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Responding to the Environment
Nervous and Hormonal Communication
Animals increase their chances of survival by responding to changes in their external
environment
They respond to changes in their internal environment
This ensures the conditions are always optimal for their metabolism
Plants increase their chances of survival by responding to changes in their environment
A​
stimulus ​a change in the internal or external environment
is
Receptors​
detect stimuli
They can be cells or proteins on a cell surface membrane
Different receptors detect different stimuli
Effectors​ cells that bring about a response to a stimulus to produce an effect
are
Effectors include muscle cells and cells found in glands
Receptors communicate with effectors via the nervous system or the hormonal system
The nervous system send information as electrical impulses
A​
neuron ​a complex network of cells
is
The nervous system is made up of neurons
There are three main types of neurone:
Sensory neurons ​
transmit electrical impulses from receptors to the central nervous system
Motor neurons ​
transmit electrical impulses from the central nervous system to effectors
Relay neurons ​
transmit electrical impulses between sensory neurons and motor neurones
A stimulus is detected by receptor cells
An electrical impulse is sent along a sensory neurone
When an electrical impulse reaches the end of a neuron neurotransmitters take the
information across to the next neuron
This then send an electrical impulse

The CNS processes the information
It decides what to do about it
It send impulses along motor neurones to an effectors

The nervous system is split into two different systems
The central nervous system is made up of the brain and the spinal cord

The nervous system communication in localised, short-lived and rapid
When an electrical impulse reaches the end of a neuron, neurotransmitters are secreted
directly onto cells
This means the nervous response is localised
Neurotransmitters are quickly removed once they’ve done this
This means the response is short-lived
Electrical impulses are really fast

This means the response is rapid
This allow animals to react quickly to stimuli
The hormonal system sends information as chemical signals
The hormonal system is made up of glands and hormones
A​
gland ​a group of cells that are specialised to secrete a useful substance
is
Hormones ​ chemical messengers
are
Many hormones are proteins or peptides
Some hormones are steroids
Hormones are secreted when a gland is stimulated
Glands can be stimulated by a change in the concentration of a specific substance
They can be stimulated by electrical impulses
Hormones diffuse directly into the blood
They’re then taken around the body by the circulatory system
They diffuse out of the blood all over the body
Each hormone will only bind to specific receptors for that hormone
These are found on the membranes of some cells
The hormones trigger a response in the target cells

The hormonal system communication is slower, long-lasting and widespread
Hormones are not released directly onto target cells
They must travel in the blood to get to them
This means the chemical communication is slower than electrical communication
They are not broken down as quickly as neurotransmitters
The effects of hormones can last for much longer

Hormones are transported all over the body
This means the response may be widespread if the target cells are widespread

Receptors
Receptors are specific to one kind of stimulus
They only detect on particular stimulus
There are many different types of receptor
Each detect a different type of stimulus
Some receptors are cells
Some receptors are proteins on cell surface membranes
A​
resting state ​when a receptor is not being stimulated
is
When a receptor is in its resting state there is a difference in charge between the inside and
outside of the cell
This is generated by ion pumps and ion channels
There is a voltage across the membrane
Voltage is also known as potential difference
When a cell is at rest its potential difference is known as its resting potential
When a stimulus is detected the cell membrane is excited
It becomes more permeable
This allows more ions to move in and out of the cell
This alters the potential difference
Generator potential ​the change in potential difference as a result of a stimulus
is
A bigger stimulus excites the membrane more
This causes a bigger movement of ions, and a bigger change in potential difference
Therefore a bigger generator potential is produced
If the generator potential is big enough it will trigger an action potential

An ​
action potential ​an electrical impulse along a neuron
is
It will only be triggered if the generator potential reaches the threshold level
Action potentials are all one size
The strength of the stimulus is measured by the frequency of action potentials
If the stimulus is too weak, the generator potential won’t reach the threshold
Therefore there is no action potential
Pacinian corpuscles ​ pressure receptors in the skin
are
They are mechanoreceptors
Mechanoreceptors ​
detect mechanical stimuli
They contain the end of a sensory neurone known as a sensory nerve ending
The sensory nerve ending is wrapped in layers of lamellae
The lamellae is connective tissue
When a pacinian corpuscle is stimulated, the lamellae are deformed
They press on the sensory nerve ending
This causes deformation of stretch-mediated sodium channels in the neuron's cell membrane
The sodium ion channels open
Sodium ions then diffuse into the cell
This creates a generator potential
If the generator potential reaches the threshold, it triggers an action potential
Photoreceptors ​ light receptors found in the eye
are
Light enters the eye through the pupil
The amount of light that enters is controlled by the muscles of the iris
Light rays are focused by the lens onto the retina
The retina lines the inside the eye, and contains photoreceptor cells
The ​
fovea ​an area of the retina where there are multiple photoreceptors
is

