Response To Stimuli Flashcards
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response in the organism
How can response to a stimulus increase an organisms chances of survival?
Organisms can detect harmful stimuli and move away from them.
Describe the response pathway:
Detection by receptors that are specific to one type of stimulus=>co-ordination=> response produced by an effector
What is taxis?
A directional response to a directional stimulus
Differentiate between positive and negative taxis
Positive - movement towards a stimulus
Negative - movement away from a stimulus
What is kinesis?
A change in the speed at which an organism moves and the rate at which it changes direction.
What is tropism?
Growth of a plant in response to a directional stimulus.
Describe the tropism of plant shoots
Positive phototropism and negative gravitropism
Describe the tropism of plant roots
Negative phototropism
Positive gravitropism
What are the three main things plants respond to?
Light, water and gravity.
What controls plants response to external stimuli?
Plant growth factors (hormone-like substances)
Why are plant growth factors NOT hormones?
The influence growth and are made by cells located throughout the plant rather than one organ
Some growth factors affect the tissue that releases them as opposed to only distant target organs.
What is the main role of IAA?
Cell elongation
Describe how positive phototropism happens in the shoot of a plant
In the condition of unilateral light;
1) cells in the top of the shoot produce IAA
2) this is initially transported evenly through all regions as it moves down the shoot
3) light causes the movement of IAA from the light side to the shaded side of the shoot
4) a greater concentration of IAA builds up on the shaded side of the shoot than the light side causing the elongation to be greater on the shaded side.
5) since the shaded side grows faster than the light side, the shoot tip bends towards the light l
What does IAA do in plant roots?
IAA inhibits cell elongation in roots so elongation of roots would occur on the light side causing the roots to bend away from the light (negative phototropism)
Explain gravitropism in plants
In the conditions of a horizontally growing root;
1) cells in the tip of root produces IAA which is then transported to all sides of the root.
2) initially, the IAA is transported to all sides of the root
3) gravity influences the movement of IAA from the upper side to the lower side of the root.
4) a greater concentration of IAA builds up on the lower side of the root than the upper side. This makes the cells on the lower side elongate less than the upper side.
5) this causes the root to bend downwards towards the force of gravity
Explain how IAA causes cell elongation
IAA increases the plasticity (ability to stretch) of cell walls.
What type of cells does cell elongation work on and why?
Young cells as mature cell walls are more rigid and do not respond
Explain acid growth hypothesis
Hydrogen ions are actively transported from the cytoplasm into spaces into the cell wall to become more plastic allowing the cell to elongate
What are the two divisions of the nervous system?
The central nervous system (brain and spinal cord)
The peripheral nervous system (pairs nerves connected to the brain and spinal cord)
What are the subdivisions of the peripheral nervous system?
Sensory neurones and motor neurones
What can the motor nervous system be divided into?
- The voluntary nervous system which carries nerve impulses to muscles under conscious thought
- The autonomic nervous system that carries nerve impulses to glands, smooth muscle and cardiac muscle and is subconscious.
Describe a reflex arc/spinal reflex
A simple response consisting of only 3 neurones- one being the spinal cord.
Why are reflexes an aid to survival?
Responses are involuntary so they require little thought.
They protect the body from harm.
They’re fast because the pathway is so short- important for withdrawal reflexes.
It is effective from birth and does not have to be learnt.
Since there is no decision making the response is rapid.