Organisms responds to changes in environments

Define stimulus

A detectable change in the an organism’s internal or external environment

Why is it important that organisms can respond to stimuli?

To increase their chance of survival

What are stimuli detected by?

Receptors

What is a taxis?

A simple response in which an organism will move its entire body towards a favourable stimulus (positive taxis) or away from an unfavourable stimulus (negative taxis)

What is a kinesis?

A non-directional response to presence and intensity of a stimulus in which an organism changes the speed of movement and the rate it changes direction so it can quickly return to a favourable environment

What is a tropism? Explain the 2 types

The growth of a plant in response to a directional stimulus

Positive tropism: grows toward stimulus

Negative tropism: grows away from stimulus

What are plant growth factors? Where are they produced? Give an example

Chemicals that regulate plant growth response to directional stimuli (tropisms)

They are produced in plant growing regions and can diffuse to other cells

Eg IAA (indoleacetic acid)

Summarise the role of growth factors in flowering plants

Specific growth factors move via phloem or diffusion from growing regions where they’re produced to other tissues where they regulate tropisms

Explain phototropism in shoots of flowering plants

• Cells in tip of shoot produce IAA

• IAA diffuses down shoot

• Light causes IAA to move to the shaded side of the shoot

• A higher concentration of IAA build up on the shaded side

• In shoots, IAA causes cell elongation

• Cells on shaded side elongate more, and faster

• This causes the shoot tip to bend towards the light

• This is positive phototropism

Explain phototropism in roots of flowering plants

• Cells in tip of root produce IAA

• IAA diffuses down root

• Light causes IAA to move to shaded side of root

• In roots, a high concentration of IAA inhibits cells elongation

• Root cells elongate more on the lighter side

• Root bends away from light

• This is negative phototropism

Explain gravitropism in shoots of flowering plants

• Cells in tip of shoot produce IAA

• IAA diffuses down shoot

• Gravity causes IAA move to the lower side of shoot - increasing concentration at lower side

• This stimulates cell elongation at lower side

• Shoots bend upwards - away from gravity

• This is negative gravitropism

Explain gravitropism in roots of flowering plants

• Cells in tip of root produce IAA

• IAA diffuses down root

• Gravity causes IAA to move to lower side of root - concentration increases at lower side

• This inhibits cell elongation at lower side

• Due to greater elongation of cells on the upper side, roots bend downwards towards gravity

• This is positive gravitropism

According to the acid growth hypothesis, explain the role of IAA in elongation growth

• IAA causes active transport of H+ ions into cell wall

• Disruption to H bonds between cellulose molecules and action of expansins make cell more permeable to water

• These cells elongate faster due to a higher turgor pressure and increased flexibility

What are the 2 major divisions of the nervous system?

• The central nervous system (CNS) - made up of brain and spinal cord

• The peripheral nervous system (PNS) - made up of pairs of nerves that originate from either the brain or the spinal cord

What is a reflex?

An involuntary response to a sensory stimulus

Outline the pathway of nerve impulses involved in a reflex arc

1. Receptor detects stimulus

2. Sensory neurone

3. Relay neurone in CNS coordinates response

4. Motor neurone

5. Response by effector

Explain 3 advantages of a reflex arc

• Involuntary - doesnt have to be learnt or considered by brain

• Fast - short neurone pathway (only 3 neurones and few synapses (which are typically slow in transmitting nerve impulses))

• Protects from harmful stimuli

What 2 features are common to all sensory receptors?

• Specific to a single type of stimulus

• Act as energy transducers - convert the energy of the stimulus into a nervous impulse known as a generator potential

What is a generator potential?

Depolarisation of the membrane of a receptor cell as a result of a stimulus

What type of stimuli do Pacinian corpuscles respond to?

Mechanical stimuli eg pressure

Where are pacinian corpuscle receptors found?

