Organisms Response To Environment: Response To Stimulis Flashcards
(35 cards)
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response in the organism
What is a response?
A physical/ chemical reaction to a stimulus
What are an organisms advantages to responding to stimuli?
- ability to carry out homeostasis
- allows organisms to be better suited to adapt to the changing environment
What are the three forms of stimulus response organisms carry out?
-taxis = the directional response to a directional stimulus through the movement of motile organisms
-kineses = the speed of movement seen in motile organisms. It’s non directional and involves change in the rate of turning
Tropism= growth of plants in response to a directional stimulus
How does each stimulus response increase an organisms chances of survival?
- taxis: the motile organism can move its whole body either towards a favourable stimulus (positive taxis) or away from a unfavourable stimulus (negative taxis) e.g. algae are photosynthetic so move towards light to increase their chances of survival as more light = photosynthesis
- kineses= if an organism crosses a sharp dividing line between favourable and non favourable conditions, it’s rate of turning increases to raise the chances of a quick return to a favourable environment
- tropisms: plant shoots grow towards light so their leaves are in most favourable conditions to capture light for photosynthesis. Plant shoots grow away from light to increase the probability roots grow into the soil where they are better able to absorb water and mineral ions
What are the stimuli that plants respond to?
- light for photosynthesis
- gravity as the plants need to be firmly anchored to the soil
- water to use for photosynthesis and other metabolic processes
Give an example of a plant growth factor
IAA which is a type of auxin
Give the sequence of events in positive phototropism of flowering plants
- Cells in the tip of the shoot produce IAA which is transported down the shoot throughout all regions as it moves down
- Light causes the movement of IAA from the light side to the shaded side
- A greater concentration of IAA builds up on the shaded side of the shoot
- The cells on the side of greater concentration of IAA elongate more
- The shaded side of the shoot elongates faster than the light side causing the shoot tip to bend downwards towards the light
Give the sequence of events detailing positive gravitropism in flowering plants
- Cells in the tip of the root produce IAA which is then transported along the root
- The IAA is initially transported to all sides of the root
- Gravity influences the movement of IAA from the upper side to the lower side of the root
- A greater concentration of IAA builds up on the lower side of the root than the upper side
- IAA inhibits the elongation of root cells so the cells on the lower side which have a lower concentration of IAA elongate less than those on the upper side
- The greater elongation of cells on the upper side cause the root to bend downwards towards gravity
Give a use of IAA in the plant cells properties
It increases the plasticity of the cell wall
What are the two major divisions of the nervous system?
- the CNS = brain and spinal chord
- the PNS = pairs of nerves that originate from either the brain or the spinal chord
What is the peripheral nervous system made up of?
- Sensory neurones: carry nerve impulses from receptors towards the CNS
- motor neurones: carry impulses away from yen cns to effectors
Give the pathway of a general reflex arc
- the stimulus causes an internal or external detectable change
- The receptors act as transducers by converting the stimulus into a series of nerve impulses which are stimulus specific.
- Sensory neurones carry the nerve impulses to the spinal chord in the CNS
- Relay neurones receive the impulses from the sensory neurones and pass them to motor neurones
- The motor neurone passes the nerve impulse to the effector
- The effector is usually a muscle or gland that stimulates the response e.g. muscle contraction
Give four reasons why reflex arcs are important
- involuntary = no decision making so brain isn’t overloaded with situations where the same response is needed
- protect the body from harm
- fast as the neurone pathway is short with only one or two synapses
- absence of the decision making process make the action rapid
Give 3 differences between nervous and hormonal coordination
- hormones are transmitted through the blood while nerve impulses are transmitted through neurones
- hormonal response is widespread while nervous response is localised
- hormonal response is slow transmission while nervous response is fast transmission
- hormonal response is long lasting while the nervous response is short lived
What is the structure of a motor neurone and describe each component it is made up of
- cell body: contains the cell organelles and larger amounts of rough er due to the neurones production of proteins and neurotransmitters
- dendrons: extensions of the cell body which subdivide into dendrites that carry nerve impulses towards the cell body
- axon: single long fibre that carries nerve impulses away from the cell body
- Schwann cells: wrap around the axon many times so layers of there membrane build up around it. They protect the axon, provide electrical insulation, carry out phagocytosis.
- myelin sheath: the covering over the axon is made of Schwann cell membranes which contain the lipid myelin
- nodes of ranvier: constrictions between adjacent Schwann cells where there is no myelin sheath
Name the three types of neurones and what their function is
- sensory neurones transmit nerve impulses from a receptor to a relay or motor neurone. Their dendron carries impulses towards cell body and the axon carries impulses away
- motor neurones transmit nerve impulses from a relay neurone to an effector. They have long axons and many small dendrites
- relay neurones transmit impulses between neurones e.g. from sensory neurones to motor neurones and have numerous short processes
What are the features of sensory receptors using the pacinian corpuscle example
- specific to a single type of stimulus: responds only to changes in mechanical pressure
- produce a generator potential by acting as a transducer: receptors convert the energy transferred by mechanical pressure into the generator potential
Describe the structure of the pacinian corpuscle
Check biology album
How does the pacinian corpuscle work?
- At resting state, the stretch mediated sodium channels of the membrane around the neurone of the pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the pacinian corpuscle has a resting potential
- When pressure is applied to the pacinian corpuscle, it’s deformed and the membrane around it becomes stretched
- The stretching widens the sodium ion channels in the membrane and sodium ions diffuse into the neurone
- Influx of sodium ions changes the potential of the membrane as it becomes depolarised, hence producing a generator potential as the threshold is breached
- The breeches threshold means the generator potential creates an action potential that is triggered at the first node of ranvier
What kind of light is seen by rod cells
Black and white because they work in low light intensity
How many rod cells are connected to each bipolar cell?
3 rod cells for every one bipolar cell hence creating retinal convergence as this allows a greater chance for the threshold value to be exceeded as a result of spacial summation
Why do rod cells respond to low intensity light?
Because the generator potential is created by the breakdown of the pigment in rod cells and there is enough energy in low intensity light to trigger this breakdown
Give a consequence of many rod cells connected to one bipolar neurone
Light received by the rod cells connected to one bipolar neurone only generates one action potential / impulse so brain can’t distinguish between separate light sources leading to low visual acuity
