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Biology
Unit Five

Chapter Nine

Sensory Reception
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Stimulus  detectable change in the internal or external environment that
produces a response
...

Effectors carry out the response
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•

Taxes  Simple response direct to environmental changes – moves whole body
either towards a favourable stimulus (positive taxis) or away from an unfavourable
one (negative taxis)
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•

Tropisms  Growth movement of a plant in response to directional stimulus –
positive grows towards and negative grows away from stimulus
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Only three neurones are used
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Stimulus – heat from object
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Receptor – temperatures receptors in skin – impulse creates in sensory neurone
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Sensory neurone – passes impulse to spinal cord
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Intermediate neurone – links sensory to motor
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Motor neurone – impulse to upper arm muscle
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Effector – contraction stimulated
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Response – hand pulled from object
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Protects body from harmful stimuli
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Control of Heart Rate
• Autonomic Nervous System 
 Sympathetic  stimulates effectors and speeds up activity – fight or flight
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• Changes to heart rate is controlled by medulla oblongata – two centres
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 Decrease heart rate through SAN by parasympathetic system
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2
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4
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Chemoreceptors  in wall of carotid arteries – responds to pH changes
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Chemoreceptors detect this – increase frequency of impulses to medulla oblongata –
heart rate increases
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Blood flow increases – more carbon dioxide removed
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


Pressure receptors in wall of carotid arteries and aorta
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Low blood pressure  impulses to medulla oblongata and SAN – increases heart rate
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Specific to a single type of stimulus – mechanical pressure
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Single sensory neurone of pacinian corpuscle is at the centre of layers of tissue –
separated by gel
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Function 
Resting state – channels are too narrow for sodium ions to pass – resting potential
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Stretching widens sodium channels – ions diffuse inwards
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Generator potential produces action potential
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Rod cells 
Cannot distinguish different wavelengths – produce black and white images
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Share a single sensory neurone – responds to low light intensity – certain threshold
value has to be exceeded before a generator potential is created
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A number of rod cells are attached to a bipolar cell – bipolar cell attached to a sensory
neurone
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Rod cells have low visual acuity
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Cone cells have their own bipolar cell to sensory neurone
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High light intensity causes pigment (iodopsin) to break down
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Chapter Ten

Chapter Ten

Coordination
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Nervous system 
Nerve cells pass electrical impulses along their length – stimulates target cells and causes
neurotransmitter secretion
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Hormonal system 
Produces hormones – transported in blood plasma to target cells
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Chemical mediators 
Chemicals released from certain cells have an effect on cells in their immediate vicinity –
typically released by injured cells
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Causes dilation of small arteries/arterioles and increased permeability of capillaries
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Affects blood pressure and neurotransmitters – affects pain sensation
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Gravity (positively geotropic) – roots grow towards pull – need to be firmly anchored
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Plant growth factors 
Exert influence by affecting growth – made by cells throughout the plant – only affects
the tissues that release them
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Positive phototropism 
Cells in the shoot tip produce IAA – transported down
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Greater concentration of IAA in shaded side causes cell elongation more on this side
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IAA causes bending of roots in direction of gravity
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Neurones
• Neurones  specialised nerve cells adapted to rapidly carrying
electrochemical changes called nerve impulses
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• Dendrons – extensions of cell body – subdivide into dendrites carry nerve
impulses towards cell body
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• Schwann cells – surround axon, protect it and provide electrical insulation –
carry out phagocytosis and nerve regeneration
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• Nodes of ranvier – gaps between adjacent schwann cells where there is no
myelin sheath
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• Has one dendron and one axon
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• Has a long axon and many short dendrites
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The Nerve Impulse
Resting Potential
• Outside of the axon is more positive than the inside
...

• More sodium ions in tissue fluid surrounding axon than potassium ions in
the cytoplasm – creates a chemical gradient
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• Most potassium ion channels are open, most sodium ion channels are
closed
...

