Response to Stimuli Flashcards
What is a stimulus ?
a stimulus is a change in the internal or external environment of an organism that produces a response in the organism.
What is a taxis ?
movement of a cell or part of an organism towards or away from a source of stimulus e.g light or chemicals.
What is a positive taxis ?
movement towards
what is a negative taxis ?
movement away
what is a phototaxis ?
movement in response to light
what is chemotaxis ?
movement in response to chemicals
Earthworms move away from light what is this an example of ?
negative phototaxis
(move away from light so they dont dry out )
Some bacteria move towards a region where glucose is concentrated what is this an example of ?
positive chemotaxis
(more glucose for respiration)
What is kinesis ?
The random movement of an organism to reach a more favourable environment.
The movement depends on the intensity and not the direction of the stimulus.
What happens when animals that demonstrate kinesis such as woodlice reach a more favourable environment ?
they slow down and change direction less often.
How do woodlice show kinesis ?
in dry conditions, they move more rapidly and change direction more often to increase their chance of moving into a different area.
What is a favourable environment for woodlice ?
moist conditions
What is a tropism ?
movement or curvature of a plant in response to a directional stimulus
Plant shoots grow towards the light. What is this an example of ?
positive phototropism
Plant roots grow towards gravity. What is this an example of ?
positive geotropism.
Plant roots grow towards water. what is this an example of ?
positive hydrotropism
What is the name of the specific growth factor that is found in plants ?
Indoleacetic acid (IAA)
Where is IAA synthesised ?
in the growing tips of roots and shoots (i.e in the meristems where the cells are dividing )
What is the effect of IAA in the shoots ?
IAA is unevenly distributed.
IAA moves from the illuminated side of a shoot to the shaded side
The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation.
This causes the shoot to bend towards the light. (positive phototaxis)
What is the effect of IAA in the roots ?
In roots, higher concentrations of IAA results in a lower rate of cell elongation.
IAA is actively transported to the region in the root tip where the amyloplasts have sunk (under the force of gravity)
The larger concentration of IAA at the lower side of the root inhibits cell elongation
As a result, the lower side grows at a slower rate than the upper side of the root.
This causes the root to bend downwards.
What are amyloplasts ?
Starch Storage organelles
How does IAA stimulate cell elongation ?
When IAA moves into the elongating region it binds to the protein receptors on the cell membranes
IAA stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall
this acidifies the cell wall (lowers the pH of the cell wall)
This activates proteins known as expansins, which loosen the bonds between cellulose microfibrils.
At the same time, potassium ion channels are stimulated to open.
This leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water potential of the cytoplasm.
This causes the cell to absorb water by osmosis (water enters the cell through aquaporins) which is then stored in the vacuole.
This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
The cell elongates
What does IAA effect ?
the elongating region of a plant
Where is the elongating region in a plant ?
The elongating region is found just prior to the tip/area of cell division
How are plant growth factors transported to the roots ?
In the phloem
from cell to cell via membrane transporter proteins
