FIRST UNIT TEST SYLLABUS- CH1, CH 2, CH 16 & CH 17CH-1

THE LIVING WORLD

All living organisms grow. Increase in mass and increase in number of individuals are twin characteristics of growth. One must remember that increase in body mass is considered as growth. Non-living objects also grow if we take increase in body mass as a criterion for growth. Mountains, boulders and sand mounds do grow. However, this kind of growth exhibited by non-living objects is by accumulation of material on the surface. In living organisms, growth is from inside. Growth, therefore, cannot be taken as a defining property of living organisms.

Reproduction, likewise, is a characteristic of living organisms. In multicellular organisms, reproduction refers to the production of progeny possessing features more or less similar to those of parents. there are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc). Hence, reproduction also cannot be an all-inclusive defining characteristic of living organisms. Of course, no non-living object is capable of reproducing or replicating by itself.

Another characteristic of life is metabolism. All living organisms are made of chemicals. These chemicals, small and big, belonging to various classes, sizes, functions, etc., are constantly being made and changed into some other biomolecules. These conversions are chemical reactions or metabolic reactions. .Metabolic reactions can be demonstrated outside the body in cell-free systems. An isolated metabolic reaction(s) outside the body of an organism, performed in a test tube is neither living nor non-living. Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things but surely living reactions. Hence, cellular organisation of the body is the defining feature of life forms.

All organisms therefore, are ‘aware’ of their surroundings. Human being is the only organism who is aware of himself, i.e., has self-consciousness. Consciousness therefore, becomes the defining property of living organisms.

we can say that living organisms are self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli.

The number and types of organisms present on earth is referred as Biodiversity.

There is a need to standardise the naming of living organisms such that a particular organism is known by the same name all over the world. This process is called Nomenclature.

Nomenclature or naming is only possible when the organism is described correctly and we know to what organism the name is attached to & this is called Identification.

For plants, scientific names are based on agreed principles and criteria, which are provided in International Code for Botanical Nomenclature (ICBN).

Animal taxonomists have evolved International Code of Zoological Nomenclature (ICZN).

The scientific names ensure that each organism has only one name. Description of any organism should enable the people (in any part of the world) to arrive at the same name. They also ensure that such a name has not been used for any other known organism.

Biologists follow universally accepted principles to provide scientific names to known organisms. Each name has two components – the Generic name and the specific epithet. This system of providing a name with two components is called Binomial nomenclature. This naming system given by Carolus Linnaeus is being practised by biologists all over the world.

The scientific name of mango is written as Mangifera indica. Let us see how it is a binomial name. In this name Mangifera represents the genus while indica, is a particular species, or a specific epithet. Other universal rules of nomenclature are as follows:

1. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.

2. The first word in a biological name represents the genus while the second component denotes the specific epithet.

3. Both the words in a biological name, when handwritten, are separately underlined, or printed in italics to indicate their Latin origin.

4. The first word denoting the genus starts with a capital letter while the specific epithet starts with a small letter. It can be illustrated with the example of Mangifera indica.

Name of the author appears after the specific epithet, i.e., at the end of the biological name and is written in an abbreviated form, e.g., Mangifera indica Linn. It indicates that this species was first described by Linnaeus.

Classification is the process by which anything is grouped into convenient categories based on some easily observable characters. For example, we easily recognise groups such as plants or animals or dogs, cats or insects.

based on characteristics, all living organisms can be classified into different taxa. This process of classification is Taxonomy.

Hence, characterisation, identification, classification and nomenclature are the processes that are basic to taxonomy.

Classification is not a single step process but involves hierarchy of steps in which each step represents a rank or category. Since the category is a part of overall taxonomic arrangement, it is called the taxonomic category and all categories together constitute the taxonomic hierarchy.

Each category, referred to as a unit of classification, in fact, represents a rank and is commonly termed as taxon (pl.: taxa).

Kingdom

Phylum or Division

Class

Order

Family

Genus

Species

Species-A group of individual organisms with fundamental similarities as a species. One should be able to distinguish one species from the other closely related species based on the distinct morphological differences. For example Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion). All the three names, indica, tuberosum and leo, represent the specific epithets, while the first words Mangifera, Solanum and Panthera are genera and represents another higher level of taxon or category.

Genus-Genus comprises a group of related species which has more characters in common in comparison to species of other genera. We can say that genera are aggregates of closely related species. For example, potato and brinjal are two different species but both belong to the genus Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several common features, are all species of the genus Panthera. This genus differs from another genus Felis which includes cats.

