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HL IB Biology Your notes

Integration of Body Systems

Contents
Integration in Living Organisms
The Nervous System
Re ex Arc & Movement Control
Epinephrine & Melatonin
Control Mechanisms
Observing Tropic Responses: Skills (HL)
Phototropism (HL)
Plant Hormones (HL)
Regulating Plant Growth & Fruit Ripening (HL)

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Integration in Living Organisms

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System Integration
Complex living organisms have evolved to make use of living, or body, systems made up of
component parts that collectively perform an overall function
Coordination of these parts is required in order for the systems to fully integrate and work together for
the whole organism
Living systems are often made up of billions of cells and so require mechanisms of cell-cell
communication within the system and with cells in a separate system in a di erent part of the organism
An example of this, found in both plants and animals, is the use of hormones; these are produced
within one body system (the endocrine system) but may have an e ect in a di erent body system
(the reproductive system)

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System Integration: Cells, Tissues, Organs & Systems

Multicellular organisms have developed a hierarchy of organisation that allows for e ective Your notes
communication and interaction with their environment
Specialised cells of the same type group together to form tissues
A tissue is a group of cells that work together to perform a particular function. For example:
Epithelial cells group together to form epithelial tissue (the function of which, in the small intestine,
is to absorb food)
Muscle cells (another type of specialised cell) group together to form muscle tissue (the function
of which is to contract in order to move parts of the body)
Di erent tissues work together to form organs. For example:
The heart is made up of many di erent tissues (including cardiac muscle tissue, blood vessel
tissues and connective tissue, as well as many others)
Di erent organs work together to form organ systems
Organ systems work together to carry out the life functions of a complete organism
At each hierarchical level there is great e ciency and complexity
Example of the hierarchy of organisation diagram

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Multicellular organisms have many levels of organisation

Emergent properties
Multicellular organisms are able to undertake functions that unicellular organisms cannot, e.g. move
over vast distances and digest large macromolecules
This is a result of properties emerging when individual cells organise and interact to produce living
organisms
Scientists sometimes summarise this with the phrase "The whole is greater than the sum of its
parts" , this phrase describes the idea that the individual systems within the organism are more
e ective when they work together
Traditionally, scientists have approached the study of biology from a reductionist perspective, looking
at the individual cells, however, due to emergent properties there is an argument that the systems
approach should be used
For example a cheetah becomes an e ective predator by integration of all its body systems

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Integration of Organs
Communication within the bodies of animals is primarily by the nervous system or the endocrine Your notes
system
Often these two systems are required to work together to maintain body processes such as digestion,
maintaining heart rate, blood glucose levels and blood pressure
These processes rely on transfer of energy and materials around the body of the organism
Transport vessels within the blood system are at times required to move materials around the body to
various tissues, for example:
Oxygen and glucose are transported to all cells of the body to facilitate respiration
Urea, produced by protein metabolism in the liver, is transported in the blood to be excreted by the
kidneys
Hormones, such as FSH and LH, are transported via the blood from the pituitary gland in the brain
to the ovaries during the menstrual cycle

The nervous system

The human nervous system consists of:
Central nervous system (CNS) – the brain and spinal cord
Peripheral nervous system (PNS) – all of the nerves in the body
It allows us to make sense of our surroundings and respond to them, and to coordinate and regulate
body functions
Information is sent through the nervous system in the form of electrical impulses – these are electrical
signals that pass along nerve cells known as neurones
A bundle of neurones is known as a nerve
The nerves spread out from the central nervous system to all other regions of the body and
importantly, to all of the sense organs
More information about the structure of the nervous system can be found here

The endocrine system

A hormone is a chemical substance produced by an endocrine gland and carried by the blood
The endocrine glands that produce hormones in animals are known collectively as the endocrine
system
A gland is a group of cells that produces and releases one or more substances (a process known
as secretion)
Glands of the endocrine system diagram

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Hormones are produced in the glands and travel round the body in the blood
Hormones are chemicals which transmit information, via the blood, from one part of the organism to
another and that bring about a change
They alter the activity of one or more speci c target organs
Hormones only a ect cells with receptors that the hormone can bind to
These are either found on the cell surface membrane, or inside cells
Receptors have to be complementary to hormones for there to be an e ect
E ects can be long lived, as long as hormones are bound to the receptors
Hormones are used to control functions that do not need instant responses
They travel more slowly in the blood compared to a nervous impulse
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Comparison of the nervous and endocrine systems table

