Biology -What is Biology



● Biology is a natural science

concerned with the study of life and

living organisms, i.e., plants and

animals. It is classified into two part:

Botany and zoology.

● Study of plants is called Botany and

study of animals is called Zoology.

Zoology and Botany are collectively

called Biology. The term ‘Biology’

was coined by Lamarck and


● The scientist who gave his thought

for the first time about the life of

plants and animals was Aristotle.

That’s why he is known as the Father

of Biology. He is also known as the

Father of Zoology.

● Theophrastus is known as Father of




● They have cellular organisation and also

respire, i.e., take in O2

and evolve CO2


● Metabolism is one of the most important

characteristic feature of living organism. It

comprises two phases that are anabolism

(constructive phase) and catabolism

(destructive phase.)

● They take nutrition for their growth.

● They have tendency to reproduce.

● They have the ability to respond to changes

in both internal and external environment

i.e., they have sensitivity. Their survival

chances are maximum.

● They move from place to place as animals or

some bacteria. Plants cannot move but

some movement can occur in plants.

The Cell

● The Cell is the basic structural and

functional unit of all known living

organisms. It is the smallest unit of

life and is often called the building

block of life.

● The branch of biology which deals

with the study of cell, is called


● Robert Hooke coined the term cell

when he saw honey-comb like

structure in the section of cork.

However, he only discovered cell wall.

● The first living cell was discovered by


Types of Cells

These are of two types

1. Prokaryotic Cells

(Pro=primitive, karyos=nucleus)

These are primitive cells, lacking a

well defined nucleus and most of

the other cell organelles, e.g.,

bacterial cell.

2. Eukaryotic Cells (Eu = true, karyos =

nucleus) These have a well defined

nucleus and membrane bound cell

organelles. These are present in

unicellular and multicellular plant and

animal cells.

Parts of Cell and their


A typical cell consists of cell wall and


Protoplasm of Cell

It is the living fluid matter present inside the

plasma membrane. The fluid present outside

the nuclear membrane is called cytoplasm and

the fluid present inside the nuclear membrane

is called nucleoplasm. Deutoplasm is the

non-living matter of the cell.

Cell Wall

It is present in plant cells, bacteria, fungi, algae

and some archaea. It is composed of cellulose

in plants and chitin in fungi. It is non-living. Its

main function is to provide shape and rigidity to

the cell.

Plasma Membrane

The cell is enclosed by a thin membrane called

the cell membrane or plasmalemma. It is

composed of proteins and phospholipid

molecules. It is elastic, living and selectively

permeable, i.e., provide passage for various



● It was discovered by R Altman in

1880 and the term mitochondria

was coined by Carl Benda.

● It is bounded by a double memb-

rane. The inner membrane has

many folds, called the cristae. Fluid

(called matrix) is present on interior,

which contains many enzymes and

coenzymes. It is a semi- autonomous

(can form its own copies) organelle

and is called power house of the

cell because in it, stepwise oxidation

of fuel occurs which results in

release of chemical energy. This

energy is stored in the form of ATP.


● These are present only in plant cells

and are of three types– chloroplasts

(green), leucoplasts (white) and

chromoplasts (of various colours

except green). Chloroplast is the site

of photosynthesis as it contains

chlorophyll, while leucoplasts are

storage plastids. Chloroplast is called

the kitchen of the cell.

● The red colour of tomatoes is due to

the presence of lycopene pigment,

i.e., chromophore.

● The colour of carrot is due to

carotene pigment.

Endoplasmic Reticulum

It was discovered by KR Porter. These

are hollow membranous system having

ribosomes (thus called Rough ER) or

no ribosomes (thus called Smooth


Rough endoplasmic reticulum is

the site of protein synthesis, while

smooth endoplasmic reticulum is the

site of synthesis of steroids and


Golgi Apparatus

It was discovered by Camillo Golgi. It

is made up of sac-like flattened

structures and play an important role

in secretion, transportation and

acrosome formation.


Ribosomes were discovered by GE Palade.