Family-The next category, Family, has a group of related genera with still less number of similarities as compared to genus and species. Families are characterised on the basis of both vegetative and reproductive features of plant species. Among plants for example, three different genera Solanum, Petunia and Datura are placed in the family Solanaceae. Among animals for example, genus Panthera, comprising lion, tiger, leopard is put along with genus, Felis (cats) in the family Felidae. Similarly, if you observe the features of a cat and a dog, you will find some similarities and some differences as well. They are separated into two different families – Felidae and Canidae, respectively.

Order- Generally, order and other higher taxonomic categories are identified based on the aggregates of characters. Order being a higher category, is the

assemblage of families which exhibit a few similar characters. The similar characters are less in number as compared to different genera included in a family. Plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales . The animal order, Carnivora, includes families like Felidae and Canidae.

Class-This category includes related orders. For example, order Primata comprising monkey, gorilla and gibbon is placed in class Mammalia along with order Carnivora that includes animals like tiger, cat and dog. Class Dicoytedon include orders like Polymoniales, Sapindales etc.Phylum-Classes comprising animals like fishes, amphibians, reptiles, birds along with mammals constitute the next higher category called Phylum. All these, based on the common features like presence of notochord and dorsal hollow neural system, are included in phylum Chordata. In case of plants, classes with a few similar characters are assigned to a higher category called Division eg. Division Angiosperm include dicotyledon & monocotyledon.

Kingdom-All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia in the classification system of animals. The Kingdom Plantae, on the other hand, is distinct, and comprises all plants from various divisions. Henceforth, we will refer to these two groups as animal and plant kingdoms.

Common Biological Genus Family Order Class Phylum/

Name Name Division

ManHomo sapiens Homo Hominidae Primata Mammalia Chordata

Housefly Musca Musca Muscidae Diptera Insecta Arthropoda

domestica

MangoMangifera

Mangifera

Anacardiaceae

Sapindales Dicotyledonae Angiospermae

indica

Wheat Triticum Triticum Poaceae PoalesMonocotyledonae Angiospermae

aestivum

TAXONOMICAL AIDS

Taxonomic studies of various species of plants, animals and other organisms are useful in agriculture, forestry, industry and in general in knowing our bio-resources and their diversity. These studies would require correct classification and identification of organisms. Identification of organisms requires intensive laboratory and field studies. The collection of actual specimens of plant and animal species is essential and is the prime source of taxonomic studies. These are also fundamental to studies and essential for training in systematics. It is used for classification of an organism, and the information gathered is also stored along with the specimens. In some cases the specimen is preserved for future studies.

Biologists have established certain procedures and techniques to store and preserve the information as well as the specimens. Some of these are explained to help you understand the usage of these aids.

Herbarium-Herbarium is a store house of collected plant specimens that are dried, pressed and preserved on sheets. These sheets are arranged

according to a universally accepted system of classification. These specimens, along with their descriptions on herbarium sheets, become a store house or repository for future use. The herbarium sheets also carry a label providing information about date and place of collection, English, local and botanical names, family, collector’s name, etc. Herbaria also serve as quick referral systems in taxonomical studies.

Botanical Gardens-These specialised gardens have collections of living plants for reference. Plant species in these gardens are grown for identification purposes and each plant is labelled indicating its botanical/scientific name and its family. The famous botanical gardens are at Kew (England), Indian Botanical Garden, Howrah (India) and at National Botanical Research Institute, Lucknow (India).

Museum-Biological museums are generally set up in educational institutes such as schools and colleges. Museums have collections of preserved plant and animal specimens for study and reference. Specimens are preserved in the containers or jars in preservative solutions. Plant and animal specimens may also be preserved as dry specimens. Insects are preserved in insect boxes after collecting, killing and pinning. Larger animals like birds and mammals are

usually stuffed and preserved. Museums often have collections of skeletons of animals too.

Zoological Parks-These are the places where wild animals are kept in protected environments under human care and which enable us to learn about their food habits and behaviour. All animals in a zoo are provided, as far as possible, the conditions similar to their natural habitats. Children love visiting these parks, commonly called Zoos (Figure 1.3).