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The Nervous System

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The Brain as Integration Organ
The structure of the brain
The brain alongside the spinal cord is part of our central nervous system
The brain is made of billions of interconnected neurones and is responsible for controlling complex
behaviours, both conscious and unconscious
Within the brain are di erent regions that carry out di erent functions
The cerebral cortex: this is the outer layer of the brain which is divided into two hemispheres. It’s
highly folded and is responsible for higher-order processes such as intelligence, memory,
consciousness and personality
The cerebellum: this is underneath the cerebral cortex and is responsible for balance, muscle
coordination and movement
The brainstem: this relays messages between the cerebral cortex, the cerebellum and the spinal
cord. A key part is the medulla which controls unconscious activities such as heart rate and
breathing
Two important glands of the brain that are integral the endocrine system are
The pituitary gland: This gland is responsible for producing many hormones including those
involved in controlling the menstrual cycle (FSH and LH)
The hypothalamus: This region of the brain is involved in regulating body temperature, it also
producing hormones which control the pituitary gland
Structures of the brain diagram

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The brain is made up of several regions

The role of the brain
The brain coordinates and processes information received
Interactions within the brain are responsible for learning and memory
The brains requires several receptors in order to receive information (this is input of information)
At a conscious level information is received by
Photoreceptors located within the retina of the eye for visual information
Chemoreceptors found in the tongue for tasting
Thermoreceptors located in the skin for detection of temperature changes
Mechanoreceptors located in the inner ear which are sensitive to sound vibrations
At unconscious level input of information is via
Osmoreceptors located in the carotid arteries and hypothalamus which detect the water
content of the blood
Baroreceptors, also located in the carotid arteries and the aorta, these sense pressure
changes of the blood
Proprioceptors which are located in muscles and joints and provide information on balance
and movement

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Examiner Tip
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You are are not required to know complex details of the brain such as the role of slow-acting
neurotransmitters.

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The Spinal Cord as Integration Centre

The spinal cord is part of the central nervous system (CNS), along with the brain Your notes
It is a neural pathway between the body and the brain, yet it can also process information
independently from the brain
This information is processed at the unconscious level and involves some re ex reactions
The spinal cord can be described as an integration centre for unconscious processes
Note that information processed at conscious level means that the cerebrum of the brain is also
involved
The spinal cord is responsible for bringing sensory information to the CNS from the body and enables
motor (muscular) information to be sent out
There are two types of tissue in the spinal cord
White matter contains mainly the axons only of neurones that carry information to and from the
brain
Grey matter contains the neurones and synapses involved in spinal cord integration processes
which create a re ex response
Sensory information enters the spinal cord along sensory neurones, this is immediately
processed and sent out as motor information along motor neurones; this pathway is called a
re ex arc
Because the brain is not involved in this pathway this is unconscious control directed by the
spinal cord alone
Re ex arc diagram

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The spinal cord of the central nervous system acts as an integration centre for unconscious processes
Your notes

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Input Through Sensory Neurones

A neural pathway begins with a receptor Your notes
A receptor is a specialised cell that can detect changes in the environment that cause a stimulus
Receptor cells are transducers – they convert energy in one form (such as light, heat or sound) into
energy in an electrical impulse within a sensory neurone
Receptor cells are often found in sense organs (e.g. light receptor cells are found in the eye)
Some receptors, such as light receptors in the eye and chemoreceptors in the taste buds, are
specialised cells that detect a speci c type of stimulus and in uence the electrical activity of a
sensory neurone
Other receptors, such as some kinds of touch receptors, are just the ends of the sensory neurones
themselves
When receptors cells are stimulated they are depolarised
If the stimulus is very weak, the cells are not su ciently depolarised and the sensory neurone is
not activated to send impulses
If the stimulus is strong enough, an action potential is initiated in the sensory neurone and the
impulse is transmitted to the CNS, speci cally the spinal cord and the cerebral hemispheres

An example of the sequence of events that results in an action potential in a

sensory neurone
The surface of the tongue is covered in many small bumps known as papillae
The surface of each papilla is covered in many taste buds
Each taste bud contains many receptor cells known as chemoreceptors
These chemoreceptors are sensitive to chemicals in food and drinks
Each chemoreceptor is covered with receptor proteins
Di erent receptor proteins detect di erent chemicals
Chemoreceptors in the taste buds that detect salt (sodium chloride) respond directly to sodium ions
If salt is present in the food (dissolved in saliva) being eaten or the liquid being drunk:
Sodium ions di use through highly selective channel proteins in the cell surface membranes of
the microvilli of the chemoreceptor cells
This leads to depolarisation of the chemoreceptor cell membrane
The increase in positive charge inside the cell is known as the receptor potential
If there is su cient stimulation by sodium ions and su cient depolarisation of the membrane, the
receptor potential becomes large enough to stimulate voltage-gated calcium ion channel
proteins to open
As a result, calcium ions enter the cytoplasm of the chemoreceptor cell and stimulate exocytosis
of vesicles containing neurotransmitter from the basal membrane of the chemoreceptor
The neurotransmitter stimulates an action potential in the sensory neurone
The sensory neurone then transmits an impulse to the brain
Diagram to show the sequence of events initiated by activated chemoreceptors in the taste
buds

