CHAPTER 3:
COORDINATION AND RESPONSE
3.1
RESPONSE AND COORDINATION
1.
Definition:
(a) Stimuli: Any changes in the environment which cause
response in our body.
There are two types of stimuli:
§
External
stimuli – from external environment of an organism
§
Internal
stimuli – from internal environment of an organism
(b)
Response:
The
way an organism reacts after detecting a stimulus.
Coordination: The process by which stimuli are
detected and eventually results in appropriate response.
1.
The
necessity of organism to respond to stimuli:
- to ensure the survival of the organism
3.1.1
Components
and pathways involved in detecting and responding to changes in the external
environment
1.
Three
main components:
(a) Receptors
-
specialized
sensory cells which detect stimuli
-
consists
of cell or organ
(b) Integrating centre
-
Consists
of brain and spinal cord
-
Integrates
and interprets impulses from receptors and then sends out impulses to effectors
to obtain certain responses.
(c) Effectors
-
The
part of the body that makes responses
-
Consists
of muscles or glands
2.
The
pathways of the impulses:
(a) Afferent pathway
-
The
pathway of impulses from a receptor to an integrating centre
(b) Efferent pathway
- The pathway of impulses from an integrating to
centre an effector
3.1.1
Components
and pathways involved in detecting and responding to changes in the internal
environment
1.
In
regulating the internal environment, negative feedback mechanism is involved.
-
When
there is a change in the internal environment, the body will respond by
producing a corrective mechanism in the opposite direction.
1.
The
regulation of changes in the internal environment is called homeostasis.
3.1.1
The
role of coordination and response is carried out by two different systems
-
coordinates
and controls body activities by sending out nerve impulses.
-
Only
found in animals
(b)
The
endocrine system
-
coordinates
and controls body activities by secreting hormones
-
found
in animals and plants
3.2
THE
ROLE OF THE HUMAN NERVOUS SYSTEM
1.
The
human nervous system is divided into two main subsystems:
(a) The central nervous
system (CNS)
-
consist
of brain and spinal cord
-
acts
as the control centre for our body activities
(b) The peripheral
nervous system (PNS)
-
includes
all nerve tissues outside the central nervous system
-
links
receptors and effectors to CNS
-
consists
of 12 pairs of cranial nerves (take impulses to and from the brain) and 31
pairs of spinal nerves (take impulses to and from the spinal cord)
1.
The
nervous system performs three functions:
(a) Sensory
-
Sensory
involves the sensory receptors which detect stimuli and initiate nerve impulses
that carry information to CNS
(b) Integrative
-
CNS
processes and integrates the information by analyzing and storing some of it
and making decision for appropriate responses.
(c) Motor
-
Motor
transmits the commands from the CNS to an effector
2.
The
role of the nervous system
(a) detects changes in
external and internal environment
(b) sends impulses to
effectors so as to respond appropriately to the changes
(c) coordinates various
functions and activities which occur inside the body
3.1.1
The
Brain
1.
The
brain consists of 100 billion neurons and weighs about 1300 – 1400 g in an
adult.
2.
It
is a highly folded structure with a large surface to accommodate more neurons
3.
Grey
matter forms the outer part of the human brain, while the interior of the brain
is made up of white matter.
4.
Grey
matter contains the cell bodies of the neurons while white matter contains
myelinated axons of the neurons.
5.
The
brain is surrounded by protective membranes called meninges. Cerebrospinal
fluid is found between these layers. This fluid supplies the neurons in the
brain with oxygen and nutrient and removes waste products.
1.
The
main parts of human brain are:
(a) cerebrum
-
the
largest and most complex part of the brain
-
divided
into two halves called hemispheres
(i)
left
hemisphere: controls movement of the right side of the body
(ii)
right
hemisphere: controls movement of the left side of the body
-
the
outer 3 mm of the cerebral hemispheres is called cerebral cortex and it is
densely packed with nerve cells.
-
functions:
(i)
directing
voluntary muscle movement
(ii)
creating
sensory perception (see, smell, hear, taste, touch)
(iii) responsible for
higher mental abilities (learning, memory, reasoning, speech etc)
(iv) coordinates other
parts of the brain
(b) cerebellum
-
It
is located at the back of the brain
-
It
also has two hemispheres
-
Functions:
(i)
Coordinates
the muscular movement
(ii)
Maintains
body balance
-
The
cerebellum receives information from the organs of balance in the ear and from
stretch receptors in muscles and tendons. Then, cerebellum coordinates the
movement of the muscles and maintains the body balance.