Key-Key is another taxonomical aid used for identification of plants and animals based on the similarities and dissimilarities. The keys are based on the contrasting characters generally in a pair called couplet. It represents the choice made between two opposite options. This results in acceptance of only one and rejection of the other. Each statement in the key is called a lead. Separate taxonomic keys are required for each taxonomic category such as family, genus and species for identification purposes. Keys are generally analytical in nature.

Manuals-It contain compiled information about area covered, Keys, description of families, genus and species.

Monograph-It gives comprehensive account of complete compilation of available information of anyone family or genus at a given time.

CH-2

BIOLOGICAL CLASSIFICATION

Aristotle was the earliest to attempt a more scientific basis for classification. He used simple morphological characters to classify plants into trees, shrubs and herbs. He also divided animals into two groups, those which had red blood and those that did not.

Linnaeus gave Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants and animals respectively.

Need to change this system –

1. did not distinguish between the eukaryotes and prokaryotes, prokaryotic bacteria and eukaryotic blue green algae were put together.

2. unicellular and multicellular organisms were placed together. Unicellular Chlamydomonas & multicellular Spirogyra were placed together under Algae.

3.Photosynthetic (green algae) and non-photosynthetic (fungi) organisms were put together under plant kingdom due to presence of cell wall.

Five Kingdoms

Characters

Monera Protista Fungi Plantae Animalia

Cell type Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic

Cell wall Noncellulosic Present in Present Present

(Polysaccharide Some with chitin (cellulose) Absent

+ amino acid)

NuclearAbsent

Present Present Present Present

membrane

BodyCellular

Cellular M u l t i c e u l l a r / Tissue/ Tissue/organ/

organisation loose tissue organ organ system

Autotrophic

Autotrophic H e t e r o t r o p h i c Autotrophic Heterotrophic(chemosyn-

(Photosyn- ( S a p r o p h y t i c / ( P h o t o s y n - ( H o l o z o i c /thetic and

Mode of thetic) and Parasitic) thetic) S a p r o p h y t i cphotosynthetic)

nutrition Hetero- etc.)and Hetero-

Trophictrophic (sapro-

phytic/para-

sitic)

R.H. Whittaker (1969) proposed a Five Kingdom Classification. The kingdoms defined by him were

named Monera, Protista, Fungi, Plantae and Animalia. The main criteria for classification used by him include cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. The table is shown above.

In Five Kingdom Classification-

All prokaryotic organisms were grouped together under Kingdom Monera. the unicellular eukaryotic organisms were placed in Kingdom Protista. Kingdom Protista has

brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba (which were earlier placed in the animal kingdom which lack cell wall).

It has put together organisms which, in earlier classifications, were placed in different kingdoms.

phylogenetic, i.e., is based on evolutionary relationships.

In this chapter we will study characteristics of Kingdoms Monera, Protista and Fungi of the Whittaker system of classification. The Kingdoms Plantae and Animalia, commonly referred to as plant and animal kingdoms, respectively, will be dealt separately in chapters 3 and 4.

Kingdom Monera

Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms. Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive. Many of them live in or on other organisms as parasites.

Bacteria are grouped under four categories based on their shape: the spherical Coccus (pl.: cocci), the rod-shaped Bacillus (pl.: bacilli), the comma-shaped Vibrium (pl.: vibrio) and the spiral Spirillum (pl.: spirilla)

Types of bacteria-

1. Archaebacteria- These bacteria are special since they live in some of the most harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens). Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions. Methanogens are present in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.

2. Eubacteria- There are thousands of different eubacteria or ‘true bacteria’. They are characterised by the presence of a rigid cell wall, and if motile, a flagellum. The cyanobacteria (also referred to as

blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs (Figure 2.2). The cyanobacteria are unicellular, colonial or filamentous, freshwater/marine or terrestrial algae. The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena. Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.Heterotrophic bacteria are most abundant and useful in many ways-a. They are decomposers.

b. Convert milk to curd.c. Used in production of antibiotics.d. Fix nitrogen in legumes.Some are pathogens causes diseases in different organism like Cholera, typhoid, tetanus citrus canker etc.

Reproduction- They reproduce mainly by fission, under unfavourable conditions, they produce spores, They show primitive type of sexual reproduction by transferring DNA from one bacterium to the other.

Smallest living cells which can survive without oxygen and lack cell wall is Mycoplasma.

Kingdom Protista

All single-celled eukaryotes are placed under Protista. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi. Being eukaryotes, the protistan cell body contains a well defined nucleus and other membrane-bound organelles. Some have flagella or cilia. Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation. It includes-

A. Chrysophytes- 1.This group includes diatoms and golden algae (desmids).

2. They are found in fresh water as well as in marine environments. They are microscopic and float passively in water currents (plankton).

