Download Biologi - Biology Chapter 5 PDF

TitleBiologi - Biology Chapter 5
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Page 1


What we have learned earlier

Plants and animals differ in their growth.

The stem grows towards light and the roots grow towards water.

The leaves of Mimosa plant fold themselves on touching.

The roots of mangrove trees grow upwards. Some plants climb on supports.

Plant hormones influence activities like growth of stem, flowering etc.

Artificial hormones can be utilized for stimulating processers such as rooting,

flowering etc.

The hormones produced by endocrine glands control physiological activities.

Over production and under-production of some hormones cause

abnormalities in growth.



Page 2


You have already learned about the
role of nervous system in controlling and
co ordinating life activities of organisms. The
activities controlled by nervous system are
those which happen quickly. But, activities
such as growth, reproduction, maintenance
of glucose level in blood, reabsorption of
water in kidneys etc. that take place regularly
also need to be regulated. The chemicals
that regulate such activities are called
hormones. The hormones co-ordinate
various activities that take place within and
between cells, by chemical means.

You may be aware of the different stages
in the metamorphosis of a butterfly. This
stepwise and systematic process is caused
by the action of certain hormones (eg: Juvenile
hormone, ecdysone). Life activities such as
cell division in invertebrates like hydra,
reproduction in flatworm are also regulated
by hormones. The hormones also control
activities like maintenance of osmotic and
ionic balance in vertebrates like fishes,
migration in birds and so on. The hormones
have been identified even in unicellular
organism like protozoa. Similarly, hormones
play a significant role in regulating several
biological activities in human beings as well.

In this chapter, we will study some of
the important hormones and their functions.
We will also study the hormones that regulate
the activities in plants viz. growth, movement,

flowering and shedding of leaves and their
beneficial use in the field of Agriculture.

Hormones in human beings

We know that the digestive glands
produce enzymes involved in digestion.
These enzymes are secreted from the glands
through specific ducts to their sites. But the
glands which secrete hormones have no
ducts. The hormones are transported to their
target sites of action through circulation.
Therefore, these glands are called endocrine

pitutory gland

adrenal gland



Figure 5.1
Various endocrine glands

thyroid gland

thymus gland

Page 9



neuro secretory cells that
produce hormones


nerve fibres


inferior part of the pituitary gland


front part of the pituitary gland

capillaries release hormones

hypothalamus v


Figure 5.5
Relation between hypothalamus and pituitary gland

Central Nerve System


Releasing hormones (through blood)

Anterior lobe of the pituitary gland Posterior lobe


Thyroid Adrenal Ovary Testis
Thyroxine Aldosterone Progesteron Testosterone Growth hormone
Calcitonine Cortisol Estrogen


Illustration IV

See illustration IV.

How many lobes does pituitary gland

What are the hormones produced by the
anterior lobe?

These hormones of the pituitory gland
influence the secretion of other glands.

(Through nerve fibres)

(Trophic hormones)

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Therefore, they are called Trophic hormones.
However, the growth hormone,
somatotrophin, acts directly on various
tissues of the body. It causes growth and
thus effects an increase in body weight.

What will happen if the production of
this hormone is deminished during the
growing stages? The growth of bones
becomes stunted and the body development
decreases accordingly. This condition is
called Dwarfism.

In some persons, somatotrophin is
produced excesively. The main cause for this
is the formation of tumour in the pituitary
gland. The symptoms vary according to the
growth stage at which the tumour develops.
If somatrophin is excessively produced
during the growing stage, it leads to
Gigantism. Such persons become very tall
and will have overweight. If the tumour
develops after growing stage is over, the
internal organs and body extremities alone
grow. This condition is known as acromegaly
(acro = tip, mega = large, by = disease).
Such persons will have large hands, feet and
jawbones. The excessive growth of facial
bones gives them a hideous look.

The posterior lobe of the pituitary gland
stores the hormones - oxytocin and
vasopressin. You know that these hormones
are produced by the hypothalamus.


The two hormones secreted by
hypothalamus are vasopressin and oxytocin.
Study the figure 5.5. How do these
hormones secreted by the specialised
neurons in hypothalamus reach the pituitary
gland? What are the functions of these
hormones? Havent' you noticed that during
summer, the urine output is low? Due to
increased sweating, the water level of the
blood is lowered. As a result the pituitory
releases vasopressin into the blood. This
hormone increases the rate of reabsorption
of water from the urine by the nephrons.
Therefore vasopressin is also called Anti
diuretic hormone (ADH).

A Clock in Animals

You know that several activities in

living beings take place in a time-

specific manner. We sleep at night and

wake up in the morning. Such rhythmic

daily activities (circardian rhythm) are

controlled by a clock working inside the

body, which can be called the biological

clock. The pineal gland situated

opposite the pituitary gland has an

important role in this respect. The

secretions of the pineal gland namely

melatonin and seratonin influence some

rhythmic activities. The secretion of

melatonin increases during night and

this induces sleep. Pineal gland is more

active in birds and other animals and is

less developed in the mammals.

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The endocrine glands play a vital
role in controlling and co-ordinating
activities of life?

The thyroid gland has an important
role in physical and mental

The complementary actions of
calcitonin and parathormone
maintain the level of calcium in the

Adrenalin prepares the body to face
emergency situations.

The islets of Langerhans in the
pancreas regulate blood sugar level.

Hormones of gonads control the
growth and functions of sex organs.

The various hormones of pituitary
gland control the secretion of
hormones from other endocrime

The pituitary gland and
hypothalamus are connected by
nerve fibres and blood vessels.

The pituitary gland is controlled by
the releasing hormones from

The under-secretion or over-
secretion of various hormones can
cause health problems.

The chemicals released by
organisms to the outside are called

Pheromones help activities such as
communication between organisms
of the same species, attracting a
mate etc.

Plant growth is effected by

Based on the influence of stimulus
in the direction of movement, plant
movements can be recognised into
tropic and nastic movements.

Plants exhibit different kinds of
tropic movements in response to
different stimuli.

Plant hormones influence activates
like cell division, elongation of cells,
flowering, ripening of fruits etc.

Synthetic plant hormones are widely
used in the field of Agriculture.


Page 18




The sugar level in the urine of a diabetic
patient is varies. Give reasons.

Parathyroid hormone has a key role in
maintaining life. Justify your opinion.

A person shows symptoms like lazyness,
aversion towards food, swelling of body,
dry skin, low heart beat etc. Suggest
the changes to be made is his diet.

Pituitary gland is connected to the
hypothalamus with nerves and blood
vessel. Name the region where the hor-
mone from he hypothalamus is mixed
with blood?

Ethylene is present in ripened fruits. What
will happene to unripened tomatoes
which are kept among ripened fruits?

Suggest an experiment to demonstrate
hydrotropism shown by roots.

It is seen that when bee hives are
attacked by enemies, the bees began to
attack their enemies following the
attacks of a single one. Give explanation
for this.

Monocot plants do not shows growth
in thickness as in dicots. Justify the

To obtain more plantlets of rose and red
mussanda, a gardener used stem cutting
of that plants. But all of them dried out
Can you help him to produce healthy

Diabetic patients feel more thirst. They
drink lots of water. Justify the statement.

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