MOT

Introduction to Molecular Orbital Theory

Valence Bond Theory fails to answer certain questions like Why He2 molecule does not exist and why O2 is paramagnetic? Therefore in 1932 F. Hood and RS. Mulliken came up with theory known asMolecular Orbital Theory to explain questions like above. According to Molecular Orbital Theory individual atoms combine to form molecular orbitals, as the electrons of an atom are present in various atomic orbitals and are associated with several nuclei.

Fig. No. 1 Molecular Orbital Theory

Electrons may be considered either of particle or of wave nature. Therefore, an electron in an atom may be described as occupying an atomic orbital, or by a wave function Ψ, which are solution to theSchrodinger wave equation. Electrons in a molecule are said to occupy molecular orbitals. The wave function of a molecular orbital may be obtained by one of two method:
1. Linear Combination of Atomic Orbitals (LCAO). 2. United Atom Method.

Linear Combination of Atomic Orbitals (LCAO)

As per this method the formation of orbitals is because of Linear Combination (addition or subtraction) of atomic orbitals which combine to form molecule. Consider two atoms A and B which have atomic orbitals described by the wave functions ΨA and ΨB .If electron cloud of these two atoms overlap, then the wave function for the molecule can be obtained by a linear combination of the atomic orbitals ΨA and ΨB i.e. by subtraction or addition of wave functions of atomic orbitals

ΨMO= ΨA + ΨB

The above equation forms two molecular orbitals

Bonding Molecular Orbitals

When addition of wave function takes place, the type of molecular orbitals formed are called Bonding Molecular orbitals and is represented by ΨMO = ΨA + ΨB.

They have lower energy than atomic orbitals involved. It is similar to constructive interferenceoccurring in phase because of which electron probability density increases resulting in formation of bonding orbital. Molecular orbital formed by addition of overlapping of two s orbitals shown in figure no. 2. It is represented by s.

Anti-Bonding Molecular Orbitals

When molecular orbital is formed by subtraction of wave function, the type of molecular orbitals formed are called Antibonding Molecular Orbitals and is represented byΨMO = ΨA - ΨB.

They have higher energy than atomic orbitals. It is similar to destructive interference occurring out of phase resulting in formation of antibonding orbitals. Molecular Orbital formed by subtraction of overlapping of two s orbitals are shown in figure no. 2. It is represented by s* (*) is used to represent antibonding molecular orbital) called Sigma Antibonding.

Fig. No. 2 Formation of Bonding and Anti-Bonding Orbital

Therefore, Combination of two atomic orbitals results in formation of two molecular orbitals, bonding molecular orbital (BMO) whereas other is anti-bonding molecular orbital (ABMO).

BMO has lower energy and hence greater stability than ABMO. First BMO are filled then ABMO starts filling because BMO has lower energy than that of ABMO.

Formation of molecular orbitals occurs by the combination of atomic orbitals of proportional symmetry and comparable energy. Therefore, a molecular orbital is polycentric and atomic orbital is monocentric. Number of molecular orbitals formed is equal to the number of atomic orbitals.

Differences between Molecular Orbital and Atomic Orbital

Molecular Orbital

Atomic Orbital

An electron Molecular orbital is under the influence of two or more nuclei depending upon the number of atoms present in the molecule. Molecular orbitals are formed by combination of atomic orbitals They have complex shapes.

An electron in atomic orbital is under the influence of only one positive nucleus of the atom. Atomic orbitals are inherent property of an atom. They have simple shapes.

Relative Energies of Molecular Orbitals

Bonding Molecular Orbitals (BMO) - Energy of Bonding Molecular Orbitals is less than that of Anti Bonding Molecular Orbitals because the attraction of both the nuclei for both the electron (of the combining atom) is increased.

Anti-Bonding Molecular Orbitals (ABMO) - Energy of Anti Bonding Molecular Orbitals is higher than Bonding Molecular Orbitals because the electron try to move away from the nuclei and are in repulsive state.

The Energies of Bonding Molecular Orbitals and Anti-Bonding Molecular Orbitals are shown in figure below:

Fig. No.3 Energies of BMO and ABMO

Energy Level Diagram

The factors upon which relative energies of molecular orbitals depend are:

(i) Energies of the Atomic orbitals combining to form Molecular Orbitals.

