6.1 and 6.2

Question 1

What is a stimulus?

Answer 1

a change in an organisms internal or external environment

Question 2

Why is important that organisms respond to stimuli?

Answer 2

Organisms increase their chance of survival, responding to internal environment allows them to maintain optimum conditions for metabolism

Question 3

What is a tropism?

Answer 3

Growth of a plant in response to a directional stimulus. Positive= towards stimulus, negative= away from stimulus

Question 4

Summarise the role of growth factors in flowering plants:

Answer 4

Specific growth factors e.g auxins (IAA) move via phloem or diffusion from growing regions (shoot/root tips) where they’re produced to other tissues where they regulate growth in response to directional stimuli (tropisms)

Question 5

How does indoleacetic acid affect cells in roots/shoots?

Answer 5

In shoots, high concenration of IAA stimulates cell elongation
In roots, high concentrations of IAA inhibits cell elongation

Question 6

Explain gravitropism in the shoots of flowering plants:

Answer 6

Cells in tip of shoot produces IAA
IAA diffuses down shoot, and moves to lower side of shoot so concentration increases. This stimulates cell elongation so shoots bend away from gravity

Question 7

Explain gravitropism in the roots of flowering plants:

Answer 7

Cells in tip of root produce IAA.
IAA diffuses down root, and moves to lower side so concentration increases. This inhibits cell elongation so shoots bend towards gravity

Question 8

Explain phototropism in shoots of flowering plants:

Answer 8

Cells in tip of shoot produce IAA
IAA diffuses down shoot and moves to shaded side, so concentration increases. This stimulates cell elongation so shoots bend towards light

Question 9

Explain phototropism in the roots of flowering plants:

Answer 9

Cells in tip of root produce IAA
IAA diffuses down root and moves to shaded side of root, so concentration increases. This inhibits cell elongation so root bends away from light

Question 10

What is taxes/tactic response?

Answer 10

Directional response, movement towards or away from a directional stimulus

Question 11

What is kinesis/kinetic response?

Answer 11

Non-directional response. Speed of movement/ rate of direction change changes in response to a non directional stimulus, depending on intensity of stimuli

Question 12

Explain the protective effect of a simple reflex:

Answer 12

Rapid, as only 3 neurones and few synapses
Autonomic as doesn’t involve conscious regions of brain so doesn’t have to be learnt
Protects from harmful stimuli, e.g escape predators or prevents damage to body tissues

Question 13

Describe how a generator potential is established in a Pacinian corpuscle:

Answer 13

Mechanical stimuli deforms lamellae and stretch mediated sodium ion channels
Na+ channels in membrane open and Na+ diffuse into sensory neurone.
Greater pressure causes more Na+ channels to open and more Na+ to enter
This causes depolarisation, leading to a generator potential.
If generator potential reaches threshold it triggers an action potential

Question 14

Why are rods more sensitive to light?

Answer 14

Several rods connect to a single neurone (retinal convergence) so there is spatial summation to reach/overcome threshold to generate an action potential

Question 15

Why are cones less sensitive to light?

Answer 15

Each cone connected to a single neurone and so no spatial summation

Question 16

Why do rods give lower visual accuity?

Answer 16

Several rods connected to a single neurone and so several rods send a single set of impulses to brain so can’t distinguish between separate sources of light

Question 17

Why do cones give higher visual acuity?

Answer 17

Each cone connected to a single neurone and so cones send separate sets of impulses to brain and can distinguish between separate sources of light

Question 18

Why do rods allow monochromatic vision?

Answer 18

Only one type of pigment (rhodopsin) so only one type of rod cell

Question 19

Why do cones allow colour vision?

Answer 19

3 types of cones- red, blue and green sensitive. Each have different pigments of iodopsin so absorb different wavelengths. Stimulating different combos of cones give a range of colours

Question 20

Define myogenic:

Answer 20

Able to contract and relax without receiving electrical impulses from nerves

Question 21

Describe the myogenic stimulation of the heart:

Answer 21

SAN acts as a pacemaker, releasing regular waves of electrical activity across atria- causes atria to contract simultaneously
Non-conducting tissue between atria and ventricles prevents impulses passing directly to ventricles, preventing immediate contraction of ventricles
Waves of electrical activity reach AVN which delays impulse, allowing atria to fully contract and empty before ventricles contract
AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue, causing ventricles to contract simultaneously from the base up

Question 22

Where are chemo- and pressure-/baro- receptors located?

Answer 22

In the aorta and carotid arteries

Question 23

Describe how the nervous system controls heart rate in response to a fall in blood pressure/pH:

Answer 23

Baroreceptors detect fall in blood pressure/ chemoreceptors detect fall in blood pH
Send impulses to medulla
Which send more frequent impulses to SAN along sympathetic neurones
So more frequent impulses sent from SAN to AVN
Cardiac muscle contracts more frequently and heart rate increases

Question 24

Describe how the nervous system controls heart rate in response to a rise in blood pressure/pH:

Answer 24

Baroreceptors detect rise in blood pressure/ chemoreceptors detect rise in blood pH
Send impulses to medulla
Which send more frequent impulses to SAN along parasympathetic neurones
So less frequent impulses sent from SAN to AVN
Cardiac muscle contracts less frequently and heart rate decreases