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Your notes

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Sensory neurons convey messages from receptor cells to the CNS

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Output Through Motor Neurones

Once an action potential has been transmitted from a sensory neurone to the CNS Your notes
The cerebrum within the brain uses the information to process movements within the body as needed;
the part of the cerebrum responsible for this is called the motor cortex
The role of motor neurones
Motor neurones are used to carry action potentials to muscles to initiate the movement required
Motor neurones terminate within a muscle within a neuromuscular junction (also known as motor end
plates)
There are multiple neuromuscular junctions spread across several muscle bres within the muscle
Neuromuscular junctions work in a very similar way to synapses
They are located between the terminal branches of a motor neurone and a muscle cell
Transmission across the neuromuscular junction
When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane,
the action potential causes calcium ions to di use into the neurone
This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the
presynaptic membrane
The ACh that is released di uses across the neuromuscular junction and binds to receptor proteins on
the sarcolemma (surface membrane of the muscle bre cell)
This stimulates ion channels in the sarcolemma to open, allowing sodium ions to di use in
In ux of sodium ions depolarises the sarcolemma, generating an action potential that passes down
the T-tubules towards the centre of the muscle bre
Action potentials stimulate muscle contraction
Action potentials travel down the T-tubules and trigger the opening of voltage-gated calcium ion
channel proteins in the membranes of the sarcoplasmic reticulum
Calcium ions di use out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the
myo brils
Calcium ions bind to troponin molecules, stimulating them to change shape
This causes the troponin and tropomyosin proteins to change position on the thin (actin) laments
The myosin-binding sites are exposed to the actin molecules
The process of muscle contraction (known as the sliding lament model) can now begin
Muscle contraction diagram

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Your notes

Action potentials from the motor neurone cross the neuromuscular junction to trigger the events
leading to muscle contraction

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The Structure of Nerves

Information is sent through the nervous system as nerve impulses – electrical signals that pass along Your notes
nerve cells known as neurones
Nerves are made up of bundles of sensory neurones or motor neurones
These may be myelinated or unmyelinated
The di erent structures of these neurones are considered in more detail here
Myelination is covered in more detail here
Structure of a Nerve Diagram

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A nerve is made up of a bundle of individual neurone cells

Transverse and Cross Section of Myelinated Neurone Diagram

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Each Schwann cell wraps its plasma membrane concentrically around the axon to form a segment of
myelin sheath about 1mm long

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Re ex Arc & Movement Control

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The Pain Re ex Arc
Re exes
Re ex responses are actions of the body that occur without conscious thought
Re exes are automatic and rapid, minimising damage to the body and therefore aiding survival
Awareness of a re ex response occurs after it has been carried out; this is because the
information takes longer to reach the conscious parts of the brain
Examples of re exes include blinking, coughing, and the pupil and knee re exes
Blinking prevents the outer surface of the eye from drying out as well as protecting it from foreign
objects
Coughing prevents food from entering the airways and removes mucus from the airways during
infection or an allergic reaction
The pupil re ex prevents damage to the eye from bright light
The knee re ex aids balance when standing upright
What is a re ex arc
A re ex arc is a pathway along which impulses are transmitted from a receptor to an e ector without
involving conscious regions of the brain
A re ex arc therefore brings about a re ex response
Sensory neurones, relay neurones and motor neurones work together in a re ex arc
Order of a re ex arc
A pain re ex arc is another example of a re ex response
The skin has receptors for pressure, touch, and pain
The receptor involved is the pain receptor called a nocireceptor
The stimulus may be a sharp pin or hot ame which is detected by the nocireceptor in the skin
of the hand
An a erent (sensory) action potential is sent along a sensory neurone to the CNS
An electrical impulse is passed to a relay neurone in the spinal cord
Relay neurones are found entirely within grey matter of the spinal cord
A relay neurone synapses with a motor neurone
A synapse is the junction between neurones; nerve impulses cross synapses by di usion
of a chemical called a neurotransmitter
A motor neurone carries an impulse to an e ector muscle in the hand
When stimulated by the motor neurone the muscle will contract and pull the hand away from
the sharp object or heat; this is the re ex response
The re ex arc for a spinal re ex is as follows:
A Re ex Arc Diagram

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Spinal re exes involve relay neurones in the spinal cord as part of a pain re ex

