Response to Stimuli

Question 1

What is a stimulus ?

Answer 1

a stimulus is a change in the internal or external environment of an organism that produces a response in the organism.

Question 2

What is a taxis ?

Answer 2

movement of a cell or part of an organism towards or away from a source of stimulus e.g light or chemicals.

Question 3

What is a positive taxis ?

Answer 3

movement towards

Question 4

what is a negative taxis ?

Answer 4

movement away

Question 5

what is a phototaxis ?

Answer 5

movement in response to light

Question 6

what is chemotaxis ?

Answer 6

movement in response to chemicals

Question 7

Earthworms move away from light what is this an example of ?

Answer 7

negative phototaxis
(move away from light so they dont dry out )

Question 8

Some bacteria move towards a region where glucose is concentrated what is this an example of ?

Answer 8

positive chemotaxis
(more glucose for respiration)

Question 9

What is kinesis ?

Answer 9

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.

Question 10

What happens when animals that demonstrate kinesis such as woodlice reach a more favourable environment ?

Answer 10

they slow down and change direction less often.

Question 11

How do woodlice show kinesis ?

Answer 11

in dry conditions, they move more rapidly and change direction more often to increase their chance of moving into a different area.

Question 12

What is a favourable environment for woodlice ?

Answer 12

moist conditions

Question 13

What is a tropism ?

Answer 13

movement or curvature of a plant in response to a directional stimulus

Question 14

Plant shoots grow towards the light. What is this an example of ?

Answer 14

positive phototropism

Question 15

Plant roots grow towards gravity. What is this an example of ?

Answer 15

positive geotropism.

Question 16

Plant roots grow towards water. what is this an example of ?

Answer 16

positive hydrotropism

Question 17

What is the name of the specific growth factor that is found in plants ?

Answer 17

Indoleacetic acid (IAA)

Question 18

Where is IAA synthesised ?

Answer 18

in the growing tips of roots and shoots (i.e in the meristems where the cells are dividing )

Question 19

What is the effect of IAA in the shoots ?

Answer 19

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)

Question 20

What is the effect of IAA in the roots ?

Answer 20

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.

Question 21

What are amyloplasts ?

Answer 21

Starch Storage organelles

Question 22

How does IAA stimulate cell elongation ?

Answer 22

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

Question 23

What does IAA effect ?

Answer 23

the elongating region of a plant

Question 24

Where is the elongating region in a plant ?

Answer 24

The elongating region is found just prior to the tip/area of cell division

Question 25

How are plant growth factors transported to the roots ?

Answer 25

In the phloem
from cell to cell via membrane transporter proteins