(c) medulla oblongata
-
It
is located under the cerebellum and connected to the spinal cord
-
Functions:
(i)
controls
and coordinates all involuntary actions: heartbeat, breathing, digestion etc.
(ii)
contains
reflex centres for vomiting, coughing, sneezing, hiccupping and swallowing
(d) thalamus
-
Functions:
(i)
Thalamus
is responsible for sorting information which goes in and comes out of the
cerebral cortex.
(ii)
Integrates
the information that is carried by sensory receptors to the cerebrum by
enchancing some signals and blocking others
(e) Hypothalamus
-
functions:
(i)
maintains
homeostasis and contains centres for regulating sleep, hunger, thirst, body
temperature, water balance and blood pressure
(ii)
controls
pituitary gland and serve as a link between the nervous system and the
endocrine system
3.1.1
Spinal
Cord
1.
The
spinal cord lines below the medulla oblongata.
2.
The
spinal cord is made up of white matter on the outside and gray matter on the
inside.
1.
Spinal
nerves emerge from the spinal cord by two short branches called roots.
-
The
dorsal root contains the axons of afferent neurones which conduct impulses to
the spinal cord from the sensory receptor. The dorsal root ganglion is composed
of cell bodies of afferent neurons.
-
The
ventral root contains the axon of efferent neurons which conduct impulses away
from the spinal cord from effectors
.
2.
The
functions of spinal cord:
(a) processes certain
types of sensory information and sends out response via the efferent neurons
(b) connects the
peripheral nervous system to the brain
(c) acts as a minor
integrating centre in producing simple reflex responses such as withdrawal of
the hand from a hot object
3.1.1
Neurone
1.
A
neurone is a bacis unit of a nerve cell. Neurones cannot be replaced if damage.
2.
The
function of neurones:
-
transmit
nerve impulses to other nerve cells or to the glands and muscles
3.
There
are three types of neurons:
(a) afferent neurons or sensory
neurones
-
carry
sensory information from receptors cells into the brain and spinal cord
(b) efferent neurons or
motor neurones
-
carry
information from the brain or spinal cord to the effectors, that is muscle or
gland cells
(c) interneurones or
relay neurones
-
convey
nerve impulses between the various parts of the brain and the spinal cord
-
transmit
nerve impulses between the afferent neurons and the efferent neurons
1.
The
structure of a typical neurone:
§
A
typical neurone has 4 distinct parts:
(b) Dendrites
-
receive
information or signals from other neurons or from the external environment and
conduct them towards the cell body
(c) Cell body
-
contains
the nucleus and other organelles
-
integrates
signals and coordinated metabolic activities
(d) Axon
-
conducts
nerve impulses away from the cell body
-
some
axons are insulated by a thick coat of material called myelin sheath to protect
and insulate the axons and helps to speed up the conduction of nerve impulses
(e) Synaptic terminals
-
located
at the far end of each axon
-
transmit
nerve impulses to muscle cells, gland cells or the dendrite of another neurone
3.1.1
Transmission
of information along neurons
-
Information
is transmitted along a neurone by way of electrical signals known as nerve
impulses.
-
Stimuli
are received by receptors
-
When
a receptor receives a stimulus, it will send out information in the form of
nerve impulses.
-
These
impulses are transmitted along the afferent / sensory neurone to the
interneurone in the central nervous system.
-
The
central nervous system integrates and interprets the information and then sends
out impulses to the effectors via the efferent / motor neurone.
-
When
nerve impulses reach an effector, a response is elicited.
3.1.1
Transmission
of information across synapses
1.
The
transmission of information across a synapse:
-
involves
the conversion of electrical signals into chemical signals in the form of
neurotransmitters
-
is
an active process which requires energy.
2.
Examples
of neurotransmitters:
-
acetylcholine,
noradrenaline, serotonin and dopamine
3.
The
functions of synapses:
(a) controlling and
integrating the nerve impulses transmitted by the stimulated receptors
(b) facilitating the
transmission of nerve impulses in one direction.
-
a
nerve impulse cannot go backward across a synapse
-
synaptic
vesicles are only present in the synaptic terminals
-
only
the presynaptic membrane can discharge neurotransmitters
-
receptors
are only present in the postsynaptic membrane.