Deep in the skin, mainly in fingers, soles of feet and external genitalia

Describe the structure of a Pacinian corpuscle

• Single sensory neurone surrounded by layers of tissue, each separated by gel

• Sensory neurone has stretch-mediated sodium channels in its plasma membrane

• Contained in a capsule

Describe how a generator potential is established in a Pacinian corpuscle

• Mechanical stimulus eg pressure deforms the Pacinian corpuscle

• This stretches and widens the Na+ channels

• Na+ ions diffuse into the sensory neurone

• This causes depolarisation, leading to a generator potential

• If generator potential reaches threshold potential, it triggers an action potential

Name the 2 types of photoreceptor cell located in the retina

Rod cells

Cone cells

Where are rod and cone cells located in the retina?

Rod cells: mostly around the periphery of the retina. NOT in central fovea

Cone cells: mainly at central fovea, fewer at periphery of retina

State why rod cells and cone cells act as transducers

They covert light energy into the electrical energy of a nerve impulse

Explain the differences in sensitivity to light for rods and cones in the retina

• Rods are more sensitive to light - several rods are connected to a single neurone in the optic nerve which means that there is a greater chance that the threshold value is exceeded to create a generator potential and then an an action potential due to spatial summation

• Also, to create a generator potential, the pigment in rod cells (rhodopsin) must be broken down - there is enough energy from low-intensity light to cause this breakdown

• Cone cells are less sensitive to light - each one is connected to a single neurone so no spatial summation. The stimulation of a number of cone cells cannot be combined to help excess the threshold value and create a generator potential

• Also, the pigment in cone cells (iodopsin) requires a higher light intensity for its breakdown and creation of a generator potential

Explain the differences in visual acuity for rods and cones in the retina

• Rods give lower visual acuity - several rods are connected to a single neurone so several rods send a single impulse to the brain so the brain cannot distinguish between the separate sources of light that stimulated each rod

• Cones give higher visual acuity - each cone is connected to a single neurone so send separate impulses to brain which are distinguished as separate sources of light

Explain the differences in sensitivity to colour for rods and cones in the retina

• Rods cannot distinguish between different wavelengths of light so only see images in black and white, due to there only being 1 type of rod with 1 type of pigment

• Cones allow colour vision as there are 3 types of cone cells each containing a specific type of iodopsin which absorb different wavelengths of light

What is the autonomic nervous system? Name and explain the 2 divisions of the autonomic nervous system

• The systems that controls the involuntary activities of muscles and glands

• Sympathetic nervous system - stimulates effectors to speed up activity (involved in ‘fight or flight’ response)

• Parasympathetic nervous system - inhibits effectors to slow down activity (involved in normal resting conditions)

Cardiac muscle is myogenic. What does this mean?

It contracts on its own accord rather than by nerve impulses

State the name and location of the 2 nodes involved in heart contraction

• Sinoatrial node (SAN) - located within the wall of the right atrium. Known as the pacemaker

• Atrioventricular node (AVN) - located in lower end of right atrium, in the wall that separates the 2 atria

Where is the Bundle of His located?

Runs through the septum

Where are the Purkyne fibres located?

In the walls of the ventricles

Describe how heartbeats are initiated and coordinated

• The SAN releases a wave of depolarisation across the atria, causing atrial systole

• The AVN releases another wave of depolarisation when the first reaches it.

• There is a non-conducive layer between the atria and ventricles which prevents the wave of depolarisation travelling down to the ventricles, allowing them to fill

• Impulse travels down Bundle of His, which conducts and passes the wave of depolarisation down the septum and branches into the Purkyne fibres in the walls of the ventricles

• This causes the ventricles to contract from the apex upwards - there’s a shot delay before this happens, whilst the AVN transmits the second wave of depolarisation

Which part of the brain controls the heart rate, via the autonomic nervous system?

The medulla oblongata

Explain the difference the 2 centres of the medulla oblongata that are concerned with heart rate

• Centre which increases heart rate - linked to the sinoatrial node by the sympathetic nervous system

• Centre which decreases heart rate - linked to the sinoatrial node by the parasympathetic nervous system

Name the 2 receptors involved in changing heart rate and state what they detect

Where are they found?

Chemoreceptors (detect changes in blood pH)

Baroreceptors (detect changed in blood pressure)

Found in aorta and carotid arteries

Why is it important for chemoreceptors to respond to changes in blood pressure?

• If blood pressure is too high, this can damage walls of arteries

• If blood pressure is too low, there may be insufficient supply of oxygenated blood to respiring cells and removal of waste

Why is it important for Baroreceptors to respond to changes in blood pH?