(ii) The extent of overlapping between the atomic orbitals. The greater the overlap, the more the bonding orbital is lowered and the anti-bonding orbital is raised in energy relative to AOs

1s Atomic Orbitals (AOs) of two atoms form two Molecular Orbitals (MOs) designated as s1s and s *1s.The 2s and 2p orbitals (eight AOs of two atoms) form four bonding MOs and four anti-bonding MOs as:

Bonding MOs: σ 2s, σ 2pz, π 2px, π 2py

Anti – Bonding MOσ: σ *2s, σ *2pz, π *2px, π *2py

Using Spectroscopy, the energy levels of these molecular orbitals are determined experimentally. The order of increasing energy of molecular orbitals obtained by combination of 1s, 2s and 2p orbitals of two atoms is →

σ1s, σ *1s, σ 2s, σ *2s, σ 2pz, π 2px = π 2py, π *2px= π *2py, σ *2pz

(Energy Increases from left to right)

The molecular orbital diagram representing this order of energy levels is shown in fig.

Fig. No. 4 Order of Energy

But experimental evidence for some diatomic molecules have shown that the above sequence of energy levels of MOs is not correct for all the molecules. For example, homonuclear diatomic molecules of second row elements like Li2, Be2, B2 , C2, N2 , the σ 2pz  MOs is higher in energy than π 2pxand π 2py MOs

For these atoms, the order is: →

σ1s, σ *1s, σ 2s, σ *2s, [π 2px = π 2py], σ 2pz [π *2px= π *2py], σ*2pz

The molecular orbital diagram representing this order of energy levels is shown in fig.

Fig. No. 5 Order of Energy Levels for Boron, Carbon, Nitrogen etc.

This kind of energy reversal is due to mixing of 2s and 2p orbitals where the energy difference is very close, that is, for B, C, and N atoms. According to the symmetry interactions, the two orbitals of the same symmetry repel each other and the lower energy orbital lowers down more while the higher energy orbital is energized more. Accordingly, σ 2s and σ 2px have same symmetry and similarly for   σ *2s and σ *2px the energy of σ 2s is lowered and that of the σ 2px becomes higher. Similarly, the energy of σ *2s lowered while that of σ *2px becomes higher. Finally, the energy of the σ *2px becomes higher than π2py and π2pz which remain unchanged in the symmetry interaction. This kind of mixing of orbitals or symmetry interaction is not applicable for O2 and F2 molecule formation because of larger energy gap between 2s and 2p orbitals for these atoms.

Rules for Filling of Molecular Orbitals

Certain rules are to be followed while filling up molecular orbitals with electrons in order to write correct molecular configurations:

1. Aufbau Principle – This principle states that those molecular orbital which have the lowest energy are filled first.
2. Pauli’s Exclusion Principle – According to this principle each molecular orbital can accommodate maximum of two electrons having opposite spins.
3. Hund’s Rule – This rule states that in two molecular orbitals of the same energy, the pairing of electrons will occur when each orbital of same energy consist one electron.

Order of energy of various Molecular Orbitals

Order of energy of various molecular orbitals is as follows:

For O2 and higher molecules →

σ1s, σ *1s, σ 2s, σ *2s, σ 2pz, [π2px = π2py], [π*2px= π*2py], σ *2pz

 

Fig. no. 6 Order of Energy for O2 and Higher molecules

For N2 and lower molecules →

σ 1s, σ *1s, σ 2s, σ *2s, [π 2px = π 2py], σ 2px [π *2px= π *2py], σ*2pz 

Fig. No. 8 Order of Energy for N2 and lower molecules 

Bond Order

It may be defined as the half of difference between the number of electrons present in the bonding orbitals and the antibonding orbitals that is,

Bond order (B.O.) = (No. of electrons in BMO - No. of electrons in ABMO)/ 2

Those with positive bonding order are considered stable molecule while those with negative bond order or zero bond order are unstable molecule.

Magnetic Behavior: If all the molecular orbitals in species are spin paired, the substance is diamagneti. But if one or more molecular orbitals are singly occupied it is paramagnetic. For Example, if we look at CO Molecule, it is diamagnetic as all the electron in CO are paired as in the figure below:

Fig. No. 9 Molecular Orbital Diagram for CO

Analysis done by Bond Order

• If value of bond order is positive, it indicates a stable molecule and if the value is negative or zero, it means that the molecule is unstable.
• The stability of a molecule is measured by its bond dissociation energy. But the bond dissociation energy is directly proportional to the bond order. Greater the bond order, greater is the bond dissociation energy.
• Bond order is inversely proportional to the bond length. The higher the bond order value, smaller is the bond length. For Example the bond length in nitrogen molecule is shorter than in oxygen molecule
• Magnetic character - If all the electrons in the molecule of a substance are paired, the substance is diamagnetic (repelled by the magnetic field). On the other hand, if there are unpaired electrons in the molecule, the substance is paramagnetic (attracted by the magnetic field).