These are minute, non-membranous particles,

composed of RNA and protein. 70 S type of

ribosomes are found in prokaryotes, while 80 S

type in eukaryotes. These are the site of protein



● Lysosomes were discovered by de Duve.

These are polymorphic organelles having

hydrolytic enzymes. These enzymes function

at (acidic) pH ~ 5. These are sometimes

called suicidal bags of the cell.

● Lysosome helps in carcinogenesis, i.e.,

conversion of a normal cell into cancerous



It was discovered by T Boveri. It is composed of

two set of centrioles and participate in the

formation of mitotic spindle during cell division.


These are non-living reservior, bounded by

a membrane called tonoplast. Pigment

anthocyanin is present in the cell vacuole,

which provide colour to flowers. It stores toxic

metabolic waste and helps in osmoregulation.


It was discovered by Robert Brown. It contains

nucleoplasm, nucleolus and chromatin

material. Nucleolus is rich in protein and RNA.

All this material is covered up by a nuclear

membrane. Chromatin is the controlling

centre of cell as it form chromosomes.


Chromosome is thread-like structure,

found in the nucleus. Bead-like

structures found on chromosome are

called genes, which are made up of

DNA and are the carrier of genetic

information from generation to

generation. Chromosomes are units of

inheritance. In some viruses e.g.

retrovirus, RNA is the genetic


Plant Cell Animal Cell

It has cell wall. Cell wall is usually


Plastids are found. Plastids are usually


Centrioles and

centrosome are


Centrioles and

centrosomes are

found in all cells.

A big vacuole is


Vacuole is absent

or very small in


Nucleic Acids

These contain the genetic

instructions used in the development

and functioning of all known living


These are of two types : DNA and RNA.


Acid (DNA)

● DNA was discovered by James

D Watson and Francis Crick, who

got Nobel Prize for this discovery.

● It is a long polymer made from

repeating units called nucleotides.

● Each nucleotide consists of a

nucleoside (i.e. nitrogenous base

and deoxyribose sugar) and a

phosphate group, joined together by

phosphodiester bonds.

● It has four bases, i.e. adenine,

guanine, cytosine and thymine.

● Adenine and guanine are the

purine bases; cytosine and thymin

1 gm glucose provides about 17 kJ energy

or 4.2 kcal energy.

● Carbohydrates are better fuel as

compared to proteins and fats as they

readily decompose to give energy.

● Main sources of carbohydrates are wheat,

maize, rice, potato etc.

The carbohydrates are categorised into

following three types


● These are simple sugars, which cannot be

hydrolysed further, e.g., ribose, glucose,

fructose, galactose etc.

● In human beings, blood glucose level is

100-120 mg/mL. Extra glucose, if any, is

converted into glycogen in the liver by a

process called glycogenesis.

● D-fructose is the sweetest of all sugars

and is found in fruit juice, honey etc.


● They release 2-10 monosaccharides on

hydrolysis, like disaccharides, e.g.

sucrose, lactose maltose etc and

trisaccharides, like raffinose etc.

● Sucrose is also called invert sugar. It

gives glucose and fructose when

subjected to hydrolysis.


● They release more than ten

monosaccharides on hydrolysis. These

are non-sugars, i.e., do not have sweet

taste, e.g., cellulose, glycogen and starch.

● Cellulose is found in plant cell wall and is

digested by ruminants (like cow, goat,

buffalo, etc), but not by carnivorous or

omnivorous animals like human beings.

Thus, it acts as roughage in case of

human beings.

Function of Carbohydrates

● Carbohydrates provide energy, that acts

as reserve food, help in the synthesis of

nucleic acid and form exoskeleton of

animals. Excessive intake of carbohydrate

results in digestive problems and obesity.


These are found in all living cells. These are

the compounds of carbon (C), hydrogen

(H), oxygen (O), nitrogen (N) and sulphur

(S). These form 15% part of human body.

Snake venom, ricin of castor and bacterial

toxins are proteinaceous in nature. Main

sources of protein are groundnuts,

soyabean, pulses, fish etc.