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Role of the Cerebellum

The cerebellum coordinates movement Your notes
This includes balance; a highly complex function that requires coordination between multiple
parts, including the eyes, semicircular canals in the ears, and many muscles
Other movements coordinated by the cerebellum are
Posture
Walking
Hand and nger movements
Eye movements
Speech
The cerebellum does not initiate movement, the motor cortex of the cerebrum is responsible for
initiating muscle contractions and therefore movement
Once the movement begins the cerebellum receives feedback signals from the area of the body that is
moving and di erent sense organs, it then sends signals to coordinate and control the movement
The structure and function of the brain as an organ is covered in more detail here

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Epinephrine & Melatonin

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Melatonin
Circadian rhythms
Many physiological processes and behavioural patterns occur in regular, daily rhythms in organisms
throughout the plant and animal kingdoms
Many animal species are only active for a speci c part of the 24−hour cycle e.g. nocturnal animals
are only active at night
Humans are diurnal meaning that we are more awake during daylight hours
Humans are adapted to live in a 24−hour cycle and many aspects of our physiology and behaviour,
including physical activity, sleep, body temperature, and secretion of hormones, follow speci c and
regular cycles throughout the 24−hour period
These daily cycles are known as circadian rhythms
In humans, many circadian rhythms are in uenced by the hormone melatonin
Melatonin is secreted by the pineal gland, which is located in the brain
Melatonin secretion increases in the evening in response to darkness and decreases at dawn in
response to light, leading to our diurnal behaviour patterns

Melatonin and sleep patterns

Although melatonin a ects many aspects of human physiology and behaviour, one of the main
circadian rhythms it controls is our sleep-wake cycle
The pineal gland secretes melatonin into the blood
Production is in uenced by the detection of light and dark by the retina of the eye
Signals are then transmitted to the pineal gland according to the amount of daylight a person
is exposed to and varies with changes in day length (this is why the pineal gland is sometimes
referred to as both an endocrine clock and an endocrine calendar)
Melatonin's target sites are found in many areas of the brain including the hypothalamus and
pituitary glands, and also in the cells of the immune system, gonads, kidney, and the cardiovascular
system, blood vessels, and intestinal tract
Increasing melatonin levels lead to feelings of tiredness and promote sleep
Decreasing melatonin levels lead to the body's preparation for waking up and staying awake
during the day
Experiments have also suggested that
Increased melatonin at night contributes to the night-time drop in core body temperature in
humans
Melatonin receptors in the kidney enable melatonin produced at night to cause the night-time
decrease in urine production in humans
Melatonin is still released in the absence of light and dark signals, but on a slightly longer cycle
than the usual 24 hours
Subjects living in the dark with no access to natural daylight still release melatonin on a roughly
24 hour cycle
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This suggests that the role of light is to reset the melatonin system every day to keep the
circadian rhythm in line with daylight hours
Your notes
Secretion of melatonin graph

The production of melatonin is in uenced by the amount of daylight a person is exposed to: melatonin
levels peak during

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Epinephrine
During situations that creates stress, fear or excitement, the neurones of the sympathetic nervous Your notes
system will stimulate the adrenal medulla (of the adrenal gland) to secrete epinephrine (also called
adrenaline)
Epinephrine is a hormone that will prepare your body for reacting to a stressful situation
This reaction is often called the " ght or ight" response
It is the e ects of epinephrine that lead to the typical symptoms we experience during
stressful situations such as increased heart rate, dry mouth, increased sweating etc
The adrenal gland diagram

The adrenal glands secrete the hormone epinephrine and prepare the body for vigorous activity
Since epinephrine is a hormone, it is transported around the body in the bloodstream
It will bind to receptors on its target organs
One of the targets of epinephrine is the SAN, leading to an increase in the frequency of excitations
This in turn, will increase the heart rate to supply blood to the muscle cells at a faster rate
More blood means more oxygen and glucose that reaches the muscle cells, which in
turn, increases the rate of aerobic respiration
This releases more energy that will be used during the response to the stressful, vigorous or
dangerous situation
Epinephrine will also stimulate the cardiovascular control centre in the medulla oblongata
This increases the impulses travelling along the sympathetic neurones a ecting the heart, further
speeding up the heart rate
Blood vessels to less important organs (such as the digestive system and skin) constrict so that more
blood can be diverted to organs that will be involved in the " ght or ight" response

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Note that blood ow to the brain remains constant, regardless of whether the body is in a state of
stress or relaxation
The brain is one of the most important organs in the body and needs a constant blood supply Your notes
in order to function properly
The changes experienced by the body during the " ght or ight" response are controlled by a
combination of nervous and hormonal responses
Epinephrine is also covered in the course with reference to the second messenger model, this can be
found here