Nerve impulses from the photoreceptors are carried from the retina to the brain by the optic
nerve
The ​
optic nerve ​a bundle of neurons
is
The eye contains a blind spot
This is where there are no photoreceptors
Therefore it is not sensitive to light
Photoreceptors convert light into an electrical impulse
Light enters the eye and hits photoreceptors
It is absorbed by light-sensitive pigments
Light bleaches the pigments
This causes a chemical change
The membrane’s permeability to sodium is changed
A generator potential is created
If it reaches the threshold a nerve impulse is sent along a bipolar neuron
Bipolar neurons ​
connect photoreceptors to the optic nerve
This takes impulses to the brain

The human eye has two types of photoreceptors
Rods and cones
Rods are mainly found in the peripheral part of the retina
Cones are found packed together in the fovea
Rods give information in monochromatic vision - black and white

Cones give information in trichromatic vision - colour
There are three types of cones:
Red-sensitive
Green-sensitive
Blue-sensitive
They’re stimulated in different proportions
This means people can see different colours
Rods are more sensitive
They are very sensitive to light
They fire action potentials in dim light
This is because many rods join one neurone
This means many weak generator potentials combine to reach the threshold and trigger an
action potential

Cones are less sensitive
They only fire action potential in bright lights
Only one cone joins to one neuron
Therefore it takes more light to reach the threshold and trigger an action potential

Rods give low visual activity as many rods join the same neurone
This means light from two objects close together cannot be told apart

Cones give high visual activity as cones are close together
Only one cone joins one neurone
When light from two point hits two cones, two action potentials go to the brain
You can distinguish two points that are close together as two separate points

Neurones
Neuronal cell membranes are polarised at rest
In a neuron's resting state the outside of the membrane is positively charged compared to the
inside
There are more positive ions outside the cell than inside

The membrane is polarised
Polarised ​when there is a difference in charge
is
The membrane’s resting potential is -70mV
The resting potential is created and maintained by sodium-potassium pumps and potassium
ions channels in a neuron's membrane
Sodium-potassium pumps use active transport to move three sodium ions out of the neuron for
every potassium ions moved in
ATP is needed to do this
Potassium ion channels allow facilitated diffusion of potassium ions out of the neuron down
their concentration gradient

The sodium-potassium pumps move sodium ions out of the neuron
The membrane is not permeable to sodium ions
They cannot diffuse back in
This creates a sodium ion electrochemical gradient, as there are more positive sodium ions
outside the cell than inside
An ​
electrochemical gradient ​a concentration gradient of ions
is

The sodium-potassium pumps move potassium ions into the neurone
The membrane is permeable to potassium ions
They diffuse back out through potassium ion channels
This makes the outside of the cell positively charged compared to the inside
Neuronal cell membranes become depolarised when they’re stimulated
A stimulus triggers sodium ion channels to open
If the stimulus is big enough it will trigger a rapid change in potential difference
This triggers action potential
The stimulus excites the neuron cell membrane
This causes sodium ion channels to open
The membrane become more permeable to sodium
Sodium ions diffuse into the neurone down the sodium electrochemical gradient
This makes the inside of the neurone less negative
Depolarisation occurs if the potential difference reaches the threshold
The sodium ion channels close and potassium ion channels open
The membrane is more permeable to potassium
The potassium ions diffuse out of the neuron, down the potassium ion concentration gradient
This starts to get the membrane back to its resting potential
Hyperpolarization occurs when potassium ion channels are slow to close
There’s a slight ‘overshoot’
This means too many potassium ions diffuse out of the neuron
The potential difference becomes more negative than the resting potential
Resting potential is reached when the ion channels are reset
The sodium-potassium pump returns the membrane to its resting potential
This is maintained until the membrane’s excited by another stimulus

