their function.

Ans. The finger-like projections produced by mucosa, called villi. The cells that line the surface of the villi produce numerous microscopic .bristle like projections called the microvilli or brush border.These cells mainly secrete mucus that acts as a lubricant and protects the

epithelial surface from excoriation and digestion.

Q15.What are crypts of leiberkuhn?

Ans.The surface area of the gastrointestinal tract is lined by invaginations of the epithelium (pits) into the submucosa. These pits of the intestine are called the crypts of Leiberkuhn. These crypts contain specialized secretory cells.

Q16.Name three accessory digestive organs in a human.

Ans.1. Tongue: It is located in the buccal cavity. Tongues are provided with

Gustatory receptors called taste buds.

2. Salivary glands: Humans have three pairs of salivary glands-parotid glands in the cheek, submandibular and sublingual, opening into the floor of the mouth.

3. Liver: It is situated in the right upper part of the abdomen. In an adult, the

liver weighs about 1.5kg.

Q17.What is peristalsis?How does it help in degestion?

Ans. When food passes into the oesophagus, a travelling wave of constriction

which pushes the luminal contents downward is called peristalsis. Peristalsis is

produced by the involuntary contraction of circular muscles. When a peristaltic

wave reaches the end of the sphincter opens allowing the passage of bulk of

food into the stomach.

Q18.Discuss the five steps involved in the processof nutrition?

Ans. Nutrition: Sum total of certain processes that enable a cell to utilize nutrients is called nutrition.The entire process of nutrition includes the following steps:Ingestion Digestion, Absorption, Assimilation and Egestion.

(a) Ingestion: The process of taking in food through the mouth is called ingestion.

The digestion of food starts from the mouth and ends in the intestine.

1. Mouth: The food is ingested through the mouth, carbohydrates, such as starch, are broken down or digested to form sugar. The saliva contains the enzyme salivary amylase that helps in the digestion of starch into sugar. The saliva also helps in lubricating the food and making it easier for swallowing.The tongue helps in rolling and pushing food into the oesophagus.

2. Oesophagus: The oesophagus or the food pipe helps in pushing the food into the stomach. The expansion and contraction of muscles of the oesophagus are called peristalsis or peristaltic movement.

3. Stomach: The stomach is a highly muscular organ. The gastric glands

present in its walls secrete gastric juices and help in the digestion of food.These juices contain hydrochloric acid (HCI) and enzymes like pepsin. HCI created an acidic medium for the activation of enzymes and kills bacteria.These enzymes break down the proteins into smaller fragments called

peptones. The muscles of the stomach help in churning the food so that it is

properly mixed with the digestive juices.

Pepsin Proteins Peptones (partly digested proteis)Pepsin Milk proteins Calcium Paracareinate

4. Small intestine: The food moves from the stomach to the duodenum. Here

emulsification of fat takes place with the help of the bile juice secreted by the

liver. The bile juice is stored in the gall bladder. The pancreas secretes trypsin, amylase and lipase which are poured into the duodenum. The food moves to the ileum, which is the lower part of the small intestine. The inner surface of the ileum contains thin finger-like projections called villi. Villi are responsible for the absorption of digested food. Blood then carries the absorbed food to a different part of the body and undigested food is pushed into the large intestine.

Pree Peptones Amino acids (completely digested proteins) Amylase

Fats ——<—<$< <> 1 Fatty acids + Glycerol

5. Large intestine: This part of the body absorbs water from the undigested

food and solid waste is lubricated to form the faeces. The faeces pass on to

the lower part of the large intestine, called the rectum, and thrown out of the

body through the anus.

Q19.Where does the digestion of starch,proteins and fats take place and what is the role played by the associated glands?

Ans. Starch: Digestion of starch takes place in the mouth. Carbohydrates

are broken down or digested to form sugar. The saliva contains an enzyme

Salivary Amylase that helps in the digestion of starch into sugar.