• Apart from chemical gradient that causes potassium/sodium movement,
there is an electrical gradient – potassium is attracted to more negative
axon inside so it’s difficult to diffuse outwards
...


The Nerve Impulse
Action Potential
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Energy from stimulus causes temporary charge reversal
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Process
Energy from stimulus causes some sodium channels to open
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As sodium ions diffuse inwards, more channels open – greater influx of ions
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Electrical gradient preventing further outward movement of potassium ions is
reversed – more channels open, more ions diffuse out
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Potassium ion channels close
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Passage of an Action Potential
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Once established, an action potential ‘moves’ rapidly along an axon – reversal of
electrical charge is reproduced at different points
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Passage along unmyelinated axon 
Resting potential – outside of the axon is more positive – high concentration of sodium
ions
...

The localised electrical circuits established by the influx of sodium ions causes the
opening of sodium channels a little further along the axon
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Repolarisation of the axon allows sodium ions to be actively transported out – returns to
resting potential
...

Action potentials can only occur at nodes of ranvier
...


Speed of the Nerve Impulse
• Factors affecting speed 
• Myelin sheath  act as an electrical insulator – action potentials jump
between nodes of ranvier
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• Temperature  affects rate of diffusion of ions – higher the temperature,
faster the nerve impulse – energy for active transport comes from
respiration – controlled by enzymes
...

• During this time, it is impossible for a further action potential to be
generated – refractory period
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It produces discreet impulses – a new action potential cannot be formed
immediately – ensures they are separated
...


All or nothing principle 
There is a certain level of stimulus, a threshold value, which triggers an action
potential
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Any stimulus above the threshold value will generate an action potential – only one
will be made
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By having different neurones with different threshold values – the brain interprets
the number and type of neurones that pass impulses
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Neurotransmitter  a chemical that transmits impulses
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One made, neurotransmitters are stored in synaptic vesicles
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Synapses act as junctions, allowing 
A single impulse to be transmitted to different neurones – a single stimulus can create
simultaneous responses
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Synapses are unidirectional  only pass impulses in one direction
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Temporal summation  a single pre-synaptic neurone releases neurotransmitters many
times over a short period
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Transmission across a Synapse
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1
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3
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5
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Cholinergic synapse  the neurotransmitter is acetylcholine – occurs in the central
nervous system and at neuromuscular junctions
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Influx of ions causes vesicles to fuse to pre-synaptic membrane – acetylcholine is
released into the synapse
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Influx of sodium ions generates a new action potential in the post-synaptic
neurone
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ATP released by the mitochondria is used to recombine acetyl and choline – they
are then stored in vesicles – sodium ion channels close in absence of acetlycholine
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Myofibrils are made up of two types of protein filaments 
Actin – thinner and consists of two strands twisted around one another
...


• Important proteins in the
muscle 
• Tropomyosin – forms a
fibrous strand around
the actin filament
...