After an action potential the neuron cell membrane can not be excited again straight away
This is because the ion channels are recovering
They cannot be made to open
Sodium ion channels are closed during repolarisation
Potassium ion channels are closed during hyperpolarization
The ​
refractory period ​the period of recovery after the action potential
is
The action potential moves along the neuron
Some of the sodium ions that enter the neuron diffuse sideways
This causes sodium ion channels in the next region of the neuron to open
Sodium ions diffuse into that part
This causes a wave a depolarisation to travel along the membrane
The wave moves away from the parts of the membrane in the refractory period
This parts cannot fire an action potential
Diagram
The refractory period produces discrete impulses
Ions channels are recovering and cannot be opened
The refractory period acts as a time delay between one action potential and the next
This makes sure that action potentials don’t overlap
Instead they pass along as discrete impulses
The refractory period ensures action potentials are unidirectional
Unidirectional ​to travel in one direction only
is
Once the threshold is reached, an action potential will always fire with the same change in
voltage
This is regardless of how big the stimulus is
If threshold isn’t reached, an action potential will not fire

A bigger stimulus will not cause a bigger action potential
It will cause a more frequent firing
Three factors affect the speed of conduction of action potentials
Some neurons have a myelin sheath
We call them myelinated
The sheath is an electrical insulator
It’s made up of a Schwann cell
Between these cells are tiny patches of bare membrane called the nodes of Ranvier
Sodium ion channels are concentrated at the nodes
In a myelinated neuron depolarisation only happens at the nodes of Ranvier
Here sodium ions can get through the membrane
The neuronal cytoplasm conducts enough electrical charge to depolarise the next node
The impulse ‘jumps’ from node to node
This is called saltatory conduction
It occurs very quickly
In a non-myelinated neuron, the impulse travels as a wave along the whole length of the axon
membrane
This occurs more slowly than saltatory conduction
However it is still relatively quick
Action potentials are conducted quicker along axons with bigger diameters
There’s less resistance to the flow of ions than in the cytoplasm of a smaller axon
With less resistance, depolarisation reaches other parts of the neuron cell membrane quicker

The speed of conduction increases as the temperature increases
Ions diffuse faster

The speed only increases up to 40o​
​
C
After that the protein begins to denature and the speed decreases

Synaptic Transmission
A​
synapse ​a junction between a neuron and the next cell
is
A​
synaptic cell ​the gap between the cells at a synapse
is
The presynaptic neuron has a swelling known as a synaptic knob
This contains synaptic vesicles filled with chemicals called neurotransmitters
When an action potential reaches the end of the neuron it causes neurotransmitters to be
released into the synaptic cleft
They diffuse across to the postsynaptic membrane
Here they bind to specific receptors
When neurotransmitters bind to receptors they can trigger:
Action potential in a neurone
Muscle contraction in a muscle cell
Hormone secretion in a gland
The receptors are only on the postsynaptic membranes
Therefore synapses ensure impulses are unidirectional
Neurotransmitters are removed from the cleft
This ensures the response doesn’t keep happening
They’re taken back into the presynaptic neuron
They can be broken down by enzymes
The products are then taken into the neurone
ACh transmits the nerve impulse across a cholinergic synapse
An action potential arrives at the synaptic knob of the presynaptic neuron
The action potential stimulates voltage gated calcium ion channel;s in the presynaptic neuron
to open

Calcium ions diffuse into the synaptic knob

The influx of calcium ions into the synaptic knob causes the synaptic vesicles to fuse with the
presynaptic membrane
The vesicles release the neurotransmitter acetylcholine into the synaptic cleft
This is known as exocytosis

ACh diffuses across the synaptic cleft
It binds to specific cholinergic receptors on the postsynaptic membrane

This causes sodium channels in the postsynaptic neuron to open
The influx of sodium ions into the postsynaptic membrane causes an action potential on the
postsynaptic membrane
This only occurs if the threshold is released
ACh is removed from the synaptic cleft
The ensures the response does not keep happening
It’s broken down by an enzyme called acetylcholinesterase - AChE
The products are reabsorbed by the presynaptic neuron
It is used here to make more ACh