Let’s take a question to understand it more clearly.

Q1: Arrange the species O2, O2-, O2+ the decreasing order of bond order and stability and also indicate their magnetic properties.

Sol. The molecular orbital configuration of
O2, O2-, O22-, O2+ are as follows:
O2 = σ1s2, σ*1s2, σ2s2, σ*2s2, σ2pz2, π2px2 = π2py2,  π*2px1 = π*2py1
Bond order = (10-6)/2 = 2, Number of unpaired electrons = 2, Therefore paramagnetic

O2- = σ 1s2, σ *1s2, σ 2s2, σ *2s2, σ 2pz2, π2px2 = π2py2, π*2px2 = π*2py1
Bond order = (10-7)/2 = 1.5, Number of unπaired electrons = 1, Therefore paramagnetic

O22- = σ1s2, σ*1s2, σ2s2, σ*2s2, σ2pz2, π2px2 = π2py2, π*2px2 = π*2py2

Bond order = (10-8)/2 = 1, Number of unpaired electrons = 0, Therefore diamagnetic

O2+ = σ1s2, σ*1s2, σ2s2, σ*2s2, σ2pz2, π2px2 = π2py2, π*2px1= π*2py0

Bond order = (10-5)/2 = 2.5, Number of unpaired electrons = 1, Therefore paramagnetic

The bond order decreases in the order is
O2+ >O2>O2->O22-
so, we conclude stability is directly proportional to bond order.

In 1971, the General conference on weights and measures, adopted mole as the seventh basic
SI unit of the amount of a substance.
The term ‘mole’ is a Latin word which means heap or pile. A mole of atom is a collection of
atoms whose total weight is equal to the gram atomic weight. As equal number of moles of
different elements contain equal number of atoms, it is very easy to express the amount of a substance in terms of moles. Just as a dozen means twelve objects, in the similar fashion, a
mole is a collection of a ‘definite number’ of particles, viz., atoms, molecules, ions or electrons.
This ‘definite number’ is called Avogadro number which is equal to 6.023  1023. Thus, a
mole can be defined as, “the amount of a substance containing as many atoms, molecules, ions,
electrons or other elementary entities as there are carbon atoms in exactly 12g of C-12”.
Now-a-days, gram-molecules and gram-atom are termed as a mole of molecules and a
mole of atoms respectively, e.g., 1 gram-molecule of chlorine and 1 gram atom of
chlorine are expressed as 1 mole of Cl2
 and 1 mole of Cl respectively.
The number of moles can be calculated by a number of formulae depending upon the data
given :
No. of moles of molecules = Wt. of substance in g
Mol. wt. of substance
No. of moles of atoms = Wt. of element in g
At. wt. of the element
No. of moles of gas = Standard molar volume (i.e. 22.4L)
Volume of the gas at STP

Mole Concept

Stoichiometry

Introduction

Stoichiometry deals with the calculation of the quantities of various reactants and products involved in a chemical reaction. The numerals used to balance a chemical equation are known as stoichiometric coefficients. These numerals are essential for solving problems based on chemical equation, hence called stoichiometric calculation.

In stoichiometric calculations, the mole relationship between different reactants and products are required as from them the mass-mass, mass-volume and volume-volume relationship between different reactants and products can be obtained.

Stoichiometric calculations can be carried out by two methods; Gravimetric Analysis and Volumetric Analysis. In the first method the amount of chemical species is determined by measurement of mass, whereas, in second method it is determined by measurement of volume. Stoichiometric calculation can be carried in terms of two important concept: (i) Mole Concept; (ii) Equivalent concept.

MOLE CONCEPT

A mole (symbol mol) is defined as the amount of substance that contains as many atoms, molecules, ions electrons or any other elementary entities as there are carbon atoms in exactly 12 gm of 12C.

The number of atoms in 12 gm of 12C is called Avogadro’s number (N0)

N0 = 6.023 x 1023

One atomic mass unit (amu) = 1/N0 = 1.66 x 10–24 gm = 1.66 x 10–27 kg

The number of moles of a substance can be calculated by various means depending on data available, as follows.

(a) 
(b) 
(c) 
(Standard molar volume at STP = 22.4 lit)

(d) 
(e) Number of moles of solute = molarity x volume of solution in litres

(f) for a compound AxBy , y moles of A = x moles of B

(g) Mole fraction = fraction of the substance in the mixture expressed in terms of mol is called its mol fraction (X)
E.g. for a mixture of substance A & B


XA + XB = 1 & XA= (1 − XB)

PRINCIPLE OF ATOM CONSERVATION (POAC)

In chemical reaction atoms are conserved, so moles of atoms shall also be conserved. This is known as principle of atomic conservation.