• If blood pH is too high, enzymes may denature

Describe how receptors in the heart responds to a decrease in blood pH

• When blood has a higher than normal concentration of carbon dioxide, its pH is lowered

• Chemoreceptors in walls of carotid arteries and aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate

• This centre increases frequency of impulses via the sympathetic nervous system to the SAN

• This increases rate of production of electrical waves by SAN, cardiac muscle contracts more and therefore increases the heart rate

• This increases blood flow leads to more CO2 being removed by lungs

• CO2 concentration returns back to normal, as does pH of blood

Describe how receptors in the heart responds to an increase in blood pH

• Chemoreceptors detect fall in blood CO2/rise in blood pH

• They send impulses to medulla oblongata

• Which sends more frequent impulses to SAN along parasympathetic neurones

• So less frequent impulses/electrical waves produced by SAN

• Cardiac muscle contracts less

• Heart rate decreases

Describe how receptors in the heart respond to an increase in blood pressure

• Baroreceptors detect rise in blood pressure

• These send more impulses to the centre in the medulla oblongata that decreases heart rate

• This centre sends more frequent impulses to SAN along parasympathetic nervous system

• Cardiac muscle contracts less

• Heart rate decreases

Describe how receptors in the heart respond to a decrease in blood pressure

• Baroreceptors detect fall in blood pressure

• These send more impulses to the centre in the medulla oblongata that increases heart rate

• This centre sends more frequent impulses to SAN along sympathetic nervous system

• Cardiac muscle contracts more

• Heart rate increases

Give 8 differences between the hormonal system and the nervous system

• Hormonal: communication is by chemicals called hormones. Nervous: communication is by nerve impulses

• Hormonal: transmission by blood system. Nervous: transmission by neurones

• Hormonal: transmission relatively slow. Nervous: transmission very rapid

• Hormonal: hormones travel to all parts of the body, but only target cells respond. Nervous: nerve impulses travel to specific parts of the body

• Hormonal: response is widespread. Nervous: response is localised

• Hormonal: response is slow. Nervous: response is rapid

• Hormonal: response is often long-lasting. Nervous: response is short-loves

• Hormonal: effect may be permanent and irreversible. Nervous: effect is usually temporary and reversible

Name and describe the structures of a motor neurone

1. Cell body - contains all the usual cell organelles. Site of production of proteins and neurotransmitters

2. Dendrite - (dendrons branch into dendrites) carry nerve impulses to cell body

3. Axon - single long fibre that carries nerve impulses away from cell body, along the neurone

4. Myelin sheath - forms a covering to the axon. Made up of Schwann cells

5. Nodes of Ranvier - gaps between Schwann cells where there is no myelin sheath

6. Axon terminal

Name 3 processes Schwann cells are involved in

• Electrical insulation

• Phagocytosis

• Nerve regeneration

Describe resting potential

The inside of an axon has a negative charge relative to outside (approx -70mV)

Explain how a resting potential (-70mV) is established across the axon membrane in a neurone

• Na+ ions are actively transported out of axon by sodium-potassium pumps

• K+ ions are actively transported into the axon by the sodium-potassium pumps

• The active transport of sodium ions is greater than that of potassium ions (3 Na+ for every 2 K+)

• So there are more Na+ ions in tissue fluid surrounding axon than in cytoplasm and more K+ ions in cytoplasm than in tissue fluid - this creates an electrochemical gradient

• Na+ ions hardly diffuse back into axon while K+ ions diffuse back out - most of the gates in sodium ions channels are closed while many gates in potassium ion channels are open. There is a difference in membrane permeability for the 2 ions

• This creates a negative charge inside axon relative to outside

When does an action potential occur?

When the neurone’s voltage reaches the threshold, generating a nerve impulse

Name the stages in an action potential

1. Depolarisation

2. Repolarisation

3. Hyperpolarisation

4. Return to resting potential

What happens during depolarisation?