Neuromuscular junctions​ synapses between motor neurons and muscle cells
are
They use the neurotransmitter ACh
They work in the same way as as the cholinergic synapse
The postsynaptic membrane has lots of folds that form clefts
These clefts store AChE, the enzyme that breaks down ACh
The postsynaptic membrane has more receptors than other synapses
When a motor neuron fires an action potential, it always triggers a response in a muscle cell

Neurotransmitters are excitatory or inhibitory
Excitatory ​
neurotransmitters depolarizes the postsynaptic membrane
This makes it fire an action potential if the threshold is reached
Inhibitory ​
neurotransmitters hyperpolarize the postsynaptic membrane
This makes the potential more negative
This prevents it from firing an action potential
Summation ​where the effect of neurotransmitter released from many neurons is added
is
together
There are two types of summation
Spatial summation
Sometimes many neurons connect to one neuron
The small amount of neurotransmitter released from each of these neurons can be enough
altogether to reach the threshold in the postsynaptic neuron
This triggers an action potential
If some neurones release an inhibitory neurotransmitter, the total effect of all the
neurotransmitters may be no action potential
Temporal summation
Two or more nerve impulses arrive in quick succession from the same presynaptic nerve
This makes an action potential more likely
This is because more neurotransmitter is released into the synaptic cleft
Both types of summation mean synapses accurately process information
This finely tunes the response
Drugs affect the action of neurotransmitters
Some drugs have the same shape as neurotransmitters
They mimic neurotransmitters action at receptors
This means more receptors are activated

These are called agonists
Some drugs block receptors
This prevents them from being activated by neurotransmitters
This means fewer receptors are activated
These are called antagonists
Some drugs inhibit the enzyme that breaks down neurotransmitters
This means there are more neurotransmitters in the synaptic cleft to bind to receptors
They’re there for longer
This can lead to loss of muscle control
Some drugs stimulate the release of neurotransmitters from the presynaptic neuron
This means more receptors are activated
Some drugs inhibit the release of neurotransmitters from the presynaptic neuron
This means fewer receptors are activated

Effectors - Muscle Contraction
Skeletal muscles are made up of long muscle fibres
Muscle fibres are large bundles of long cells
It is the muscle used for movement
The ​
sarcolemma ​the cell membrane of muscle fibre cells
is
Parts of the sarcolemma fold inwards across the muscle fibre
They stick to the sarcoplasm
The ​
sarcoplasm ​a muscle cell’s cytoplasm
is
These folds are known as transverse tubules
They help to spread electrical impulses throughout the sarcoplasm
This ensures the reach all parts of the muscle fibres
The ​
sarcoplasmic reticulum ​a network of internal membranes
is

These run through the sarcoplasm
The sarcoplasmic reticulum stores and releases calcium ions which are needed for muscle
contraction
Muscle fibres have multiple mitochondria to provide ATP needed for muscle contraction
They are multinucleate - contain multiple nuclei
Muscle fibres have myofibrils
A ​yofibril ​a long, cylindrical organelle
m
is
They’re made up of proteins
These are highly specialised for contraction

Thick myofilaments are made of myosin proteins
Thin myofilaments are made of actin proteins
These move past each other to make muscles contract
They are made of alternating dark and light bands
Dark bands contain the thick myosin filaments
They have some overlapping thin actin filaments
These are called A-bands

Light bands contain thin actin filaments only
These are called I-bands
A myofibril is made up of short units called sarcomeres
The ends of each sarcomere are marked with a Z-line
In the middle of each sarcomere is an M-line
The M-line is the middle of the myosin filaments
Around the M-line is the H-zone
The H-zone contains myosin filaments only

Muscle contraction is explained by the sliding filament theory
Myosin and actin filaments slide over one another to make the sarcomeres contract
They filaments themselves do not contract
The simultaneous contraction of multiple sarcomeres mean the myofibrils and muscle fibres
contract
Sarcomeres return to their original length as the muscle relaxes

Myosin filaments have globular heads that are hinged
This allows them to move back and forth
Each myosin heads has a binding site for actin, and a binding site for ATP
Actin filaments have binding sites for myosin heads
These are called actin-myosin binding sites
Two other proteins called tropomyosin and troponin are found between actin filaments
These proteins are attached to each other
They help filaments move past each other

In a resting muscle the actin-myosin binding site is blocked by tropomyosin
This is held in place by troponin