Question 25

Describe resting potential:

Answer 25

Inside of axon has negative charge relative to outside as more positive ions outside compared to inside

Question 26

Explain how a resting potential is established across the axon membrane in a neurone:

Answer 26

Na+/K+ pump actively transports 3 Na+ out of axon and 2 K+ into axon This creates an electrochemical gradient as higher conc. of K+ inside and higher conc. of Na+ outside Different membrane permeability as membrane more permeable to K+ (move out by facilitated diffusion) than Na+ (closed channels)

Question 27

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential:

Answer 27

-70mV: Na+ channels open, so membrane permeability to Na+ increases Na+ diffuse into axon down electrochemical gradient, causing depolarisation -55mV: If threshold potential reached an action potential is generated as more voltage gated Na+ channels open so more Na+ diffuse in rapidly 40mV: Voltage gated Na+ channels close, and voltage gated K+ channels open so K+ diffuse out of axon K+ channels are slow to close so slight overshoot where too many K+ diffuse out Resting potential restored by Na+/K+ pump

Question 28

What is the all or nothing principle?

Answer 28

For an action potential to be produced, depolarisation must exceed threshold potential. Action potentials produced are always the same magnitude- bigger stimuli increase frequency of action potentials

Question 29

How do action potential pass along mon-myelinated neurones?

Answer 29

Action potential passes as wave of depolarisation. Influx of Na+ ion one region increases permeability of adjoining region to Na+ by causing voltage gated Na+ channels to open so adjoining region depolarises

Question 30

How do action potentials pass along myelinated neurones?

Answer 30

Myelination provides electrical insulation. Depolarisation of axon at nodes of Ranvier only, resulting in saltatory conduction so there is no need for depolarisation along whole length of axon

Question 31

Suggest how damage to myelin sheathe can lead to slow responses and/or jerky movement?

Answer 31

Less/no saltatory conduction so depolarisation occurs along whole length of axon, and nerve impulses take longer to reach neuromuscular junction so delay in muscle contraction. Ions/depolarisation may pass/leak to other neurones, causing wrong muscle fibres to contract

Question 32

What is the refractory period?

Answer 32

Time taken to restore axon to resting potential when no further action potential can be generated as Na+ channels are closed/inactive/will not open

Question 33

Why is the refractory period important?

Answer 33

Ensures discrete impulses produced (action potentials don't overlap) Limits frequency of impulse transmission at a certain intensity so prevents over reaction to stimulus Also ensures action potentials travel only in one direction

Question 34

What are the three factors that affect the speed of conductance?

Answer 34

Myelination- depolarisation at nodes of Ranvier only (saltatory conduction) so impulse doesn't travel whole length of axon Axon diameter: bigger diameter means less resistance to flow of ions in cytoplasm Temperature: Increases rate of diffusion of Na+ and K+ as more kinetic energy, however proteins/enzymes could denature at too high a temp

Question 35

What is a synapse?

Answer 35

A junction between two neurones/neurone and an effector

Question 36

What are cholinergic synapses?

Answer 36

Synapses that use the neurotransmitter acetyl choline

Question 37

Describe transmission across a cholinergic synapse:

Answer 37

Pre-synaptic neurone: Depolarisation of presynaptic membrane causes opening of voltage gated Ca2+ channels so Ca2+ diffuse into presynaptic neurone/knob This causes vesicles carrying ACh to move and fuse with presynaptic membrane releasing ACh into synaptic cleft by exocytosis Post synaptic membrane: ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane, causing Na+ channels to open Na+ diffuses into post synaptic knob causing depolarisation. If threshold met, then action potential initiated

Question 38

What happens to ACh after synaptic transmission?

Answer 38

Hydrolysed by acetylcholinesterase into ethanoic acid and choline. Products are reabsorbed by presynaptic neurone to stop overstimulation

Question 39

Explain how synapses result in unidirectional impulses?

Answer 39

Neurotransmitter only made in/released from presynaptic neurone Receptors only on postsynaptic neurone

Question 40

Explain summation by synapses:

Answer 40

Addition of a number of impluses converging on a single post synaptic neurone causing rapid buildup of neurotransmitter so threshold more likely to be reached

Question 41

Describe spatial summation:

Answer 41

Many presynaptic neurones share one postsynaptic neurone and so collectively releases sufficient neurotransmitter

Question 42

Describe temporal summation:

Answer 42

One pre-synaptic neurone release neurotransmitter many times over a short time so sufficient neurotransmitter to reach threshold

Question 43

Describe inhibition by inhibitory synapses:

Answer 43

Inhibitory neurotransmitters hyperpolarise the membrane as Cl- channels open (Cl- move in) and K+ channels open (K+ out) Inside of axon has a negative charge below resting potential So more Na+ required to enter for depolarisation which reduces likelihood of threshold being met

Question 44

What effect do drugs have on a synapse?

Answer 44

Some stimulate the nervous system leading to more action potential - similar shape to neurotransmitter - stimulate release of more neurotransmitter - inhibit enzyme that breaks down neurotransmitter so Na+ continues to enter Some inhibit the nervous system, leading to fewer action potenitals - inhibit release of neurotransmitter (e.g prevent opening of Ca2+ channels) - Block receptors by mimicking shape of neurotransmitter