Transcript Phototropism & Plant Hormones - mvhs

Phototropism and Plant Hormones
AP Biology
Unit 5
Tropisms
• Tropism = how a plant responds to a
particular stimulus (light, gravity, touch,
etc.)
• Phototropism = growth in response to light
• Gravitropism = growth in response to
gravity
• Thigmotropism = response to touch
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Auxin
• The plant hormone auxin (also known as
indoleacetic acid) plays a large role in many
tropisms
Slide 3 of 10
Phototropism
• How a plant grows in
response to light
– When light is placed on one
side of a plant, it will bend
towards the light as it grows
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Phototropism
More growth
• Why is the plant
bending?
Less growth
– At a cellular level, it
means that cells on one
side of the plant are
growing faster than the
other.
– Cells on the shaded
side are growing faster
than the lit side
Slide 5 of 10
Phototropism Experiments
• Several scientists
performed experiments
to study the cause of
phototropism
– Darwin & Darwin:
showed the plant only
exhibited phototropism
when the tip was
exposed  the area that
senses light is in the tip
region
Slide 6 of 10
Phototropism Experiments
• Boysen & Jensen
– Separated the tip from the
rest of the plant and put
mica or gelatin under it
– Mica is impermeable to
substances, gelatin is not
– The tip with gelatin still
caused the plant to bend
 signal (chemical) is
being passed down the
plant from the tip
Slide 7 of 10
Phototropism Experiments
• Went
– Removed tips and placed
them on agar blocks to
transfer chemical to agar
– When agar block is
placed on one side of the
cut tip, the plant grew
curving away from the
side the agar was on 
hormone has diffused
from tips into blocks that
affected growth
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Auxin and Phototropism
• Later studies determined that
the chemical at work was
auxin
• When light is shone on one
side, it causes auxin to move
AWAY from the light to the
shadier side
• Since the concentration of
auxin is higher on the shadier
side, it grows more on that
side–plant bends towards the
light
Slide 9 of 10
Note: some recent experiments
indicate other molecules might
also contribute to phototropism
Image taken without permission from http://bcs.whfreeman.com/thelifewire/default.asp
Plant Hormones
• Many other plant hormones have also since
been identified
• Compounds that move throughout the plant
• Control plant growth and development by
affecting
–
–
–
–
Cell division, elongation, or differentiation
Enzyme activity
Gene expression
Properties of membranes
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