Plants response to the environment - Hormones

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Effect of the hormone gibberelins?
Regulate growth and developmental processes such as stem elongation, germination, flowering and leaf and fruit senescene.
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Effect of the hormone auxin?
Involved in tropisms in which they stimulate cell elongation, bud formation and root initiation. Inhibits leaf abscission.
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Effect of the hormone cytokinin?
Stimulate cell division (cytokinesis) and effect leaf senescene.
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Effect of the hormone abscisic acid?
Inhibits seed germination and growth; causes stomatal closure when the plant is stressed by low water availability.
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Effect of the hormone ethene?
Promotes fruit ripening.
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Define phototropism
Shoots grow towards light (positively phototropic), which enables them to photosynthesise.
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Define geotropism
Roots grow towards gravity. Anchors them into soil and helps them to take up water needed for support to keep cells turgid as a raw material for photosynthesis and to help cool the plant. Will also be minerals e.g. nitrates in water.
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Define chemotropism
On a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
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Define thigmotropism
Shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
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State the 3 ways hormones can move around a plant
Diffusion, active transport, mass flow in the phloem sap or in the xylem vessels.
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Define tropism
A growth response to a directional stimulus. The response may either be in the direction of the stimulus (positive) or away from it (negative).
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Where does growth only happen in plants?
Meristems
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Define apical meristems
Located at the apex of the roots and shoots and are responsible for making the grow longer.
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Define lateral bud meristems
Found in the buds. These could give rise to side shoots.
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Define lateral meristems
Found in a cylinder near the outside of roots and shoots, responsible for them getting wider.
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Define intercalary meristems
Located between the nodes. Growth between the nodes is responsible for the shoots getting longer.
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Where does cell division happen?
Closest to the apex.
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Where does cell elongation happen?
Just behind the apex
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Where are auxins produced?
At the apex
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The effect of auxins
Stimulate shoot growth by causing cell elongation. Extent which cells elongate=proportional to conc of auxins. Auxin increases stretchiness of cell wall by promoting active transport of hydrogen ions by ATPase enzyme on plasma membrane to cell wall.
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Effect of auxin continued
Resulting low pH provides optimum conditions for wall-loosening enzymes to work. Enzymes break bonds in cellulose so cells become less rigid and can expand as cell takes in water.
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What causes phototropism?
In phototrophic response, shoot bends towards light. Happens because shaded side elongates faster than illuminated side. 2 enzymes phototropin 1 and 2 activity promoted by blue light (400-450nm). More phototropin 1 on light side, less on dark side.
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Describe leaf abscission
Cytokinins stop leaves of deciduous trees senescing (dying+turning brown) by making leaves acts as sink for phloem transport. Drop in cytokinin production=sensescene begins/nutrients dwindles. Usually followed by leaves being shed (abscission).
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Effect of leaf senescene
Leaf senescene causes auxin production at tip of leaf to drop. Makes cells in abscission zone more sensitive to growth substance ethene. Drop auxin conc=increases ethene. Increases cellulose production which digest walls of cells in abscission zone.
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Effect of the hormone auxin?

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Involved in tropisms in which they stimulate cell elongation, bud formation and root initiation. Inhibits leaf abscission.

Card 3

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Effect of the hormone cytokinin?

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Card 4

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Effect of the hormone abscisic acid?

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Card 5

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Effect of the hormone ethene?

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