CONTROL AND COORDINATION
Endocrine Glands and their Functions
We salivate when we see delicious food;
we feel tense or anxious while making important decisions; we digest different
types of food that we ingest; children grow taller during the early years of
their life; voices of boys crack during puberty; lactation begins during
motherhood. Do you know why these changes occur? How does the body control
these functions? Is there another system governing these complex functions? Let
us explore.
Hormones
A hormone is a chemical messenger that
regulates physiological processes in living organisms. It is secreted by a
gland.
The regulation of the physiological
processes, control and coordination by hormones is governed by the endocrine
system. The nervous system, along with the endocrine system in our body,
controls and coordinates the physiological processes.
Characteristics of hormones:
· Hormones act as chemical messengers.
· They are secreted by living cells/tissues or organs called glands.
· They are secreted in very small quantities by glands.
· They act upon specific cells, tissues, or organs called the target sites.
· They are generally slow in action, but have long-lasting effects.
· They either accelerate or inhibit a reaction.
Glands
A cell, tissue, or an
organ that secretes hormones required for a specific function is called a gland.
You are familiar with the pancreas, pituitary gland, and thyroid gland. Glands
are mainly divided into two broad categories -endocrine and exocrine.
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Do you know that the term ‘hormone’ was
first introduced by Ernest Starling? Secretin was the first hormone to be
discovered. Ernest Starling and William Bayliss are credited for its discovery.
Importance of hormones
We have often heard people complaining
that they suffer from sugar. Do you know which condition they are referring to?
How do we treat this condition?
They are referring to the condition
called diabetes. It is a chronic condition characterized by high levels
of sugar (glucose) in the blood.
What is the reason for the increase of
sugar in the blood? How are other activities such as sleep, hunger, stress,
body temperature etc. regulated? Let us explore.
Hormones in the human body
There are different
kinds of hormones found in the human body. However, we will discuss only a few.
Pituitary Gland
It is a tiny pea-sized gland located
near the hypothalamus of the brain. It is known as master gland as functions of
many other endocrine glands are regulated by the pituitary gland. The gland is
divided into two lobes- anterior lobe also called adenohypophysis and
posterior lobe also called neurohypophysis.
Functions of the Pituitary Gland
· It
influences the secretion of other glands like thyroid gland, adrenal gland,
mammary glands, gonads etc.
· It
controls the excretion of water from kidneys and contraction of muscles of
uterus.
· It
also influences the development of testes and ovaries.
· It secretes the growth hormone (GH) that is involved in the growth and development of the human body.
The secretion of the growth hormone
should be properly maintained in the body. Over secretion of the growth hormone
prior to puberty (in children) results in abnormal growth of the body known as gigantism.
This is characterised by the excessive growth and increase in height of the
person. On the other hand, its low secretion results in retarded growth and
this condition are called dwarfism.
The oversecretion of the growth hormone
in adults causes a disease called acromegaly. This results in the
excessive growth of the bones, especially in jaws, nose, hands and legs.
Thyroid Gland
The thyroid gland is situated close to
the trachea in the neck. It secretes a hormone called thyroxin. It regulates
carbohydrate, fat, and protein metabolism in our body. It indirectly affects
the growth of the body by promoting the growth and differentiation of tissues.
Hence, it is also known as the personality hormone.
Thyroid disorders:
The deficiency (hypothyroidism) or
excess secretion (hyperthyroidism) of this hormone results in certain
disorders.
Hypothyroidism- The condition in
which thyroid gland does not produce enough thyroxin to meet the normal
requirements of the body. This condition causes abnormalities like simple
goitre, myxoedema and cretinism.
Simple goitre- It is usually caused
by the deficiency of iodine in the diet and results in swelling in the neck region due
to enlargement of the thyroid gland. People living near coastal regions usually
do not suffer from this disease as seawater and soil are rich in iodine.
People suffering from goitre are suggested to increase their intake of iodine
with food and water.
Myxoedema- It is mainly
caused in adults due to under activity of the thyroid gland and symptoms
include low metabolic rate, loss of mental and physical vigour, increase in weight,
thickening of the skin and a lower rate of the heartbeat. This condition is cured by
giving proper doses of thyroxin to the person.