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Control Mechanisms
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Control of the Endocrine System
A hormone is a chemical messenger produced by an endocrine gland and carried by the blood
They are chemicals which transmit information from one part of the organism to another and
bring about a change
They alter the activity of one or more speci c target organs
Hormones are used to control functions that do not need instant responses
The endocrine glands that produce hormones in animals are known collectively as the endocrine
system
A gland is a group of cells that produces and releases one or more substances (a process known
as secretion)
Control of the endocrine system is primarily by the hypothalamus and the pituitary gland

The hypothalamus
The hypothalamus monitors the blood as it ows through the brain and, in response, releases
hormones or stimulates the neighbouring pituitary gland to release hormones
The hypothalamus plays an important role in some homeostatic mechanisms
Hypothalamus functions include
Regulating body temperature
The hypothalamus monitors blood temperature and initiates a homeostatic response if
this temperature gets too high or too low
Osmoregulation
Cells in the hypothalamus monitor the water balance of the blood and releases the
hormone ADH if the blood becomes too concentrated
ADH increases absorption of water in the kidneys
Regulating digestive activity
The hypothalamus regulates the hormones that control appetite as well as the secretion
of digestive enzymes
Controlling endocrine functions
The hypothalamus causes the pituitary gland to release hormones that control a variety of
processes e.g. metabolism, growth and development, puberty, sexual functions, sleep,
and mood

The pituitary gland

The pituitary gland is located below the hypothalamus
Its role is to produce a range of hormones
Some of these directly in uence and regulate processes in the body while some stimulate the
release of further hormones from other endocrine glands
The pituitary gland is divided into two sections; the anterior pituitary and posterior pituitary
The anterior pituitary produces and releases hormones

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The posterior pituitary stores and releases hormones produced by the hypothalamus e.g. ADH and
oxytocin
Your notes

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Feedback Control of Heart Rate

There are several circumstances that can cause an individual's heart rate to increase, such as during Your notes
exercise
The brain is involved in this response of the heart however it does not require any thinking and is under
unconscious control
There is a speci c region of the brain that plays a vital role in controlling the heart rate
This cardioregulatory centre in the brain is called the medulla
The medulla is found at the base of the brain near the top of the spinal cord
The medulla is made up of two distinct parts:
The acceleratory centre, which causes the heart to speed up
The inhibitory centre, which causes the heart to slow down
Both centres are connected to the sinoatrial node (SAN) by nerves
These speci c nerves are di erent from the nerves that control conscious activities. They make up
the autonomic nervous system
The autonomic nervous system is self-controlling
Control of heart rate diagram

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Your notes

The location of the medulla helps to keep it protected from harm. It has an essential function as a
cardioregulatory centre.
Increasing heart rate
Once the acceleratory centre has been activated impulses are sent along the sympathetic neurones
to the SAN
Norepinephrine is secreted at the synapse with the SAN
Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces
This results in an increased heart rate
Decreasing heart rate
Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones
to the SAN
Acetylcholine is secreted at the synapse with the SAN
This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces
This reduces the elevated heart rate towards the resting rate

Chemoreceptors and baroreceptors

Exercise causes several internal conditions to change, creating internal stimuli:
Carbon dioxide concentration in the blood increases
There is an initial fall in blood pressure caused by the dilation of muscle arterioles
These internal stimuli can be detected by chemoreceptors and baroreceptors (pressure)
receptors located in the aorta (close to the heart) and in the carotid arteries (they supply the head with

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oxygenated blood)
Chemoreceptors detect changes in blood pH and oxygen and carbon dioxide levels
Baroreceptors monitor changes in blood pressure Your notes
Location of the baroreceptors and chemoreceptors

Baroreceptors are located on the arch of the aorta and on the enlargement of the carotid arteries called
the sinus. Chemoreceptors are located near the baroreceptors but on the outside of the aorta and
carotid arteries
These receptors release nerve impulses that are sent to the acceleratory and inhibitory
centres (coordinators)
The frequency of the nerve impulses increases or decreases depending on how stimulated the
receptors are:
Lower frequency impulses activate the inhibitory centre to slow down the heart rate and stroke
volume
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Higher frequency impulses activate the acceleratory centre to speed up the heart rate and stroke
volume
Your notes

The processes involved in the control of the heart rate. The internal stimuli are detected by
chemoreceptors and baroreceptors that send impulses to coordinators (accelerator centre or
inhibitory centre). The coordinators send signals to the e ector (SAN) which produces a speci c
response.

