14 Response to stimuli
14.1 Survival and response
Stimulus response
Stimulus: detectable change in internal/external environment of an organism
leading to response in organism
o Ability to respond to stimuli increases chance of survival, breed,
reproduce, pass on alleles ∴ always a selection pressure favouring
organisms with more appropriate responses
o Detected by receptors: specific to 1 type of stimulus
o Coordinator: formulates suitable response to stimulus at molecular level/involve behaviour of whole organism
(eg hormones are a means of communication in large multicellular organisms; relatively slow process in
plants/animals)
Nervous system is a more rapid communication in animals: has many diff receptors and control
effectors, which are linked by a coordinator
stimulus → receptor → coordinator → effector → response
Taxes
Simple response, direction determined by direction of stimulus
o Motile organism responds directly to environmental changes by moving whole body towards favoured
stimulus/away from unfavoured
Classified by direction of movement/nature of stimulus
o Positive taxis: towards stimulus
o Negative taxis: away from stimulus
o Positive phototaxis: algae moving towards light ∴ psis and make food
o Negative phototaxis: earthworms move away from light ∴ in soil, better conserve of water, food, away from
predators
o Positive chemotaxis: some bacteria move towards area of high glucose concentration ∴ source of food
Kineses
Response where organism changes speed as it moves and rate at which changes direction
If sharp difference btw favourable/unfavourable, rate of turning increases ∴ increase chances of quick return to
favourable
o If goes far into unfavourable, rate of turning decreases ∴ moves in long straight lines before it turns (sharply)
bringing organism into favourable area
Important when stimulus less directional eg humidity and temp don’t have clear gradient btw
extremes
Eg woodlice: lose water from body in dry conditions
o Damp → dry: move more rapidly and change direction more often ∴ more likely to move back into damp area
o Damp: slow down and change direction less often ∴ more likely to stay in damp area
o After some time in damp area, rapidly changing direction: move rapidly in straight lines, increasing chances of
going through dry and into new damp ∴ more time spent in favourable damp area, prevent drying out and
increase chances survival
Tropisms
Growth in part of plant in response to directional stimulus
o Almost all cases: grows towards (positive response)/away from (negative response) stimulus
o Positive phototropism: plant shoots grow towards light ∴ favoured for psis
o Negative gravitropism: shoots away from gravity
Roots do opposite, vice v, grow into soil to absorb water and mineral ions
14.2 Plant growth factors
Plants respond to:
o Light: shoots grow towards light (positively phototropic) bc light is needed for photosynthesis.
o Gravity: plants need to be firmly anchored in the soil, roots are sensitive to gravity and grow in the direction of
its pull (positively gravitropic)
o Water: almost all plant roots grow towards water (positively hydrotropic) in order to absorb it for use in
photosynthesis and other metabolic processes. as well as for support.
Plant growth factors: involved in responses to external stimuli
o exert their influence by affecting growth and, they may be made by cells located throughout plant rather than
in particular organs
o unlike animal hormones. some plant growth factors affect tissues that release them rather than acting on a
distant target organ
o Produced in small quantities
o Eg indoleacetic acid (IAA), belongs to a group of substances called auxins, also controls plant cell elongation
Control of tropisms by IAA
, o Unilateral light: light shining from the side
Phototropism in flowering plants
Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
Light causes the movement of lAA from the light side to the shaded side of the shoot.
Greater concentration of IAA builds up on the shaded side of the shoot than on the light side
IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot ∴ cells
on this side elongate more
Shaded side of the shoot elongates faster than the light side causing the shoot tip to bend towards the light
IAA controls bending of roots in response to light: high IAA conc of 10 parts/million increases shoot cell elongation by
200% but decreases (inhibits) root cell elongation by 100%
o Root cell elongation greater on light side ∴ roots bend away from light (-ve phototropic)
Gravitropism in flowering plants
Response to horizontally-growing root to gravity:
o Cells in the tip of the root produce IAA, which is then transported along the root
o IAA is initially transported to all sides of the root
o Gravity influences the movement of IAA from the upper side to the lower side ol' rhe
root
o A greater concentration of IAA builds up on the lower side of the root than on the
upper side
o IAA inhibits the elongation of root cells and there is a greater concentration of IAA
on the lower side, the cells on this side elongate less than those on the upper side
o Relatively greater elongation of cells on the upper side compared to the lower side
causes the root to bend downwards towards the force of gravity
Shoots: greater conc IAA on lower side increases cell elongation ∴ elongate more than upper
side and grows upwards away from force of gravity
Role of IAA in elongation growth
Transport of IAA in 1 direction, away from tip of shoots/roots where they ae produced
Acid growth hypothesis – IAA increases plasticity (ability to stretch) of cell walls (only in young
cells, mature cells more rigid): involves active transport of H + from cytoplasm to cell wall spaces
∴ cell wall more plastic allowing cell to elongate by expansion
Stimuli causes uneven distribution of IAA
14.3 Reflex arc
Simplest type of nervous response
Nervous organisation
CNS central nervous system: brain and spinal cord
PNS peripheral nervous system: pairs of nerves originating from brain/spinal cord
o Sensory neurones: carry nerve impulses (electrical signals) from receptors to CNS
o Motor neurones: carry nerve impulses from CNS to effectors
Voluntary nervous system: carry nerve impulses to body muscles under conscious control
Autonomic nervous system: carry nerve impulses to glands, smooth muscle, cardiac muscle under
subconscious
Spinal cord
Column of nervous tissue in back, inside vertebral
column for protection
Reflex arc
Reflex: involuntary response to a sensory stimulus
Reflex arc: pathway of neurones involved in a reflex,
contain 3 neurones (1 in spinal cord ∴ spinal reflex)
o stimulus: heat from hot object