Structure of Skeletal Muscle
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Slow-twitch fibres 
Contract more slowly and provide less powerful contractions over a long period
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Adapted for aerobic respiration to avoid a build-up of lactic acid
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Supply of glycogen to provide a source of metabolic energy
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Numerous mitochondria to produce ATP
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Adaptions 
Thicker and more numerous myosin filaments
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Store of phosphocreatine – can rapidly generate ATP from ADP
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• There are many junctions along the muscle – this ensures that contraction of a
muscle is rapid and powerful
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• Gives control over the force that the muscle exerts
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Contraction of Skeletal Muscle
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Evidence for sliding filament theory 
I-band becomes narrower
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H-zone becomes narrower
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Myosin  Made up of two proteins 
Fibrous protein arranged into a filament made up of several hundred molecules –
tail
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Actin 
Globular protein – molecules arranged into long chains – twisted around one
another
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Contraction of Skeletal Muscle
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Sliding filament mechanism 
Actin and myosin slide past each other during muscle contraction
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Ions cause the synaptic vesicles to fuse with the pre-synaptic membrane – release
acetylcholine
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Muscle contraction 
Action potential travels deep into the fibre through the T-tubules – branch
throughout the sarcoplasm
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Action potential opens calcium ion channels on the sarcoplasmic reticulum and
calcium ions flood into the muscle cytoplasm
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Contraction of Skeletal Muscle
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Muscle contraction 
ADP is attached to the myosin heads – allows it to bind to actin and form a crossbridge
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ATP attached to each myosin head – detached from actin
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Myosin head, once more with ADP attached, then reattaches itself further along the
actin filament
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Reabsorption of calcium ions allows tropomyosin to block actin filament again
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Phosphocreatine  cannot supply energy directly – can regenerate ATP – stored in
muscle and reserves a supply of phosphate
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Importance 
Enzymes and proteins are sensitive to changes in pH and temperature
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Maintain a constant temperature and blood glucose levels
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Receptor – detects any deviation from the set point
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• Effector – brings about changes needed to return the system from the set
point
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Regulation of Body Temperature
• Thermoregulation  regulating body temperature
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• Environment – by conduction, convection and radiation
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• Environment – by conduction, convection and radiation
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• Convection – occurs in fluids – heat causes fluid to expand and move,
carrying with it the heat absorbed
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Regulation of Body Temperature
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Endotherms  an animal maintaining its body temperature by physiological
mechanisms
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Body temperature remains relatively constant despite fluctuations
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Compromise between high temperature and energy needed to maintain it
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Control of body temperature 
Within the hypothalamus, there is a heat gain centre (activated by a fall in blood
temperature) and a heat loss centre (activated by a rise in blood temperature)
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There are thermoreceptors on the skin to measure skin temperature
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Relatively large bodies with smaller extremities (e
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ears)
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Regulation of Body Temperature
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Response to a cold environment 
Vasoconstriction – the diameter of the arterioles near the surface of the skin is made
smaller – less blood reaches the surface – most of the blood entering the skin passes
beneath the insulating the layer of fat so little heat is lost
...

