Response to stimuli
Organisms can only survive if they successfully
1. Find favourable living conditions
2. Find food
3. Avoid being eaten
Growth responses in plants
Plants don’t possess a nervous system thus respond to stimuli differently
Indoleacetic acid
 Synthesised at the root/ shoot tips, mainly affecting the elongating region of the plant
o Found just prior to the tip/area of cell division
 As it moves into the elongating region, it binds to protein receptors on cell membranes,
stimulating ATPase proton pumps to pump H+ ions from the cytoplasm into the cell wall
 This lowers pH, activating expansins (proteins) which loosen the bonds between microfibrils
in the cellulose cell walls
 This causes the cell wall to loosen and the cells to be more easily stretched when the cells
become more turgid
 K+ channels are stimulated to open, causing K+ to move into the cytoplasm and decrease the
water potential
 The cell absorbs water by osmosis, which is stored in the vacuole
 This increases the internal pressure of the cell causing the cell wall to stretch
o The cell elongates, growing towards the stimulus
Growth response towards a stimulus is called tropism
 Phototropism is a growth response to light
 Gravitropism is a growth response to gravity
Phototropism
 The concentration of IAA determines the rate of cell elongation
 Uneven growth occurs due to unequal distribution of IAA
 IAA moves to the shaded side causing elongation and the plant to bend towards the light
 Growth towards light is called positive phototropism
Gravitropism
 Some plant cells are able to detect gravity:
o Columellar cells near the root tips have heavy organelles called amyloplasts
o They are densely packed with starch, sinking to the bottom of the cell
o When a root is moved from vertical to horizontal, these organelles sink
 IAA is actively transported to the region in the root tip where amyloplasts have sunk
 The lower side grows at a slower rate than the upper side of the root, causing the root to
bend downwards
 When roots grow in the same direction as gravity, it is called positive gravitropism
Taxis and kinesis
Taxes and kineses are simple responses to enable mobile organisms to stay in favourable conditions
Kinesis
 A non-directional response to a stimulus
 Rate of movement is affected by the intensity of the stimulus
 E.g. planarians moving in random directions to find darkness again
Taxis
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A directional response to a stimulus
Moves directly away or towards the stimulus
E.g. Euglena detecting light and using its flagellum to swim towards light
Taxes and kinesis can be investigated with choice chambers to create subsections with different
abiotic conditions
Reflex arcs
Three types of neurone
 Sensory neurones carry impulses from receptors to the CNS
 Relay neurones are found in the CNS and connect the sensory and motor neurones
 Motor neurones carry impulses from the CNS to the effectors
Reflex arcs are automatic, not involving conscious thought, thus are rapid
The Pacinian Corpuscle
 Are a type of receptor found deep in the skin
 They respond to changes in pressure
 A generator potential is established when a stimulus is detected
Structure
 Found at the ends of sensory neurone axons
 Made up of many layers of membrane separated
by gel
 The gel between layers contains positively
charged Na+ ions
 The section of the axon surrounded by layers of
membrane contains stretch-mediated sodium ion
channels which open when sufficient pressure is
applied
Generator potentials
 When stretch-mediated sodium ion channels open, sodium ions enter the axon via
facilitated diffusion
 This influx of ions changes the electrical potential difference, leading to depolarisation
 This triggers action potentials which travel along the sensory neurone to the CNS
The eyes
Structure
 Roughly 125 million rods and 7 millions cones on the retina
 Cornea – transparent lens that refracts light
 Iris – controls how much light enters the eye
 Lens – focuses light onto the retina
 Optic nerve - sensory neurone that carries impulses to the brain from the eye
 Pupil – hole that allows light to enter
The eye contains two types of receptor cells:
 Rod cells – sensitive to light intensity
 Cone cells – sensitive to different wavelengths of visible light
Sensitivity to light & colour
 Rod cells contain rhodopsin
 Cone cells contain iodopsin
 The breakdown of these optical pigments results in a generator potential
 Rod cells are very sensitive to light
o This allows distinguishing between light and dark objects in low light
 Cones are less sensitive to light
 There are 3 types: red, blue and green
o The combined effect allows us to see all colours
Visual acuity
Acuity is the ability to distinguish two separate points, resulting in the resolution
Once a receptor is stimulated it can send impulses to the brain which interprets the pattern to form
an image:
1. Synapses connect rod and cone cells to bipolar neurones
2. The bipolar neurones connect to ganglion cells via synapses
3. The ganglion cells have axons that extend to the optic nerve
4. The way that rods and cones are connected to the optic nerve affects visual acuity
Rod cells
 Have lower visual acuity
 Multiple rod cells synapse with a single bipolar cell
 Many bipolar cells synapse with a single ganglion cell
o Approximately 100 rod cells are connected to a ganglion cell
Cone cells
 Have higher visual acuity
 One cone cell synapses with a single bipolar cell
 A single bipolar cell synapses with a single ganglion cell
o If two cones are stimulated to send an impulse, the brain can interpret these as two
different spots of lights
Summation
 Each rod is very sensitive to light however one rod is unlikely to produce a large enough
generator potential to stimulate a bipolar cell
 When a group of rods are stimulated at the same time, the combined generator potentials
are sufficient to reach the threshold and stimulate bipolar cells
o This additive effect of rods is called summation
Myogenic stimulation of the heart
Myogenic means that the heart beats without any external stimulus
1. The sinoatrial node is a group of cells in the wall of the right atrium which initiates a wave of
depolarisation that causes the atria to contract.
2. The annulus fibrosus is a region of non-conducting tissue which prevents the depolarisation
from spreading straight to the ventricles.
3. The wave is instead carried to the atrioventricular node
4. The atrioventricular node passes the stimulation along the bundle of
His
5. The bundle of His carries the wave of
excitation along its’ two conducting
fibres called Purkyne tissue
6. Purkyne fibres spread around the ventricle and
initiate the depolarisation from the apex of the
heart (the bottom)
7. Ventricles contract and blood is forced
out of the pulmonary artery and aorta
The autonomous nervous system
The cardio regulatory centre of the brain is called the medulla
Made up of two parts:
Acceleratory centre:
 Causes heart to speed up
 Activated impulses are sent down the sympathetic neurones to the San
 Noradrenaline is secreted at the synapse with the SAN
 Noradrenaline causes the SAN to increase the frequency of waves it produced
 Increases heart rate as a result
Inhibitory centre:
 Activated impulses are sent down 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 waves produced
 Decreases heart rate towards resting rate
The effects of exercise:
 Carbon dioxide concentration of the blood increases
 Initial fall in blood pressure due to the dilation of muscle arterioles
 Chemoreceptors and pressure receptors detect these internal stimuli
These are found in the aorta or carotid arteries
 Receptors release impulses that are sent to the acceleratory/ inhibitory centres
 Frequency of impulses increases/decreases depending on the stimulation