3. Most of them are photosynthetic.

4. In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth’. Being gritty this soil is used in polishing, filtration of oils and syrups. Diatoms are the chief ‘producers’ in the oceans.

B. Dianoflagellates - 1. They are mostly marine and photosynthetic.

2. As per the presence of pigment, they appear yellow, brown, blue or red.

3. Cell wall has stiff cellulosic plates and have to flagella.

4. Red dianoflagellate called Gonyaulax multiply very rapidly which make sea appear red (red tides) and release toxin which kill fishes and other marine animals.

C. Euglenoids- 1. Mostly they are fresh water organisms.

2. They have protein rich layer called Pellicle instead of cell wall which makes their body flexible.

3. They are photosynthetic in presence of sunlight otherwise behave as heterotroph.

4. Their pigments are identical to higher plants. Eg- Euglena.D. Slime Moulds-1. They are saprophytic in nature.

2. Under suitable conditions, they form an aggregation called Plasmodium which can grow several feet but under unfavourable condition, plasmodium differentiates and form fruiting bodies having spores.

3. Spores have true walls which is extremely resistant and survive for many years and help in reproduction.

E. Protozoans- They are heterotrophs. Four major groups of protozoans-

i. Amoeboid protozoans: These organisms live in fresh water, sea or moist soil. They move & capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites.

ii. Flagellated protozoans: The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diaseases such as sleeping sickness. Example: Trypanosoma.

iii. Ciliated protozoans: These are aquatic, actively moving organisms because of the presence of thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium .

iv. Sporozoans: This includes diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on human population.

Kingdom Fungi- The fungi constitute a unique kingdom of heterotrophic organisms. They show a

great diversity in morphology and habitat. We can see fungi on a moist bread and rotten fruits. The

common mushroom we eat and toadstools are also fungi. White spots seen on mustard leaves are due

to a parasitic fungus. Some unicellular fungi, e.g., yeast are used to make bread and beer. Other fungi

cause diseases in plants and animals; wheat rust-causing Puccinia is an important example. Some are

the source of antibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water, soil and on

animals and plants. They prefer to grow in warm and humid places. we keep food in the refrigerator to

prevent food from going bad due to bacterial or fungal infections.

With the exception of yeasts which are unicellular, fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium. Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae. Others have septae or cross walls in their hyphae. The cell walls of fungi are composed of chitin and polysaccharides.

Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes. Those that depend on living plants and animals are called parasites. They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.

Reproduction in fungi can take place by vegetative means – fragmentation, fission and budding. Asexual reproduction is by spores called conidia or sporangiospores or zoospores, and sexual reproduction is by oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruiting bodies. The sexual cycle involves the following three steps:

(i) Fusion of protoplasms between two motile or non-motile gametes called plasmogamy.

(ii) Fusion of two nuclei called karyogamy.(iii) Meiosis in zygote resulting in haploid spores.

When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi the fusion of two haploid (n) cells immediately results in diploid cells (2n). However, in other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n, i.e., two nuclei per cell) occurs; such a condition is called a dikaryon and the phase is called dikaryophase of fungus. Later, the parental nuclei fuse and the cells become diploid. The fungi form fruiting bodies in which reduction division occurs, leading to formation of haploid spores

Kingdom Fungi is divided into four classes-

1.Phycomycetes-

Habitat and mode of nutrition- Found in aquatic habitat and on decaying wood in moist and damp places or as obligate parasite (can live only as parasite).

Structure- Mycellium is aseptate and coenocytic(multinucleated).

Reproduction- Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile). These spores are endogenously (inside) produced in sporangium. Sexual reproduction-A zygospore is formed by fusion of two gametes. These gametes are similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).

Some common examples are Mucor, Rhizopus (the bread mould mentioned earlier) and Albugo (the parasitic fungi on mustard).

2. Ascomycetes-

Habitat and mode of nutrition- They are saprophytic, decomposers, parasitic or coprophilous (grow on dung).

Structure- Mycellium is branched and septate.

Reproduction- Asexual reproduction conidia produced exogenously (outside) on the special mycelium called conidiophores. Conidia on germination produce mycelium.

