Responses to External Stimuli - B15 Flashcards
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response.
What does an organism’s ability to respond to stimuli determine?
-The ability to respond to stimuli is a characteristic of life and increases the chances of survival for an organism.
-Those that survive are more likely to reproduce and raise offspring, therefore passing on favourable alleles.
-There is always a selection pressure, favouring those with ore appropriate responses.
What are stimuli detected by?
-Stimuli are detected by receptors with are specific to one stimulus.
-We have lots of different receptors that detect different stimuli e.g. baroreceptors for detecting blood pressure and rods and cones in the eye to detect light and colour.
What are effectors?
Effectors are cells that causes a response to a stimulus. These include muscle and glands e.g. pancreatic cells.
How do receptors and effectors communicate?
Receptors communicate with effectors via the nervous and/or hormonal system.
Why is the nervous system rapid?
-The nervous system is a rapid means of communication with a nervous system having many different receptors and effectors.
-each receptor and effector is linked to a central coordinator of some type, which coordinates information between appropriate receptors and effectors.
What is a taxis?
-A taxis is a simple directional response
-direction of movement is determined by the direction of the stimulus
How do motile organisms respond to stimuli?
Motile organisms respond by moving their whole body ether towards(positive taxis) or away(negative taxis)
What is kinesis?
Kinesis is a non-directional response to a stimulus
What do organisms do in kinesis and why is it useful?
-once in a favourable condition, the organisms slow down and turn more often
-this is useful because if an organism crosses a line between favourable to unfavourable conditions its chance of returning back to favourable condition increases.
What is a tropism?
-The growth of part of a plant in response to a directional stimulus
-again there is positive(towards) and negative(away) tropisms, depending on the part of the plant and the stimulus
What tropisms do plant shoots show?
plant shoots are demonstrate positive phototropism and negative gravitropism - ensures the leaves are always in favourable conditions for photosynthesis
What tropisms do plant roots show?
plant roots demonstrate negative phototropism and positive gravitropism - ensure the roots grow into the soil where they are better able to absorb water and mineral ions
What stimuli do plants have to respond to?
-plants have no nervous system, yet they still need to respond to stimuli such as:
1)light - shoots grow towards light as light is needed for photosynthesis
2) gravity - plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull.
3)water - roots are positively hydrotropic in order to absorb water for photosynthesis and other metabolic processes, as well as for support.
What are plant growth factors?
-plant responses to external stimuli involve ‘hormone-like substances’ called plant growth factors:
1)they affect the growth and are produced by cells located throughout the plant rather than a specialised organ
2)plant growth factors affect the tissues that release them rather than acting on distant target organs
-IAA controls cell elongation among other things
What are auxins?
-auxins are a group of growth factors(including IAA) which speed up or slow down growth by controlling cell elongation
-they are produced in growing regions of the plant e.g. root and shoot tips
-they have the ability to diffuse backwards to nearby tissue in order to cause cell elongation
-if the tip of the shoot/root us removed, no auxin will be available and shoot stops growing.
-auxins stimulate growth in shoots but high conc. in roots inhibits growth.
What is IAA?
-made at the tip of the root/shoot. As it diffuses into the growing region, it binds to protein receptors on cell membranes
-scientists believe IAA causes H+ to be released. This lowers the pH, affecting bonding, including the H bonds between microfibrils of cellulose
-as a result, the plant cell walls weaken and the plant cells can be easily stretched and elongate.
Describe how phototropism occurs in plant shoots.
1) cells in shoot tip produce IAA which diffuses backwards
2) IAA initially is transported evenly throughout all regions
3)light causes IAA to move from the light side to the shaded side
4)greater conc. of IAA on the shaded side vs light side
5) IAA causes elongation of shoot cells, so shaded side cells elongate more
6) shaded side elongates faster than the light side cause if the shoot to bend towards the light.
Describe phototropism in flowering plant roots.
-in roots, high conc. of IAA inhibits cell elongation
-if a root comes into contact with light(assuming from above):
1) the IAA moves to the shaded side
2)inhibits cell elongation
3)topside elongates faster
4)root grows away from the light source
What happens to IAA in roots?
-columella cells near the root tip contain organelles called amyloplasts which are packed with starch
-they are heavy and sink to the bottom of cells they are in. This enables the cells to detect the direction of gravity
-IAA is actively transported to the side of the root to which the amyloplasts sink
Explain gravitropism in roots.
-the response to a horizontally growing root:
1)cells in the tip produce IAA which diffuses backwards
2)IAA is initially evenly spread
3)gravity influences the movement of IAA from the upper side to the lower side
4)greater conc. of IAA in lower side
5) IAA inhibits cell elongation in roots-lower side elongates less than the upper side
6)root bends downwards towards the force of gravity
Explain gravitropism in shoots.
In shoots, greater conc. of IAA inhibits cell elongation the lower side, increase cell elongation, causing the lower side to elongate faster, moves away from the force of gravity
