Whole of Unit/Module 6 - Organisms respond to changes in their internal and external environments for AQA A level Biology based on class notes, specification, textbooks, websites, videos and revision sessions. Summarised into bullet point notes that are quick and to the point with just information ...
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?
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