Plant Hormones (Edexcel International A Level (IAL) Biology): Flashcards
Exam code: YBI11
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Phytochrome
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Phytochrome
A phytochrome is a plant pigment that reacts to different wavelengths of light to bring about plant responses such as germination and flowering.
What are the two forms of phytochrome, and which is the active form?
PR is the inactive form, which absorbs red light (660 nm). PFR is the active form, which absorbs far-red light (730 nm).
In the absence of red light, the unstable PFR gradually converts back into .
In the absence of red light, the unstable PFR gradually converts back into PR.
How does phytochrome influence transcription in plant cells?
In red light, PR is converted to PFR, which moves into the nucleus and binds to the protein PIF3. The PFR–PIF3 complex acts as a transcription factor that initiates transcription of genes controlling growth and development.
In long-day plants, how do high levels of PFR bring about flowering?
High levels of the active form PFR act as a transcription factor, activating expression of genes that stimulate flowering.
True or False?
Seeds exposed to far-red light will germinate.
False.
Far-red light converts PFR back into PR, which prevents germination; a burst of red light forms PFR and triggers germination.
Indoleacetic acid (IAA)
IAA is a type of auxin, a plant growth factor that brings about responses such as phototropism by altering the transcription of genes involved in cell growth.
How does IAA cause a shoot to bend towards a light source?
IAA is transported to the shaded side of the shoot. The higher IAA concentration there increases the rate of cell elongation, so the shoot bends towards the light.
Why does IAA have opposite effects on elongation in shoots and roots?
In shoots, a high IAA concentration increases cell elongation. In roots, a high IAA concentration inhibits cell elongation, so the lower side grows more slowly and the root bends downwards.
IAA brings about phototropism by altering the of genes that code for proteins involved in cell growth.
IAA brings about phototropism by altering the transcription of genes that code for proteins involved in cell growth.
How do gibberellins bring about germination in barley seeds?
Gibberellins made by the embryo stimulate the aleurone layer, increasing the transcription of genes coding for amylase. The amylase hydrolyses starch into maltose to provide energy for the embryo.
Gibberellins
Gibberellins are plant growth regulators that control seed germination, stem elongation, flowering and fruit development.
Endosperm
The endosperm is a starch-containing energy store surrounding the embryo of a cereal grain.
Aleurone layer
The aleurone layer is a protein-rich layer on the outer edge of the endosperm that synthesises amylase in response to gibberellin.
What is the role of gibberellin in germinating cereal grains?
Gibberellin stimulates the aleurone layer to produce amylase, which hydrolyses starch in the endosperm into maltose for the developing embryo.
Why is the embryo end of each grain cut off and discarded before the assay?
The embryo naturally produces gibberellin. Removing it lets the investigator control the gibberellin concentration the grain is exposed to.
Why are the grain halves placed in sodium hypochlorite solution?
To sterilise the grains, killing microorganisms that could compete or act as other sources of starch that would affect the results.
Why are the grains washed with distilled water until the chlorine smell has gone?
To remove all of the sodium hypochlorite, which could otherwise interfere with amylase activity and the results.
Why are the petri dish lids taped only lightly, so the dishes are not airtight?
To allow oxygen to enter for respiration during germination, while still preventing contamination of the agar.
How is starch detected on the agar, and what does a clear zone show?
Iodine stains any remaining starch blue-black. A clear zone around a grain shows where amylase has digested the starch — a larger clear zone means more amylase was produced.
The larger the clear zone around a cereal grain, the amylase the grain has produced.
The larger the clear zone around a cereal grain, the more amylase the grain has produced.
True or False?
Areas of agar that stain blue-black with iodine are where amylase has digested the starch.
False.
Blue-black staining shows that starch is still present; the clear (unstained) zones are where amylase has digested the starch.
What is measured and plotted to show the effect of gibberellin on amylase production?
Measure the diameter of the clear zone around each grain, then plot gibberellin concentration against clear-zone diameter.
Why are grains placed with their cut side facing downwards onto the starch agar?
So that the amylase diffuses out of the cut surface directly into the starch agar, digesting the starch beneath the grain.
By which two systems is coordination brought about in animals?
By the nervous system and the hormonal (endocrine) system.
Stimulus
A stimulus is a change in the internal or external environment that is detected by receptor cells.
Hormone
A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to alter the activity of specific target organs.
What is the pathway of a nervous response?
stimulus → receptor → sensory neurone → CNS → motor neurone → effector
What is the pathway of a hormonal response?
stimulus → receptor → hormone (carried in the blood) → effector
How does the transmission of nervous signals differ from hormonal signals?
Nervous signals travel as electrical impulses along neurones; hormonal signals travel as chemicals carried in the blood.
Compare the speed of nervous and hormonal responses.
Nervous responses are fast; hormonal responses are slower.
Compare the duration of nervous and hormonal responses.
Nervous responses are short-lived; hormonal responses are longer-lasting.
Compare the target of nervous and hormonal responses.
Nervous responses act on a precise, localised target; hormonal responses are more widespread, affecting one or more target organs around the body.
True or False?
Hormonal responses are generally faster and shorter-lived than nervous responses.
False.
Hormonal responses are slower and longer-lasting than nervous responses.
Effector
An effector is a muscle or gland that carries out a response to a stimulus.
Because they are slower than nerve impulses, hormones are used to control functions that do not need an response.
Because they are slower than nerve impulses, hormones are used to control functions that do not need an instant response.
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