Cretinism- It is found in
children born with improperly functioning thyroid gland. Its symptoms include
stunted growth, retarded mental development, bow legs, defective teeth,
protrusion of the tongue and loose skin. These effects can be prevented by
providing proper treatment in time.
Hyperthyroidism- It is caused due
to the excess secretion of thyroxin. It results in high metabolism, protrusion
of the eyeballs, high BP, nervous tension, irritability, profuse swelling,
weight loss and fatigue. This can also be cured by giving proper treatment in
time. Thyroxine hormone not only plays an important role in humans but also in
frogs. It is required for the completion of the life cycle of frogs. The process of
conversion of the larva into an adult is called metamorphosis. This process is
controlled by the thyroxine hormone in frogs. In the absence of this hormone,
tadpoles cannot develop into adult frogs. Parathyroid Gland
There are four parathyroid glands
present on the backside of thyroid glands that secrete parathyroid hormone or
parathormone (PTH). This hormone regulates the level of calcium ions in the
bloodstream.
The deficiency of this hormone results
in painful muscle cramps. On the other hand, its over secretion removes
calcium from the bones making them soft and spongy.
Adrenal Glands
The adrenal (suprarenal glands) gland,
is situated at the anterior part of the kidneys. Each adrenal gland consists of
two parts- outer cortex and an inner medulla.
The adrenal cortex secretes two hormones- Aldosterone
and cortisol while adrenal medulla secretes hormone- adrenaline
(epinephrine) and noradrenaline (norepinephrine).
In case of danger, emergency, or stress,
adrenaline is secreted in large quantities to prepare the body to face
the situation. For this reason, it is also known as ‘emergency hormone’. This
hormone is secreted even in normal situations but in small quantities.
Adrenaline is secreted
directly into the blood and is transported to different parts of the body. The
specific tissues/organs on which this hormone acts also includes the heart.
This results in faster heartbeats. Hence, more oxygen is supplied to the
muscles. The breathing rate also increases due to the contractions of the diaphragm
and rib muscles.
Pancreas
Pancreas is a
mixed gland i.e. both exocrine and endocrine in function. The cells of the pancreas
which are endocrine in function are known as islets of Langerhans and
secrete two hormones- insulin and glucagon.
Insulin regulates the blood sugar level
in the human body. It increases the permeability of the cell membrane for
glucose and accelerates the passage of glucose into the cells from the bloodstream.
In case of deficiency of insulin, the
level of glucose in blood increases and is excreted through urine. This
condition is known as diabetes mellitus. Other symptoms of the disease
include frequent urination and thirst. The treatment of diabetes mellitus
includes oral pills or injections of insulin.
Glucagon is antagonistic hormone to
insulin and is secreted when blood glucose levels are low. It helps in the conversion of glycogen into glucose. This glucose is released into the blood
and supplied to the tissues.
Gonads
Gonads (testes and ovaries) are both
involved in producing gametes and are also endocrine (produce hormones) in
function. Testes secrete testosterone and ovaries secrete the hormones estrogen
and progesterone. These are also known as male sex hormones and
female sex hormones respectively.
Testosterone is responsible
for the growth and development of male secondary sexual characters.
Deficiency of testosterone results in
under-sexed individuals whose masculine characters are not properly developed.
Estrogen is responsible
for the development of secondary sexual characters in females like the development
of mammary glands etc.
Deficiency of estrogen causes
infertility.
Progesterone helps
in the regulation of the menstrual cycle and maintaining pregnancy.
Components of Nervous
System
Do you know which organs make up the
nervous system?
The nervous system is made up of the
brain, spinal chord, and nerve cells or neurons.
Let us first study
about the structure of the functional units of the nervous system i.e., the neurons.
Structure of a neuron
The three main parts of a neuron are the
axon, dendrite, and cell body. The axon conducts messages away from the
cell body. The dendrite receives information from the next cell and
conducts it towards the cell body. The cell body contains the nucleus,
mitochondria, and other organelles. It is mainly concerned with the maintenance and
growth of the cell.