Function of Proteins

● These are important for the growth and

repair of the body (75% of our body is

protein only). However, in the

deficiency of carbohydrates, these acts

as the source of energy. Protein also

control the development of genetic


● Deficiency of protein causes

Kwashiorkor (a disease in which hands

and legs of children get slimmed and the

stomach comes out) and Marasmus (a

disease in which muscles of children are

loosened). Kwashiorkor occurs in

children between 1 to 5 years of age and

marasmus in children below 1 year.

Lipids (Fats)

● These are also the compounds of carbon

(C), hydrogen (H) and oxygen (O).

Chemically, these are the ester of

glycerol and fatty acids. These are

present in cytoplasm, cell wall etc.

● The main source of fats are ghee, butter,

almond, cheese, egg yolk, meat,

soyabean etc.

● Fats are digested by enzymes called

lipases in the small intestine. Generally,

at 20°C, these are in solid state but if

their state is liquid at this temperature,

these are termed as oils.

● Fatty acids are of two types- Saturated

and Unsaturated. Saturated fatty acids

are found in coconut oil and palm oil,

while unsaturated fatty acids are found

in fish oil and vegetable oil.

● Excess of saturated fats raises the level

of blood cholesterol and may cause

arteriosclerosis. This may lead to

heart attack.

Function of Lipids

The main functions of lipids are

● These provide twice the energy than

that from carbohydrates.

● These remain under the skin and

prevents the loss of heat from the body.

Malnutrition Effects

● Deficiency of fat results in dryskin and weight


● If fat is in excess, the body gets fatty and

result in several heart diseases and high

blood pressure.

● The skin fat, in case of whales and seals,

forms a thick layer called the blubber. It acts

as reserve food and also maintains the body



● It was first discovered by FG Hopkin.

However, the term vitamin was coined by C


● Vitamin is an organic compound, which

cannot be synthesised in sufficient quantities

by an organism and must be obtained from

the diet.

● They provide no calories, they only regulate

chemical reactions occurring in the

metabolism of the body.

These are divided into two groups

1. Fat soluble vitamin, viz. vitamin-

A, D, E and K.

2. Water soluble vitamin, viz.

vitamin-B and C.

● Vitamin-B12 contains cobalt.

Vitamin-D is synthesised in our skin

by the action of ultraviolet rays of

the sunlight. Vitamin-K is

synthesised in our colon by the


● Water soluble vitamins normally do

not show hypervitaminosis (this

disease occurs due to excess intake

of vitamins) as excess of these

vitamins is normally excreted

through urine.

● In balanced diet, all the important

nutrients (like carbohydrate,

protein, fats, vitamins etc.) are

available in sufficient quantity.

Biological Evolution

Origin of Universe

● The universe is made up of matter and

energy. Scientists believe that it was

formed about 10 to 13 billion years ago

as a vast, dense, red-hot and rotating

gaseous cloud of cosmic dust called the

“primaeval matter or ‘ylem’.

● Two hypothesis i.e., Big -Bang

(universe formation through a very big

explosion) and Nebular (universe

formation by the condensation of

gaseous cloud) were given to explain

origin of Earth.


● Life originated on Earth about 3.5

billion years ago. Some philosophical

theories like special creation,

spontaneous generation, Biogenesis,

panspermia (life originated in the form

of pansperms from some unknown

part of the universe) or cosmozoic and

catastrophism (life orginated suddenly

from inorganic matter) were given to

explain origin of life.

● The first scientific account of the

origin of life was given by Russian

scientist AI Oparin in his book ‘Origin

of life’.

● The primitive atmosphere contained

hydrogen, methane, ammonia and

water vapour. In it, oxygen and ozone

were absent. Thus, it was believed that

life is originated from inorganic

substances by a series of complex


● Hydrogen atoms were most numerous

and most reactive in the primitive

atmosphere. First, these combined

with all available oxygen atoms,

forming water and leaving no free

oxygen atoms. Thus, the primitive

atmosphere was ‘reducing’ unlike the

present ‘oxidising’ atmosphere. This

was also supported by Miller and Urey

in 1953.