This principle is helpful in solving problems of nearly all stoichiometric calculations e.g.

KClO3(s) → KCl(s) + O2(g)

Applying POAC for K atoms

Moles of K atoms in KClO3 = Moles of K atoms in KCl

Since one mole of KClO3 contains 1 mol of K atom. Similarly 1 mol of KCl contains one mole of K atoms.

(Mass-mass relationship)

Applying POAC for ‘ O ‘ atoms

Moles of O atom in KClO3 = Moles of O atom in O2

3 x nKClO3 = 2 x nO2

(Mass volume relationship of reactant and product)

In this way applying POAC we can break the chemical equation into a number arithmetic equations without balancing the chemical equation.

Moreover number of reactions and their sequence from reactants to products are not required. It is important to note that POAC can be applied for the atoms which remain conserved in chemical reaction.

Know these facts

*All mammals have 7 cervical vertebrae except 6 in manatee, 8 in ant bear (Tamandua) and 9 in sloth (Bradypus).

*Elephants can hear infrasonic sounds of 14 hertz. from hundreds of km and whales can hear ultrasonic sounds of 123,000 hertz from thousands of km.

*Each second brain receives 100 billion impulses and fires 5000 commands but we are conscious of one millionth activity of our brain.

*Human brain loses about 100,000 neurons each day but the loss is insignificant because of the presence of billions of neurons in brain.

*Snakes auditory lobes are highly enlarged to hear faint sounds received through ground as they possess no external ear.

* Other than mammals, in all vertebrates spinal cord has more control over body functions than the brain.

*In humans brain is marvelously packed as 1800 sq cm of brain surface area is packed in only 15% of skull space.

*Nerve impulse travels in our body @ 400 km per hour.

*Crab eating frog (Rana cancerivora) is the only amphibian that can survive in the intertidal marine water.

*Monotremes and marsupials do not possess aprostate gland.

*In sharks and caecilian kidney is calledopisthonephros because it extends up to the posterior end of the body.

*Birds have only the left ovary and oviduct.

*Systolic blood pressure decreases in arteries from 130 mmHg in systemic arch, 30 mmHg in arterioles, 20 mmHg in capillaries, 12 mmHg in venules, 5 mmHg in smaller veins and zero mmHg in larger veins.

*Aortic blood pressure in giraffe is 260/160 to supply brain sitting on top of the long neck, but in brain it is only 120/70.

*Largest RBCs (75 microns) are found in the urodele, Amphiuma, and the smallest (2.5 microns) in the musk deer.

*Whales store air in huge nasal chambers and not in lungs.

*Elephant’s trunk has 40,000 muscles.

*No animal breathes free air. Oxygen from air must diffuse through fluid to the blood and hence a water film is necessary over the respiratory surface in all animals.

*Sharks jaws are loosely attached to the skull, so that when shark bites its jaws come out of the oral cavity to seize prey.

*Plethodont salamanders possess a tongue that is three-fourth the length of the body.

*Tongue of an elephant weighs 12 kg while that of a whale weighs 1500 kg and 50 persons can stand on it.

*Incisors of rodents and elephants continue to grow throughout life. A single tusk of an African elephant weighs about 100 kg.

*In hind gut fermenters such as horses, elephants and rabbits, large amount of food eaten goes undigested and hence they must eat large quantities to obtain enough nutrients.

*Gastric juices in snake’s stomach are so strong that they dissolve even the bones of the swallowed prey.

*In humans intestine is 28 feet long whereas in cow it is 165 feet long.

*Batrachotoxin and bufotoxin are such potent poisons secreted by frogs and toads that only 200 microgram can kill a man.

*Penguins breed in Antarctic and hatch their eggs at minus 50 degrees.

*Density of sweat glands is highest in the skin of Indians (738/sq cm) followed by Negros (709/sq cm), while in Europeans it is only 550/sq cm.

*The American pronghorn(Antilocapra americana) is the only antelope that sheds its horn annually.

*Antlers are found only in male deer and are shed after breeding season is over. Reindeers are the only deers that possess permanent antlers in both males and females.

In echidna and duckbilled platypus both male and female possess active mammary glands and feed young babies.

Sharks retain 100 times more urea in blood as compared to mammals. Their organs cannot function without such a high amount of urea in body.

Humans can sweat away as much as 3 litres of water per hour in summer. A person can collapse if loss of body fluid is 10% of body weight.

