3.6 Organisms Respond To Changes In Internal And External Environments Flashcards
What is a stimulus and why is it important to respond to them?
•A detectable change in the environment. These changes can be detected by cells that are called receptor.
•Increases chance of survival
What is kinesis?
•organism moves to an area with harmful stimuli= increase the rate it changes direction to return to the favourable conditions quickly
•if it is in an area with beneficial stimuli it decreases speed
•non-directional response to unfavourable conditions
What is taxis?
A directional response, where they move towards or away from a stimulus
E.g -ve chemotaxis = moving away from beneficial chemical
/ +ve phototaxis = moving towards light
Why do organisms respond to temperature and humidity via kinesis?
Less directional stimuli where there is often no clear gradient from one extreme to the other
What is the effect of IAA in the shoot tip?
•in unilateral light, IAA will diffuse towards the shaded side
•it causes the cells to elongate more and this will bend the top to the light source
=positive phototropism
What is the effect of IAA in the root tip?
•IAA will move to the lower side of the root
•this will inhibit cell elongation so that roots anchor into the soil
= positive gravitropism and negative phototropism
What are the events involved in a response?
1.stimulus= change in internal/external environment
2.receptor= detects stimulus and respond by producing action potential in neurones
3.sensory neurone= carries impulses from receptor to CNS
4.coordinator=in CNS where info is interpreted
5.motor neurone= impulse from CNS to effector (gland/muscle)
What are the features of the endocrine system?
•widespread action
•hormones transported in blood
•long lasting response
•slow effect
What are the features of the nervous system?
•localised in a specific area
•neurotransmitters (chemical coordinator)
•short lived
•rapid effect
•impulses directly to target cells
What is a reflex action?
•rapid, involuntary response to a stimulus
•innate (not learned)
Why are reflexes important?
•increase survival
•leave brain free to carry out complex responses
•protect body from harm
•fast as neurone pathway is short
What are the structures of a neurone and their functions?
•cell body
•dentrites: receive info and carry it towards cell body
•axon: transmit impulses away from cell body
•myelin sheath: fatty material which insulates the axon so no loss of impulses or crossing over occurs
•Schwann cells wrap around axon to form myelin sheath, gas between called nodes of ranvier
•axonites
How is a resting potential established?
•3Na+ out and 2K+ into the axon by active transport via the Na-K pump
•electrochemical gradient produced = K+ diffuse out and Na+ in by facilitated diffusion
•membrane is more permeable to K+ as more K+ channels so more K+ moves out = -70 mv inside the axon
What are the steps involved in an action potential?
Depolarisation:
•neurone stimulated opens voltage gates Na+ channels
•Na+ flood down conc. gradient into axon = +40 mv in axon
Repolarisation:
•+40 mv reached, Na+ voltage gated channels close
•K+ voltage gated channels open and K+ flood down conc. gradient out of axon so -70 mv in axon
Hyper polarisation:
•too many K+ move out of axon so temporarily more -ve than -70 mv
What is the all-or nothing principle?
•if depolarisation < -55mv= no action potential
•all at -55 mv will trigger a depolarisation to +40 mv max
•bigger stimuli increases the frequency of action potentials
This is important as only large stimuli are responded to for increased survival