Starch Malltose

Proteins: The gastric glands present in the stomach secretes gastric juice which contains (HCI) hydrochloric acid and enzymes like pepsin. These enzymes breakdown the proteins into smaller fragments called peptones.Pepsin

Proteins Peptones (partly digested proteins)Pepsin Milk proteins s Calcium Paracareinate

Fat: Fat is digested in the duodenum (small intestine) with the help of the bile juice secreted by the liver. The bile juice is stored in the gall bladder. The pancreas secretes trypsin, amylase and lipase which are poured into the duodenum.

A167

INTRODUCTION:Dear students we have studied the whole chapter no.16, DIGESTION &

ABSORPTION, its diagrams, NCERT questions, and revision of another extra

questions in the previous Daily Dose assignments. As a part of revision we will

now discuss about all the important words and definitions that we come across in

this chapter.

IMPORTANT WORDS & DEFINITIONS IN THE CHAPTER IMPORTANT WORDS:NUTRIENTS: The materials required by the living organisms to sustain their life.

DIETITION or DIETETIST: A specialist associated with the needs and problems of nutrition.

AUTOTROPHS: Organisms which prepare their own food.

HETEROTROPHS: Organisms which take up readymade organic food from plants or animals.

HERBIVORES: Plant feeding heterotrophs e.g. rabbit, cow, etc.

CARNIVORES: These eat upon animal food e.g. lion, tiger.

OMNIVORES: These take plant and animal food e.g. human beings,cockroaches, etc.

CANNIBALS: These eat upon members of their own species e.g. certain snakes, cockroaches, etc.

DETRIVORES: These feed chiefly upon dead organic matters present in the mud e.g. earthworms.

SCAVENGERS or CARRION EATERS: These feed upon dead organisms e.g. vultures, crow, hyena, etc.

COPROPHAGOUS: Animals feed upon faeces e.g. pigs, dung beetles etc.

ORBICULARIS ORIS MUSCLE: A specific muscle associated with lip.

BUCCAL VESTIBULE: The space between the gums and checks where food is stored temporary for some time.

: Anterior part of the palate.

SOFT PALATE: Posterior part of the palate.

UVULA: Posterior outgrowth of soft palate which hangs down in the form of finger like processes.

PHARYNX: Common path for air and good.

NASOPHARYNA: Upper part of phyarynx which is related to the nasal chamber.

OROPHARYNAX: Lower part of the pharynx which is related to the oral cavity.

SULCUS TERMINALIS: A v- shaped sulcus which divides dorsal surface of tongue into two unequal parts.

APICAL- FORAMEN: An aperture at the base of pulp cavity through which blood capillaries and nerve fibres enter inside the teeth.

DIASTEMA: The empty space of canines in gums of herbivore animals.

MONOPHYODONT: The teeth which appear only once in life. E.g. premolars and last molars in humans.

DIPHYODONT: The teeth which appear twice in life. e.g. incisors,canines, 1% and 2" molars.

THECODONT: The teeth which are present in bony socket of jaw. e.g.human and crocodile.

HETERODONT: When the teeth are of different type in mammals on the basis of structure and function.

MALT: Mucosa Associated Lymphoid Tissue.

LACTEALS: The lymphatic vessels of the small intestine which absorb digested fats.

CRYPTS OF LIEBERKUHN (COL): Small slit like space is found at the base of villi is called Crypts of Lieberkuhn.

CHYME: The food when mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular walls and is called chyme.

FALCIFORM LIGAMENT: A ligament which separates the left and right lobe of liver.

COMMON HEPATIC DUCT: Right and left hepatic ducts join to form common hepatic duct.

GLISSON’S CAPSULE: A thin fibrous connective tissue sheath which covers each hepatic lobule.

ISLETS OF LANGERHAN’S: The group of endocrine cells which secrete hormones in pancreas.

PEPSIN: Protease or proteolytic enzyme of gastric juice.

RENNIN: Milk coagulating factor.