• A stimulus opens some Na+ channels, causing Na+ to enter cell by facilitated diffusion down their electrochemical gradient

• Membrane potential becomes less negative

• Once threshold (-55mV) is reached, voltage-gated Na+ channels open

• This causes a significant influx of Na+ ions, and a reversal of charge across the membrane - membrane potential rises to about +40mV and outside becomes relatively negative

What happens during repolarisation?

• Once membrane potential reaches around +40mV, voltage-gated sodium channels close - neurone is at peak depolarisation

• Voltage-gated potassium channels open due to electrical gradient (inside more positive than outside)

• K+ ions diffuse out of axon

What happens during hyperpolarisation and how does this lead to restoring resting potential?

• There’s an overshoot when K+ diffuse out - p.d becomes more negative than resting potential

• The gates on the potassium ion channels now close

• Sodium-potassium pump starts to act again to restore resting potential

Draw and label a graph showing an action potential

Describe the all-or-nothing principle

• For an action potential to be produced, depolarisation must exceed threshold potential

• Action potentials produced are always the same size/peak at same potential

• Bigger stimuli increase frequency of action potentials (not size)

Explain 2 ways in which organisms can perceive the size of a stimulus

• Larger stimuli generate more impulses in a given time so raises membrane to threshold potential more quickly

• Different neurones have different threshold values - the brain interprets the number and type of neurone that pass impulses and thereby determines its size

Describe the passage of an action potential along an unmyelinated axon

• The action potential passes as a wave of depolarisation

• Stimulus leads to influx of Na+ ions in one region of axon - this region depolarises

• The localised electrical current established by this influx causes the opening of sodium voltage-gated channels further along the axon

• This region depolarises whilst the section behind begins to repolarise

Describe the passage of an action potential along a myelinated axon

• Myelin sheath provides electrical insulation

• Action potentials only occur at nodes of Ranvier

• Action potentials jump from node to node in a process known as saltatory conduction

• So no need for depolarisation along whole length of axon

Do action potentials pass along unmyelinated axons or myelinated axons faster? Why?

• Myelinated axons

• Action potentials don’t happen across the whole length of the axon due to saltatory conduction

Name the 3 factors that affect the speed at which an action potential travels

• Myelination

• Axon diameter

• Temperature

Explain how myelination affects speed of conductance

• The myelin sheath provides electric insulation

• It prevents an action potential forming in the part of the axon covered in myelin

• Depolarisation happens at Nodes of Ranvier only (saltatory conduction)

• This increases the speed of conductance

Explain how axon diameter affects the speed at which an action potential travels

• The greater the diameter, the faster the speed of conductance

• This is due to less leakage of ions and less resistance to flow of ions

Explain how temperature affects the speed at which an action potential travels

• Higher temp = higher nerve impulse

• Increases rate of diffusion of Na+ and K+ during depolarisation and repolarisation as more kinetic energy

• Increases rate of respiration so more ATP for active transport to re-establish resting potential

• However, proteins/enzymes denature at a certain temperature

What is the refractory period?

• The time taken to restore axon to resting potential when no further action potential can be generated

• As sodium voltage-gated channels are closed

Explain 3 reasons why the refractory period is important

• Ensures discrete impulses are produced - action potentials don’t overlap

• Ensures that action potentials are propagated in one direction only

• Limits number of action potentials that pass along an axon in a given time - prevents over reaction to stimulus

What is a synapse?

The point where one neurone communicates with another neurone or with an effector

By what means do synapses transmit action potentials?

As neurotransmitters which diffuse across the synapse

Describe the structure of a synapse

• Presynaptic neurone - releases neurotransmitter. It’s axon ends in synaptic knob which contains lots of mitochondria, endoplasmic reticulum (for manufacture of neurotransmitter) and synaptic vesicles (stores neurotransmitter)

• Synaptic cleft - gap between neurons

• Postsynaptic neurone - has specific receptor proteins to neurotransmitter to receive it

Explain why synaptic transmission is unidirectional

• Only presynaptic neurone contains vesicles of neurotransmitter at its axon terminal

• Only postsynaptic membrane has complementary receptors at its dendrites

Define summation

The rapid build-up of neurotransmitter in the synapse, allowing threshold to be reached and an action potential to be generated

Name the 2 types of summation

• Spatial summation

• Temporal summation

Explain the importance of summation

Low frequency action potentials release insufficient neurotransmitter to exceed threshold

Describe spatial summation

Many presynaptic neurones collectively release sufficient neurotransmitter to one postsynaptic neurone to reach threshold and trigger an action potential

Describe temporal summation

A single presynaptic neurone releases neurotransmitter many times over a short period so there is sufficient neurotransmitter to reach threshold and trigger an action potential

What is a cholinergic synapse?