Arrangement of neurons
Neurons are arranged end to end, forming
a chain. This helps in the continuous transmission of impulses. Each neuron
receives an impulse through its dendrite and transmits it to the next neuron in
a sequence through its axon.
Neurons are not
connected. Synapse or a small gap occurs between the axon of one neuron
and dendron of the next neuron.
A synapse in the muscle fibre is also
known as the neuromuscular junction. Let us discuss the working of a synapse
in detail.
Nerve A nerve is a collection of nerve
fibres (or axons) enclosed in a tubular medullary sheath. This sheath acts as
an insulation and prevents mixing of impulses in the adjacent fibres. Types
of neurons
Neurons are of three types.
How does a nerve impulse travel?
The dendrite end of the
neuron collects information and triggers a chemical reaction, which results in
an electric impulse. This impulse is transmitted from the dendrite to the cell
body and then to the axon. From the axon, the impulse travels to its end, where
the electrical impulse sets off the release of some more chemicals. These
chemicals cross the synapse and start a similar electrical impulse in the
dendrite of the next neuron. In this way, impulses are transmitted from one
neuron to another to finally reach the brain. Under normal conditions, the
outer side of the nerve fibre consists of positive charge as more Na+ ions are
present outside the axon membrane. The neuron is then said to be in a polarised
state. On stimulation, the membrane becomes more permeable and Na+ ions move
inside causing depolarization. Such a region is known as an exciting region. The
point of depolarisation behaves as the stimulus for the neighbouring area and this
goes on. In the meantime, the previous area becomes repolarised due to active
transport (using ATP) of Na+ ions with the help of the sodium pump.
In a similar manner, impulses are
transmitted from the brain to muscle glands.
Human Brain - Structure and Function
The body performs various activities. All
these activities are controlled by the brain. How does the brain control all
activities? Are there any divisions in the brain, which take over the control
of different activities?
Do you know which organs make up the
nervous system?
Let us explore.
The nervous system is divided into - central
nervous system (CNS) and peripheral nervous system (PNS). The CNS consists
of the brain and spinal chord while the PNS consists of the nerves that
connect the central nervous system to different parts of the body.
The central nervous system receives
information from all parts of the body and also sends information to the
muscles. Communication between the CNS and body parts is facilitated by
the nerves of PNS.
The important
components of the nervous system are:
The Central Nervous System
The central nervous system consists of
the brain and the spinal cord. The brain is enclosed in a bony box called the cranium
and spinal cord is protected by the vertebral column. The brain and
spinal cord are externally covered by a protective covering called meninges.
It is made up of three layers namely duramater (outer layer), arachnoid
(middle layer), piamater (inner layer). The space between meninges
is filled by a watery fluid called cerebrospinal fluid (CSF). This
fluid
flows from the
brain to the spinal cord and then back to the brain. It acts as a shock absorber and
protects the brain from injuries. It also provides nutrients to the cells in the brain
and spinal cord.
Human Brain
The brain is the main coordinating
centre of the body. It is a part of the nervous system, which controls and
monitors every organ of the body. The weight of the brain of an adult is about
1400 grams.
Different regions of the brain
The brain is divisible
into three main regions—forebrain, midbrain, and hindbrain.
Forebrain
It is the main thinking part of the
brain. It consists of the cerebrum, thalamus and hypothalamus. The forebrain
has sensory regions, which receive sensory impulses from various receptors. It
also has motor regions, which control the movement of various muscles such as
leg muscles. There are separate areas in the forebrain specialized for hearing,
smelling, seeing, general sensations such as pain, touch, taste, etc.
Cerebrum: The cerebrum
is the largest part of the brain and constitutes four-fifths of its weight. It
is divided by a deep cleft into two equal parts called left and right cerebral
hemispheres.
The cerebrum has two regions, an outer
cortex and inner medulla. The inner cortex is made up of cytons
(nerve cell body) that give it a greyish appearance, so it is also called as grey
matter. The medulla is composed of nerve fibres (axons and dendrites) that
give it an opaque white appearance due to the presence of myelin sheath covering,
so is also called a white matter.
The cortex is
provided with ridges called convolutions that increase the surface area of the
cerebrum. The well-developed cortex is responsible for the high degree of
intelligence of the humans.