Digestive system in vertebrates

Digestive system includes alimentary canal and the associated glands. Alimentary canal or gut shows four distinct regions, namely, ingressive zone that includes mouth, lips and jaws for capturing and handling food; progressive zone includes oral cavity, pharynx and oesophagus through which food passes with little digestion; degressive zone contains stomach and small intestine in which digestive process is accomplished and the egressive zone includes colon and rectum where undigested food stays before expulsion and excess water from it is absorbed. Each part of the alimentary canal is described below one by one in a sequence.

 MOUTH

Mouth is the anterior opening of the alimentary canal, which may be terminal, ventral or slightly dorsally directed. It is guarded by suckers in cyclostomes and by jaws and teeth in gnathostomes. Lips are horny in fishes but fleshy and suctorial in mammals. Birds and turtles possess horny beak in place of lips.

ORAL CAVITY

This is the anterior most chamber of alimentary canal meant for handling food. Depending upon the kind and size of food, its size is highly variable and it contains three important organs for handling food material, namely, oral glands, tongue and teeth.

ORAL GLANDS

Cyclostomes possess mucous glands in oral cavity but Petromyzon also has salivary glands that secrete an anticoagulant enzyme. Fishes and perennibranch amphibians have no particular oral glands except simple mucous glands. In tailed amphibians and apoda oral glands are almost nonexistent but in anurans there are. Oral glands are poorly developed in turtles and crocodiles but well developed in lizards and snakes. Poison glands of snakes are modified labial glands and those of Heloderma,the only poisonous lizard that has fangs in the lower jaw, has sublingual glands modified as poison glands.

Birds possess sublingual glands that open into the floor of the oral cavity. Mammalian oral cavity is very wet as it contains two types of glands – salivary glands and mucous glands. The compound acinar submaxillary or submandibular glands lie in the posterior part of the lower jaw. Sublingual glands are smaller than the other two salivary glands. Molar glands are mucous glands that are well developed in herbivores and open near the upper molars. Another kind of mucous glands are Orbital glands which occur in cat and dog family.

TONGUE

Tongue is a fleshy and highly mobile organ in the oral cavity that is used in various ways in vertebrate groups.

Cyclostomes possess a thick and fleshy primary tongue on the floor of the oral cavity. In fishes the tongue is primary and merely a fleshy fold on the floor of anterior end of pharynx supported by the extension of hyoid arch.

In perennibranch urodeles such as Necturus,tongue is similar to fishes and is not put to much use. Frogs and toads having a predilection for insectivorous diet are gifted with a highly flexible tongue that consists of a basal primary tongue and the anterior glandular and muscular secondary tongue. Turtles and crocodiles being amphibious in nature have a small non-protrusible tongue but snakes and lizards possess a highly movable tongue that is bifurcated at the apex and supplied with olfactory cells. 

Bird tongue is short and hard and practically lacks muscles and lateral lingual swellings. Such incapable tongue is of no handicap to these beaked creatures as the food does not stay in the mouth for longer duration. However, some birds are gifted with long and flexible tongue such as woodpeckers.

Mammalian tongue is the best developed of all vertebrates, except in the aquatic cetaceans. It is derived from 5 portions—paired fleshy ridges of hyoid arch, a median secondary tongue called tuberculum impar and paired lateral lingual swellings, which provide it extraordinary mobility and flexibility in the oral cavity. Tongue of anteaters is suitably long and sticky to feed on termites which they dig out by their strong fossorial front legs.

 TEETH

Teeth are hard bony structures in the oral cavity that are variously modified to capture, tear, cut or grind food material before it is swallowed. Epidermal teeth are hard cornified epidermal structures of rare occurrence, as in the buccal funnel of cyclostomes and on the edges of tadpole jaws. 

 TYPES OF TEETH

Polyphyodont dentition involves replacement of teeth from time to time several times in lifetime so that jaws are never left without teeth.

Diphyodont dentition is a characteristic of mammals in which milk teeth appear in the young ones but as they grow and jaw becomes larger, milk teeth are replaced by larger permanent ones.

Monophyodont teeth appear only once in lifetime and if they fall they are never again replaced by new ones. 

Based on the type of attachment of teeth on the jaw bone the following three types are found in vertebrates:

Acrodont teeth are attached on the top surface of the jaw bone as in fish and amphibians.

Pleurodont teeth are attached on the inner side of the jawbone that brings larger surface area of tooth in contact with jawbone and hence attachment is stronger, as in lizards and urodeles.

Thecodont dentition is found in mammals in which root of the tooth is firmly fixed in a socket of the jawbone, making the attachment strongest in vertebrates. 

Based on the kinds of teeth found there are two types of dentition:

Homodont dentition is found in the majority of vertebrates such as fish, amphibia and reptiles in which all teeth are functionally of the same type, although their size may be variable depending on the location.