Raising of hair – hair erector muscles in the skin contract – the thicker layer of still air
acts as an insulator
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Decrease in sweating
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Response to a warm environment 
Vasodilation – the diameter of the arterioles near the surface of the skin becomes larger
– warm blood passes close to the skin – heat from blood radiated away
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Lowering of body hair – the hair erector muscles in the skin relax – thinner insulating
layer – more heat lost
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Regulation of Body Temperature
• Ecotherms  an animal that uses the environment to regulate its body
temperature
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• Taking shelter – shade prevents overheating when the sun’s radiation is at
its peak
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• Warmth from the ground – they press their bodies against areas of hot
ground
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• Colour variation – darker colours absorb more heat whereas lighter colours
reflect more heat
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Carried in the blood plasma to the cells on which they act (target cells) which have
complementary receptors
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Second messenger model 
Used by adrenaline and glucagon
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These bind to specific receptors on target cells – forms a hormone-receptor
complex
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The second messenger causes a series of chemical changes that produce the
required response
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g
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Regulation of Blood Glucose
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Role of the pancreas 
Situated behind the stomach
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Made up of mainly enzyme-producing cell
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There are two types of Islets of Langerhans 
α-cells  larger and produce glucagon
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Regulation of blood glucose 
Glucose is the main substrate for respiration – mammals must contain a relatively
constant level
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Normal level  90mg per 100cm³
Blood glucose comes from three sources 
Diet – breakdown of carbohydrates
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Gluconeogenesis – production of new glucose from sources other than carbohydrates
e
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glycerol and amino acids
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Insulin is a globular protein – 51 amino acids
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When it binds it bring about 
A change in the tertiary structure of the glucose transport protein channels –
change shape and open – more glucose is released into the cells
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An activation of the enzymes that convert glucose to glycogen and fat
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Increasing the respiratory rate of the cells – uses up more glucose so more is taken
from the blood
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Increasing the rate of conversion of glucose into fat
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Glucagon and α-cells
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α-cells detect a fall in blood glucose levels – secrete glucagon into the blood plasma
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They respond by 
Activating an enzyme that converts glycogen into glucose
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α-cells then reduce their secretion – negative feedback
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At times of excitement or stress, adrenaline is produced by the adrenal glands above
the kidneys
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Inactivating an enzyme that synthesises glycogen from glucose
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Type One (Insulin Dependent)  body unable to produce insulin – maybe the result of
an autoimmune response whereby the body’s immune system attacks its own cells –
begins in childhood and develops quickly
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Control of Type One 
Controlled by injections of insulin
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Too much will cause low blood glucose levels
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Control of Type Two 
Controlled by regulating the intake of carbohydrate in the diet and match this to exercise
taken
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Other drugs slow rate at which body absorbs glucose from the intestine
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Example (thermoregulation) 
Levels too high – thermoreceptors in hypothalamus send nerve impulses to the heat
loss centre in the hypothalamus
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Blood cools – thermoreceptors detect set point and cease impulses – negative
feedback
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Positive feedback 
Occurs when the feedback causes the corrective measures to remain turned on
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Occurs most often when there is a breakdown of control systems
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• Follicle-stimulating hormone (FSH)  released from the pituitary gland –
stimulates the development of follicles in the ovary – stimulates the follicles
to produce oestrogen
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• Oestrogen  produced by the ovaries – causes the rebuilding of the uterus
lining after menstruation and stimulates LH production
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• The menstrual cycle is controlled by the interaction of these hormones and
their negative and positive feedback loops
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From day 1, FSH is released to stimulate the follicles in the ovary to grow and
mature – each contains an egg
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As follicles grow, more oestrogen is produced – reaches a critical point at day 10 –
stimulates FSH and LH release – positive feedback
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After ovulation – LH stimulates the empty follicle to develop into a structure called
the corpus luteum – secretes progesterone and small amounts of oestrogen
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If egg is not fertilised, the corpus luteum degenerates and so no longer produces
progesterone
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FSH release resumes – cycle repeats
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Sequence of nucleotide bases on mRNA is called the genetic code
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Few amino acids have a single codon
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Stop codons – don’t code for amino acid – mark the end of a polypeptide chain
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Universal code – same codon codes for the same amino acid in all organisms
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Bases 
Adenine, Guanine, Cytosine & Uracil
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Manufactured when DNA forms a mirror copy of part of one of its strands
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Acts as a template upon which proteins are built
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Transfer RNA (tRNA) 
Relatively small molecule – around 80 nucleotides
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At opposite end, sequence of three other bases known as the anticodon – for each
amino acid there is a different sequence of organic bases on the anticodon
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Lines up amino acids on mRNA template during protein synthesis
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Complementary section of the sequence is made in the form of pre-mRNA –transcription
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mRNA is used as a template to which complementary tRNA attaches and the amino acids they carry
are linked to form a polypeptide – translation
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Enzyme RNA polymerase moves along one of the two DNA strands (template strand) – nucleotides
on this strand join with individual complementary nucleotides from the pool
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As RNA polymerase moves along, DNA strands re-join behind it – only about 12 bases pairs are
exposed at any one time
...