A synapse that uses the neurotransmitter acetylcholine (ACh)

Describe transmission across a cholinergic synapse

• Depolarisation of presynaptic membrane causes opening of voltage-gated Ca2+ channels

• Ca2+ diffuses into presynaptic neurone

• Influx of Ca2+ ions causes synaptic vesicles to fuse with presynaptic membrane and release acetylcholine into synaptic cleft by exocytosis

• ACh diffuses across synaptic left and binds to receptors on post-synaptic membrane

• This causes Na+ channels to open

• Na+ ions diffuse into postsynaptic neurone, causing depolarisation

• If threshold is met, action potential generated

Explain what happens to acetylcholine after synaptic transmission and why this is useful

• It is hydrolysed by acetlycholinesterase

• Hydrolysed into acetyl (ethanoic acid) and choline

• These are reabsorbed by the presynaptic neurone - diffuse into it

• Reformed when needed using ATP released by mitochondria which is stored in synaptic vesicles

• Useful as it stops overstimulation - if not removed, it would keep binding to receptors and continuously generate a new action potential

What are inhibitory synapses?

Synapses that make it less likely that a new action potential will be created on the postsynaptic neurone

Describe inhibition by inhibitory synapses

• Presynaptic neurone releases a type of neurotransmitter that binds to chloride ion channels on postsynaptic neurone

• Causes chloride ion protein channels to open

• Cl- ions move into postsynaptic neurone by facilitated diffusion

• Binding of neurotransmitter also causes the opening of K+ protein channels

• K+ ions move out of postsynaptic neurone into synapse

• Inside of axon has a more negative charge relative to outside - below resting potential (hyperpolarisation)

• More Na+ required to enter for depolarisation

• Reduces likelihood of threshold being met/action potential formation

Describe the structure of a neuromuscular junction

A synapse that occurs between a motor neurone and a muscle

• Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane

Give 4 similarities between neuromuscular junctions and cholinergic synapses

• They both have neurotransmitters that are transported by diffusion

• They both have receptors, that on binding with the neurotransmitter, causes an influx of Na+ ions

• They both use a sodium-potassium pump to repolarise the axon

• They both use enzymes to breakdown the neurotransmitter

Give 4 differences between neuromuscular junctions and cholinergic synpases

• Neuromuscular junction is always excitatory, cholinergic synapses may be excitatory or inhibitory

• Neuromuscular junction only links neurones to muscle, cholinergic synapse links neurones to other neurones or to effectors

• In neuromuscular junctions, the action potential ends here (they are the end of a neural pathway), in cholinergic synapses, a new action potential ma be produced along another neurones

• In neuromuscular junctions, acetylcholine binds to receptors on membrane of muscle fibre, in cholinergic synapses, acetylcholine binds to receptors on membrane of postsynaptic neurone

How might drugs increase synaptic transmission?

• May be similar shape to neurotransmitter

• May stimulate release of more neurotransmitter

• May inhibit enzyme that breaks down neurotransmitter (eg AChe)

How might drugs decrease synaptic transmission?

• May inhibit release of neurotransmitter

• May block receptors by mimicking shape of neurotransmitter

• May decrease permeability of postsynaptic membrane to ions

• May hyperpolarise postsynaptic membrane

Name the 3 types of muscle in the body and where they are located

• Cardiac: exclusively found in heart, striated, involuntary

• Smooth: walls of blood vessels and intestines, non-striated, involuntary

• Skeletal: attached to incompressible skeleton by tendons, striated, voluntary

What does the phrase ‘antagonistic pair of muscles’ mean?

