Control and coordination in plants
Plants produce hormones to coordinate growth and responses to
light (phototropism ) and gravity
(gravitropism or geotropism)
Auxin is the hormone. An unequal distribution of auxin
causes unequal growth rates in roots and shoots
In shoots , auxin stimulates growth. Light from one side
makes auxin move to the shaded side , which grows faster, so the
shoot bends towards the light : positive phototropism. This gets
the leaves more light for photosynthesis
In roots , auxin inhibits growth. Gravity makes auxin
collect on the lower side , which grows slower, so the root
bends downwards : positive gravitropism. This gets the root into
the soil for water and anchorage
Shoots grow away from gravity (negative gravitropism): auxin on the
lower side makes it grow faster and turn up
(HT) Gibberellins initiate seed germination
(HT) Ethene controls cell division and the ripening of
fruit
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Auxin, phototropism and gravitropism a shoot lit from one side light auxin builds up on the SHADED side those cells ELONGATE more, so the shoot bends TOWARDS the light — positive phototropism a root lying on its side gravity auxin builds up on the LOWER side in a ROOT, auxin INHIBITS elongation — so the lower side grows less and the root bends DOWNWARDS (positive gravitropism) Auxin is a plant hormone made at the tips. It always collects on the shaded or lower side — what differs is the effect: in a SHOOT auxin makes cells grow longer, so the shoot bends towards light and away from gravity; in a ROOT auxin stops cells growing, so the root bends away from light and towards gravity. A shoot lit from one side with auxin concentrated on the shaded side and the shoot bending towards the light; a root lying on its side with auxin on the lower side and the root bending down
The mechanism is always where the auxin is and what it does
there : in a shoot, more auxin on the shaded side, more growth
there, so it bends towards the light. In a root, more auxin on the lower
side, less growth there, so it bends down. Say which side grows
more, comparatively
"Positive phototropism" or "phototropism" is credited; "tropism" alone is
not, and "negative phototropism" for a shoot cancels the mark
The rest of this topic is in the full Exam Guide The complete AQA GCSE Biology guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• Control and coordination in plants• Required Practical 8: light or gravity and seedling growth• Uses of plant hormones [Higher Tier]Read the free biology topics →