NEET Biology Digestion and Absorption Notes

This NEET Biology notes page on Digestion and Absorption (Class 11 Biology, Chapter 16) covers the nutrition types of heterotrophs, the four steps of holozoic nutrition, the human alimentary canal, teeth and dental formulae, salivary glands, the stomach and ruminant stomach, the small intestine with its enzymes, absorption of food, and the caecum, colon and rectum.

Key Points to Pay Special Attention To

Terms and Definitions
1. Coprophagous
2. Galactosemia
3. Chylomicrons
4. Sanguivorous

Differences
1. Thecodont and acrodont teeth
2. Nucleosidases and nucleotidases
3. Bunodont and lophodont teeth
4. Chyme and chyle

Common Names
1. Wisdom teeth
2. Intrinsic factor

Functions
1. Ptyalin
2. Oxyntic cells
3. Rennin
4. Enterogastrone
5. Bile salts

Dental Formulae
1. Rat
2. Child
3. Rabbit
4. Horse

At a Glance

• Teeth & dental formulas
  • Salivary glands
  • Oesophagus
  • Stomach
  • Duodenum
  • Ileum
• Colon
  • Rectum

Nutrients and Types of Nutrition

The chemical substances of food are called nutrients and are mainly used for growth, development and energy production. These nutrients fall into two categories.
(I) Organic Nutrients: Carbohydrates, proteins, fat and vitamins.
(II) Inorganic Nutrients: Minerals and water (For details please refer 'Vitamins and Minerals')

The carbohydrates, proteins and fats are three major energy sources and are called macro nutrients or proximate principles of food. (For details please refer 'Biomolecules')

Autotrophs: These organisms synthesize their food from inorganic raw materials, for example plants.

Heterotrophs: These include most of the protozoans and animals which feed on high energy organic molecules. They are of four categories.
(i) Parasitic: Generally consume liquid food derived from the body of the host. For example, tapeworm, Ascaris and leech.
(ii) Symbiotic: Live in association with other animals and obtain food material from each other. For example, bacteria E. coli present in the human gut provide vitamins to the human body and in turn consume simple food materials from the intestine.
(iii) Saprophytic: Feed on dead organic matter by digesting them outside the body. For example, housefly and spider.
(iv) Holozoic: Consume whole or part of a plant or animal food in liquid or solid form (for example, animals). They can be classified into three major categories.
(a) Herbivores: Consume plant material, for example rabbit and cows.
(b) Carnivores: Consume flesh or animal meat, for example lion and tiger.
(c) Omnivores: Consume both plants and animals, for example cockroach, crow, rat and man.

Special Categories of Heterotrophs
(i) Sanguivores: Feed on blood of vertebrates, for example leech, louse, bedbug, female mosquito and vampire bat. Male mosquitoes feed on flower nectar and do not transmit malaria, yellow fever or dengue.
(ii) Insectivores: Feed on insects, for example frog, lizard and ant-eaters.
(iii) Larvivores: Feed on larvae, for example Gambusia fish, which feeds on mosquito larvae.
(iv) Coprophagous: Feed on faecal matter, for example pig and rabbit.
(v) Detritivores: Feed directly on decaying organic matter, for example earthworm.
(vi) Frugivores: Feed on fruits, for example birds, bats and monkeys. Most of the birds are grainivores.
(vii) Cannibals: Feed on the members of the same species.
(viii) Osmotrophs: Absorb predigested food material through skin by osmosis, for example tapeworm.
(ix) Filter-feeders: Pick up food particles from the water current, created by ciliary or flagellar beating, for example sponges, Paramecium and Unio.

Holozoic Nutrition

Holozoic nutrition includes four important steps.
(i) Ingestion: The process of feeding. Depending upon the size of food, animals with holozoic nutrition can be classified into two groups. Microphagy is feeding on microorganisms, for example Paramecium. Macrophagy is feeding on larger organisms, for example higher animals.
(ii) Digestion: The enzymatic conversion of complex food material like protein, carbohydrate, lipid and nucleic acid into simpler food materials by hydrolysis (break down by the addition of water). Digestion can be intracellular, for example in Amoeba, Paramecium and sponges, or extracellular in a cavity, for example in higher animals.
(iii) Absorption: Simpler food materials are taken up by the cells directly or indirectly through the blood or lymph, as in higher organisms.
(iv) Egestion: The removal of undigested waste from the body.