3.1.1
Voluntary
action and involuntary action
|
Voluntary Action
|
Type of action
|
Involuntary Action
|
|
|
Characteristic
|
|
|
|
Involvement of
Central Nervous System
|
|
|
|
Rate of action
|
|
|
|
Awareness
|
|
|
|
Effector
|
|
|
|
Examples
|
|
3.1.2
Reflex
Action and Reflex Arc
1.
A
reflex action is a rapid response which is obtained automatically without
conscious thought
2.
Examples:
(a) rapid and automatic
withdrawal of hand when it accidentally touches a hot object.
(b) Automatic closing of
eyes when an insect flys towards the eye
3.
Reflexes
provide a means for immediate withdrawal from dangerous stimuli.
The pathway of a transmission of nerve impulse which
produces a reflex action is called a reflex arc.
1.
Reflex
arc in the withdrawal of hand when touching a hot object:
- When the hand accidentally touches a hot
object, pain receptors in the skin will sense the stimulus and produce a nerve
impulse.
- The nerve impulse is transmitted via the
sensory neurone and then across a synapse to the interneurone in the grey
matter of spinal cord.
- From the interneurone, the impulse crosses a
synapse again to the motor neurone.
- The motor neurone sends the impulse to the
effector organ, which is the biceps. The biceps contract and the hand withdraw
from the hot object.
2.
Reflex
arc for the knee-jerk response
-
When
the tendon below the kneecap is tapped by a rubber-headed hammer, the
quadriceps (thigh muscles) are stretched.
-
The
stretching stimulates the stretch receptors in the quadriceps to produce a
nerve impulse.
-
The
nerve impulse is transmitted via the sensory neurone to the spinal cord.
-
Within
the grey matter of spinal cord, the
nerve impulse crosses a synapse to the motor neurone.
-
When
the impulse arrive at the quadriceps, the quadriceps contract, jerking leg
forward
3.1.1
The
autonomic nervous system
3.1.1
Diseases
of the nervous system
1.
Parkinson’s
Disease
-
An
accelerating disorder of the CNS that typically affects victims around the age
of 60 years old and above.
-
Due
to the low level of dopamine ( a neurotransmitter) in the brain
-
Effects
of Parkinson’s disease
(a) tremors or trembling
of the arms, jaws and face
(b) difficulty in
maintaining a normal posture and causes impaired balance and coordination
(c) results in weakness
of the muscles
2.
Alzheimer’s
Disease
-
A
neurological disease which affects victims around the age of 60 years old and
above.
-
Main
cause of the disease is still unknown, but other factors like genetic,
environmental and aging process do affect.
-
Effects
of Alzheimer’s disease
(a) increasing loss of
memory and unable to care for oneself
(b) confused and
forgetfully
(c) lose their way
(d) lose ability to read,
write, eat, walk and talk
3.2
THE
ROLE OF HORMONES IN HUMAN
-
Hormone
= an organic chemical compound produced
by an endocrine gland and is transported by blood to its target organ to obtain
a certain response.
-
Endocrine
gland = a ductless gland that synthesizes and secretes hormones.
3.1.1
The
need of the endocrine system
-
The
endocrine system complements the nervous system in maintaining homeostasis.
-
The
differences between the nervous system and the endocrine system
|
The
nervous system
|
Aspects
|
The
endocrine system
|
|
|
Definition
|
|
|
|
Function
|
|
|
|
Type
of signal
|
|
|
|
Route
of transmission
|
|
|
|
Speed
of transmission
|
|
|
|
Destination
|
|
|
|
Effect
of action
|
|
3.1.2
The
human endocrine system
-
The
human endocrine system comprises glands that contain hormone-secreting cells.
-
Hormones
which are no longer useful are neutralized by the liver and excreted by the
kidneys.
-
Organs
which certain hormones act upon are called target organs.
-
A
hormone may have a few target organs.
-
The
functions of hormones can be divided into three main categories:
3.1.1
Regulation
of hormones secretion
-
An
endocrine gland releases its hormone more frequently when stimulated.
-
If
there were no stimulation, the level of hormone would decrease in the blood as
it is excreted or inactivated.
-
Hormone
secretion is normally regulated to prevent over-production and
under-production.
-
Hormone
secretion is regulated by:
(a) signals from the
nervous system
(b) other hormones
(c) the level of certain
substances in the body
1.
The
regulation of hormone secretion by signals from the nervous system
-
The
hypothalamus receives nerves impulses from the receptors which then sends the
nerve impulses to stimulate the pituitary glands to release the hormones.