● In Miller-Urey experiment, a mixture

of water, hydrogen, methane and

ammonia was cycled through an

apparatus and the organic compound,

amino acids were obtained.

Organic Evolution

More and more creation of organism by

gradual changes from low category animal to

higher animal is called organic evolution.

There are several evidences regarding

organic evolution.

Evidences from morphology

and anatomy

On the basis of morphology (outer

appearance) and anatomy (inner structure),

several evidences have been described as


Homologous Organs

● The organs which are similar in basic

structure and origin but dissimilar in

function are called homologous organs,

e.g., wings of bat, cat’s paw, front foot of

horse, human’s hand and wings of birds.

● These show divergent evolution.

Analogous Organs


● The organs which are similar in shape and

function but dissimilar in their origin and

development. e.g., wings of insects, birds

and bats, eyes of octopus and mammals.

● They show convergent evolution.

Vestigial Organs

● These are degenerate, non-functional

organs which were functional earlier.

● Human body has been described to possess

about 90 vestigial organs. Some of these

are muscles of ear pinna, canine teeth and

third molar teeth, body hairs, vermiform

appendix, nictitating membrane of eye,

caudal vertebral (coccyx or tail bone) etc.

Atavism or Reversion

It is the sudden reappearance of some

ancestral features. Appearance of thick body

hair, large canines, monstral face, short

temporary tails, extra nipples etc are

examples of atavism.

Evidence from Connecting


Connecting link is one which exhibit

characteristics of more than one groups.


● These are unicellular animals, i.e., made

up of only one cell.

● In these, all the metabolic activity like

digestion, respiration, excretion and

reproduction takes place in unicellular


● Respiration and excretion take place

through diffusion.

e.g., Amoeba, Plasmodium, Euglena etc.


● These are found in marine water and have

porous body. The pores are called ostia.

These are multicellular animals.

● Their skeleton is made up of minute

calcareous or siliceous spicules.

e.g., Sycon, Sponge etc.


● These are aquatic animals, have thread-like

structures called tentacles around the

mouth which help in holding the food.

● They have specialised cnidoblast cell to

help in catching the food.

● Phenomenon of polymorphism (many

forms) and metagenesis (alternation of

generation) are associated with


e.g., Hydra, Jelly fish, Sea Anemone etc.

● Hydra has a tendency of regeneration of

body organs.

Phylum– Platyhelminthes

● Animals of this phylum have

alimentary canal with single opening,

anus is absent.

● Excretion takes place by flame cells.

● There is no skeletal system such as,

respiratory system, circulatory system


● These are hermaphrodite animals

(males are not separated from


e.g., Planaria, Liver fluke, Tape worm



● These are long, cylindrical,

unsegmented worms.

● Their alimentary canal is complete in

which mouth and anus both are


● There is no circulatory and respiratory

system but nervous system is

developed. Excretion takes place

through protonephridia.

● They are unisexual.

● Most forms are parasitic but some are

free living in soil and water.

e.g., Ascaris, Threadworm, etc.

¡ Threadworm is found mainly in the

anus of child. Due to which children

feel itching and often vomits. Some

children urinate on the bed at night.


● Their body is long, thin, soft and

metamerically (truly) segmented.

● Alimentary canal is well-developed.

● These are the first to have proper organ


● Nervous system is normal and blood

(called haemolymph) is red (iron rich


● Their blood flows in closed vessels.

● Like in earthworm, there are five pairs of

blood vessels called as heart.

● They respire through skin, in some

animals respiration takes place through


● Excretion by nephridia.

● They move through setae made up of


e.g. Earthworm, Nereis, Leech etc.


● Arthropoda is the largest phylum

(contains maximum number of animals

and its existence is recorded for

maximum period over the Earth).

● Jointed leg is their main feature.

● Their body is divided into three

parts–head, thorax and abdomen.