Survey of the Digestive Systems in Different Animal Groups

In free living protozoans, like Amoeba and Paramecium, ingestion occurs with the help of pseudopodia or cilia, and digestion is intracellular (inside food vacuole).

In parasitic protozoans, like Plasmodium and Trypanosoma, the food material is taken from the body of the host.

In porifers, digestion is completely intracellular.

In coelenterates, digestion is partly extracellular (in gastrovascular cavity) and partly intracellular (in nutritive cells). Egestion occurs through the same opening (mouth) since coelenterates have a blind-sac body plan.

In aschelminthes (for example Ascaris), the alimentary canal has two openings, that is mouth and anus. The body plan in such case is 'tube within tube'.

In annelids, arthropods, molluscs etc., the alimentary canal is very well developed. The digestive tract in earthworm includes mouth, buccal cavity, pharynx, oesophagus, gizzard, stomach, intestine (having typhlosole), rectum and anus. In cockroach the important parts of the gut are mouth, buccal cavity, pharynx, oesophagus, crop, gizzard, mid gut, hind gut and anus.

Digestive System in Human

The human digestive system includes two main components: (1) alimentary canal or gut and (2) digestive glands. The length of the human intestine (small and large) is about 4 m in live conditions. After death it becomes about 7 m. The small intestine is the longest part of the alimentary canal.

The digestive glands can be extrinsic (outside alimentary canal), for example salivary glands, pancreas and liver, or intrinsic (inside alimentary canal), for example gastric glands and intestinal glands.

Alimentary Canal: Mouth, buccal cavity, pharynx*, oesophagus, stomach, small intestine (duodenum, jejunum and ileum), large intestine (caecum, colon and rectum), anal tube and anus.
(*Pharynx is the common passage for food as well as air.)

[Figure: Fig. 16.1 Digestive system of human, showing nasal cavity, soft palate, buccal cavity, epiglottis, tongue, trachea, oesophagus, diaphragm, gall bladder, liver, duodenum, stomach, spleen, pancreas, transverse colon, ascending colon, caecum, appendix, ileum, colon, descending colon, pelvic colon, rectum and anus. Inset shows the stomach with cardiac notch, fundus, cardiac stomach, pyloric sphincter, body of stomach and pylorus (antrum and canal). Labels in parenthesis belong to the respiratory system.]

Mouth and Buccal Cavity

The buccal cavity is lined with stratified squamous epithelium and contains two important structures, teeth and salivary glands (openings).

Teeth

Teeth are ecto-mesodermal in origin. The enamel is ectodermal, whereas dentine and pulp cavity are mesodermal in origin.

A tooth is differentiated into three parts:
(1) Crown: The exposed part of the tooth.
(2) Neck: The part of the tooth embedded in the gums.
(3) Root: Embedded in the sockets of jaw bones.

[Figure: Fig. 16.2 C.S. of a tooth, labelled enamel, dentine, pulp cavity, gum, neck, crown, root, cement, and blood vessels and nerves]

The enamel, which forms the covering over the crown, is the hardest part in the body and is secreted from the ameloblast layer (ectodermal). This part is secreted before the eruption of the tooth.

The dentine forms the bulk of the tooth. It is secreted by the odontoblast layer, forming the covering around the pulp cavity. This layer is mesodermal in origin.

The pulp cavity is usually closed after the development of teeth. In rabbits and rats the pulp cavity of incisor teeth remains open throughout life, and these teeth keep growing. The tusks in elephant, which are modified incisors, also have open root and keep growing throughout life.

The joint between teeth and jaw bone is an immovable joint or fixed joint.

Types of Teeth

Heterodont teeth are of four types:
1. Incisors: Chisel shaped, for biting or gnawing.
2. Canines: Pointed, for tearing flesh.
3. Premolars: For chewing or crushing the food.
4. Molars: For chewing or crushing the food.

The premolars and molars are called cheek teeth.

Dental Formula

The dental formula is represented by the teeth of half of the upper jaw and half of the lower jaw. In dental formula, I = incisors, C = canines, Pm = premolars and M = molars.

Calculation for adult human: upper jaw (2 + 1 + 2 + 3) × 2 = 16, lower jaw (2 + 1 + 2 + 3) × 2 = 16, total 32.
Calculation for milk dentition: (2 + 1 + 0 + 2) × 2 = 10 for each jaw, total 20.