Heterodont dentition occurs in mammals in which there are 4 functionally different types of teeth, namely, flat incisors for cutting, long and pointed canines for tearing flesh and large and broad premolars and molars with flat grinding surface. 

There are also some other type of teeth as follows:

Secodont teeth have sharp cutting edges that function like scissors to cut flesh.

Bunodont teeth are small with smaller cusps or tubercles on the surface for handling soft diet as in man, monkeys, rodents etc.

Brachydont teeth are smaller and low crowned suitable for feeding on soft diet.

Hypsodont teeth possess larger crown that can resist wear and tear of feeding on tough and fibrous diet as in ungulates.

Selenodont teeth, as found in horses and other ungulates, deposit silica around cusps and in the depressions of the grinding surface.

Lophodont teeth are found in elephants which feed on the roughest diet that any mammal can feed on.

DEVELOPMENT OF TOOTH

Teeth develop over the jaw bone where certain malpighian cells start actively multiplying forming a mass of cells called dental lamina or enamel organ. A dental papilla made of group of dermal cells appears below the dental lamina that supplies nourishment to the growing mass of cells. Cells of the outer layer of dermal papilla arrange themselves in a row and get differentiated into odontoblast cells.

Epidermal cells of the dental lamina that cover the growing dentine are called ameloblasts. The tooth gradually grows outwards and eventually gets exposed by penetrating through the skin covering the jaw bone. The dental papilla inside the pulp cavity remains active along with its blood supply and nerve intact. This development of tooth is identical to the development of dermal scales in fishes. Hence shark teeth are also called modified placoid scales.

COMPARATIVE ACCOUNT OF DENTITION

Cyclostomes have only epidermal teeth. Some fishes are toothless such as sturgeons, sea horse and pipe fish and others like lung fishes and Chimaera have teeth modified into crushing plates. Majority of fishes possess Polyphyodont, Acrodont and homodont dentition suitable for seizing prey.

In amphibians teeth are located on jaw bones, palatine and vomer bones and are Polyphyodont. Tadpoles lack true teeth and their jaws have horny epidermal ridges which are used to scrape algae on which they feed.

Among reptiles, turtles lack teeth and have horny beak. In others, teeth are generally confined to jaw bones but in snakes and lizards may occur on palatine and pterygoid bones. Fangs of snakes are modified upper maxillary teeth.

Modern birds lack teeth but Archaeopteryx had thecodont dentition and so were the toothed birds Ichthyornis and Hesperornis.

DENTITION IN MAMMALS 

Mammals as a rule possess heterodont, diphyodont and thecodont dentition. However, some mammals lack teeth as given below in detail.

Among monotremes, the spiny anteater or echidna (Tachyglossus and Zaglossus) lacks teeth.

Edentates, as the name suggests are toothless such as the giant anteater of South America. Among the aquatic Cetacea baleen whales have no teeth, such as the blue whale, Balaenoptera musculus  and, the whalebone whale, 

Among humans, and astonishingly, males in “Bhudas” tribe of Hyderabad Sindh in Pakistan are genetically so predisposed that they never grow teeth all their lives.

The Dental Formula

Mammals have heterodont dentition having four types of teeth meant for different function in handling food in the oral cavity. Incisors in front are flat teeth designed for cutting food into pieces and the canines next to them are generally long and pointed spike-like used for tearing flesh by carnivore animals. Premolars and molars are located on the posterior side of the jaw, have flat surface with tubercles called cusps and are used for grinding food of plant origin. They are therefore well developed in herbivore animals. Number and arrangement of teeth in mammals is specific in different groups of animals so much so that mammalian orders can be identified by their teeth and dental formula, which is written for one half of the upper and lower jaw as follows:

3 – 1 – 4 – 3 x 2 = 44. This dental formula belongs to horse and pig.

 3 – 1 – 4 – 3

PRIMATES.  Primates are basically arboreal animals whose ancestors were insectivores and some of them still continue with their original diet.  

UNGULATES. Ungulates belong to two orders, the even-toed Artiodactyla and the odd-toed Perissodactyla that includes horses and rhinoceroses. They are all herbivores and fleet footed grazers and browsers, with teeth adapted for grinding tough vegetation. Musk deer also has large upper canines hanging on either side of the jaws. Hippopotamus has large canines too with sharp edges meant for defense against the lurking crocodiles in their amphibious habitat.

Pigs and horses have full set of 44 teeth as depicted in the dental formula given above. Premolars and molars are similar in shape and size, have flat grinding surface, with silica deposits between cusps. Such grinders are called selenodont and are designed to grind tough grasses. 