-
Pituitary
gland is the master endocrine gland because it secretes several hormones that
control other endocrine glands.
-
The
hypothalamus has a cluster of specialized nerve cells called neurosecretory
cells.
-
The
cell bodies of the neurosecretory cells are located in the hypothalamus .The
posterior pituitary contains the axons and synaptic terminals of the
neurosecretory cells.
-
Neurosecretory
cells produce:
(i)
Hypothalamic
releasing hormones and Hypothalamic inhibiting hormones which carried in the
bloodstream to the anterior pituitary
Ø
Hypothalamic
releasing hormones: stimulate the secretion of anterior pituitary hormones
Ø
Hypothalamic
inhibiting hormones: suppress the secretion of anterior pituitary hormones
(ii)
ADH
and axytocin are hormones synthesized in the cell bodies but stored in the
posterior pituitary. Nerve impulses that arrives at the synaptic terminals in
the posterior pituitary trigger the release of both these hormones.
2.
The
regulation of hormone secretion by other hormones
-
Example:
The secretion of thyroxine is regulated by the thyroid-stimulating hormone.
-
Examples
of hormones regulated by the secretion of other hormones:
(i)
luteinising
hormone (LH)
2.
The
regulation of hormone secretion by the level of certain substances in the body
-
Substances
in the blood such as glucose can control the secretion of hormones.
3.1.1
The
involvement of both the nervous system and the endocrine system in a ‘fight or
flight’ situation
3.1.1
Hormonal
imbalances and related diseases
|
Hormone
|
Excess
of hormone
|
Deficiency
of hormone
|
|
Growth
hormone
|
|
|
|
Hormone
|
Excess
of hormone
|
Deficiency
of hormone
|
|
Thyroxine
|
|
|
|
Insulin
|
|
|
|
Antidiuretic
Hormone
(ADH)
|
|
|
3.1.1
Hormone
usage in medicine
1.
Growth
Hormone
-
Growth
hormone can be produced in large quantities through genetically-engineered
bacteria
-
Growth
hormone is used to treat patients who are suffering from stunted growth or
dwarfism
-
Growth
hormone and thyroxine can be used to treat children with cretinism
2.
Insulin
-
Insulin
is extracted from cultured bacteria using advanced technology in genetic
engineering
-
Insulin
is used to treat patients with diabetes mellitus
-
Insulin
is injected into the bloodstream to regulate the blood glucose level in a
patient’s body
3.
Oestrogen
-
Oestrogen
can be used to treat problems in the development of secondary sexual
characteristics, problems in menstrual cycle or menopause
-
Oestrogen
is also used to prevent osteoporosis in the treatment called Hormone
Replacement Therapy (HRT) for women undergoing menopause
-
A
mixture of oestrogen and progesterone is used to prepare contraceptive pills.
The mixture of hormones in the pill changed the balance of hormones that
regulates the menstrual cycle. This changes prevent ovulation and thus prevent
pregnancy.
4.
Progesterone
-
Progesteron
is injected into the body of a pregnant woman so that the lining of the uterine
wall can be strengthened and thickened to prevent miscarriage.
5.
Antidiuretic
hormone
-
Antidiuretic
hormone is used to treat diabetes insipidus so that adequate amount of water
will be retained in the body.
3.2
HOMEOSTASIS IN HUMAN
1.
Homeostasis
= The process of maintaining optimal
physical and chemical conditions in the internal environment.
2.
Examples
of physical factors: body temperature , blood pressure and the osmotic pressure
3.
Examples
of chemical factors: partial pressure of carbon dioxide and oxygen, and the
blood glucose level.
4.
The
homeostasis occurs through a negative feedback mechanism.
5.
Examples
of process that are regulated by homeostasis through negative feedback
mechanism:
(a) Regulation of blood
osmotic pressure (Osmoregulation)
(b) Regulation of body
temperature (Thermoregulation)
(c) Regulation of blood
sugar level
3.2.1 Regulation of blood
osmotic pressure
3.2.1.1
The
Human Kidney
The position of kidney The longitudinal section of kidney
1.
The
kidneys filter blood and produce urine which exits the body through the
ureters, urinary bladder and urethra.
2.
The
kidney has an outer light-red region called the renal cortex and an inner
darker red-brown region called the renal medulla.
3.
Urine
formed in the kidney drains into the renal pelvis.
4.
The
renal artery supplies oxygenated blood and nutrients to the kidney; the renal vein
carries away filtered blood to the body.
5.