The total number of cheek teeth (premolars and molars) in an adult human is 20.

In children the premolars are absent and molars are present in place of premolars. The total number of teeth is 20.

The number of teeth which erupt only once in human are 8 premolars and 4 molars, that is 12.

The temporary or milk teeth begin to erupt when a child is about 6 months old, and all teeth (20) are developed by the end of 24 months. The permanent teeth begin to replace milk teeth in the 6th year of age and all teeth (32) are completed by 24 years of age. (Remember that the numerical values are the same for months and years.)

Each third molar of the upper and lower jaw (total 4 teeth) is called a wisdom tooth. These teeth are vestigial in human. They erupt late and are not used for crushing the food material. The total number of teeth in a 16-17 year old individual will be 28.

In rabbit the canines are absent, and the space between incisors and premolars is called diastema.

In rat the total teeth are 16 with dental formula 1003/1003.

The heterodont type of teeth are maximum (44) in horses and pigs with dental formula 3143/3143. The number of teeth in toothed whales is maximum, but they are homodont type.

The 4th premolar of the upper jaw and the 1st molar of the lower jaw form carnassial teeth in dogs, and are used for crushing the bones and tearing the tendons.

The terms bicuspid and tricuspid, used for heart valves, are also used for cheek teeth depending upon the number of cusps (projections over the surface of teeth).

The number of roots in each molar of the upper jaw is 3, while in each molar of the lower jaw it is 2.

Cheek Teeth Based on Surface Projections
(1) Lophodont teeth: Such teeth bear transverse ridges for crushing the food material, for example elephant and rabbit.
(2) Bunodont teeth: Such teeth bear rounded projections or tubercles, for example human, monkey and pig.
(3) Hypsodont teeth: The projections in such teeth are triangular or pyramidal in shape, for example horses and cattle.

Salivary Glands

In human there are 3 pairs of salivary glands (the infraorbital glands are absent). In rabbit the salivary glands are two pairs (the submaxillary glands are absent and infraorbital are doubtful). In frog the salivary glands are lacking.

A typical mammal has 4 pairs of salivary glands: parotid gland, submaxillary or submandibular gland, sublingual gland and infraorbital or zygomatic gland.

[Figure: Fig. 16.3 Salivary glands in human, showing the parotid gland, submandibular gland and sublingual gland, with the tongue, epiglottis, glottis, trachea, gullet and oesophagus]

(1) Parotid gland
It is present near the ear (pinna). It is the largest salivary gland but produces only about 20% of saliva. It contains the enzyme ptyalin (α-amylase). Ptyalin is lacking in predators, for example lion and tiger. The viral infection of these glands causes mumps. Its secretion is poured into the buccal cavity through Stensen's duct.

(2) Submaxillary (submandibular) gland
It is present on the lower side of the upper jaw. It produces about 70% of saliva secretion. Its secretion reaches the buccal cavity through Wharton's duct.

(3) Sublingual gland
It is present below the tongue. It is the smallest salivary gland and produces only about 5% of the secretion. Its secretion passes through the duct of Rivinus.

The poison glands of snakes such as cobra, viper and krait are modified parotid glands.

The salivary glands are absent in fishes, amphibians and aquatic mammals.

Saliva

Saliva is about 1.5 litres per day with a pH of 6 to 7 (slightly acidic). On standing or heating, it releases CO2 and becomes alkaline.
Mucus of saliva helps in lubrication.
Saliva causes denaturation of raw proteins without hydrolyzing them.
Thiocyanate ions, present in saliva, act as antimicrobial agents to prevent infection by microbes that often enter along with food.
Saliva also has lysozymes.
Ptyalin of saliva acts on boiled starch and converts it into maltose (a disaccharide).
Ptyalin: Starch (boiled) → Maltose

The digestion of carbohydrate (starch) is the first step in the digestion process, and begins in the buccal cavity.

In human the buccal cavity and pharynx are separated by a soft palate (uvula), while in frog the uvula is absent and the common cavity is called buccopharynx.

The chewed food, mixed with saliva, called bolus, is swallowed into the oesophagus through the gullet. During swallowing (a simple reflex), the tongue blocks the mouth, the soft palate closes the internal nostrils, and the epiglottis closes the glottis due to rising of the larynx.