The information obtained through sense
organs is stored in the cerebrum and used when needed. This ability to store
information helps in retaining the memory.
A certain part of the cerebrum primarily
controls intelligence, learning, memory, emotions, consciousness, thinking, and
the ability to articulate speech. The forebrain is also known as the main
thinking part of the brain.
In the cerebrum, the nerves that come from
the right side of the body are connected to the left side of the cerebral
hemisphere and the nerves that come from the left side of the body are
connected to the right side of the cerebral hemisphere. Therefore, organs of
the right side of the body are controlled by left hemisphere and organs of the
left side are controlled by the right hemisphere. Thus, injury in the left side
of cerebral hemisphere results in the paralysis of organs on the right side of the
body and vice-versa.
Diencephalon
It is the part of the forebrain located
below the cerebrum. It includes both thalamus and hypothalamus.
Thalamus is situated
between cerebral cortex and midbrain. It receives the nerve impulse from sense
organs and transmits them to the upper region. It coordinates the sensory and
motor signalling.
The hypothalamus contains many
areas that control the body temperature, urge for eating and drinking, etc.
Some regions of the cerebrum along with hypothalamus are involved in the
regulation of sexual behaviour and expression of emotional reactions such as
excitement, pleasure, fear, etc.
Midbrain
It is the small region of the brain that
connects the cerebrum with the hindbrain. It has regions that are concerned with
the sense of sight and hearing. Some regions of the midbrain transmit motor
impulses to the limbs.
Hindbrain
It consists of three parts namely pons
varoli, cerebellum and medulla oblongata.
Pons varoli consists of the
nerve fibres that connect various portions like cerebrum, cerebellum and
medulla oblongata of the brain. It has the control centres for facial
expression, respiration
and mastication
etc. Among the twelve pairs of cranial nerves, four pairs originate from the
pons varoli.
The cerebellum, which is a part
of the hindbrain, is responsible for maintaining the posture and equilibrium of
the body. It also coordinates the contraction of voluntary muscles, according
to the directions of the cerebrum.
The medulla is the posterior-most part of the brain and is connected to the spinal cord. Most involuntary
actions such as heartbeat, blood pressure, movement of food in the alimentary
canal, salivation, etc. are controlled by the medulla of the hindbrain.
Spinal Cord
It is the continuation of the medulla
oblongata and runs through the vertebral column. The spinal cord is made up of
two similar halves fused together to form a central canal containing the
cerebrospinal fluid. The outer portion of the spinal cord is known as the white
matter, which consists of nerve fibres and the inner portion contains the
cell bodies of neurons and is known as the grey matter. There are thirty-one pairs of spinal nerves that arise from the spinal cord. These nerves are
divided into branches that reach to several parts of the body like, heart,
lungs, stomach, urinary bladder, sex organs etc. The movement of limbs in the
body is controlled by the spinal cord through reflex actions.
The spinal cord tapers
at the end at the last vertebrae where from a collection of nerve roots
originate, which are horsetail-like in appearance and hence called the cauda
equina.
Protection
to the brain and spinal cord The brain, being an important organ, requires
protection. Therefore, it is enclosed in a bony box called the cranium. The
brain inside the brain box is also surrounded by a fluid-like material, which
acts as a shock absorber and thus, provides further protection to the brain. The spinal cord is protected by a bony, vertical rod with several curves called the
vertebral column.
Peripheral Nervous System
It consists of the nerves arising from the brain and the spinal cord, which links the CNS to the rest of the body. It consists of two types of nerves.
· Cranial nerves: There are 12 pairs of cranial
nerves and they emerge from the brain and reach the organs in the head region.
· Spinal nerves: There are 31 pairs of spinal nerves that emerge from the spinal cord and reach various parts of the body.
Messages are transferred from the brain
to the spinal cord and then to the rest of the body and similarly messages from
the rest of the body reach the spinal cord from where they are transferred to
the brain. The spinal cord also controls all reflex actions.
Autonomic Nervous System
The autonomic nervous system helps to
carry out the orders of the medulla, which controls the vital body functions.
It consists of two networks:
· Sympathetic system: The sympathetic nerves lead to
all vital internal organs and glands. They regulate the actions of smooth
muscles such as that of the stomach, intestine, and the heart.