TITANIC CONTRACTIONS: Forceful contractions of musculature of stomach to send the chime into duodenum.

SATIETY CENTRE: Located in hypothalamus and sends signals to stop the feeding.

LACTOSE INTOLERANCE: Feeling of flatulence, intestinal pain and diarrnoea in old persons due to excess intake of milk whose lactose is fermented to form gases and acids.

DYSPHAGIA: Difficulty in swallowing.

APPENDICITIS: Abnormal elongation of vermiform appendix.

COMPOUND STOMACH: Four chambered stomach found in ruminating mammals like cattle.

DEGLUTITION: Swallowing of food bolus into the oesophagus.

DENTAL FORMULA: Number and arrangement of teeth in one-half of each jaw.

MICELLE: Water- soluble droplets formed of fatty acids, monoglycerides and bile salt during absorption of fats.

RUMINANTS: Cud-chewing mammals like cow.

IMPORTANT DEFINITIONS:

DIGESTION: The process of conversion of complex food substances into simple and absorbable forms is called digestion.

NUTRITION: Sum total of the processes by which living organisms obtain those substances from food which are necessary for their growth and maintenance is called nutrition.

HOLOZOIC NUTRITION: It is the type of nutrition in which organisms feed exclusively on solid organic food material.

INGESTION: The process of taking in of food is called ingestion.

ABSORPTION: Passing of digested food through the stomach of small intestine into the blood and lymph.

ASSIMILATION: Use of simple food components in different body cells.

EGESTION: It is the process of elimination of undigested food as faeces.

INTRACELLULAR DIGESTION: Digestion inside the cell (mainly in vacuole) is called as intracellular digestion.

INTERCELLULAR DIGESTION: Digestion outside the cell is called intercellular digestion.

EMULSIFICATION: It is the process of breaking down large fat droplets

into fine emulsion by bile juice of the liver.

GLYCOGENSIS: The conversion and storage of extra amount of glucose into glycogen is called glycogensis.

GLYCOGENOLYSIS: The conversion of glycogen again into glucose when glucose level in blood falls down is called glycogenolysis.

GLUCONEOGENSIS: The process of conversion of non-carbohydrate compounds (amino acids, fatty acids) into glucose.

GLYCONEOGENSIS: The process of synthesis of glycogen from lactic acid.

DETOXIFICATION: The conversion of toxic substances into non —toxic substance by liver.

HAEMOPOIESIS: The formation of blood cells is called haemopoesis.

PERISTALSIS: It is the involuntary movements of gut wall which move food.

MALNUTRITION: Malnutrition is improper or under-nutrition.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A_VERY SHORT ANSWER TYPE QUESTIONS:

a) MCQs:

1. Animals which feed upon own faeces are called:

(a)Cannibals

(b)Detrivores

(d)Scavengers

2. The teeth which are present in the bony socket of jaw are called:

(a)Monophyodont

(b)Diphyodont

(d)Heterodont

3. Abnormal elongation of vermiform appendix is known as:

(a)Dysphagia

(b)Appendicitis

(c)Lactose intolerance

(d)None of the above

4. The process of use of simple food components in different body

cells is called as:

(a)Ingestion

(b)Assimilation

(c)Absorption

(d)Egestion

5. Which is known as milk coagulating factor?

(a)Pepsin

(b)Rennin

(c)Pepsinogen

(d)Trypsin

b) FILL UPs:

1. Satiety centre is located in..............and sends signals to stop the feeding.

2. The process of formation of blood cells is called............0..0..

3. Cud-chewing mammals like cow are known asg................

c) TRUE/FALSE:

1. Organisms which eat upon members of their own species are called

cannibals.

2. Difficulty in digestion is known as dysphagia.

3. Glycogensis and glycogenolysis both are same process.

ANSWER KEY: PART -A

a) MCQs:

1. (c) Coprophagous

2. (c)Thecodont

3. (b) Appendicitis

4. (b) Assimilation

5. (b) Rennin

b) FILL UPs:

1. Hypothalamus

2. Haemopoisis

3. Ruminants

c) TRUE/ FALSE:

1. True

2. False

3. False

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Expand MALT.