Oesophagus

The oesophagus has both voluntary and involuntary muscles and runs through the neck and thorax to reach the abdominal cavity by piercing the diaphragm.
Its lining does not secrete digestive enzymes.
Mucus, secreted from goblet cells or glands, is added into the food.
Salivary digestion continues in the oesophagus.
Peristalsis in the gut begins from this part. During peristalsis, the circular and longitudinal muscles contract alternately to push the food further into the gut.

Stomach

The stomach is differentiated into three main parts: fundic stomach, body of stomach and pyloric stomach. The fundic as well as body of the stomach are for digestion and contain gastric glands (simple, branched and tubular type).

These glands contain three types of cells:
(1) Mucus cells: Secrete mucus which acts as a lubricant. Mucus also prevents the digestion of stomach by proteolytic enzymes and injury to the stomach by acid.
(2) Oxyntic (parietal) cells: Secrete HCl and Castle's intrinsic factor.*
(3) Peptic, zymogen or chief cells: Secrete digestive enzymes.

*Castle's intrinsic factor is secreted from oxyntic cells and helps in the absorption of vitamin B12. Vitamin B12 is also known as Castle's extrinsic factor.

The secretion of gastric juice (about 2.5 litres per day) is stimulated by the hormone gastrin, secreted from the pyloric mucosa.

Functions of HCl
It inactivates ptyalin and maintains pH 1 to 2 (strongly acidic).
It is germicidal and kills microbes.
It activates pro-enzymes.
The secretion of HCl is stimulated by histamine, acetylcholine and gastrin.

Enzymes of Gastric Juice

(1) Pepsin: Pepsinogen (proenzyme) → (HCl) → Pepsin (active). Pepsin acts on proteins to form proteoses and peptones.
Pepsin can digest collagen protein (maximum protein in the body) of white fibres.
Rennin is absent in adult human and probably in children also. The curdling of milk in human is caused by pepsin. Rennin is present in calves but is absent in adult cows.
Protein digestion in human is normal even in the absence of pepsin enzyme.

(2) Rennin: Prorennin (proenzyme) → (HCl) → Rennin (active). Rennin acts on milk protein: calcium paracaseinate (casein) → curdling of milk.

(3) Gastric lipase: Fat → Fatty acids + Glycerol (triglycerides). Gastric lipase is secreted in small amounts. Most lipase is mainly secreted in pancreatic juice.

The semi-digested food in the stomach is called chyme.

Emptying of Stomach
Carbohydrate-rich food leaves the stomach first, followed by the protein-rich food. The fat-rich food takes the maximum time for emptying the stomach.

When food reaches the duodenum, a part of the small intestine, the hormone enterogastrone, secreted from the duodenum, inhibits the secretion of gastric juice. VIP (vasoactive intestinal polypeptide) stimulates the secretion of electrolytes and water from the intestine and also inhibits gastrin secretion.

Cud-Chewing Animals (Ruminants)
Animals such as cows, buffaloes and camels have a four-chambered stomach specialized for cellulose digestion. It contains: (1) rumen, (2) reticulum, (3) omasum and (4) abomasum.
The rumen is the first and the largest part of the stomach. Rumen and reticulum both contain bacteria and protozoans for cellulose digestion. (Remember that the cellulase enzyme is not secreted from the gut wall of any vertebrate.)
The abomasum is the true stomach, as it secretes gastric juice for the digestion of proteins.

[Figure: Fig. 16.4 Stomach of a ruminant, showing oesophagus, rumen, reticulum, omasum, abomasum and duodenum]

Small Intestine

The duodenum contains Brunner's glands (compound tubular glands), which secrete mucus for neutralizing the acidity of food.

Two digestive juices are added into the duodenum through the sphincter of Oddi (at the opening of the ampulla of Vater): (i) bile juice and (ii) pancreatic juice.

Bile Juice (about 500 ml per day)
It is a greenish fluid which is synthesized in the liver and stored in the gall bladder. The gall bladder concentrates the bile juice and also alters its pH. The release of bile juice from the gall bladder is stimulated by the hormone cholecystokinin (CCK) of the duodenal mucosa. pH is 7.4 (slightly alkaline).
It contains bile pigments, bile acid (cholic acid), cholesterol, bile salts and phospholipids.
There is no digestive enzyme in bile juice.
Bile pigments are excretory products formed from non-iron haem. They include bilirubin and biliverdin. The bilirubin in the intestine breaks up into stercobilin and urobilin (urochrome). The colour of urine is due to urobilin, and the colour of faecal matter is due to stercobilin. (Remember that during jaundice or hepatitis, bilirubin, a toxic chemical, is not excreted out and gets deposited in the body tissues.)
Gall-stones are called cholelithiasis. The stones are of two types: (1) cholesterol stones and (2) calcium bilirubinate stones. The maximum of the stones (up to 80%) are of cholesterol, formed by supersaturation of bile.