Splicing 
Exons code for proteins, introns do not
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Mutations can affect the splicing of pre-mRNA
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• tRNA with complementary anticodon moves to the ribosome and pairs with
mRNA – tRNA carries an amino acid
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• Ribosome moves along the mRNA – only two tRNA are paired at a time
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• Ribosome moves to third codon
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• Synthesis continues until the ribosome reaches a stop codon
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Secondary structure folds – forms tertiary structure
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Gene Mutation
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Mutation  any change to the quantity or the structure of the DNA of an organism
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Nonsense mutation 
If a base change results in the formation of a stop codon
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Silent mutation 
If the substituted base still codes for the same amino acid
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Causes of mutations 
Spontaneous mutations occur without outside influence
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Mutagenic agents (mutagens) increase the mutation rate
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Tumour suppressor genes – slow cell division
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A gene mutation can cause proto-oncogenes to mutate into oncogenes
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Oncogene may code for growth factor that is produced in excessive amounts
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Tumour suppressor genes 
Inhibit cell division
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If mutated, it becomes inactivated
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Some genes are permanently expressed, others are switched on and off when needed
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Totipotent cells  cells that can mature into any body cell
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Breaking down mRNA before its genetic code is translated
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Few totipotent cells exist in mature animals – these are adult stem cells
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Stem cells are found in the intestine lining, skin and bone marrow
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Mature plants have many totipotent cells
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Regulation of Transcription & Translation
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Transcriptional Factors
Transcriptional factors stimulate genes to begin transcription
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When it binds, transcription begins
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• The inhibitor molecule prevents transcription
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• Once inside, it combines with a receptor on the transcriptional factor
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• The transcriptional factor can now combine with DNA
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• One of the two SiRNA strands combine with an enzyme
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• The enzyme cuts the mRNA
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• The gene is not expressed
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• SiRNA can be used to block genes that cause disease
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• The resulting organism is a genetically modified organism
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Insertion – of DNA fragment into a vector
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Identification – of the host cells that have successfully taken up the gene by
use of gene markers
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• Retroviruses  group of viruses whose genetic information is in the form of
RNA
...


Producing DNA Fragments
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Process of isolation 
A cell that readily produces the protein is selected
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Reverse transcriptase makes DNA from RNA – the resulting DNA is called
complementary DNA (cDNA)
...

• Restriction endonucleases  enzymes that cut up viral DNA – used by
bacteria as a defense mechanism
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• Some cut between two opposite base pairs – leave straight edges called
blunt ends
...


In Vivo Gene Cloning
The Use of Vectors
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If the same restriction endonuclease is used to cut DNA in both organisms, then the
fragments produced will have complementary ends
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Insertion 
A vector is used as a carrying unit and joined to the DNA fragment
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Most common vector is a plasmid
...

When DNA fragments are mixed with the opened-up plasmids, they may become
incorporated into them
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Introduction into host cells 
Plasmids must be reintroduced into bacterial cells – transformation
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Not all bacteria take up the plasmids because 
Only a few do (<1%)
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In Vivo Gene Cloning
The Use of Vectors
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Testing for DNA fragment 
Genes for the production of enzymes that break down antibiotics are found in the
plasmids
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Process 
All bacteria are grown on a medium that contains ampicillin
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Those not resistant will die
...


Gene markers 
This involves using a second, separate gene
...

May make a fluorescent protein
...


In Vivo Gene Cloning
The Use of Vectors
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Antibiotic-resistance markers 
The gene that was cut out was responsible for resistance to a second antibiotic – bacteria with the
required gene won’t be resistant
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Each will grow into a genetically identical colony
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Colonies killed must have taken up the required gene
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Fluorescent markers 
A more rapid method is the transference of a gene from a jellyfish into the plasmid
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The gene to be cloned is transplanted into the centre of the GFP gene
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Those that fluoresce do not have the gene to be cloned
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Lactase will turn a particular colourless substrate blue
...

If a plasmid with the required gene is present in the bacterial cell, it will not produce lactase and
change the colour of the colourless substrate
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• Addition of primers – Mixture is cooled to 55°c – primers join to their
complementary bases – primers provide starting sequences for DNA
polymerase
...

• Synthesis of DNA – temperature is increased to 72°c – optimum
temperature for DNA polymerase to add complementary nucleotides
...


Advantages of In Vitro & In Vivo
• In Vitro 
• Extremely rapid – within a matter of hours, a 100 billion copies can be
made
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In Vivo 
Useful to introduce a gene into another organism using vectors
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Very accurate
...