· Parasympathetic system: This system is made up of the vagus and the pelvic nerves.
The sympathetic system speeds up the
body functions and prepares the body for combat and escapes while the
parasympathetic system counteracts to that of the sympathetic system and slows
down the body functions.
Responses of the Nervous System
What happens when the following takes place?
·
Bright
light is focused on our eyes
·
We
accidentally touch a flame
· We are hungry and we think about our favourite meal
For all the situations mentioned above, the response would be quick and automatic. We would
·
close
our eyes immediately when bright light is focused on our eyes
·
withdraw
our hand from the flame
· start salivating on thinking about our favourite meal
This automatic action or response
provoked by a stimulus is known as a reflex action.
The responses of the nervous system can
be classified into voluntary, involuntary, and reflex actions.
The actions that can be controlled
voluntarily are called voluntary actions. The signal or message for
these actions is passed to the brain. Therefore, they are consciously
controlled.
On the other hand, the movement of food
in the alimentary canal or the contraction and relaxation of the blood vessels
are involuntary actions i.e.they cannot be consciously controlled.
The reflex actions, however, show
sudden responses and do not involve any thinking. This means that unlike
involuntary actions, these actions are not under the control of the brain.
Reflex arc
When we accidentally touch a hot object,
we withdraw our hands immediately without thinking. If we do not do this, our
hands will burn.
The sensory nerves detect the heat. They
are connected to the nerves, which move the muscles of the hand. Such a
connection of detecting the signal from the nerves (input), and responding to
it immediately (output) is called a reflex arc. In other words, it is the
pathway along which nerve impulse travels during the reflex action.
A reflex arc makes
instant and automatic responses possible. It connects the input nerve and
output nerve and meets in a bundle in the spinal chord. In fact, nerves from
all over the body meet in a bundle in the spinal cord, on their way to the
brain. Therefore, the information input reaches the brain.
The reflex arc consists of five distinct
parts and these are:
1. Receptor: It includes
sense organs that receive a stimulus.
2. Sensory neuron: It conducts the
nerve impulse from the receptor to the spinal cord or brain.
3. Association neuron: It helps to
transmit nerve impulse from sensory neuron to motor neuron.
4. Motor neuron: It transmits
nerve impulse to the effector organs like muscles or glands.
5. Effector: It includes
muscles or glands where the action takes place in response to the stimulus.
Types of Reflexes
Ivan Pavlov classified all reflex
responses in two categories − Unconditional and conditional reflexes.
Unconditional Reflexes − These are the
inborn, unconscious responses to a given stimuli which are transferred to the
next generation as well.
Some of the examples of such
unconditional responses are suckling of the mother’s breast by a new born body
blinking of eyes when an object is brought very close to the eyes.
Condition Reflexes − Such responses
are acquired during the life time of an individual. These responses are
different for different organisms. These responses can be easily induced or
lost depending upon the environmental conditions. Pavlov's Experiment on a
Dog In this experiment the Russian famous biologist, Ivan Pavlov tested the
conditional reflexes. He used a dog as his experiment subject and tested for the
secretion of saliva in response to ringing of a bell. Under normal condition,
dog will not secret saliva on listening the ringing of a bell or any other
sound. In his experiment, Pavlov brought food and rang the bell simultaneously
for a prolonged period of time.
After an adequate
period of training, it was observed that the dog started secreting saliva just
by listening to the bell's ringing.
Conditional reflexes are controlled by
cerebral cortex.
Some of the examples of conditional or
acquired reflex are learning, playing the piano, typing on a computer, etc.
Functions and Regulation of Hormone
A feedback mechanism regulates the action
of the hormones
The glucose present in the blood is broken
down to produce the energy required for the body. If it is present in an excess
amount in the blood, then it is converted into glycogen.
How does the body know when to convert
glucose into glycogen or to breakdown glycogen into glucose?
Hormones control most physiological
reactions.
How are the timing and the number of
hormones released regulated? Is there any mechanism to control the system?