● Circulatory system is open type.

Cockroach’s heart has 13 chambers.

● Trachea or book lungs, body surface are

respiratory organs.

e.g., Cockroach, Prawn, Crab, Bug, Fly,

Mosquito, Bees, etc.

¡ Insects generally have six feets and four


¡ Ant is a social animal which reflects

division of labour.

¡ Termite is also a social animal which

lives in colony.


● Their body is soft and divided into head

and muscular foot.

● Mantle is always present in it, which

secretes a hard calcareous shell.

● Their alimentary canal is well-


● Respiration takes place through gills or

ctenidia. Blood is colourless.

● Excretion takes place through kidneys.

e.g., Pila, Aplysia (Sea rabbit), Doris (Sea

lemon), Octopus (Devil-fish), Sepia


¡ Eyes of octopus are similar to chordate


Phylum– Echinodermata

● All the animals in this group are marine.

They have water vascular system.

Brain is not developed in nervous


● They have a special capacity of


● These are the only invertebrate animals

which contain proper bone like

structures (ossicles).

e.g., Star fish, Sea urchin, Sea cucumber



● They have notochord. A dorsal hollow

tubular nerve cord and paired

pharyngeal gill slits at some stage of their

life span.

● In advanced forms, notochord changes

to vertebral column, nerve cord develops

to brain and spinal cord and pharyngeal

gill slits to structures of jaw attachment.

● This phylum is sub-divided into two

sub-phylum, i.e., Protochordata and


Some main classes of phylum– Chordata

are as follows


● These are aquatic animals (cold-blooded

animals). Their heart pumps only

impure blood and have two chambers.

● Respiration takes place through gills.

e.g., Trygon, Scoliodon, Torpedo etc.

Amphibia (First land


● These are found both on land and water.

All of them are cold-blooded.

● Respiration takes place through gills,

skin and lungs.

● They have three chambered heart.

e.g., Frog, Necturus, Toad, Icthyophis,


Reptilia (First true land


● These are crawling animals.

● These are cold-blooded and contains two

pair of limbs.

● The skeleton is completely flexible.

● Respiration takes place through lungs.

● They have 3



chambered heart (four

chambered in crocodile).

● Their eggs are covered with shell made up

of calcium carbonate.

e.g., Lizard, Snake, Tortoise, Crocodile,

Turtle, Sphenodon etc.

¡ Cobra is the only snake which makes


¡ Heloderma is the only poisonous lizard.

¡ Sea snake is also called Hydrophis

belcheri. It is the world’s most poisonous


Aves (Aerial Vertebrates


● The animals of this group are

warm-blooded tetrapod vertebrates with

flight adaptation.

● Their fore-feet are modified into wings to


● They respire through lungs.

● Birds have no teeth, beak helps in feeding.

● They have a single ovary and pneumatic

bones. e.g., Crow, Peacock, Parrot etc.

¡ Flightless birds are Kiwi and Emu.

¡ Largest bird is Ostrich.

¡ Smallest bird is Humming bird.

¡ Largest zoo in India is Alipur (Kolkata)

and the largest zoo of the world is Cruiser

National Park in South Africa.


● These are warm-blooded animals.

● Tooth comes twice in these animals


● There is no nucleus in their red blood cells

(except in camel and llama).

● Skin of mammals contains hair.

● External ear is present.

Mammalia is divided into three


● Prototheria It lays eggs, e.g.,


● Metatheria It bears the immature

child, e.g., Kangaroo.

● Eutheria It bears the well developed

child, e.g., Humans.

They give birth to young ones, but

Echidna and Platypus are the egg

laying mammals.



Integumentary System

The human skin (integumentary) is

composed of a minimum of three major

layers of tissue, the epidermis, dermis

and hypodermis.


The top layer of skin is made up of

epithelial cells and does not contain

blood vessels.


● It gives elasticity to the integument,

allowing stretching and conferring

flexibility, while also resisting

distortions, wrinkling and sagging.