Within
each kidney, the renal artery divides into the smaller vessels that eventually
deliver blood to the afferent arteriole.
6.
Each
afferent arteriole divides into a tangled capillary network called the
glomerulus.
7.
The
capillaries of glomerulus reunite to form an efferent arteriole.
8.
Each
efferent arteriole divides to form a network of capillaries around the kidney
tubules.
3.1.1.1
The
nephron
1.
The
functional unit of kidney is called the nephron.
2.
Each
human kidney consists of about one million nephrons.
3.
A
nephron consists of three major parts:
(i)
the
glomerulus
(ii)
Bowman’s
capsule
(iii)
the
renal tubule which made up of
-
the
proximal convoluted tubule
-
the
loop of Henle
-
the
distal convoluted tubule
4.
The
distal convoluted tubule of several nephrons join to form a collecting duct.
5.
To
produce urine, the nephron performs three basic processes:
(i)
Ultrafiltration
(ii)
Reabsorption
(iii)
Secretion
3.1.1.2
The
formation of urine
(I)
Ultrafiltration
in Bowman’s capsule
-
The
kidneys receive blood from the aorta, so the blood is under relatively high
pressure when it reaches the nephron.
-
This
pressure is maintained because the afferent arteriole has a larger diameter
than the efferent arteriole.
-
As
blood enters the glomerulus, ultrafiltration takes place.
~ The high pressure
forces fluid through the filtration membrane into the capsule space.
-
The
fluid that enters the capsule space is called the glomerulus filtrate.
-
The
glomerulus filtrate has the same composition as blood plasma, but does not
contain red blood cells and plasma protein because the red blood cells and
plasma protein are too large to pass through the filtration membrane.
-
The
content of glomerulus filtrate:
~ water, glucose, amino acids, salts, urea
and other molecules
-
Each
minute about 125cm3 of filtrate enters the nephron.
(I)
Reabsorption
-
After
the filtrate leaves the Bowman’s capsule, its volume and composition are
adjusted in the renal tubule.
-
Useful
substances from the filtrate are reabsorbed into the blood capillaries that
around the tubule.
-
At
the proximal convoluted tubule:
(a) 75% - 80% of water is
reabsorbed back to the blood capillaries by osmosis.
(b) 100% of the filtered
glucose and amino acids, 65% of sodium
ion, and a large quantity of other ions such as potassium, chloride,
bicarbonate, calcium and magnesium are reabsorbed back to the blood capillaries
by active transport.
-
At
the loop of Henle:
(a) About 15% of water is
reabsorbed back to the blood capillaries by osmosis.
(b) Sodium ions and
chloride ions are actively transported into the blood capillaries.
-
At
the distal convoluted tubule and the collecting tubule
(a) The amount of water
and inorganic ions (salts) that will be reabsorbed from the filtrate depends on
the body’s needs and is controlled by the endocrine system.
(b) About 99% of water
has been reabsorbed into the bloodstream. Only 1% of the water in the filtrate
actually leaves the body as urine.
(II) Secretion
-
Secretion
is a process by which waste and excess substances that were not initially
filtered are secreted from the blood capillaries into the renal tubule.
-
Secreted
substances include:
(a) urea
(b) hydrogen ion
(c) potassium ion
(d) ammonia
(e) creatinine
(f)
toxinxs
(g) certain drugs
-
Secretion
occurs via passive diffusion and active transport
-
Secretion
takes place all along the renal tubules and collecting duct but it is
relatively active at the distal convoluted tubule.
-
Secretion
helps to:
(a) eliminate and
increase the rate of waste removal from the body
(b) regulate blood levels
of certain ions
3.1.1.1
The
role of the kidney in homeostasis (Osmoregulation)
3.1.1.1
Consequences
of impaired kidney function
1.
Kidneys
that are damaged by disease or injury fail to carry out ultrafiltration at the
glomerulus, and thus unable to:
-
regulate
the blood osmotic pressure
-
filter
the blood
-
remove
the unwanted waste products
2.
Methods
to treat the impaired kidney function:
(a) Haemodialysis
-
Haemodialysis
is the treatment for the patients with impaired kidney function to filter the
blood by dialysis using an artificial kidney machine.
(b) Kidney transplant
-
A
kidney transplant is a procedure that replaces the impaired kidney of a patient
with a healthy kidney from a donor.
3.1.2
Regulation
of blood sugar level
1.
The
pancreas is the gland that is responsible for maintaining blood sugar level in
the normal range of 75-110mg/100ml.