Sexual reproduction- Sexual spores are called ascospores which are produced endogenously in sac like asci (singular ascus). These asci are arranged in different types of fruiting bodies called ascocarps.

Some examples are Aspergillus, Claviceps and Neurospora. Neurospora is used extensively in biochemical and genetic work. Many members like morels and truffles are edible and are considered delicacies.

3 . Basidiomycetes-

Habitat and mode of nutrition- They grow in soil, on logs and tree stumps and in living plant bodies as parasites.

Structure- Mycellium is branched and septate.

Reproduction- Asexual reproduction- The asexual spores are generally not found, but vegetative reproduction by fragmentation is common. Sexual reproduction-The sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes. The resultant structure is dikaryotic which ultimately gives rise to basidium. Which on meiosis produce four basidiospores. The basidiospores are exogenously produced on the basidium (pl.: basidia). The basidia are arranged in fruiting bodies called basidiocarps.

Some common members are Agaricus (mushroom) (Figure 2.5c), Ustilago (smut) and Puccinia (rust fungus).

4 . Deuteromycetes-

Habitat and mode of nutrition- Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling.

Structure- Mycellium is branched and septate.

Reproduction- Asexual reproduction- by asexual spores known as conidia.

Sexual reproduction- Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known. When the sexual forms of these fungi were discovered they were moved into classes they rightly belong to. It is also possible that the asexual and vegetative stage have been given one name (and placed under deuteromycetes) and the sexual stage another (and placed under another class). Later when the linkages were established, the fungi were correctly identified and moved out of deuteromycetes. Once perfect (sexual) stages of members of dueteromycetes were discovered they were often moved to ascomycetes and basidiomycetes.

V IRUSES , V IROIDS , P RIONS AND L ICHENS

In the five kingdom classification of Whittaker there is no mention of lichens and some acellular organisms like viruses, viroids and prions. These are briefly introduced here.

All of us who have suffered the ill effects of common cold or ‘flu’ which is caused by viruses. Viruses did not find a place in classification since they are not considered truly ‘living’, if we understand living as those organisms that have a cell structure. The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell. Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host. So viruses are connecting link between living and non-living.

The name virus that means venom or poisonous fluid was given by Dmitri Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco Viruses are smaller than bacteria because they passed through bacteria-proof filters.

M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid).

W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell. Viruses are obligate (can only live as) parasites.

Characteristics of viruses-

viruses contain genetic material, that could be either RNA or DNA and a coat of protein. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. In general, viruses that infect plants have single stranded RNA and viruses that infect animals

have either single or double stranded RNA or double stranded DNA. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double

stranded DNA viruses . The protein coat called capsid made of small subunits called capsomeres, protects the nucleic

acid. These capsomeres are arranged in helical or polyhedral geometric forms. Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also

caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Viroids :

In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.

It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.

The RNA of the viroid was of low molecular weight.

Prions :

In modern medicine certain infectious neurological diseases were found to be transmitted by an agent consisting of abnormally folded protein.

The agent was similar in size to viruses. These agents were called prions. The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE)

commonly called mad cow disease in cattle and its analogous variant Cr–Jacob disease (CJD) in humans.

Lichens :

Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which are

autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for

its partner. So close is their association that if one saw a lichen in nature one would never imagine that they

had two different organisms within them. Lichens are very good pollution indicators – they do not grow in polluted areas.

NOTE- PREPARE DIAGRAM NO. 2.1, 2.2 & 2.6(b)

CH-16

DIGESTION AND ABSORPTION

DIGESTIVE SYSTEM

The human digestive system consists of the alimentary canal and the associated glands.

Alimentary Canal

The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.

The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and a muscular tongue.

Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont. Majority of mammals including human being forms two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of dentition is called diphyodont.

An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (I), canine I, premolars (PM) and molars(M).

Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human

2123

is 2123 .

The hard chewing surface of the teeth, made up of enamel, helps in the mastication of food. The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum. The upper surface of the tongue has small projections called papillae, some of which bear taste buds.

The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx.

A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing.

The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.

A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach.

The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a cardiac portion into which the oesophagus opens, a fundic region, body (main central region) and a pyloric portion which opens into the first part of small intestine.

Small intestine is distinguishable into three regions, a ‘C’ shaped duodenum, a long

coiled middle portion jejunum and a highly coiled ileum. The opening of the stomach into the duodenum is guarded by the pyloric sphincter. Ileum opens into the large intestine. It consists of caecum, colon and rectum.