INSECTIVORES. In insectivores such as shrews, hedgehogs and moles, all teeth are pointed and grinding teeth possess peg-like cusps for crushing hard exoskeleton of insects on which they feed. In the mole genera Scalopus andCondylura milk teeth are retained throughout life.

CHIROPTERA. In bats milk teeth are shed before birth and they are born with permanent teeth. Insectivorous bats have conical cusps on the grinding teeth for crushing insects.

 RODENTIA. Rodents include rats, mice, squirrels and guinea pigs which possess chisel-shaped front incisors for gnawing nuts and hard objects. These teeth grow throughout life due to the wide opening of the pulp cavity but they are worn out equally fast.

 LAGOMORPHA. In rabbits also the upper incisors are chisel shaped adapted for gnawing, canines are absent and cheek teeth are modified for grinding. As in other herbivores, a big diastema is present between incisors and premolars.

PROBOSCIDIA.  The order includes elephants which have upper incisors modified as long tusks which are used for digging roots of plants, for removing barks from trees or for offence and defense. Premolars and molars are alike in appearance and they have broad and lophodont surface, in which cusps unite to form circular lophs of ridges with silica deposits in the depressions. 

CARNIVORES.  Carnivores include cats, tigers, lions, dogs, wolves, jackals and bears and the aquatic seals, sea lions and walruses. Their canines are long and pointed, dagger like for tearing flesh of the prey. Canines of walruses are modified into long tusks. Carnassial teeth are enlarged teeth with pointed cusps. These teeth also have sharp cutting edges and are called secodont teeth used for shearing flesh.

CETACEA.  There are no teeth in Mysticeti or baleen whales which possess baleen plates hanging from the palate that are suitable for straining planktons from sea water. In Odontoceti or toothed whales, teeth are homodont and monophyodont which are used to seize fish or other prey.

 MARSUPIALS. Generally marsupials retain milk teeth except the last premolars. Herbivores have a diastema and premolars and molars modified for grinding.

PHARYNX

Pharynx is part of the alimentary canal between oral cavity and oesophagus and is primarily concerned with respiration. In fishes pharynx exhibits paired gill pouches containing gill lamellae and gill slits opening to the exterior, whereas in terrestrial vertebrates trachea opens into the pharynx. Nasal passage opens into the oral cavity in all other vertebrates except mammals and crocodiles in which nasal passage opens far backwards into the pharynx, allowing the oral cavity to handle food while breathing can go on uninterrupted. At this time larynx is pulled forward to lie against a flap of tissue called epiglottis, which closes the tracheal opening called glottis. The nasal opening is closed by the soft palate and uvula which is a fleshy elongation hanging downward from the soft palate. In all other vertebrates except mammals as long as the food remains in the oral cavity breathing has to stop. 

 OESOPHAGUS

Oesophagus is a narrow tube that connects pharynx with stomach and is generally as long as the length of neck. It has no serous coat and inner mucosa bears longitudinal folds that give it enormous power of distension to allow large food to pass through it by peristalsis. Fish oesophagus is very short bearing longitudinal folds but in birds and mammals it may be very long as in giraffe where it has to match the whole length of neck.

STOMACH

Stomach is a muscular chamber or a series of chambers that serves for storage of food swallowed, macerating and churning it into pulp by peristalsis and secrete and mix certain digestive juices with it for digestion of nutrients. Primary function of stomach continues to be storage of large quantity of food that has been swallowed.

Cyclostomes.  In larval cyclostomes stomach is ciliated which is quite useful in pushing detrivorous diet backwards on which they feed but in adult lampreys stomach is indistinguishable.

 Fishes. In most of the fishes stomach continues to be narrow and long to adjust inside the elongated body cavity but in elasmobranchs it is J-shaped and measures about half the length of the entire digestive tract.

 Amphibia. Urodeles have straight stomach with hardly any digestive function assigned to it. But in frogs and toads cardiac end is wide and pyloric small.

Reptiles. Snakes and lizards have elongated stomachs that fit inside their elongated abdominal cavity but in turtles the stomach is narrow U-shaped tube. Crocodiles have highly specialized stomach that is highly curved. Except for the tortoises, digestive glands are strongly developed in the stomachs of reptiles.

Birds. Bird stomach is modified greatly into a glandular proventriculus and horny gizzard that is necessary to grind seeds and other types of food that they need to swallow whole in the absence of teeth.

Mammals. In monotremes stomach is sack-like but lacks glands and in ungulates and cetaceans the glands occur in pyloric portion only. Stomach is a large sac meant to accommodate large quantity of food that must be swallowed quickly when available before the arrival of predators and competitors. 