2.
The
islets cells in the pancreas produce and secrete hormones insulin and glucagons
directly into the bloodstream.
3.
Defects
in the production, release and reception of insulin by target cells can lead to
diabetes mellitus.
3.1.1
Regulation
of body temperature
3.1
PRACTISING
A HEALTHY LIFESTYLE
- The types of
drugs that are commonly abused and their effects are:
|
Types
of drug
|
Effects
|
|
Stimulants
|
|
|
Depressants
|
|
|
Hallucinogens
|
|
|
Narcotic
|
|
- The effects of
alcohol:
(a) Slows down the
function of the central nervous system, especially the transmission of nerve
impulses in the brain.
(b) Slows down reflexes
and disrupts coordination, causing unclear speech, loss of balance, blurred
vision and poor judgement of distance.
3.2
PLANT
HORMONES
1.
In
plants, hormones play an important role in growth processes such as:
(a) seed germination
(b) growth of roots
(c) development of fruits
2.
There
are five types of hormones in plants:
(a) Auxins, gibberellins
and cytokines are plant hormones that stimulate growth
(b) Ethylene and abscisic
acid are plant hormones that inhibit growth
3.
Functions
of plant hormones:
|
Types
of hormones
|
Function
|
Uses
in agriculture
|
|
Auxins
|
(a)
Stimulate growth and cell elongation
(b)
Stimulate the growth of adventitious roots
(c)
Stimulate ovaries in flowers to form fruits without
being fertilizer (parthenocarpy)
(d)
Inhibit the growth of weeds in high concentration
(e)
Prevent the developing fruits and leaves from falling
off prematurely.
(f)
Inhibit growth of lateral buds into shoots
|
(a)
To promote growth in plants
(b)
To promote the growth in adventitious roots
(c)
The stigma and ovaries of certain flowers are sprayed
with auxins to form fruits without being fertilized.
(d)
Used as herbicide (weed killer) in high concentration
(e)
Sprayed on fruits a few days before harvest to ensure
the fruits do not fall off the plant before they are ripe.
(f)
Used on potatoes to suppress the growth of lateral buds
on the potatoes.
|
|
Ethylene
|
(a)
Stimulate the ripening of fruits
(b)
Stimulate the falling off (abscission) of leaves and
fruits
|
(a)
Sprayed in air-tight storage rooms with fruits to
stimulate the ripening of fruits
(b)
Sprayed on fruits before harvest to promote abscission
|
|
Cytokinins
|
(a)
Stimulate cell division and cell elongation in the
presence of auxins
(b)
Delay the aging and death of leaves
|
(a)
Use in tissue culture to promote cell division and
growth
(b)
Sprayed on vegetables to ensure that the leaves are
green and fresh during packaging
|
|
Types
of hormones
|
Function
|
Uses
in agriculture
|
|
Gibberellins
|
(a)
Stimulate cell division and cell elongation in shoots
(b)
Stimulate the production of enzyme in germination of
seeds
|
(a)
To increase the size of plants
(b)
Rubbed on seeds to speed up germination
|
|
Abscisic
acid
|
(a)
Inhibits growth of shoots and promote dormancy in seeds
(b)
Stimulates the closing of stomata in water shortage
|
(a)
Used to prevent shoot growth and germination of seeds
in winter
(b)
Conserve water by slowing down transpiration through
the closure of stomata
|
4.
Effects
of Auxins on Growth Response
-
Auxins
are produced in the apical meristems of the shoot tip and the root tip in the
cell division zone.
-
From
the apical meristems, the auxins diffuse into the zone of cell elongation to
promote growth and elongation of cells.
-
In
the shoot tip, a high concentration of auxins stimulate cell elongation.
In the root tips, a high concentration of auxins inhibits
cell elongation.
1.
Role
of auxins in tropism
(A) Phototropism
§
Phototropism
= the growth of shoots and roots in response to light
§
If
the growth response is towards the light, then its is called positive
phototropism; if the growth response is away from the light, then its is called
negative phototropism.
i. The coleoptile bends
and grows towards the light, showing positive phototropism.
ii. The root bends away
from the light, showing negaitive phototropism.
(A) Geotropism
§
Geotropism
= the growth response of shoot tips and root tips towards gravity
i.
The
shoot bends away from the gravity and grows upwards, showing negative
geotropism.
ii.
The
root bends towards the gravity and grows downwards, showing positive
geotropism.
Include biological diagrams for more illustration
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