B5 - Responses to External Stimuli

Detectable change in internal/external environment of an organism that leads to a response

Reflexes
Taxes + kineses
Instincts

Simple directional response
Direction of movement determined by direction of stimulus

+ive taxis
-ive taxis

Motile organisms move body towards stimulus

Motile organisms move body away from stimulus

Non-directional response
Non-favourable: rapid movement and direction changes
Favourable: slows down and turns more often

Growth of part of plant in response to a directional stimulus

Shoots:
+ive phototropism
Roots:
-ive phototropism
+ive gravitropism

Control cell elongation

Tips of shoots / roots
Where mitosis occurs

Made in tips shoots / roots
Diffuses down shoot / root

IAA stimulates cell elongation
Goes to dark side
Shoot bends towards light

IAA inhibits cell elongation
Root grows away from light
IAA goes down (towards gravity)
Amyloplasts (starch grains) in root tip fall to bottom
IAA move to where amyloplasts are