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Feedback Control of Ventilation Rate

Ventilation rates in the body are controlled by cells called respiratory centres which are located in the Your notes
medulla of the brain
At rest, action potentials, produced at random, travel to the diaphragm and intercostal muscles to
initiate contraction and therefore ventilation; this occurs at a stable and slow pace
During exercise, higher levels of carbon dioxide are produced due to an increase in respiration
Waste carbon dioxide produced during respiration di uses from the tissues into the blood
This waste carbon dioxide is transported around the body in di erent ways
In the blood plasma in the form of hydrogen carbonate ions (HCO3−); around 85 % of carbon
dioxide is transported in this way
Around 5 % of carbon dioxide dissolves directly in the blood plasma
Bound to haemoglobin as carbaminohaemoglobin; this accounts for around 10 % of carbon
dioxide transport in the blood

pH changes in the blood

Carbon dioxide released as a waste product from respiring cells di uses into
the cytoplasm of red blood cells
Inside red blood cells, carbon dioxide combines with water to form carbonic acid
(H2CO3)
CO2 + H2O ⇌ H2CO3
Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between
carbon dioxide and water
Without carbonic anhydrase, this reaction proceeds very slowly
The plasma contains very little carbonic anhydrase hence H2CO3 forms more slowly in
plasma than in the cytoplasm of red blood cells
Carbonic acid dissociates readily into hydrogen ions (H+) and hydrogen carbonate ions (HCO3−)
H2CO3 ⇌ HCO3– + H+
Hydrogen ions can lower the pH of the blood so their presence is detected by chemoreceptors in the
medulla
Action potentials are sent at a higher rate to the diaphragm and intercostal muscles of the lungs to
increase ventilation rates and the volume of air being moved into and out of the lungs
The pH of the blood can then return to normal and respiratory centres will stop sending action
potentials to the diaphragm and the intercostal muscles, ventilation rates will return to normal resting
rates
This is an example of negative feedback

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Control of Peristalsis in Digestive System

Peristalsis is series of muscle contractions in the walls of the oesophagus or small intestine that pass Your notes
like a wave along the alimentary canal
This wave forces the bolusof food along the alimentary canal
These contractions are controlled unconsciously by the autonomic nervous system, speci cally
the enteric nervous system (ENS)
The ENS consists a web of sensory neurones, relay neurones and motor neurones embedded
in the tissues of the alimentary canal
Peristalsis is controlled by circular and longitudinal muscles which initiate a mechanism called the
peristaltic re ex
These muscles are smooth muscle (not striated)
The bolus of food is detected by stretch receptors (sensory neurones of the ENS) as the alimentary
canal becomes distended
An action potential is passed to relay neurones of the ENS which synapse with two di erent motor
neurones
One motor neurone releases an excitatory neurotransmitter which stimulates
Longitudinal muscles to contract behind the bolus to reduce the length of that section the
oesophagus or the small intestine
This forces the food forwards through the alimentary canal
Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small
intestine
This prevents the food moving backwards towards the mouth
A second motor neurone releases an inhibitory neurotransmitter which stimulates smooth muscle
ahead of the bolus to relax and open the lumen of the alimentary canal
Control of peristalsis diagram

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Your notes

The control of peristalsis is initiated by the bolus and is carried out by two di erent motor neurones

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Observing Tropic Responses: Skills (HL)

Your notes
Observing Tropic Responses
Plant growth can be a ected by factors in the external environment such as light, gravity, water, and
the presence of objects
These growth responses are known as tropisms
Tropisms can be towards a stimulus; positive tropisms, or away from a stimulus; negative
tropisms
Tropisms enable plants to maximise their chances of survival e.g.
Growing towards light ensures a maximum rate of photosynthesis
Growing away from or towards gravity ensures that seedlings grow the right way up
Growing towards water enables roots to maximise their water uptake
Growing up and around an object may allow a plant to gain height quickly and so maximise
light absorption for photosynthesis
Tropisms are regulated by chemicals produced in plants known as plant hormones
Examples of tropisms include
Phototropism
Plant response to light
Plant stems grow towards light; this is positive phototropism
Gravitropism
Plant response to gravity
Plant stems grow away from gravity; this is negative gravitropism
Plant roots grow towards gravity; this is positive gravitropism
Gravitropism is also known as geotropism
Tropisms can be investigated and the responses to a variety of stimuli can be observed or measured
Data collected can either be through
Qualitative diagrams of seedling growth
Quantitative measurements of the angle of curvature of seedlings
There are many ways to investigate tropic responses of seedlings
Plants may be grown in varying light sources using photographic lters e.g. red, blue green
Shoot tips can be removed or covered up
Seedling radicle may be placed in varying positions e.g. facing up, facing down, facing horizontally

Examiner Tip
You should have had the opportunity to gather data on tropic responses in seedling growth as part of
your learning of this course.