Produces transformed bacteria that can be used to produce large quantities
of gene products
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Benefits 
Increasing crop and animal yield
...

Introducing resistance to disease and pests
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Making vaccines and producing medicines
...

Bacterial cells can now have the human insulin gene incorporated into them to
produce mass quantities of insulin
...


•

Use of Recombinant DNA Technology
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Genetically modified plants 
Modifying tomatoes so that the softening enzyme is no longer produced
...

Plants that produce plastics – this is a possibility being currently explored
...

Those affected are at risk of blood clots
...


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Process 
Mature eggs are removed from female goats and fertilised
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These genetically transformed eggs are implanted into female goats
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The anti-thrombin is extracted and purified
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Cystic fibrosis 
Caused by a mutant recessive alleles in which three bases, AAA, are deleted
...

The deletion of one amino acid results in the protein being unable to transport
chloride ions
...

In cystic fibrosis, water won’t enter the mucus, causing it to be thick and dry
...

Breathing difficulties
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Accumulation in sperm ducts, leading to infertility in men
...

Gene supplementation – one or more copies of the healthy gene are added
alongside the defective gene
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The adenoviruses are made harmless by altering the gene involved in replication
...

Adenoviruses with the gene are introduced into the nostrils of the patients
...


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Wrapping the gene in lipid molecules 
CFTR genes are isolated and inserted into bacterial plasmid vectors
...

Plasmids are extracted and wrapped in a lipid molecule to form a liposome
...

The liposomes pass across the phospholipid bi-layer of the cell
...

• Prevents the production of antibodies and cell-mediated immune
responses
...

• Normally, the enzyme ADA destroys toxins that would otherwise kill white
blood cells
...

The ADA gene is inserted into a retrovirus
...

The retroviruses inject a copy of the normal ADA gene into the T-cells
...


Locating & Sequencing Genes
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DNA probes 
A short, single-stranded section of DNA that has an easily identifiable label, such as radioactive or
fluorescent labels
...

The DNA being tested is split into two strands
...

Site at which the probe binds can be identified by the radioactivity or fluorescence it emits
...

Sanger method uses modified nucleotides that cannot attach to the next base in the sequence –
terminator bases
...

Each test tube will contain one type of terminator base – test tube one will only contain the
terminator adenine
...

Depending on where the terminator binds, synthesis may end after a few nucleotides or after many
...

These fragments can be identified as the attached primer at the beginning of the fragment will emit
radiation or fluoresce
...


Locating & Sequencing Genes
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Gel electrophoresis 
DNA fragments are placed on to an agar gel and a voltage is applied
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Smaller fragments therefore move further
...

The fragment distance is compared against a known reference called a DNA ladder
...

This can be done for the fragment with two nucleotides, then three and so on
...


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Only fragments up to around 500 bases can be sequenced
...


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Restriction mapping 
Cutting DNA with a series of different restriction endonucleases
...

The distance between the recognition sites can be determined by the patterns of
fragments that are produced
...

• Potential parents who are at risk can obtain advice from a genetic
counsellor
...

• Mutations of both alleles must be present to inactivate the gene
...

• Genetic counsellors help individuals to understand the results and
implications of the screening to make appropriate decisions
...


Genetic Fingerprinting
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In DNA, introns contain repetitive sequences of DNA called core sequences
...

The more closely related two individuals are, the more similar the core sequences
will be
...


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Extraction 
Extract the DNA by separating it from the rest of the cell
...


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Digestion 
DNA is cut into fragments by restriction endonucleases
...


Genetic Fingerprinting
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Separation 
Gel electrophoresis is used to separate the fragments according to length
...

The single strands are transferred to a nylon membrane
...

The process is carried out with different probes, each of which binds with a different
core sequence
...

The film is exposed by the probes, either by radiation or fluorescent light
...

The pattern of bands is unique to every individual, except identical twins

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