The endocrine glands secrete hormones
depending upon the need of the organism. The number of hormones secreted should
be in an accurate amount. The regulation of the quantity of the hormones and
the timing of its release are controlled by feedback mechanisms.
There are two types of feedback
mechanisms—positive and negative feedback.
Positive
feedback: In
this mechanism, the response accelerates after the feedback. The effect is
further intensified in the same direction. It helps in speeding up the process
occurring in various body systems. It is the opposite of negative feedback.
Negative feedback: In this
mechanism, the information given by the feedback causes a reverse response. It
occurs when the system needs to slow down or completely stop a process.
Illustrations to understand the two
types of mechanisms
1. Childbirth
Uterine contractions occur during the
onset of labour pain. These contractions stimulate the release of a specific
hormone called oxytocin (from the pituitary gland), which intensifies
the contractions. The contractions further stimulate the production of oxytocin
and this cycle stops only after the birth of the baby.
This is an example of positive feedback.
2. Insulin
When you consume a carbohydrate-rich
diet, it is digested into glucose. The glucose is then absorbed by the blood.
This results in an increase of blood-sugar level and leads to the stimulation
of the pancreas to secrete insulin. Insulin stimulates the target cells to take
up the extra glucose from the blood. This glucose is either used during
respiration or stored as glycogen. Thus, the level of glucose in the blood is
maintained. This is an example of negative feedback.
The negative feedback
loop is shown in the following illustration.
3.
Thyroid Stimulating Hormone (TSH)
The negative feedback loop can also be observed in the case of the regulation of thyroid hormones. In this particular regulation, the hypothalamus secretes Thyroid Releasing Hormone (TRH) which stimulates anterior pituitary to produce TSH. TSH further activates the cells of thyroid glands and consequently, thyroid hormones are released in the blood. A gradual rise in the concentration of thyroid hormones serves as an inhibitory signal for hypothalamus and it stops producing TRH. In response to inhibition of TRH, TSH production is also lowered and eventually, the secretion of thyroid hormones is lowered. Some Interesting Facts:
Do you know that insulin was first extracted from dog pancreas in 1921 at the University of Toronto?
Human insulin (Humulin) is produced by using human genes in the bacterium called E .coli.
Functions of Hormones
To regulate the metabolic activities
To regulate the morphogenic activities such as growth, development, etc.
To regulate mental activities, growth maturation, reproductive activities, etc.
To control the activities of other endocrine glands
To maintain homeostasis
Movement
in Plants
In
animals, control and coordination is governed by the nervous system. However,
plants do not have a nervous system.
Then,
how do plants respond to stimuli?
Plants
respond to stimuli by showing movement.
Have you ever seen any movement in plants?
When you touch a sensitive plant such as touch- me- not (Mimosa pudica), the plant folds its leaves and droops
When a seed germinates, the root grows down in the soil and the stem grows up in the air.
In
the first example, touch is the stimulus and the plant responds by folding its
leaves. Therefore, the plant shows movement by folding its leaves.
In
the second example, the seed germinates and shows directional movement.
In the first example, movement is
independent of growth i.e. there is no growth involved. However, in the second
example, the movement of the seedling is caused by growth. If the seedling is
prevented from growing, then it will not show any movement.
Thus,
plants exhibit both growth-dependent and growth-independent movements.
Growth-independent
movements
In
plants (like animals), the information is carried from cells by
electro-chemical means. However, there is no specialized tissue for the
conduction of information. In fact, plants change their shape by changing the
amount of water in them. This results in swelling and shrinking. This change of
shape results in movement.
Growth-dependent
movements
You
must have seen plants such as peas and grapes with tendrils. Movement in these
plants occurs in the growing stem of the tendrils. When the tendrils come in
contact with a supporting object, they coil and cling around it. Plants respond
to stimuli slowly by growing in a particular direction. This type of growth is
directional.
Nastic
movements in plants
Nastic
movements are the movements in plants that take place in response to the
environment stimulus. One of the main feature of the nastic movement is that
the direction of the movement is independent of the direction of stimulus.