NOS: Students should be able to distinguish between qualitative and quantitative

observations and understand factors that limit the precision of measurements and
their accuracy.
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There are two types of experiment, which in turn obtain two kinds of results:
Qualitative experiments are used to obtain qualitative results
Observations are recorded without collecting numerical data Your notes
For example, the starch test using iodine is a qualitative test - a colour change is recorded
Other common qualitative measurements include smells, tastes, textures, sounds and
descriptions of the weather or of a particular habitat
Qualitative results can't be processed mathematically (there isn't any numerical data) but
the observations can be analysed
The observations may be compared to a standard or other experimental work
Qualitative results are most often recorded in the form of words, short
sentences and descriptions, such as describing a colour change, making a note of someone's
opinion, describing the appearance or behaviour of an organism, or describing a chemical
reaction
Quantitative experiments are used to obtain quantitative results
Numerical data is collected and recorded
For example, recording the percentage cover of a plant species using a quadrat - a numerical
value (a percentage) is recorded
Other common quantitative measurements include temperature, pH, time, volume, length and
mass
In order to collect numerical data, a quantitative experiment must use apparatus that measures or
collects this type of data
Quantitative results must be processed using mathematical skills prior to analysis
Simple calculations work out means and rates
Further calculations are done to obtain information surrounding means (standard deviation
and standard error)
Statistical tests are performed to better understand the results (chi-squared and t-test etc.)
Quantitative results must all be recorded to the same number of decimal places but processed
data can be recorded to the same number of decimal places or to one more decimal place than
the raw data
For example, the mean of 11, 12 and 14 can be recorded as 12 or 12.3 but not 12.3333333
Reaching valid conclusions from qualitative and quantitative results
It could be argued that qualitative results can be more subjective (i.e. in uenced by the person
making the observations), but in fact, both types of results are subject to bias and error
Tools and systems for data gathering and recording are important for both
Care should be taken when making qualitative observations to keep them as objective as possible
(i.e. not allowing observations to be in uenced by the person making them)
In terms of scienti c research (and especially in biological experiments sometimes), one type of results
is not necessarily better than the other
The value of qualitative and quantitative data depends on the thing being observed and
the purpose of the experiment
Sometimes it’s important and very useful to use both
In the example table below, both qualitative and quantitative observations have been recorded whilst
observing tropic responses of seedlings and both sets of observations can be useful in drawing

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conclusions (although as always, the validity of any conclusions drawn can be increased
by repeating the experiments and gathering more data)
Your notes
Qualitative observations Quantitative observations

10 seedlings have grown at a 30o angle

Seedlings have grown toward the light
toward the light

Seedlings in green light have not grown as Seedlings in green light have grown by an
well as seedlings in blue light average of 0.5cm

Seedlings in the dark did not grow and In the dark seedling length reached an
were long and tangled average of 15.6cm

Precision and Accuracy

The certainty of any conclusions made from an experiment are impacted by the precision and
accuracy of measurements and data
It is a very common mistake to confuse precision with accuracy – measurements can be precise
but not accurate if each measurement reading has the same error
Precision refers to the ability to take multiple readings with an instrument that are close to each other,
whereas accuracy is the closeness of those measurements to the true value
Increasing Precision
Precise measurements are ones in which there is very little spread about the mean value, in other
words, how close the measured values are to each other
If a measurement is repeated several times, it can be described as precise when the values are very
similar to, or the same as, each other
The precision of a measurement is re ected in the values recorded – measurements to a greater
number of decimal places are said to be more precise than those to a whole number
Random errors cause unpredictable uctuations in an instrument’s readings as a result
of uncontrollable factors, such as environmental conditions
This a ects the precision of the measurements taken, causing a wider spread of results about the
mean value
To reduce random error:
Repeat measurements several times and calculate an average from them
Increasing Accuracy
A measurement is considered accurate if it is close to the true value
Systematic errors arise from the use of faulty instruments used or from aws in the experimental
method
This type of error is repeated consistently every time the instrument is used or the method is followed,
which a ects the accuracy of all readings obtained
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To reduce systematic errors:

Instruments should be recalibrated, or di erent instruments should be used
Corrections or adjustments should be made to the technique Your notes
Increasing Reliability
The reliability of an experiment can be described as the consistency of the results
Reliability of an experiment can be increased by taking measurements carefully and accurately
Using measuring instruments with the appropriate degree of precision can reduce random errors
Performing several trials and calculating an average of the data means the e ect of outliers will be
reduced. Repeating trials also allows you to see random errors and anomalies that can be disregarded

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Phototropism (HL)
Your notes
Phototropism
Plant shoots are positively phototropic, meaning they grow towards light (negative tropisms are
those where the plant grows away from the stimulus)
This ensures plants maximise the amount of light they can absorb for photosynthesis
Phototropism a ects shoots and the top of stems
This is an important mechanism of plants due to their immobility
Positive Phototropism Diagram

Positive phototropism leads to plants growing toward a light source

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Plant Hormones (HL)

