5.4.1: Control and coordination
Not started yet — this one needs some love.
Plants produce hormones to coordinate and control growth and responses to light and gravity.
Phototropism is a growth response to light.
Gravitropism or geotropism is a growth response to gravity.
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
In shoots, more auxin usually causes more cell elongation, so the shoot bends as one side grows faster.
In roots, auxin can reduce cell elongation, so roots and shoots may respond differently.
Higher tier: gibberellins are important in initiating seed germination.
Higher tier: ethene controls cell division and ripening of fruits; the mechanisms of gibberellins and ethene are not required.
The seedling practical investigates the effect of light or gravity on newly germinated seedlings using length measurements and labelled biological drawings (see RP8: Seedling growth).
Common exam mistakes
Do not confuse tropism with trophic or "trophism"; tropism is a growth response to a stimulus.
Phototropism is a response to light, while gravitropism or geotropism is a response to gravity.
Auxin does not simply make the whole shoot bend; unequal auxin distribution causes one side to grow faster than the other.
In RP8: Seedling growth answers, link the factor tested, such as light direction or gravity, to the observed growth response.
For biological drawings of seedlings, include clear labels and measurements rather than an unlabelled sketch.