Caecum is a small blind sac which hosts some symbiotic micro-organisms. A narrow finger-like tubular projection, the vermiform appendix which is a vestigial organ, arises from the caecum.

The caecum opens into the colon. The colon is divided into four parts – an ascending, a transverse, descending part and a sigmoid colon. The descending part opens into the rectum which opens out through the anus.

Wall of alimentary canal- Four layers are present from oesophagus to rectum.

Serosa- it is the outermost layey made of mesothelium having some connective tissue, Muscularis- made of smooth muscles arranged into inner circular & outer longitudinal layer. Submucosa- made of loose connective tissue having nerves, blood & lymph vessel. Mucosa- Innermost layer which form irregular folds in stomach & villi in small intestine.

The cells lining the villi produce many microscopic projections called microvilli which increases surface area. Villi are supplied with a network of capillaries ana a large lymph vessel called lacteal.Gastric gland of stomach & crypt of Lieberkuhn in between bases of villi is made from mucosa.

DIGESTIVE GLANDS- Three glands are Salivary glands, Liver and Pancreas.

1. Salivary glands- Three pairs are- Parotid-present in cheek, Sub maxillary/ sub mandibular- in lower jaw and sub lingual- below the tongue. They secrete salivary juice into the buccal cavity.

2. Liver- It is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes. The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The duct of gall bladder (cystic duct) along with the

hepatic duct from the liver forms the common bile duct. The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.

3. Pancreas- is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘C’ shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.

DIGESTION OF FOOD-

The process of digestion is accomplished by mechanical and chemical processes.-

1) MOUTH- The buccal cavity performs two major functions, mastication of food and facilitation of swallowing. The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus. The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis. The gastro-oesophageal sphincter controls the passage of food into the stomach. The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase and lysozyme.

Salivary amylase. About 30 per cent of starch is hydrolysed here by this enzyme (optimum Ph 6.8) into a disaccharide – maltose. Lysozyme present in saliva acts as an antibacterial agent that prevents infections.

Salivary AmylaseStarch → Maltose

Ph 6 .8

2) STOMACH-The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely –

(i) mucus neck cells which secrete mucus;

(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and

(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12).

The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme. The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach.

Function- Pepsin- It converts proteins into proteoses and peptones (peptides).

Mucus and bicarbonates -They lubricate and protect mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. HCl provides the acidic Ph (Ph 1.8) optimal for pepsins.

Rennin- is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.

NOTE-Various types of movements are generated by the muscularis layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.

The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. Pancreatic juice and bile are released through the hepato-pancreatic duct.

3)PANCREAS- The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.

Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:

Proteins Trypsin/Chymotrypsin

Peptones → Dipeptides

Proteoses Carboxypeptid

ase

Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

Amylase

Polysaccharides ( starch) → Disaccharides

Fats are broken down by lipases with the help of bile into di-and monoglycerides.

Lipases

Fats → Diglycerides → Monoglycerides

Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides

Nucleases

Nucleic acids → Nucleotides → Nucleosides

4 LIVER-The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.

5 SUCCUS ENTERICUS-The intestinal mucosal epithelium has goblet cells which secrete mucus. The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus alongwith the

bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (Ph 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also help in this.

The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.

Dipeptidases

Dipeptides → Amino acids

Maltase

Maltose → Glucose + Glucose

Lactase

Lactose → Glucose + Galactose

Sucrase

Sucrose → Glucose+ Fructose

Nucleotidases Nucleosidases

Nucleotides → Nucleosides → Sugars + Bases

Lipases

Di and Monoglycerides → Fatty acids + Glycerol

The breakdown of biomacromolecules mentioned above occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine. The undigested and unabsorbed substances are passed on to the large intestine.

6) LARGE INTESTINE- No significant digestive activity occurs in the large intestine. The functions of large intestine are:

(i) absorption of some water, minerals and certain drugs;

(ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defaecation.

NEURAL & HORMONAL CONTROL OF DIGESTIVE SYSTEM-

The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.

Gastric and intestinal secretions are also, similarly, stimulated by neural signals. The muscular activities of different parts of the alimentary canal can also be moderated by

neural mechanisms, both local and through CNS. Hormonal control of the secretion of digestive juices is carried out by local hormones produced

by the gastric and intestinal mucosa.

A BSORPTION OF D IGESTED P RODUCTS-

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.