Ruminant (cud-chewing) mammals have a complex stomach having four parts, namely, rumen or pounch, reticulum with honey-comb like rough lining, omasum or psalterium and abomasum or rennet. Gastric juice is secreted by the lining of abomasums and pylorus for further digestion. Carnivore stomach is clearly divided into cardiac, fundus and pyloric portions, of which fundus always remains empty and accommodates gases. The so called hourglass stomach is found in primates and rodents in which cardiac and pyloric parts are divided by a constriction.

INTESTINE

It is part of the alimentary canal between stomach and cloaca or anus and the primary site of digestion and absorption. Digestion is alkaline as bile and pancreatic juices are released into duodenum which is attached to the pyloric part of stomach.

Cyclostomes. Intestine is straight, slightly enlarged on the posterior side to form rectum and terminates into anus that opens in a cloacal depression. 

 Fishes. Intestine of fishes is short, wide and almost straight, although some teleosts possess a spirally twisted intestine. Lungfishes also have a cloacal caecum to increase the absorptive surface area. Bony fishes do not have a spiral valve but have many hollow finger-like pyloric caeca instead, attached between pyloric stomach and duodenum, which perform the same function of increasing the digestive area of intestine.

 Amphibia. In limbless amphibians, intestine is almost straight and is not differentiated into small and large intestine. In urodeles and anurans small intestine is long and coiled but large intestine is short and straight and separated from small intestine by an ilio-colic valve. 

Reptiles. Small intestine is elongated, coiled and uniform in diameter and large intestine is almost straight. Ilio-colic valve is present between the two portions.

Birds. Length of intestine increases in birds but large intestine is short, straight and terminates into cloaca. Most of the birds possess one or two colic caeca that increase the absorptive surface of intestine. 

Mammals. Intestine is more elaborate; the small intestine is divided into duodenum, jejunum and ileum and the large intestine differentiated into colon and rectum. Jenjunum (means empty) is about 8 feet long in man and has leaf-like large villi. Ileum is about 12 feet long and has finger-like one millimeter long villi for aiding absorption. Carnivore intestine measures only 5-6 times the length of body while in herbivores it is 20-28 times the body length. In carnivores including man caecum is reduced and appendix rudimentary. Rectum is small and opens to the exterior by anal opening, except in monotremes where there is a shallow cloaca.

 LIVER

The main function of liver is assimilation and treatment of food after it has been digested and absorbed in intestine. New types of proteins and fats are synthesized in liver. But liver also produces bile juice that contains bile pigments. Often a gall bladder that opens by cystic duct into the bile duct for quick release whenever required.

Cyclostomes. Liver is unusually small and single lobed in lampreys but bilobed in hagfishes. Gall bladder is absent in lampreys but present in hagfishes but in larval stages both gall bladder and bile ducts are present in all.

 Fishes. Liver is large and lobed in fishes. Gall bladder is always present except in some sharks.

 Amphibia. Liver is large for the body size, lobed and with a gall bladder.

 Reptiles. There is no important deviation from amphibians in reptiles. In sakes there is a single elongated lobe. Gall bladder is always present in reptiles.

Birds. Liver is always lobed. Gall bladder is sometimes absent as in pigeon, in which bile ducts open directly.

Mammals. There are many more variations in mammals as compared to other groups. There are two main lobes which are subdivided into as many as 6-7 sub-lobes. Gall bladder is generally absent in those whose diet does not include much fat. 

 PANCREAS

This is the second largest digestive gland that arises from the endoderm of embryonic gut and can be divided into head, body and tail portions that lie in the loop of duodenum. It performs both exocrine and endocrine functions; the former contributes to about 99% of the secretions that are responsible for the digestion of proteins, fats and carbohydrates in small intestine.

Cyclostomes. There is no well-defined pancreas in adult lampreys but pancreatic tissue is embedded in liver and intestine. Hagfishes possess small pancreas that lies near the bile duct.

 Fishes. Cartilaginous fishes have 2-lobed, well-defined pancreas that has a single duct. In lungfishes and some teleosts pancreas is diffused and its endocrine portion is separated.

Amphibia, reptiles & Aves. There is no noteworthy feature in these groups. There is a well-developed pancreas with one or several ducts which may open in duodenum either directly or may join the bile duct.

Mammals. Generally two pancreatic ducts are present in mammals. Both ducts are functional in horse and dog. When bile duct joins the pancreatic duct there is a sphincter of Boyden at the junction. A small sac-like hepatopancreatic ampulla or ampulla or Vater is found in man in which bile duct and pancreatic duct open.

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