2. Write difference between intercellular and intracellular digestion.

3. Write function of islets of langerhans.

PART: C LONG ANSWER TYPE QUESTIONS:

1. How the process of Glycogensis, Glycogenolysis, Gluconeogensis and

Glyconeogensis are different from one another.

A168

INTRODUCTION

DIGESTION AND ABSORPTION:Digestion is the process of breaking down complex food particles into simpler forms to absorb nutrients.The digestive system of humans comprises an alimentary canal. There are various glands associated with it.

ALIMENTARY CANAL:

Structure

Histology

Digestive Glands

Digestion of Food

Absorption

Disorders

THE ALIMENTARY CANAL HAS THE FOLLOWING PARTS:

MOUTH:Teeth have THECODONT attachment, i.e. embedded in a jaw bone socket.

DIPHYODONT: Two sets of teeth during a lifetime. The deciduous or milk teeth (20) are replaced by permanent adult teeth (32).

HETERODONT: There are four different types of teeth. Incisors (8, for biting), Canines (4, for tearing), Premolars (8, for crushing and grinding),molars (12, for making smaller pieces).The DENTAL FORMULA is 2123/2123, it represents the upper and

lower half of the jaw.TASTE BUDS are present on the upper surface of the tongue

in papillae.The tongue is attached to the oral cavity by the frenulum.

PHARYNX: Gives passage to food and air. The epiglottis (a cartilaginous flap) blocks the entry of food into glottis, the opening of trachea (windpipe)

OESOPHAGUS: It is a long tube that opens into the stomach, which is regulated by

a muscular gastro-oesophageal sphincter.

STOMACH: It is divided into four parts:

1. Cardiac where oesophagus opens

2. Fundic

3. Body- central region

4. Pyloric- opens in the small intestine

SMALL INTESTINE: It is the longest part of the alimentary canal. Pyloric sphincter

connects the stomach to the small intestine. It has three parts:

1. Duodenum

2. Jejunum

3. Ileum

LARGE INTESTINE: It also has three parts:

1. CAECUN- it is a blind sac, which hosts microbes and has vermiform appendix (a vestigial organ) attached to it

2. COLON- it consists of four parts; ascending, transverse, sigmoid and

descending.

3. RECTUM. opens out of the body through the anus.

HISTOLOGY OF HUMAN ALIMENTARY CANAL:From OESOPHAGUS to RECTUM there are four layers present in the wall of the alimentary canal:

1. SEROSA is the outermost layer, which is made up of mesothelium and connective tissues.

2. MUSCULARIS is made up of smooth muscles, outer longitudinal and inner circular.

3. SUBMUCOSA is made up of loose connective tissues and contains lymph, blood and nerves. It contains glands in the duodenum part.

4. MUCOSA is the innermost layer and contains irregular folds and gastric

glands in the stomach area. Villi and microvilli and crypts of Lieberkuhn are

present in the small intestine, which greatly increase the surface area of

absorption. Mucus is secreted from the goblets cells of mucosal epithelium,

which helps in lubrication.

DIGESTIVE GLANDS

SALIVARY GLANDS: Saliva is secreted into the buccal cavity by three pairs of

the gland; parotid- cheek, submaxillary or sub-mandibular- lower jaw and

sublingual- below the tongue.

LIVER: It is the largest gland (1.5 — 2 kg), present below the diaphragm. There

are two lobes present in the liver. The hepatic cells secrete bile juice, which is

stored in the gallbladder. Hepatic lobules contain hepatic cells and are the

structural and functional unit of the liver. Hepatic lobules are covered by

Glisson’s capsule (a sheath of connective tissue).THE BILE DUCT is formed from the cystic duct (coming from the gallbladder)

and hepatic duct (from the liver).

