IGCSE, Biology, coordination and response, plants, invertebrates.

2.  Why is this plant
growing to one side?
 How would it benefit
from this behavior?
 What happens inside
the plant to produce
such a behavior?

3.  Plants grow when the
tips divide producing
more cells then the
cells start to elongate.
 Plant cells produce a
hormone-like
chemical called
Auxins that affect cell
growth.
 P.S. Auxin is a greek
word which means to
increase.

4. Lets try to understand how Auxins work from these experiments.

5. 1 , 2 and 3 are diagrams of the same plant shoot but at different times during the experiment.
Use this experiment’s result to figure what causes a shoot to grow to one side.

6.  Which part of the shoot causes response to light?
 Tip of shoot
 If cut the shoot grows straight.
 What is the nature of auxins?
 Chemical.
 It can diffuse through gelatin block, not through mica
 What changes do they cause to plant shoot cells?
 Grow faster, bent towards light.
 As shown by Control, shoot covered by transparent
cap and gelatin block.
 When are auxins most active? light or dark?
 In dark
 When tip was covered grows faster.

7.  Unlike animals plants respond slower, by growing,
this is called tropism.
 Growing towards light is positive phototropism
while growing away is negative phototropism.
 Shoots grow toward light to place their leaves in
the best conditions for photosynthesis and their
flowers for pollination and seeds for dispersal.
 Roots grow away from light, to find soil to attach
to, and absorb minerals and water.
 Auxins stimulate growth of shoot cells, but inhibit
growth of root cells.

8. .

11. Phototaxis,
chemotaxis and
hydrotaxis.
The response of small
invertebrates by moving
towards or away from
light, chemicals and
humidity.
A choice chamber is used
to investigate invertebrate
behavior towards
light/dark conditions and
humid/dry conditions.
Many woodlice are used to
avoid errors due to chance,
to get more reliable
results.
Humid/dry
Dark/light

12.  Invertebrates usually prefer moist conditions,
can you explain why?
 What word is used to describe that response?
 To avoid drying out and to find food.
 Positive hydrotaxis
 Invertebrates usually prefer dark conditions.
Can you explain why?
 What word is used to describe that response?
 To hide from predators, and avoid drying out.
 Negative phototaxis.