Your notes
Phytohormones in Plants
Phytohormones are plant hormones that regulate their growth, development, reproductive
processes, longevity, and even death
There are many chemicals which act as phyohormones in plants, some examples include:
Auxins which result in cell elongation
Abscisic acid which suppresses the growth of plants
Cytokinins which increase the rate of cell division
Ethylene which promotes fruit ripening
Gibberellin which control cell elongation, seed germination, owering and dormancy
Brassinosteroids which regulate growth, development, and responses to stresses
Plant hormones are sometimes referred to as plant growth regulators

Maintaining Phytohormone Concentration Gradients

Auxins are a group of plant hormones that in uence many aspects of plant growth
A common auxin is known as IAA (indole-3−acetic acid)
In shoots, auxin is produced in cells at the growing tip before moving away into the surrounding tissues
Auxin has an important role in regulating shoot growth
In shoots, auxin causes cells to elongate, leading to stem growth
Note that in roots, auxin inhibits cell growth; the opposite e ect to that in shoot cells
Note that at very high concentrations, auxin can also inhibit shoot growth

Auxin e ux carriers
Auxin enters cells by simple di usion, however, to exit the cell (and therefore move to the next cell), it
requires membrane proteins called auxin e ux carriers to exit the cell
The term 'e ux' refers to an outward ow of a substance; in this case auxin is pumped out of one
cell and into another
E ux carriers are a type of protein called PIN3 proteins
Plant cells can distribute auxin e ux carriers on one side of the cell to encourage one way movement
of auxin
The process requires ATP so is a type of active transport
These e ux carriers or pumps are important in establishing an auxin gradient across a stem or root in
response to a stimulus such as light or gravity
E.g. Light is thought to a ect the expression of genes that code for the PIN3 protein e ux
pumps; light shining on one side of a stem more than the other can therefore lead to an uneven
distribution of e ux pumps, creating an auxin gradient

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Cell Growth by Auxin

Light a ects the growth of plant shoots in a response known as phototropism Your notes
The concentration of auxin determines the rate of cell elongation within the stem
A higher concentration of auxin causes an increase in the rate of cell elongation
If the concentration of auxin is not uniform across the stem then uneven cell growth can occur
When light shines on a stem from one side, auxin is transported, by PIN3 proteins, from the illuminated
side of a shoot to the shaded side
An auxin gradient is established, with more auxin on the shaded side and less on the illuminated side
The higher concentration of auxin on the shaded side of the shoot causes a faster rate of cell
elongation, and the shoot bends towards the source of light
E ect of Auxin Diagram

Higher concentrations of auxin on the shaded side of a stem increases the rate of cell elongation so that
the shaded side grows faster than the illuminated side
Controlling growth by elongation
Auxin molecules bind to a receptor protein on the cell surface membrane
Auxin stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell
wall (across the cell surface membrane)
This acidi es the cell wall (lowers the pH of the cell wall)

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This activates proteins known as expansins, which loosen the bonds between cellulose micro brils
At the same time, potassium ion channels are stimulated to open
This leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water Your notes
potential of the cytoplasm
This causes the cell to absorb water by osmosis (water enters the cell through aquaporins)
This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by
expansin proteins)
The cell elongates
Cell growth by auxin diagram

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Your notes

The role of auxin (IAA) in controlling growth by elongation

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Regulating Plant Growth & Fruit Ripening (HL)

Your notes
Regulating Root & Shoot Growth
Two plant hormones, auxin and cytokinin, interact together to ensure integration of root and shoot
growth
Auxin is responsible for cell elongation and is produced in the shoots
Cytokinin is responsible for cell division and is produced in the roots
Both hormones must be transported to the areas of the plant where they are not produced
Cytokinin is transported in the xylem tissue as direction is always from root to shoot
Auxin is transported in the phloem sap from shoot to root
At certain concentrations the two hormones work together to ensure root and shoot growth is
regulated; their activities can termed ‘complementary' due to the integration of their signaling
At low concentrations auxin limits the action of cytokinin
An increase in cytokinin level counteracts this inhibitory e ect and leads to an inhibition of auxin
signaling
At higher concentrations of both hormones, these interactions between cytokinin and auxin are
prevented

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Positive Feedback in Fruit Ripening

The production of ethylene in fruits is an example of a positive feedback loop Your notes
In positive feedback loops the original stimulus produces a response that causes the factor
to deviate even more from the normal range
They enhance the e ect of the original stimulus
Ethylene (named ethene by International Union of Pure and Applied Chemistry, IUPAC) is a gas
produced by fruit during the later stages of fruit ripening
The gas can di use from one fruit to adjacent fruit which triggers further release of ethylene
The e ect is that all fruit ripens at the same time
Ethylene Positive Feedback Loop Diagram

The production of ethylene is an example of a positive feedback loop

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