● Nails grow 1 mm per week on an


● Protein, keratin stiffens epidermal

tissue to form finger nails.


● It is made up of adipose tissue.

It performs several important functions

1. Protect against invasion by

infectious organisms.

2. Protect the body from dehydration.

3. Maintain homeostasis.

4. Act as a receptor for touch, pressure,

pain, heat and cold.

5. Protect the body against sunburns by

secreting melanin.

6. Generate vitamin-D through

exposure to ultraviolet light.

7. Store water, fat, glucose and


Animal Nutrition and


Animals are not able to synthesise their

own food, therefore they obtain it from

outside environment for their nutritional


Mineral Nutrients

● These are metals, non-metals and their

salts other than the four

elements—carbon, hydrogen, nitrogen

and oxygen and constitute about 4% of

total body weight.

● Milk, eggs, meat, fruit, food, vegetables

etc are the sources of minerals.

Minerals are of two types

1. Macronutrients These are required in

large amount, e.g., calcium (Ca),

phosphorus (P), potassium (K) etc.

2. Micronutrients These are required in

very small amount (less than 1 g), e.g.,

iodine (I), iron (Fe), zinc (Zn) etc.


About 70% of the human body consists of

water. Two-third of water exists inside

cells, the other one-third is outside the

cells in tissue fluid and blood plasma. It is

essential for digestion, transportation,

excretion and to regulate body


Many factors affects the health of human

body. One of them is adulteration.

Food Adulteration

Addition of undesirable, cheap and

harmful substances in the food is called

food adulteration.

Indian Standards Institution (ISI) Mark

and Agmark (Agricultural marketing) are

given by the Bureau of Indian Standards

after testing the purity and quality of

material and food respectively.

Food Item/Stuff Adulterant

Milk, curd and


Water and urea

Sweets Saccharin, harmful colour

Ghee Vanaspati and animal


Cereals Stones, sand and grit

Dhania powder Powdered horse dung

Haldi powder Lead chromate

Pulses Metanil yellow

Edible oils Argemone oil

Black pepper Papaya seed



The human digestive system consists of

alimentary canal and digestive glands.

The alimentary canal consists of mouth,

(having teeth and tongue) oesophagus,

stomach, small intestine and large



● With the help of teeth the food is

chewed. Teeth are of four types

Incisors (for cutting)

Canines (for tearing)

Premolars (for chewing)

Molars (for chewing and grinding)

● The number of teeth are different in

different animals. These are represented

by dental formula as

I C Pm M – Upper half jaw

I C Pm M – Lower half jaw

Where, I − Incisors, C − Canines,

Pm − Premolars and M − Molars.

● Premolars and molars are called cheek

teeth. Milk teeth do not include molar


● In humans, first teeth come in between

6 and 8 months. By the age of 6, milk

teeth are gradually replaced by permanent


● Hardest part in the body is tooth enamel.

● In elephants, the tusks are the incisors of

upper jaw.

● Maximum number of teeth are present in

horse and pig.

Mammal Dental


Total Number

of Teeth

Man (child) 2102/2102 × 2 20

Man (adult) 2123/2123 × 2 32

Horse 3143/3143 × 2 44

Dog 3142/3143 × 2 42

Cow and


0033/3133 × 2 32

Cat 3131/3121 × 2 30

Rabbit 2033/1023 × 2 28

Mouse 1003/1003 × 2 16


● Saliva, secreted by the salivary glands, is

mixed with the chewed food by the


● Tongue also contains taste buds due to

which we sense bitter, sour, salty or sweet


Digestion of Food

Digestion in Mouth

In mouth, salivary amylase acts on



(Complex form)






Digestion in Stomach

● The food passes down through the

oesophagus into stomach.

● Now food is mixed with gastric juice

and hydrochloric acid which

disinfect the food and creates acidic


● Pepsin digests proteins and converts

them into peptones.

● Rennin convert milk into curd.

● Digested food now is called chyme.

Digestion in Duodenum

● Chyme moves to duodenum.