It is carried out by passive, active or facilitated transport mechanisms. Small amounts of monosaccharides like glucose, amino acids and some electrolytes like

chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients. Transport of water depends upon the osmotic gradient.

some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called the facilitated transport.

Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.

ABSORPTION OF FATTY ACID-

Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. They are first incorporated into small droplets called micelles which move into the intestinal mucosa.

They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi.

These lymph vessels ultimately release the absorbed substances into the blood stream.

NOTE-Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine.

ASSIMILATION-The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation.

EGESTION-The remaining substances solidified into coherent faeces in the rectum & initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

D ISORDERS OF D IGESTIVE S YSTEM

The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pin worm, etc.

Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.

Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.

Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.

Constipation: In constipation, the faeces are retained within the colon as the bowel movements occur irregularly.

Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.

PEM-

In many underdeveloped countries of South and South-east Asia, South America, and West and Central Africa, dietary deficiencies of proteins and food calories are very common.

Protein-energy malnutrition (PEM) may affect large sections of the population during drought, famine and political turmoil.

This happened in Bangladesh during the liberation war and in Ethiopia during the severe drought in mid-eighties.

PEM affects infants and children to produce Marasmus and Kwashiorkar.Marasmus –

It is caused by a simultaneous deficiency of proteins and calories. It is found in infants less than a year in age, if mother’s milk is replaced too early by other foods

which are poor in both proteins and caloric value. This often happens if the mother has second pregnancy or childbirth when the older infant is

still too young. In Marasmus, protein deficiency impairs growth and replacement of tissue proteins; extreme

emaciation of the body and thinning of limbs results, the skin becomes dry, thin and wrinkled. Growth rate and body weight decline considerably.

Growth and development of brain and mental faculties are also impaired.

Kwashiorkar-

It is produced by protein deficiency unaccompanied by calorie deficiency. It results from the replacement of mother’s milk by a high calorie-low protein diet in a child more

than one year in age. Like marasmus, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth and

brain development. But unlike marasmus, some fat is still left under the skin; moreover, extensive oedema and

swelling of body parts are seen.

CH-17

BREATHING AND EXCHANGE OF GASES

R ESPIRATORY O RGAN

ANIMAL RESPIRATORY ORGAN

Lower invertebrates like sponges, simple diffusion over their entire body surface coelenterates, flatworms, etc. Earthworms Moist cuticle Insects Tracheal tubes aquatic arthropods, molluscs & fishes gills Amphibians Lungs &moist skin Reptiles, Birds & Mammals Lungs Human Respiratory System

We have a pair of external nostrils opening out above the upper lips. It leads to a nasal chamber through the nasal passage.

The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air.

The pharynx opens through the larynx region into the trachea. Larynx is a cartilaginous box which helps in sound production and hence called the sound box.

During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.

Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.

The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.

Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.

The branching network of bronchi, bronchioles and alveoli comprise the lungs (Figure 17.1). We have two lungs which are covered by a double layered pleura, with pleural fluid between them. It reduces friction on the lung-surface. The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.

The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.

The conducting part perform the following function- i) transports the atmospheric air to the alveoli, ii) clears it from foreign particles iii) humidifies air iv) brings the air to body temperature.

Exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air. The lungs are situated in the thoracic chamber which is an air-tight chamber. The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum,

laterally by the ribs and on the lower side by the dome-shaped diaphragm. The anatomical setup of lungs in thorax is such that any change in the volume of the thoracic

cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Respiration involves the following steps:

Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.

Diffusion of gases (O2 and CO2) across alveolar membrane. Transport of gases by the blood. Diffusion of O2 and CO2 between blood and tissues. Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2

MECHANISM OF BREATHING

Breathing involves two stages : 1) Inspiration-In which atmospheric air is drawn in 2) Expiration- In which the alveolar air is released out.

The movement of air into and out of the lungs is carried out by creating a pressure gradient (differencein pressure) between the lungs and the atmosphere.

Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.

MECHANISM OF INSPIRATION

When muscles of diaphragm & external inter-costal muscles contracts

Volume of thoracic chamber increases

Pulmonary volume also increases

Intra-pulmonary pressure decreases than atmospheric pressure

Air from atm. rushes in the lungs

MECHANISM OF EXPIRATION

When muscles of diaphragm & external inter-costal muscles relax

Volume of thoracic chamber decreases

Pulmonary volume also decreases

Intra-pulmonary pressure increases than atmospheric pressure

Air from lungs rushes out in the atm.

