Biology : Plant And Animal Responses

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13 Biology

AS 91603

Plant and Animal Responses


Growth responses

Tropisms Millar

Tropisms are directional plant growth responses. The plant detects a stimulus and grows toward or away from it.

The two most important are phototropism and gravitotropism (aka geotropism).

Plant shoots (a newly growing shoot is referred to as a coleoptile) grow toward light. This is positive phototropism.

The reason for this is to move them into the light they need for photosynthesis (i.e. to survive). So this lets them grow in a way that improves their survival chance

The mechanism behind this:

The tip of the coleoptile produces a chemical called auxin

• auxin is a hormone that stimulates the cells below the tip (called the zone of elongation) too expand, up to three times their original length

• exposure to light inhibits the production of auxin (it alters an oxidation process to change it into something useless)

So on the side of the plant light shines on, much less auxin is produced. On the side with less light, more is able to be produced

• all the auxin produced travels down the plant into the zone of elongation

• the dark side has more auxin, so the dark side elongates more

• this bends the plant toward the light (referred to as curvature)

Geotropism: geotropism is the movement relative to gravity. Plants roots do positive geotropism and the stems do negative geotropism

This lets the plants roots grow down into the

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