• They work in opposition to eachother - when one contracts, the other relaxes

• They can only pull so they work in pairs to move bones around joints

Describe the gross structure of skeletal muscle

Muscle fibres are made up of millions of myofibrils

Myofibrils are bundles of fused cells that share nuclei and cytoplasm (sacroplasm) and there is a high number of mitochondria and sarcoplasmic reticulum

Sarcolemma (cell membrane) folds inwards to form traverse (T) tubules

Each muscle fibre is surrounded by endomycium - connective tissue that provides structural support and has many capillaries

Describe the ultrastructure of a myofibril

• Made up of 2 types of long protein filaments, arranged in parallel - myosin (thick filament) and actin (thin filament)

• Arranged in functional units called sarcomeres (one sarcomere is the distance between adjacent Z-lines)

What 2 types of protein filament are myofibrils made up of?

• Actin - thinner and consists of 2 strands twisted around one another

• Myosin - thicker and consists of long rod-shapes tails with bulbous heads

Explain the banding pattern to be seen in myofibrils

• I-bands - light bands containing only thin actin filaments

• A-bands - dark bands containing an overlap of actin and myosin filaments

• H-zone - found at centre of A-band (contains only myosin)

• Z-line - found at centre of I-band (boundary between sarcomeres)

Describe the changes that occur to a sarcomere when a muscle contracts

• I-bands get shorter/narrower

• Z-lines move closer together (sarcomere shortens)

• H-zone gets shorter/narrower

• A-band stays the same

Describe the proteins involved in the sliding-filament mechanism

Myosin is made up of 2 types of protein:

• A fibrous protein arranged into a filament made up of several hundred molecules (makes up tail)

• A globular protein formed into 2 bulbous structures at each end (makes up heads)

• Actin is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand

• Tropomyosin forms long thin threads that are wound around actin filaments

Describe muscle stimulation according to the sliding filament theory

• An action potential reaches neuromuscular junctions

• This causes voltage-gated calcium ion protein channels to open and Ca2+ ions to diffuse into synaptic knob

• These Ca2+ ions cause synaptic vesicles to fuse with presynaptic membrane and release acetylcholine into synaptic cleft

• Acetylcholine diffuses across synaptic cleft and binds with receptors on muscle sell surface membrane, causing it to depolarise

Describe muscle contraction according to the sliding filament theory

• The action potential travels down sarcolemma via T tubules, causing Ca2+ ions to release from sarcoplasmic reticulum

• Ca2+ ions diffuse into myofibrils

• Ca2+ ions bind to tropomyosin molecules, that were blocking myosin binding sites on actin filament, causing them to move and expose myosin binding sites

• Allows myosin head, with ADP attached, to bind to binding site on actin - forming actinomyosin cross bridges

• Once attached, myosin heads change their angle, pulling actin filament along as they do so and releasing a molecule of ADP

• A new ATP molecule attaches to myosin head, to detach it from actin filament

• Ca2+ ions then activate ATPase which hydrolyses ATP to ADP, providing energy for myosin head to return to its original position

• Myosin head, with ADP molecule attached, reattaches itself further along actin file amen an cycle repeats as long as concentration of Ca2+ ions in myofibril remains high

How does sliding filament action cause a myofibril to shorten?

• Myosin molecules are joined tail to tail in oppositely facing sets

• Each set of myosin heads move in opposite directions

• Actin filaments which they are attached to therefore also move in opposite directions - towards eachother

• This shortens the distance between adjacent Z-lines

Describe muscle relaxation according to the sliding filament theory

• Ca2+ ions are actively transported back into endoplasmic reticulum using energy from the hydrolysis of ATP

• This allows tropomysoin to block the binding site on the actin filament again

• Myosin heads unable to bind - no actinomyosin cross bridges can form

Explain the role of phosphocreatine in muscle contraction

• A source of inorganic phosphate

• Rapidly phosphorylates ADP to regenerate ATP

• When oxygen for aerobic respiration is limited

• Runs out after a few seconds so used for short bursts of vigorous exercise where generating ATP anaerobically is also required

What are the 2 types of muscle fibre?

• Slow-twitch fibres

• Fast-twitch fibres

Compare the location of slow and fast twitch skeletal muscle fibres

• Slow twitch fibres are found mostly in areas of constant, sustained contractions eg calf muscle

• Fast twitch fibres are found mostly in areas of short-term, rapid, powerful contraction eg biceps