NOTE- A healthy human breathes 12-16 times/minute. The volume of air during breathing can be estimated by SPIROMETER.

RESPIRATORY VOLUMES AND CAPACITIES-

Tidal Volume(TV)- In normal respiration, the amount of air we inspire or expire is called tidal volume. It is approx..500 mL it means 6000 to 8000 mL of air per minute is inspired or expired.

Inspiratory Reserve Volume (IRV)- Additional volume of air which can be inspired by forceful inspiration is called IRV which is approx. 2500 mL to 3000 mL

Expiratory Reserve Volume (ERV)- Additional volume of air which can be expired by forceful expiration is called ERV which is approx. 1000 mL to 1100 mL

Residual Volume (RV)- Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL

By adding up a few respiratory volumes described above, one can derive various pulmonary capacities, which can be used in clinical diagnosis.

Inspiratory Capacity (IC)- Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume (TV+IRV).

Expiratory Capacity (EC)- Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV).

Functional Residual Capacity (FRC)- Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.

Vital Capacity (VC)- The maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration.

Total Lung Capacity (TLC)- Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.

E XCHANGE OF G ASES- It occur at two places in the body.

1) Alveoli- Which is the primary sites of exchange of gases. 2) between blood and tissues.

O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion.

Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide. Partial pressures of these two gases in the atmospheric air and the two sites of diffusion are given in Table.

TABLE Partial Pressures (in mm Hg) of Oxygen and Carbon dioxide at Different Parts Involved in Diffusion in Comparison to those in Atmosphere

Respiratory Atmospheric Alveoli Blood Blood Tissues

Gas Air (Deoxygenated) (Oxygenated)

O2 159 104 40 95 40

CO2 0.3 40 45 40 45

Condition which enhance exchange of gases-

The data given in the table clearly indicates a concentration gradient (difference in concentration) for oxygen from alveoli to blood and blood to tissues.

A gradient for Carbon di oxide is in the opposite direction i.e. from tissue to blood and from blood to alveoli.

Solubility of CO2 is 20-25 times higher than O2. The diffusion membrane is made is made up of three major layers- i) thin squamous epithelium

of alveoli. Ii) endothelium of alveolar capillaries iii) basement substance in between them. Total thickness is less than a millimetre.All these factors are favourable for diffusion of O2 from alveoli to tissue and CO2 from tissue to alveoli.TRANSPORT OF GASES- Blood transport oxygen & carbon di oxide. Oxygen- 97% by RBCs of blood & 3% dissolved in plasma of blood.carbon di oxide- 20-25% by RBCs, 70% as bicarbonate ion & 7% dissolved in plasma of blood as carbonic acid.TRANSPORT OF OXYGEN- Binding of oxygen with haemoglobin is related to- Partial pressure of O2, Partial pressure of CO2, hydrogen ion conc. & temperature.In alveoli, Partial pressure of O2 is high, Partial pressure of CO2, hydrogen ion conc. & temperature is low which is favourable for binding of oxygen & form oxyhaemoglobin.In tissue, Partial pressure of O2 is low, Partial pressure of CO2, hydrogen ion conc. & temperature is high which is favourable for dissociation of oxygen form oxyhaemoglobin.Every 100 mL of oxygenated blood can deliver around 5 mL of oxygen under normal physiological conditions.A sigmoid curve is obtained when percentage saturation of haemoglobin with oxygen is plotted against the partial pressure of oxygen(pO2) & is called Oxygen dissociation curve,

TRANSPORT OF CARBON DI OXIDE- CO2 is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO 2. When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place, i.e., CO2

which is bound to haemoglobin from the tissues is delivered at the alveoli. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

Carbonic anhydrase Carbonic anydrase

CO2 + H2o H2CO3 HCO3- + H+

At the tissue site where partial pressure of CO2 is high due to catabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3

– and H+,.

At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2O. Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2.

Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.R EGULATION OF R ESPIRATION Human beings have ability to maintain and moderate the respiratory rhythm as per the demands of the body tissues & is done by the neural system in the following ways-

A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation.

Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre. Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.

A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions. Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessary adjustments in the respiratory process by which these substances can be eliminated.

Receptors associated with aortic arch and carotid artery also can recognise changes in CO2

and H+ concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

D ISORDERS OF R ESPIRATORY S YSTEM

Asthma -In this, there is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.

Emphysema- It is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.

Occupational Respiratory Disorders- In certain industries, those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.