This prevents myofilaments from sliding past each other
This is because the myosin heads cannot bind to the actin-myosin binding site of the actin
filaments
When an action potential for a motor neuron stimulates a muscle cell, it depolarises the
sarcolemma
Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum
This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm
Calcium ions bin d to troponin
This causes it to change shape
This pulls the attached tropomyosin out of the actin-myosin binding site of the actin filament
This exposes the binding site
This allow the myosin head to bind
An actin-myosin cross bridge is formed
This is the bond formed when a myosin head binds to an actin filament

Calcium ions active the enzyme ATPase
This breaks down ATP into ADP + P​
i

This provides energy needed for muscle contraction
The energy released from ATP moves the myosin head
This pulls the actin filament along in a rowing action

ATP provides the energy to break the actin-myosin cross bridge
This means the myosin head detaches from the actin filament after it has moved
The myosin head then reattaches to a different binding site further along the actin filament
A new actin-myosin cross bridge is formed
The cycle is repeated
Many actin-myosin bridges from and break very rapidly
This pulls the actin filament along
This shortens the sarcomere
This causes the muscle to contract
The cycle will continue as long as calcium ions are present and bound to troponin

When the muscle stops being stimulated, calcium ions leave their binding site on the troponin
molecules
They are moved by active transport back into the sarcoplasmic reticulum
The troponin molecules return to their original shape
This pulls the attached tropomyosin molecules with them
This means the tropomyosin molecules block the actin-myosin binding sites again
Muscles are not contracted
This is because no myosin heads are attached to actin filaments
Therefore there are no actin-myosin cross bridges
The actin filaments side back to their relaxed position
This lengthens the sarcomere
Much energy is needed when muscles contract
This means ATP is used up very quickly
IUt has to be continually generated so exercise can continue
This occurs in three ways:
Most ATP is generated via oxidative phosphorylation in the cell’s mitochondria
Aerobic respiration only works when there’s oxygen
Therefore it is good for long periods of low-intensity exercise

ATP is made rapidly by glycolysis
The end product of glycolysis is pyruvate
This is converted to lactate by lactate fermentation
Lactate can quickly build up in the muscles
This causes muscle fatigue
Anaerobic respiration works without oxygen
Therefore it is good for short periods of hard exercise
ATP is made by phosphorylating ADP
This is done by adding a phosphate group taken from PCr
PCr is stored inside cells and the ATP-PCr system generated ATP very quickly
PCr runs out after a few seconds
The ATP-PCr system is anaerobic and alactic
This means it does not use oxygen, or produce lactate
Therefore it is good for short bursts of vigorous exercise
Skeletal muscles are made up of two types of muscle fibre
Slow twitch
Fast twitch
Different muscles have different proportions of slow and fast twitch fibres
They both have different properties
Slow twitch muscle fibres

Fast twitch muscle fibres

Muscle fibres that contract slowly

Muscle fibres that contract very quickly

Muscles you use for posture have a high
proportion of them

Muscles you use for fast movement have a
high proportion of them

Good for endurance activities

Good for short bursts of speed and power

Can work for a long time without getting
tired

Get tired very quickly

Energy is released slowly through aerobic
respiration
Lots of mitochondria and blood vessels
supply the muscles with oxygen

Energy is release quickly through aerobic
respiration using glycogen
There are few mitochondria or blood vessels

Reddish in colour as they’re rich in
myoglobin
A red coloured protein that stores oxygen

Whitish in colour as they don’t have much
myoglobin

Responses in animals
Control of heart rate involves the brain and autonomic nervous system
The heart is myogenic
The sinoatrial node initiates a heartbeat
It sends a wave of electrical impulses across the atria
This causes atrial contraction
The atrioventricular delays the electrical impulses
This allows the atria to empty before the ventricles contract
The AVN sends a wave of electrical impulses down the Purkyne fibres
This causes the ventricles to contract from the base up, in a ventricular systole
The rate at which the SAN fires is unconsciously controlled by the medulla
Animals need to alter their heart rate to respond to internal stimuli
Stimuli are detected by pressure receptors and chemical receptors
Baroreceptors ​ pressure receptors in the aorta and the vena cava
are
They’re stimulated by high and low blood pressure
Chemoreceptors ​ chemical receptors in the aorta, carotid artery and in the medulla
are
They monitor the oxygen, carbon dioxide and pH levels in the blood
Electrical impulses from receptors are sent to the medulla along sensory neurones
The medulla processes the information and sends impulses to the SAN along sympathetic or
parasympathetic neurons