PANCREAS: It performs exocrine (enzymes) as well as endocrine (hormones insulin and glucagon) function. The pancreatic juice is alkaline.SPHINCTER OF ODDI is present at the opening of the hepato-pancreatic duct

(bile duct + pancreatic duct) into the duodenum.

DIGESTION OF FOOD:The digestion of food starts from the mouth itself. The masticated food mixed with saliva makes a small mass of food called a bolus. The bolus moves to pharynx and oesophagus by the process of deglutition (swallowing).

There are various enzymes that get mixed with the food at different parts of

the alimentary canal and facilitate digestion, are as follows:

ABSORPTION:The bio-macromolecules are broken down in the duodenum part of the small intestine and get absorbed in jejunum and ileum.The undigested food moves to the large intestine, where some water and minerals get absorbed.The absorption of food occurs by the process of diffusion due to concentration gradient or by facilitated transport by a carrier protein.Water is transported by the osmotic gradient.

Amino acids, glucose, monosaccharides, Na* are absorbed in the blood by active

transport against the gradient and it requires energy.Micelles form chylomicrons (protein-coated fat globules), which are transported into lacteals (lymph vessels) in the villi and released in the bloodstream.

PROTEIN ENERGY MALNUTRITION (PEM) causes Marasmus and Kwashiorkor in

infants and children.

MARASMUS- It is due to a deficiency of protein and calories intake. The condition is characterised by thinning of limbs, extreme emaciation, dry and wrinkled skin and mental impairment.

KWASHIORKOR-: It is due to a deficiency of protein. There is wasting of muscles,

but some fat is still present under the skin. Tne condition is characterized by swelling and extensive edema of body parts.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A_VERY SHORT ANSWER TYPE QUESTIONS:

(A) MCQs:

1. Which of the following is not true of intestinal villi?

a. They possess microvilli

b. They increase the surface area

c. They are supplied with capillaries and the lacteal vessels

d. They only participate in digestion of fats

2. Hepato-pancreatic duct opens into the duodenum and carries

a. Bile

b. Pancreatic juice

c. Both bile and pancreatic juice

d. Saliva

3. Which of the following is not a common disorder associated with the digestive system?

a. Tetanus

b. Diarrhoea

c. Jaundice

d. Dysentery

4. A gland not associated with the alimentary canal is

a. Pancreas

b. Adrenal

c. Liver

d. Salivary glands

5. The liver is the largest gland and is associated with various functions. Choose which is not correct from the following.

a. Metabolism of carbohydrate

b. Digestion of fat

c. Secretion of a hormone called gastrin

(B) FILL UPS:-

1. Stomach has ------------------------- like structure.

2. eoneenencccnncnennane=-=-- Secretes saliva.

3. Digested food is absorbed by the walls of small intestine through---——

(C) TRUE / FALSE:-

1. Marasmus is a kind of PEM in adults.

2. There are four pairs of salivary glands in humans.

3. Villi and Microvilli help in absorption of digested foods in small.

ANSWER KEY: PART-A

(A) MCQs:

1: (d) They only participate in digestion of fats.

2: (c) Both bile and pancreatic juice.

3: (a) Tetanus.

4: (b) Adrenal.

5: (d) Secretion of a hormone called gastrin.

(B) FILL UPS:

1. bag like

2. salivary glands

3. villi

(C) TRUE / FALSE:

1. False

2. False

3. True

PART: B SHORT ANSWER TYPE QUESTIONS:

1. What is the pancreas? Mention the major secretions of the pancreas that

are helpful in digestion.

2. Name the part of the alimentary canal where major absorption of digested

food takes place. What are the absorbed forms of different kinds of food

materials?

Chapter 16 Digestion And Absorption

Plus One

Friday 5 February 2021

Chapter 16 Digestion And Absorption

Chapter 16 Digestion And Absorption

Menu

Pages

Labels

Popular Posts