Bile Salts
They are the sodium salts of cholic acid, that is sodium glycocholate and sodium taurocholate.
They cause emulsification of fats, in which larger particles of fat are converted into smaller particles, and the surface area is increased for better digestion.
They increase lipase activity and also help in the absorption of fat products.
The peristalsis is also stimulated by these salts.

Pancreatic Juice (about 1500 ml per day)
It is secreted from the acinar cells of the exocrine part of the pancreas. Its secretion is stimulated by the hormones secretin and CCK. Secretin increases the amount of bicarbonates (fluid) in pancreatic juice, while CCK increases the amount of enzymes. Both hormones are secreted from the duodenal lining. The pH of pancreatic juice is about 8.
Composition: trypsinogen, chymotrypsinogen, procarboxypolypeptidases, nucleases, amylopsin and steapsin, etc.

Functions of Pancreatic Enzymes

(1) Trypsin: Trypsinogen (proenzyme) → (Enterokinase) → Trypsin (active). Trypsin acts on proteins, proteoses and peptones to form polypeptides.
(The enterokinase is secreted from the ileum.)
Trypsin is present throughout the animal kingdom, whereas pepsin is present only in vertebrates. Trypsin cannot cause curdling of milk but can coagulate blood proteins in sanguivorous animals. It also cannot digest collagen protein.

(2) Chymotrypsin: Chymotrypsinogen (proenzyme) → (Trypsin) → Chymotrypsin (active). Chymotrypsin acts on proteins or proteoses and peptones to form polypeptides. (Chymotrypsinogen, like pepsin, can cause curdling of milk, but does it in alkaline medium.)

(3) Carboxypolypeptidases: Procarboxypolypeptidases (proenzyme) → (Trypsin) → Carboxypolypeptidases (active). They act on polypeptides to form oligopeptides and dipeptides. (Carboxypolypeptidases cleave the polypeptide chain from the side of the carboxylic group.)

(4) Nucleases: Nucleic acid (RNA or DNA) → Nucleotides.

(5) Amylopsin (α-amylase): Starch (amylose, boiled or unboiled) → Maltose.

(6) Steapsin (pancreatic lipase): Fats → Fatty acids + Glycerol.

Intestinal Juice (succus entericus) (about 1500 ml per day; pH 8 to 9)
The secretion of intestinal juice is stimulated by the hormone enterocrinin. The juice contains:
(1) Enterokinase (or enteropeptidase): Converts inactive trypsinogen into active trypsin.
(2) Aminopolypeptidases: Polypeptides → Oligopeptides + Dipeptides. (These are different from carboxypolypeptidases, and cleave the polypeptide chain from the side of the amino group.)
(3) Dipeptidases: Dipeptides → Amino acids. (The term erypsin is used for both aminopolypeptidases and dipeptidases. Thus erypsin can digest polypeptides into amino acids.)
(4) Nucleotidases: Nucleotides → Nucleosides + Phosphoric acid.
(5) Nucleosidases: Nucleosides → Nitrogenous bases + Sugar (pentose).
(6) Lipase: Secreted in small amounts. Converts fats into fatty acids and glycerol.
(7) Maltase: Maltose → Glucose + Glucose (disaccharide to two monosaccharides).
(8) Lactase: Lactose → Glucose + Galactose (disaccharide to two monosaccharides). The enzyme lactase is absent in certain persons, who cannot digest milk sugar. They can, however, take curd, in which lactose is converted into lactic acid by bacterial enzymes. Such persons have no problem in digesting milk protein, casein. (Lactose, the least sweet sugar, is present only in milk.) (Galactocemia is a congenital mental disorder due to excess of galactose. Children with this disorder are advised to take a milk-free diet.)
(9) Sucrase (invertase): Sucrose → Glucose + Fructose (two molecules of monosaccharides). Sucrose is cane sugar or table sugar and is also known as invert sugar. (Remember that fructose is the sweetest sugar in nature.)