For
example, the movement of organs like leaves and petals that are directed by the
touch as in the leaflets of touch me not plant, wherein the plant droops when
touched from any side. The various kinds of nastic movements shown by plants
are: Thigmonasty: It is the movement of a plant in reponse to an
external stimuli like touch, vibration etc. For example, Mimosa plant
responds by "folding up of leaves" when touched. The sensitive part
of the plant involved in this type of movement is called pulvinus which is a
soft, swollen structure present at the base of the leaf.
Photonasty: Some
plants show movement in plant part in response to the light. For example,
flowers of sunflower and lotus open in the morning. Thermonasty: It is the
movement of plant parts in response to the rise and fall in temperature. For
example, the flower of crocus and tulip open with a rise in temperature and close
with a drop in temperature.
Tropism in Plants
Tropic movements in plants
Tropism is the response to stimuli that
comes from one direction.
If the movement of the plant part is
towards the stimulus, then it is known as positive tropism. If the
movement of the plant part is away from the stimulus, then it is known as negative
tropism.
Types of tropisms
Phototropism
The growth movement in plants in
response to light stimulus is known as phototropism. For example, the
flower head of a sunflower is positively phototropic as it moves from East to
West, along with the movement of the Sun.
In the above activity,
the shoots show positive phototropism, while the roots show negative
phototropism.
Geotropism
The growth movement in
plants in response to the force of gravity is known as geotropism. In
geotropism, the roots of the plant always grow downwards, while the shoots
always grow upwards, away from the earth.
Chemotropism
The growth movement in plants in
response to chemical stimuli is known as chemotropism. For example, the
growth of the pollen tube towards the ovule in the ovary (through the stigma and
style) is an example of positive chemotropism.
Hydrotropism
The growth movement in plants in
response to water is known as hydrotropism. For example, the roots of
some plants grow towards the water source, even when the water source is not
present directly below it. Thigmotropism The growth movement in plants
in response to a touch stimulus or contact with a solid object is known as
thigmotropism. For example, in some plants, the coiling of tendrils occurs when
they come in contact with objects for support.
Plant Hormones
Phytohormones
In plants, growth, development, and
response to the environment is controlled and coordinated by a special class of
chemical substances known as phytohormones. These hormones are produced
in one part of the plant body and are translocated to other parts. For example,
a hormone produced in the roots is translocated to other parts where they are
required.
Thus, the growth hormones of plants are
known as phytohormones. These are naturally occurring organic
substances. They are synthesized in minute quantities in one part of the plant
body and are translocated to other parts where they are required.
Types of phytohormones
There are five
major types of phytohormones: auxins, gibberellins, cytokinins, abscisic acid,
and ethylene. These phytohormones are either growth promoters such as auxins,
gibberellins, cytokinins, and ethylene, or growth inhibitors such as abscisic
acid.
Auxins
When the growing parts of a phototropic
plant detect sunlight, auxins (synthesized at the shoot tips) help the cells
grow longer. When light falls on one side of the plant, the auxins generally
diffuse towards the shaded side of the shoot. This stimulates the cells in the
shaded area to grow longer than the corresponding cells of the illuminated
region. This results in the curvature of the plant stem tip towards the light.
Gibberellins
They are produced in the roots of a
plant. They promote stem elongation by promoting cell division in the
inter-nodal region.
Cytokinins
They promote cell division. Therefore,
they are present in greater concentration in those areas of the plants where
rapid cell division occurs. For example, the tip of the shoot.
Abscisic acid
It promotes seed dormancy by inhibiting
cell growth. It is involved in the opening and closing of stomata. It is also
responsible for the shedding of leaves.
Ethylene
It regulates fruit ripening. It is
produced during the ripening of fruits.
Fruit ripening
Ethrel (Ethephon) liquid is sprayed on
plants to facilitate fruit ripening. It contains a dilute solution of
2-chloroethyl phosphonic acid, which breaks down to release ethylene. It helps
in the artificial ripening of commercially grown fruits such as pineapples,
mangoes, bananas etc.
How is over-ripening prevented?
CO2 in high concentration prevents over-ripening of fruits as it inhibits the production of ethylene.
Do you know that a
sixth category of plant hormones has been recently discovered? This new
category is popularly known as steroids. Steroids are not only involved in
growth, but also in the regulation of plant activities such as responses to
stress, invading pathogens, etc.
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