3.6.1 Stimuli + Response

stimulus

a change in internal or external environment of an organism

plant growth factor/auxin

chemicals that regulate plant growth responses to directional stimuli

-produced in tips of shoots

-diffuse backwards/down to stimulate shoot growth by cell elongation

-stimulate growth in shoot + inhibit growth in roots

example = IAA - moves by diffusion + active transport over short distances, moves via phloem over long distances

tropism

growth response of plant to directional stimulus

-positive = towards stimulus

-negative = away from stimulus

phototropism

growth response to light

-shoots are positively phototropic + grow towards light

-roots are negatively phototropic + grow away from light

1) IAA made in shoot tip then light is shone down

2) IAA diffuses down to shaded side

3) so cells on shaded side elongate faster

4) shoot bends towards light (opposite for roots)

gravitropism

growth response to gravity

-shoots are negatively gravitropic + grow upwards

-roots are positively gravitropic + grow downwards

1) IAA made in root tip + gravity causes build up on lower side

2) IAA inhibits elongation of root cells

3) cells on upper side elongate faster

4) root bends downwards (opposite for shoots)

taxis

directional movement towards a positive stimulus or away from a negative stimulus

kinesis

non-directional (random) movement in response to stimulus

-intensity of stimulus affects response

simple reflex arc

stimulus → receptor → sensory neurone → CNS (relay neurone) → motor neurone → effector → response

advantages

-rapid + fast → only 3 neurones involved

-protection against damage

-help escape from predators

sensory receptors

-respond to specific stimuli

-act as transducers which establish a generator potential → convert energy into electrical energy

different potentials

1) resting potential - generated by Na⁺ - K⁺ pump and channels

2) generator potential - change in potential difference due to stimulus

3) action potential - when threshold met

Pacinian corpuscles

mechanoreceptors deep in skin that respond to mechanical stimuli e.g. pressure

response to stimulus:
1) pressure deforms lamellae causing stretch-mediated Na⁺ channels in membrane to open

2) causes more Na⁺ ions to diffuse, increasing the potential

3) so depolarisation of membrane = generator potential created → action potential

photoreceptors

-receptors that detect light

-located in the fovea of retina in eye

2 types: rod cells = found in peripheral parts of fovea, only give info in black + white

cone cells = found packed together in central fovea, give info in colour

properties of ROD CELLS

-pigment = rhodopsin - requires lower light intensity to break down

-higher sensitivity = multiple rod cells connected to 1 bipolar neurone = spatial summation

-120 million per eye

-1 type only

properties of CONE CELLS

-pigment = iodopsin - requires higher light intensity to break down

-high visual acuity = 1 cone cell connected to 1 bipolar neurone so no retinal convergence

-6 million per eye

-colour vision = 3 types - red, blue, green = 3 peaks at different wavelengths

autonomic nervous system

controls the involuntary activities of internal muscles + glands

-splits into 2: sympathetic - ‘fight or flight’, stimulates effectors so speeds up activity
parasympathetic - ‘rest + digest’, inhibits effectors so slows down activity

myogenic stimulation of heart

-sinoatrial node (SAN) + atrioventricular node (AVN)

1) SAN (pacemaker) releases electrical activity/wave of depolarisation

2) so atria contract at the same time

3) layer of non-conducting tissues causes delay

4) AVN passes electrical activity after the short delay

5) this is via Purkyne tissue in the bundle of His

6) causes ventricles to contract from apex upwards so spreads throughout

baroreceptors

-pressure receptors that detect changes in blood pressure

-in aorta + carotid arteries

if HIGH blood pressure

1) baroreceptors in aorta/carotid arteries detect + increase impulses along sensory neurone to medulla

2) medulla increases impulses to SAN via parasympathetic neurone

3) this causes release of acetylcholine which binds to receptors on SAN, decreasing impulses to AVN

4) this causes decrease in heartbeat

for LOW = sympathetic neurone, release of noradrenaline, increases heart rate

chemoreceptors

-chemical receptors that detect change in pH of blood

-found in aorta, carotid arteries, medulla

if HIGH CO₂ conc (exercise)

1) rate of respiration increases so CO₂ conc increases

2) chemoreceptors in aorta/carotid arteries detect low pH + increase impulses along sensory neurone to medulla

3) medulla increases impulses to SAN via sympathetic neurone

4) causes release of noradrenaline which binds to SAN receptors

5) heart rate increases