Stimulus

Receptor

Neurone and neurotransmitter

Effector

Response

High blood
pressure

Baroreceptors detect
high blood pressure

Impulses are sent to the medulla,
which sends impulses along
parasympathetic neurones
...
These secrete
noradrenaline, which binds to
receptors on the SAN

Cardiac
muscles

Heart rate
speeds up to
increase
blood
pressure to
normal

High blood O​
2 or
​
pH levels
Low blood CO​
2
levels

Chemoreceptors detect
chemical changes in
the blood

Impulses are sent to the medulla,
which sends impulses along
parasympathetic neurones
...
These secrete
noradrenaline, which binds to
receptors on the SAN

Cardiac
muscles

Heart rate
increases to
return O​
2,
​
CO​ pH
and
2​
levels back to
normal

A​
reflex ​when the body responds to a stimulus without making a conscious decision to
is
respond
Information travels really fast from receptors to effectors
This is because you do not have to spend time deciding how to respond
Simple reflexes help organisms to avoid damage to the body as they are rapid
A​
reflex arc ​a pathway of neurons inking receptors to effectors in a reflex
is
If there’s a relay neuron involved in the simple reflex arc, it is possible to override the reflex

Sensory Neurone

Motor Neurone

Interneuron

Simple

organisms have

simple responses to keep them in a favourable environment
Tactic ​
responses occur when organisms move towards or away from a directional stimulus
Kinetic ​
responses occur when organisms’ movement is affected by a non-directional stimulus
A​
chemical mediator ​a chemical messenger that acts locally
is
Chemical mediators are secreted from cells that are all over the body
Their target cells are right next to where the chemical mediator’s produced
This means they stimulate a local response
They only have to travel a short distance to their target cell
This means they produce a quicker response than hormones
A​
histamine ​a chemical mediator that’s stored in mast cell and basophils
is
It’s released in response to the body being injured or infected
It increases the permeability of the capillaries nearby
This allows more immune system cells to move out of the blood to the infected or injured area
Prostaglandins ​ a group of chemical mediators that are produced by most cells of the
are
body
They’re involved in things such as inflammation, fever and blood pressure regulation
Survival and responses in plants
Plants need to respond to stimuli as well
Flowering plants increase their chances of survival by responding to changes in their
environment
They sense the direction of light and grow towards it
This maximises light absorption for photosynthesis
They can sense gravity
This allows roots and shoots to grow in the right direction
Climbing plants have a sense of touch
This allow they to find things to climb and reach the sunlight

A​
tropism ​a plant’s growth response to a directional stimulus
is
Plants respond to stimulus by regulating their growth
A​
positive tropism ​growth towards the stimulus
is
A​
negative tropism ​growth away from the stimulus
is
Phototropism ​the growth of a plant in response to light
is
Shoots are positively phototropic
This means they grow towards lights
Roots are negatively phototropic
This means they grow away from lights
Geotropism ​the growth of a plant in response to gravity
is
Shoots are negatively geotropic
This means they grow upwards
Roots are positively geotropic
This means they grow downwards
Plants do not have a nervous system
Therefore they cannot respond using neurones
They do not have a circulatory system
Therefore they cannot respond using hormones
Plants respond to stimuli using growth factors
Growth factors ​ chemical that speed up or slow down plant growth
are
Growth factors are produced in the growing regions of the plant
They move to where they’re needed in other parts of the plant
Gibberellin is a growth factor that stimulates flowering and seed germination
Auxins are growth factors that stimulate the growth of shoots by cell elongation
This is where cell walls become loose and stretchy

This results in the cell getting longer
High concentration of auxins inhibit growth in roots
Indoleacetic Acid is an important auxin
It’s produced in the tips of shoots in flowering plants
IAA is moved around the plant to control tropisms
It moves by diffusion and active transport over short distances
It moves through the phloem over long distances
This results in different parts of the plants having different amount of IAA
The uneven distribution of IAA means there’s uneven growth of the plant
IAA moves to the more shaded parts of the shoots and roots according to phototropism
This results in uneven growth
IAA moves to the underside of the shoots and roots according to geotropism
This results in uneven growth



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