14.1 Survival and response
Stimulus response
Stimulus: detectable change in internal/external environment of an organism
leading to response in organism
o Ability to respond to stimuli increases chance of survival, breed,
reproduce, pass on alleles ∴ always a selection pressure favouring
organisms with more appropriate responses
o Detected by receptors: specific to 1 type of stimulus
o Coordinator: formulates suitable response to stimulus at molecular level/involve behaviour of whole organism
(eg hormones are a means of communication in large multicellular organisms; relatively slow process in
plants/animals)
Nervous system is a more rapid communication in animals: has many diff receptors and control
effectors, which are linked by a coordinator
stimulus → receptor → coordinator → effector → response
Taxes
Simple response, direction determined by direction of stimulus
o Motile organism responds directly to environmental changes by moving whole body towards favoured
stimulus/away from unfavoured
Classified by direction of movement/nature of stimulus
o Positive taxis: towards stimulus
o Negative taxis: away from stimulus
o Positive phototaxis: algae moving towards light ∴ psis and make food
o Negative phototaxis: earthworms move away from light ∴ in soil, better conserve of water, food, away from
predators
o Positive chemotaxis: some bacteria move towards area of high glucose concentration ∴ source of food
Kineses
Response where organism changes speed as it moves and rate at which changes direction
If sharp difference btw favourable/unfavourable, rate of turning increases ∴ increase chances of quick return to
favourable
o If goes far into unfavourable, rate of turning decreases ∴ moves in long straight lines before it turns (sharply)
bringing organism into favourable area
Important when stimulus less directional eg humidity and temp don’t have clear gradient btw
extremes
Eg woodlice: lose water from body in dry conditions
o Damp → dry: move more rapidly and change direction more often ∴ more likely to move back into damp area
o Damp: slow down and change direction less often ∴ more likely to stay in damp area
o After some time in damp area, rapidly changing direction: move rapidly in straight lines, increasing chances of
going through dry and into new damp ∴ more time spent in favourable damp area, prevent drying out and
increase chances survival
Tropisms
Growth in part of plant in response to directional stimulus
o Almost all cases: grows towards (positive response)/away from (negative response) stimulus
o Positive phototropism: plant shoots grow towards light ∴ favoured for psis
o Negative gravitropism: shoots away from gravity
Roots do opposite, vice v, grow into soil to absorb water and mineral ions
14.2 Plant growth factors
Plants respond to:
o Light: shoots grow towards light (positively phototropic) bc light is needed for photosynthesis.
o Gravity: plants need to be firmly anchored in the soil, roots are sensitive to gravity and grow in the direction of
its pull (positively gravitropic)
o Water: almost all plant roots grow towards water (positively hydrotropic) in order to absorb it for use in
photosynthesis and other metabolic processes. as well as for support.
Plant growth factors: involved in responses to external stimuli
o exert their influence by affecting growth and, they may be made by cells located throughout plant rather than
in particular organs
o unlike animal hormones. some plant growth factors affect tissues that release them rather than acting on a
distant target organ
o Produced in small quantities
o Eg indoleacetic acid (IAA), belongs to a group of substances called auxins, also controls plant cell elongation
Control of tropisms by IAA
, o Unilateral light: light shining from the side
Phototropism in flowering plants
Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
Light causes the movement of lAA from the light side to the shaded side of the shoot.
Greater concentration of IAA builds up on the shaded side of the shoot than on the light side
IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot ∴ cells
on this side elongate more
Shaded side of the shoot elongates faster than the light side causing the shoot tip to bend towards the light
IAA controls bending of roots in response to light: high IAA conc of 10 parts/million increases shoot cell elongation by
200% but decreases (inhibits) root cell elongation by 100%
o Root cell elongation greater on light side ∴ roots bend away from light (-ve phototropic)
Gravitropism in flowering plants
Response to horizontally-growing root to gravity:
o Cells in the tip of the root produce IAA, which is then transported along the root
o IAA is initially transported to all sides of the root
o Gravity influences the movement of IAA from the upper side to the lower side ol' rhe
root
o A greater concentration of IAA builds up on the lower side of the root than on the
upper side
o IAA inhibits the elongation of root cells and there is a greater concentration of IAA
on the lower side, the cells on this side elongate less than those on the upper side
o Relatively greater elongation of cells on the upper side compared to the lower side
causes the root to bend downwards towards the force of gravity
Shoots: greater conc IAA on lower side increases cell elongation ∴ elongate more than upper
side and grows upwards away from force of gravity
Role of IAA in elongation growth
Transport of IAA in 1 direction, away from tip of shoots/roots where they ae produced
Acid growth hypothesis – IAA increases plasticity (ability to stretch) of cell walls (only in young
cells, mature cells more rigid): involves active transport of H + from cytoplasm to cell wall spaces
∴ cell wall more plastic allowing cell to elongate by expansion
Stimuli causes uneven distribution of IAA
14.3 Reflex arc
Simplest type of nervous response
Nervous organisation
CNS central nervous system: brain and spinal cord
PNS peripheral nervous system: pairs of nerves originating from brain/spinal cord
o Sensory neurones: carry nerve impulses (electrical signals) from receptors to CNS
o Motor neurones: carry nerve impulses from CNS to effectors
Voluntary nervous system: carry nerve impulses to body muscles under conscious control
Autonomic nervous system: carry nerve impulses to glands, smooth muscle, cardiac muscle under
subconscious
Spinal cord
Column of nervous tissue in back, inside vertebral
column for protection
Reflex arc
Reflex: involuntary response to a sensory stimulus
Reflex arc: pathway of neurones involved in a reflex,
contain 3 neurones (1 in spinal cord ∴ spinal reflex)
o stimulus: heat from hot object
