6.1.1 Survival and response
● Stimulus: detectable change in the internal/external environment that leads to a
response in an organism
○ Organisms increase their chance of survival by responding to stimuli
Tropisms:
● IAA: indoleacetic acid, an auxin produced in shoots and roots. The distribution of
IAA controls plant tropisms
○ Promotes elongation of cells in shoots, inhibits growth in roots
● Gravitotropism:
○ IAA is produced in root tips, which initially get transported evenly around the
root
○ Gravity causes the movement of IAA from the upper to lower side of the root,
causing the concentration of IAA on the lower side to increase
○ As IAA inhibits cell elongation in the roots, the higher concentration of IAA on
the lower side causes those root cells to elongate less
○ Therefore the upper side elongates more, causing the root to bend down
● Phototropism:
○ IAA is produced in shoot tips, which initially get transported evenly around
the shoot
○ Light causes the movement of IAA from the light to shaded side of the shoot,
causing the concentration of IAA on the shaded side to increase
○ As IAA promotes cell elongation in the shoots, the higher concentration of
IAA on the shaded side causes those shoot cells to elongate more
○ Therefore the shaded side elongates faster than the light side, causing the
shoot to bend towards the light
Simple responses:
● Taxes: movement in response to the direction of the stimulus
○ Positive taxis: movement toward the stimulus, e.g. photoautotrophic algae
moving toward light for photosynthesis
○ Negative taxis: movement away from the stimulus, e.g. earthworms moving
away from light to conserve water
● Kineses: random movement at which the rate is determined by the intensity of the
stimulus but not its direction
○ e.g. woodlice moving randomly until they reach dark, damp conditions
Reflex arc:
● Only 3 neurones involved, response is rapid, short-lived, localised, involuntary
● Receptor detects stimulus, e.g. hot object, generating an electrical impulse
● Impulse sent along sensory neurone to relay neurone in CNS, then along motor
neurone to effector where it is stimulated
● Response carried out by the effector, e.g. muscle contracting
● Stimulus > receptor > sensory > relay (CNS) > motor > effector > response
, 6.1.2 Receptors
Pacinian corpuscle:
● Specific to only one stimulus, pressure
● Acts as a transducer by converting mechanical energy into an electrical impulse
known as a generator potential
○ When pressure is applied, the corpuscle membrane gets stretched
○ This opens the stretch-mediated sodium ion channels, allowing sodium ions
to diffuse into the neurone
○ Influx of sodium ions leads to depolarisation of the membrane, producing a
generator potential
Photoreceptors:
● Rod cells: more at the periphery of the retina, none at the fovea
○ No colour: rhodopsin only detects one wavelength of light, results in
monochromatic images
○ Sensitive to light: rhodopsin gets bleached by small amounts of light energy.
Multiple rod cells synapse with one bipolar cell in retinal convergence, so
many weak generator potentials can combine in spatial summation to reach
threshold potential.
○ Low visual acuity: as multiple rod cells synapse with one bipolar cell in retinal
converge, only one impulse may be sent to the brain when any of these rod
cells are stimulated.
● Cone cells: concentrated at the fovea
● Stimulus: detectable change in the internal/external environment that leads to a
response in an organism
○ Organisms increase their chance of survival by responding to stimuli
Tropisms:
● IAA: indoleacetic acid, an auxin produced in shoots and roots. The distribution of
IAA controls plant tropisms
○ Promotes elongation of cells in shoots, inhibits growth in roots
● Gravitotropism:
○ IAA is produced in root tips, which initially get transported evenly around the
root
○ Gravity causes the movement of IAA from the upper to lower side of the root,
causing the concentration of IAA on the lower side to increase
○ As IAA inhibits cell elongation in the roots, the higher concentration of IAA on
the lower side causes those root cells to elongate less
○ Therefore the upper side elongates more, causing the root to bend down
● Phototropism:
○ IAA is produced in shoot tips, which initially get transported evenly around
the shoot
○ Light causes the movement of IAA from the light to shaded side of the shoot,
causing the concentration of IAA on the shaded side to increase
○ As IAA promotes cell elongation in the shoots, the higher concentration of
IAA on the shaded side causes those shoot cells to elongate more
○ Therefore the shaded side elongates faster than the light side, causing the
shoot to bend towards the light
Simple responses:
● Taxes: movement in response to the direction of the stimulus
○ Positive taxis: movement toward the stimulus, e.g. photoautotrophic algae
moving toward light for photosynthesis
○ Negative taxis: movement away from the stimulus, e.g. earthworms moving
away from light to conserve water
● Kineses: random movement at which the rate is determined by the intensity of the
stimulus but not its direction
○ e.g. woodlice moving randomly until they reach dark, damp conditions
Reflex arc:
● Only 3 neurones involved, response is rapid, short-lived, localised, involuntary
● Receptor detects stimulus, e.g. hot object, generating an electrical impulse
● Impulse sent along sensory neurone to relay neurone in CNS, then along motor
neurone to effector where it is stimulated
● Response carried out by the effector, e.g. muscle contracting
● Stimulus > receptor > sensory > relay (CNS) > motor > effector > response
, 6.1.2 Receptors
Pacinian corpuscle:
● Specific to only one stimulus, pressure
● Acts as a transducer by converting mechanical energy into an electrical impulse
known as a generator potential
○ When pressure is applied, the corpuscle membrane gets stretched
○ This opens the stretch-mediated sodium ion channels, allowing sodium ions
to diffuse into the neurone
○ Influx of sodium ions leads to depolarisation of the membrane, producing a
generator potential
Photoreceptors:
● Rod cells: more at the periphery of the retina, none at the fovea
○ No colour: rhodopsin only detects one wavelength of light, results in
monochromatic images
○ Sensitive to light: rhodopsin gets bleached by small amounts of light energy.
Multiple rod cells synapse with one bipolar cell in retinal convergence, so
many weak generator potentials can combine in spatial summation to reach
threshold potential.
○ Low visual acuity: as multiple rod cells synapse with one bipolar cell in retinal
converge, only one impulse may be sent to the brain when any of these rod
cells are stimulated.
● Cone cells: concentrated at the fovea