In the small intestine, the major part of digestion occurs in the duodenum, and the digested simple substances are absorbed in the region of the jejunum and ileum.

Absorption of Food

The maximum of the digested food material, including water, is absorbed in the ileum part of the small intestine.
The presence of villi and microvilli in the small intestine increases the absorptive surface area by 600 times.
Water and water-soluble products are absorbed by the blood capillary network of villi, while fat-soluble substances (insoluble in water) are absorbed by lymph vessels.

Absorbed in blood capillaries: water, minerals and salts, water-soluble vitamins, amino acids, monosaccharides.
Absorbed in lymph vessels: fatty acids, glycerol, monoglycerides and fat-soluble vitamins.

The fructose, a hydrophilic sugar, is absorbed by facilitated diffusion using a transporter protein, while glucose is absorbed by active transport.

The fat-soluble vitamins and most of the digestive end products of fat, being hydrophobic, are not absorbed into the blood capillaries. They are mixed with bile salts and phospholipids to form micelles. (Fatty acids containing less than 10 to 12 carbon atoms can, however, pass directly through mucosal cells into the portal blood as free or unesterified fatty acids.)

The other fat products, like fatty acids (more than 12 C atoms), glycerol, sterols and fat-soluble vitamins, are absorbed from micelles by facilitated diffusion into enterocytes.

Within enterocytes the fats are resynthesized to form protein-coated particles called chylomicrons. These chylomicrons pass into lacteals (lymph vessels of villi), because the permeability of lymph vessels is greater than the permeability of blood vessels.

Due to absorption of chylomicrons in the lymph, the fluid becomes white and is called chyle.

About 90 to 95% of bile salts are absorbed from the ileum part of the small intestine.

Caecum and Vermiform Appendix

Both structures are meant for cellulose fermentation in rabbit and rat, etc. In human these structures are vestigial, as humans cannot digest cellulose.
Both structures in human, however, contain lymph nodules and help in lymphocyte production.

Colon

The colon includes ascending colon, transverse colon (the largest part of colon), descending colon and pelvic colon.
It contains three bands of longitudinal muscles, called taenia coli, which form pockets or pouches called haustra for water and mineral absorption.
About 1 to 2 litres of fluid reaches the colon.
No digestive enzyme is secreted from the colon.
E. coli bacteria survive on undigested waste of the colon and produce vitamins B1, B2, B12, folic acid and vitamin K for human use.

Rectum

The rectum stores undigested food. The solid waste contains about 30% bacteria and about 30% dry constituents of undigested food. The undigested food contains 10 to 20% fat and very low protein content.
This waste is removed from the body by egestion or defaecation through the anal opening.

Connecting Concepts: Practice Questions from the Chapter

1. Differentiate between:
(i) Dental formulae of human adult and child
(ii) Enamel and dentine
(iii) Sanguivorous and coprophagous animals
(iv) Digestion in sponges and cnidarians
(v) Lophodont teeth and bunodont teeth
(vi) Proximate and protective principles of food
(vii) Cannibals and carnivores

2. Fill in the blanks with suitable words or statement:
(i) ..................... teeth in dogs are used for tearing tendons and crushing bones.
(ii) ..................... layer secretes enamel.
(iii) Teeth in frog are ..................... , ..................... and ..................... type.
(iv) The space between incisors and premolar teeth of rabbit is called .....................
(v) In a person of 17 years the number of cheek teeth is ---------------

3. Name the following:
(i) Salivary glands in human
(ii) The HCl secreting cells in stomach
(iii) The hormone that inhibits the secretion of gastric juice
(iv) Digestive juice that does not have enzymes
(v) Enzyme that causes curdling of milk in acidic medium
(vi) Enzyme that activates trypsinogen
(vii) Chemical responsible for the absorption of vitamin B12
(viii) Component of bile that causes emulsification of fat

4. Differentiate between:
(i) Carboxypolypeptidases and aminopolypeptidases
(ii) Lactase and lactose
(iii) Trypsin and pepsin
(iv) Secretin and CCK
(v) Nucleosidases and nucleotidases
(vi) Micelles and chylomicrons
(vii) Bolus and chyme
(viii) Active transport and facilitated transport
(ix) Gastro-oesophageal and ileo-caecal valve