● Food is mixed with bile (liver) to breakdown

fats into smaller globules.

● Trypsin acts upon proteins and break them

into polypeptides. Amylase converts starch

into simple sugar.

● Lipase convert fats into fatty acids and


Digestion in Intestine

● Food passes into ileum and mixes with

intestinal juice, where

Maltase converts maltose into glucose

Lactase converts lactose into glucose

and galactose

 Sucrase converts sucrose into glucose and


Trypsin digests the peptides into amino


Absorption and Assimilation of

Digested Food

● Ileum’s internal surface has finger-like

folds called villi.

● It helps in absorption of food.

● Intestinal juice is alkaline in nature.

● pH of saliva, gastric juice, pancreatic

juice and intestinal juice is respectively

6.8, 2.0, 7.0 and 8.5.

Ejection of Unwanted Food

● Digested food passes into large intestine.

● Large intestine cannot absorb food, but

absorbs much of the water.

● The remaining semi solid waste is called

faeces and is passed into rectum.

● It is expelled out through anus.


● Roughage is another term for dietary

fibres e.g., Natural food, dalia etc.

● It does not provide energy but only helps

in retaining water in the body.




● It is the largest gland of the human body

and secretes bile juice, which is stored in

gall bladder.

● It regulates the quantity of glucose in the

blood by converting extra glucose (if any)

into glycogen or glycogen (during

deficiency of glucose) is converted into


● It destroys dead RBC and regulates body

temperature. It converts excess of amino

acid into ammonia (which is converted

into urea by Ornithine cycle). Urea

comes out from the body through kidney.

● If there is any obstruction in bile duct,

liver cells stop taking bilirubin from the

blood, consequently it spreads

throughout the body which is called


● Liver is an important body organ in

investigation of a person’s death that has

been due to poison in food.

Human Respiratory System

● Overall passage of air in humans is as


Nostrils → Pharynx → Larynx → Trachea

→ Bronchi → Bronchioles → Alveoli →

Cells→ Blood.

● The human respiratory system is shown

in the following diagram.

Types of Respiration

Aerobic Respiration

● The respiration taking place in the

presence of oxygen is known as aerobic


● This respiratory oxygen oxidises the

substance into carbon dioxide, water

and energy, as follows

Glucose + O2 →Carbon dioxide

+ Water + Energy.

● In this process, each glucose molecule is

converted into two molecules of pyruvic

acid by the process, called glycolysis. It

takes place in the cytoplasm of the cell.

The pyruvic acid formed, releases energy

with the formation of carbon dioxide and

water (in Kreb’s cycle which occur in


Anaerobic Respiration

● The respiration taking place in the

absence of oxygen is known as

anaerobic respiration.

● It is found in endoparasites like

roundworm. In this process, the

respiratory substances are incompletely

oxidised to carbon dioxide and alcohol.

Glucose →


Carbon dioxide +

ethyl alcohol

muscles +Energy (in plants)

Glucose → Lactic acid

+ Energy (in animals)



External Respiration

(Breathing or Ventilation of


● It involves inspiration and expiration of


● Inspiration is the process of intake of air.

During inspiration, muscles of the

diaphragm contract and diaphragm


● The lower ribs are raised upward and

outwards. The chest cavity enlarges, the

air pressure in the lungs gets decreased

and air rushes into the lungs.

● Expiration is breathing out of air. During

expiration, relaxation of muscles of the

ribs and diaphragm takes place.

● Diaphragm again become dome-shaped.

Chest cavity is reduced and air is

forced outwards through nose and


● Breathing rate in humans is 18 20 – times

per min.

● The exchange of gases, i.e., oxygen and

carbon dioxide takes place due to the

difference in their partial pressures.

Internal Respiration

(Oxidation of Food)

● It is a complex process in which food is

broken down to release energy.

● Transportation of oxygen takes place by

haemoglobin of blood. Whereas

transportation of only 10-20% carbon

dioxide takes place by haemoglobin of


● Approximately 400 ml water is lost

through breathing everyday.

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