14: Responses to stimuli Flashcards
What is a stimulus?
Detectable change in the internal or external environment of an organism that leads to a response
What is a receptor?
Something which detects the presence of a stimulus
Specific to a type of stimulus
What is a response produced by?
Effector
Response at molecular level or whole organism
What is a coordinator?
Formulates a suitable response to a stimulus
Coordination is at a molecular or organ level
What does the ability to respond to stimuli do?
Increases the chances of survival for an organism
Allows for alleles to be passed on and by natural selection
What are the two types of response to a stimuli?
Hormonal response
Nervous response
How does coordinators work?
Connects information from each receptor with appropriate effector
What is the sequence of events starting with a stimulus?
stimulus -> receptor -> coordinator -> effector -> response
What is a taxis?
Simple response whose direction is determined by moving its whole body
In which direction is a taxis done?
Either whole body is turned towards a favourable stimulus or away form an unfavourable stimulus
What are the simplest forms of response to stimuli?
Taxes
Kinesis
Tropisms
What is a positive taxis?
Movement of the organism towards the stimulus
What is a negative taxis?
Movement of the organism away from the stimulus
What is an example of positive phototaxis?
Single-celled algae move towards light, increases survival rate as they are photosynthetic cells
What does positive phototaxis mean?
Positive - towards stimulus
Phototaxis - response to light
What does positive chemotaxis mean?
Positive - towards stimulus
Chemotaxis - response to a chemical concentration
What is kinesis?
Form of response in which the rate of movement and rate of direction change is altered
What occurs in kinesis when an organism goes to an unfavourable environment?
Rate of turning increases
Increases chance of return to a favourable environment
What occurs to the rate of turning in kinesis after a long period of time in the unfavourable environment?
Rate of turning slowly decreases so it moves in a straight line before it turns very sharply
Brings into favourable conditions
When is kinesis important?
When the stimulus is less directional
What types of stimulus are thought as non-directional?
Temperature
Gradient
What occurs to woodlice when they move from damp to dry areas?
Move more rapidly and change direction more often
Once back in damp they move slower and change direction less often
What is a tropism?
Growth of part of a plant in response to a directional stimulus
What is positive and negative tropism?
Positive - plant grows towards the stimulus
Negative - plant grows away from the stimulus
What is a phototropism?
Tropism when light is involved
What tropism do plant shoots show?
Positive phototropism
Negative gravitotropism
Grow towards light and away from gravity
Allows leaves in most favourable position for photosynthesis
What tropism do plant roots show?
Negative phototropism
Positive gravitotropism
Increase probability the roots grow in the soil, where they can absorb water and mineral ions
Do plants have nervous systems?
None present
What do plants respond to?
Light
Gravity
Water
How do plants respond to light?
Shoots grow towards light
As light needed for photosynthesis
(positive phototropism)
How do plants respond to gravity?
Roots respond to gravity and grow in direction of its pull
positive gravitotropism
How do plants respond to water?
Almost all plant roots grow towards water to absorb for use in photosynthesis and other metabolic processes
(positively hydrotropic)
How do plants respond to external stimuli?
Involves hormone-like substances or plant growth factors
Why is the term plant growth factors descriptive?
Exert influence by affecting growth, made by cells throughout plant rather than particular organs
Can affect the tissues that release them rather than acting on specific organ (like animal organ)
How much of plant growth factors are produced?
Small quantities are produced
What is indoleacetic acid (IAA)?
Plant growth factor which is an auxin
Controls plant cell elongation
How does IAA affect phototropism in plants?
Cell in tip of shoot produces IAA which is transported down the shoot, which build up on shaded side of the shoot
Greater [IAA] causes shoot cells on shaded side to elongate
Shaded side elongates, causing shoot tip to bend towards the light
How does IAA work in the roots?
High [IAA] results in inhibition of elongation of shaded side
Causes light side to grow more and bends away from the light
How is IAA used in gravitotropism?
Cells in root tip produce IAA
Gravity causes IAA to move from the upper to lower side of the root
IAA inhibits elongation so greater elongation on upper side causes root to bend downwards to force of gravity
What happens to IAA in shoots for gravitotropism?
Greater [IAA] on lower side increases cell elongation
Causes this side to elongate more than upper side
Shoots grow upwards away from the force of gravity
How is IAA transported?
IAA always transported away from shoots and roots where its produced
When does IAA work?
Only works on young cell walls where cells can elongate
Known to increase the cells ability to stretch
Why do older cells of plants not respond to IAA?
Rigidity of cells increase
So cannot stretch/elongate
What is the theory by which IAA increases plasticity of cells?
Acid growth hypothesis
What is the acid growth hypothesis?
Active transport of hydrogen ions from cytoplasm into spaces in the cell wall
This causes cell wall to become more plastic and allows elongation
Do plants respond to stimuli quickly?
Elongation occurs rapidly
Due to IAA uneven distribution
What are the two major divisions of the nervous system?
Central nervous system (CNS)
Peripheral nervous system (PNS)
What are the constituents of the CNS?
Brain and spinal cord
What are the constituents of the PNS?
Made of pairs of nerves that originate from either the brain or spinal cord
What does the peripheral nervous system consist of?
Sensory neurones
Motor neurones
What is a sensory neurone?
Carries nerve impulse (electrical signals) from receptors to the CNS
What is a motor neurone?
Carries nerve impulses away from the CNS to the effectors
What is the motor nervous system divided into?
Somatic nervous systems
Autonomic nervous system
What is the somatic nervous system?
Carries nerve impulses to body muscles and is under voluntary (conscious) control