Plant Responses OCR A2 Biology

Keywords and quick revision of plant responses has pictures :)

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Biology F215
Plant Responses
Key Words
Key words Definition
Abiotic Non-living components of the environment.
Abscisic acid (ABA) Plant hormone that inhibits seed germination and growth, and causes
stomatal closure when water is limited.
Abscission The shedding of leaves from deciduous trees.
Apical dominance The growing apical bud at the tip of the shoot inhibits growth of lateral buds
further down the shoot.
Apical meristems Immature cells still capable of dividing, located at the tips (or apices) of roots
and shoots.
Auxins Plant hormones that promote cell elongation, inhibit growth of side shoots
and inhibit leaf fall.
Biotic Living components of the environment.
Chemotropism A directional plant response in which chemicals are the stimulus. For example,
pollen tubes grow down the style to the ovary in a flowering plant.
Cytokinins Plant hormones that promote cell division.
Deciduous The shedding of plant structures that are no longer needed. Often associated
with the seasonal shedding of leaves.
Ethene A gas produced by flowers, stems and leaves that causes the ripening of fruit
and ageing of flowers
Geotropism A directional plant response to gravity. Also known as gravitropism. Root
growth is an example.
Gibberellins Plant hormones that promote stem elongation and seed germination.
Hormones Chemical messengers that are transported away from their site of
manufacture to target cells (although some act within the cell of
manufacture). In plants they are often referred to as plant growth regulators.
Intercalary meristems Immature cells still capable of dividing, located between the nodes (where the
leaves and buds branch off the stem). They are responsible for growth
between the nodes leading to the shoot getting longer.
Lateral bud meristems Immature cells still capable of dividing, located at the tips (or apices) of buds
on the stem. They may give rise to side shoots.

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Lateral meristems Immature cells still capable of dividing, found in a cylinder near the outside of
roots and shoots. They are responsible for the roots and shoots getting wider.
Nastic movements Non-directional responses to stimuli such as touch, temperature or humidity.
Not dependant on the direction of the stimulus.
Photoperiodism A plant's ability to flower in response to changes in the photoperiod (the
relative lengths of day and night).
Phototropins Photoreceptor proteins that mediate a plant's response to light.…read more

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What stimuli to plants respond to?
Plants will move in response to stimuli
o This stimuli can be either biotic (living) or abiotic (non-living)
The response to this stimuli can be
Tropic (directional)
Nastic (non-directional)
What are tropic responses?
Tropic responses or tropisms are directional plant growth responses
o These tropic movements can be either
Positive (towards the stimulus)
Negative (away from the stimulus).…read more

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An example of this is flowers opening and closing in
response to day length and temperature fluctuations
Thigmonastic: where the response is the same regardless of
the direction of the stimulus
The movement is the same once triggered, e.g.…read more

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Promote growth of stems
Promote seed germination
Cytokinins are the third group of plant hormones
o Cytokinins
Promote cell division
Abscisic acid (ABA) is the fourth type of plant hormone
o Inhibits seed germination
o Causes stomatal closure during water stress
Ethene is the final type of plant hormone
o Promotes fruit ripening
o Promotes flower ageing
How do plants grow?
Plant cell walls limit their ability to divide and therefore grow
o This means that growth can only happen in particular places in the…read more

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Auxin binds to its receptor and promotes the active transport of hydrogen ions into the cell wall by an
o The low pH provides optimum conditions for wall-loosening enzymes to work (expansins)
Expansins and low pH disrupts hydrogen bonds within the cellulose
These enzymes break the bonds within the cellulose
o The wall becomes less rigid and expands as the cell takes in water
This increases the plasticity (stretchiness) of the cell wall
Allowing the cell to elongate
How do plants move towards light?…read more

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What is apical dominance?
The presence of the auxin-producing apex (shoot tip) normally inhibits the development of lateral buds
(side shoots) is a phenomenon known as apical dominance
o What are the advantages of this?
It allows plant to direct most of their assimilates to the apical bud to give growth in
It is important to gain height over competitors to ensure maximum light absorption
Gardeners exploit this property by taking off the apex to make the plant bushier
One hypothesis is that a…read more

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This means normally cytokinins are concentrated
in the shoot tip ­ but when the tip is removed
the cytokinins spread more evenly throughout
the plant
What are gibberellins?
Gibberellins are a family of plant hormones that promote stem elongation and seed germination
o Plants that are genetically dwarf, such as some pea plants, are homozygous for genes that code
for enzymes in the pathway that produces gibberellins
How are gibberellins involved in pea plant height?
The gene, Le/le, codes for the last enzyme in the…read more

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This research confirmed that GA1 caused stem elongation
How is plant growth caused by gibberellins?
Cause growth at the internodes by stimulating cell elongation
by loosening cell walls
o They also increase cell division by stimulating the
production of a protein that controls the cell cycle
The internodes of dwarf plants have fewer
and shorter cells than tall plants
What are the commercial uses of plant hormones?
Fruit drop and development
Weed killer
Growth in height
Lateral growth
What are the commercial uses…read more

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Sugar cane stores sugar in the cells of the internodes
Gibberellin stimulates growth between nodes
Sugar yield is increased
o In the first growing season sugar beet produces a large storage root containing 15-20% sucrose
In the second year the energy stored in the root is used to produce flowers for
Seed companies add gibberellins to induce seed production in the first year
Plant breeding
o Sugar beets as above
o Conifers spend a long time as juveniles, before becoming reproductively active
Gibberellins will…read more



These notes are well presented and give a good account of plant responses with a useful glossary of key terms. Use them as your notes, build on them or use them to create revision cards from. Everything is here - definitions, examples and annotated diagrams.

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