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Summary CCEA Biology As1 and As2 notes
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- CCEA
CCEA biology As1/As2 notes -Enzymes -Molecules -Principles of exchange -Adaptations of organisms -Continuity of cells -Molecules, Cells and systems -Nucleic acids
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Plant Co-ordination and Control.Stimulus:
Any change in an organisms environment to which they respond.
The main stimulus plants respond to is light;
1. The direction of light (phototropism)
2. The duration of light (photoperiodism)
In some plant species flowering process is a response to temp.
Most plant species respond to the duration of light/day length.
Photoperiodism:
The response of a plant to the relative lengths of daylight and darkness.
Affected by the period of illumination (photoperiod.)
Flowering plants classified in accordance with the photoperiod in which they flower.
Long day plants - flower during long summer days or live-in temperate regions. They need a short
night (i.e. short dark period).
Short day plants - winter-flowering or live near the equator. They need a long night (i.e. long dark
period).
Control of flowering in long day and short-day plants:
Photoreceptor involved in photoperiodism, is the light-sensitive pigment called PHYTOCHROME.
Found in leaves of flowering plants.
Detects + responds to wavelengths of light → inducing/inhibiting flowing.
Primarily involved in the timing of flowering in many plant species.
, The Two inter-convertible forms (each with a different absorption peak):
1. Phytochrome 660 (P660 or PR): Absorbs RED light with an absorption peak of 660nm.
2. Phytochrome 730 (P730 or PFR): Absorbs FAR-RED light with an absorption peak of
730nm.
Sunlight is mainly made of red light.
When P660 absorbs red light it is converted rapidly to P730 .
When P730 absorbs far red light it is converted rapidly back into
P660 .
In darkness P730 (unstable) will be converted slowly back into
P660
Removal of PFR/P730 is needed for SDPs to flower, A critical length
of night (dark period) is required to remove P730, they need a
long night as conversion of P730 - P660 is slow,
Non-removal of PFR/P730 allows LDPs to flower. Therefore a short
night is required.
Key points:
PFR/P730 is the physiologically active form of phytochrome (it has a stimulatory OR inhibitory
effect). There is either sufficient P730 accumulated to promote flowering in LDPs or there is
too much P730 to allow flowering to occur in SDPs (high P 730 inhibits flowering in SDPs, SDPs
will only flower when levels of P730 are low/ levels of P660 are high)
Sunlight contains much more red light (light of
wavelength 660nm), than far red light (730nm), so
during the day, PR is rapidly converted to PFR. which then
accumulates. Therefore as day length gets longer in
Spring/early summer, the proportion of phytochrome
that exists in the P730 form increases, eventually
reaching a critical level that promotes flowering in long
day plants and inhibits flowering in short day plants.
The plant measures the length of day (or night), by
monitoring the relative concentrations of the
phytochrome PR and PFR at any one time. In daylight, the level of PFR predominates (Rapid
conversion from PR).
The length of the night/dark period is critical in determining whether flowering occurs in both LDPs
and SDPs as it determines how much PFR/P730 is converted back to PR/P660. There is a critical
length of night (dark) required to remove P730.
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