OCR A2 Biology F215, Plant Responses

Some notes on plant responses and plant growth.

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  • Created on: 10-04-12 16:18
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Plant Responses
Plants respond to the biotic (living) and abiotic (non-living) components of their environment
as well as external stimuli.
Responding to the environment in this way may help the plant avoid stress or being eaten
and to live long enough to reproduce.
Tropisms are directional growth responses of plants:
o Phototropism ­ shoots grow towards light (for photosynthesis)
o Geotropism ­ roots grow towards the pull of gravity (for support)
o Chemotropism ­ on a flower pollen tubes grow down the style, attracted
by chemicals, towards the ovary so fertilisation can take place.
o Thigmotropism ­ shoots of climbing plants, such as ivy, wind around other
plants or solid structures (for support).
Hormones coordinate plant responses to environmental stimuli and are produced by a
variety of tissues in the plant.
When hormones reach their target cells they bind to receptors on the plasma membrane.
Specific hormones have specific shapes, which can only bind to receptors with
complementary shapes.
Hormone Effects
Auxins Promote cell elongation, inhibit growth of side shoots and inhibit leaf
abscission (leaf fall). It increases the stretchiness of the of the cell wall
by promoting the active transport of hydrogen ions, by an ATPase
enzyme on the plasma membrane, into the cell wall. The resulting low
pH provides optimum conditions for wall-loosening enzymes (expansins)
to work. These enzymes break bonds within the cellulose so the walls
become less rigid and can expand. It also acts on the cells in the
abscission zone
Cytokinins Promote cell division. Stop the leaves of deciduous trees ageing by
making sure the leaf acts as a sink for phloem transport, meaning that it
gets a good supply of nutrients.
Gibberellins Promote seed germination and growth of stems.
Abdcisic Acid Inhibits seed germination and growth and causes stomata to close when
the plant is stressed by low water availability.
Ethene Promotes fruit ripening.
Plant Growth
The cell wall around a plant cell limits its ability to divide and expand. This means that growth
only happens in certain places in the plant, where there are groups of immature cells capable
of dividing ­ meristems:
o Apical meristems ­ tips or apices of roots or shoots (responsible for roots
and shoots getting longer).
o Lateral bud meristems ­ buds (can give rise to side shoots).
o Lateral meristems ­ cylinder near the outside of roots and shoots
(responsible for roots and shoots getting wider).
o Intercalary meristems ­ between the nodes (responsible for the shoot
getting longer).

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Apical dominance ­ the growing apical bud at the tip of the shoot inhibits growth of lateral
buds further down the shoot.…read more


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