6. RESPONSE TO STIMULI Flashcards
(75 cards)
What is a Stimuli?
a change in the internal or external environment
Why do Organisms need to Respond to Stimuli?
for survival (predator/prey awareness homeostasis)
How do Simple Organisms Respond to Stimuli?
Taxis and Kinesis
What is Taxis?
directional response to a stimuli (towards or away from)
What is Kinesis?
- non-directional movement from an unfavourable area to a favourable area
- organism moves rapidly and randomly in unfavourable area until they reach favourable area where they move slowly and less randomly
- so spends more time in favourable area, less time in unfavourable area
Example of Response to Stimuli in Plants?
Tropism
What is Tropism?
- directional growth in plants in response to a stimuli
- towards = positive, away = negative
- light = photo, water = hydro, gravity = geo
- shoot shows positive phototropism and negative geotropism
- root shows positive geotropism and positive hydrotropism
- controlled by a Plant Growth Factor = Indoleacetic Acid (IAA) - auxin
What is a Plant Growth Factor?
- equivalent to animal hormones
- difference: made by cells throughout the plant, only affects cells locally, affects growth
What are the affects of IAA?
promotes growth in the shoot, inhibits growth in the root
How does positive phototropism in the shoot take place?
- normally: shoot tip produces IAA, sending it down both sides causing the shoot to grow forwards
- if light is present on one side, the IAA redistributes to the opposite side (shaded side)
- this causes the opposite side to grow faster
- so the shoot bends towards the light
How does negative geotropism in the shoot take place?
- if gravity is present on one side, the IAA redistributes to the same side
- this causes the same side to grow faster
- so the shoot bends away from gravity towards the light
How does positive geotropism/hydrotropism in the root take place?
- if gravity/water is present on one side, the IAA redistributes to the same side
- this causes the same side to grow slowly, so the opposite side grows faster
- so the root bends towards the gravity/water
Evidences for Tropism (positive phototropism in shoot)?
- removing or covering shoot tip prevents tropism [tip causes tropism]
- placing micin (prevents movement of chemicals e.g. IAA) across shoot inhibits tropism [tropism caused by movement of chemicals]
- placing gelatine (prevents movement of electrical signals) across shoot does not affect tropism [tropism not caused by movement of electrical signals]
- if shoot tip is moved to one side, that side grows faster and the shoot bends the other way [IAA promotes growth in shoot]
- when in light or darkness the overall levels of IAA remain the same [light does not inhibit or breakdown IAA but rather redistributes it]
Response to Stimuli in Mammals?
uses Nervous System, Hormonal System (nervous and hormonal systems coordinate response to stimuli)
Job of Nervous System?
coordinate response to certain stimuli – response is fast, short acting, localised
Pathway of Nervous System?
stimuli to receptor to sensory neurone to spinal cord to brain to spinal cord to motor neurone to effector for response
What does a Receptor do?
- detects stimuli & converts stimuli energy into nerve impulse (acts as a transducer – converts one type of energy into another)
- each type of stimuli has a specific receptor
- uses stimuli energy to send Na+ ions into the start of the sensory neurone
- 2 examples of receptors: Pacinian Corpuscle, Retina of Eye
What does a Pacinian Corpuscle do?
- touch receptor
- found in skin, fingers and toes
- responds to pressure/touch
- structure = corpuscle (several layers of tissue) wrapped around the start of a sensory neurone
- process = pressure applied, corpuscle compressed, stretch-mediated Na+ channels opened, Na+ ions move into the start of the sensory neurone
How does the Retina of the Eye work?
- detects light so the brain can generate an image
- detected by retina (located at back of eye)
- made of Cone and Rod cells
- Cone Cells detect high light intensity only, produces colour image, with high visual acuity
- Rod Cells can detect low light intensity, produces black and white image, with low visual acuity
- Cone Cells located in centre of retina (fovea) – site of high light intensity
- Rod Cells located in periphery of retina
Properties of Cone Cells in Retina?
- made of Iodopsin Pirgment which is only broken down at high light intensity
- one cone cell connects to one bipolar neurone which connects to one sensory neurone (therefore no summation of light can take place so only detects high light intensity)
- but because one cone cell connects to one bipolar neurone which connects to one sensory neurone, each stimuli can be distinguished = high visual acuity
Properties of Rod Cells in Retina?
- made of Rhodopsin Pigment which can be broken down at low light intensity
- a few rod cells connect to one bipolar neurone which connects to one sensory neurone (therefore summation of light can take place so can detect low light intensity)
- but because a few rod cells connect to one bipolar neurone which connects to one sensory neurone, the stimuli will be merged together = low visual acuity
What is the Central Nervous System (CNS)?
- made of brain and spinal cord
- brain = analyses and coordinates response to stimuli
- spinal cord = connects brain to sensory and motor neurones
What is the Peripheral Nervous System (PNS)?
- made of the sensory and motor neurone
- a neurone transmits a nerve impulse
- sensory neurone takes nerve impulse from receptor to CNS
- motor neurone takes nerve impulse from CNS to effector
- sensory neurone has its cell body in the middle and has a dendron and axon
- motor neurone has its cell body at the start and only has a long axon
What are the 2 different types of Motor Neurone?
- Voluntary (Somatic) and Involuntary (Autonomic) Motor Neurones
- Somatic supplies skeletal muscle = under conscious control
- Autonomic supplies cardiac muscle, smooth muscle, glands = under subconscious control
- Autonomic can be divided into Sympathetic and Parasympathetic (have opposite effects)