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A2 Biology Revision Notes Responding to the Environment


aWhy plants respond to the environment

Animals respond to the biotic (living) and abiotic (non-living) components of their environment. Plants
also respond to external stimuli. Responding to the environment in this way may help the plant to
avoid stress of avoid being…

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A2 Biology Revision Notes Responding to the Environment


How plants respond to the environment

Auxins

Auxins stimulate shoot growth by cell elongation. The extent to which cells elongate is
proportional to the concentration of auxins
Auxin increases the stretchiness of the cell wall by promoting active transport of hydrogen
ions,…

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A2 Biology Revision Notes Responding to the Environment


This increases production of cellulose, which digests the walls of the cells in the abscission
zone

Controlling plant growth

Apical dominance

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

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A2 Biology Revision Notes Responding to the Environment


Researchers compared GA concentrations of tall pea plants and dwarf pea plants, which
were otherwise genetically identical. They found that plants with higher GA concentrations
were taller
To show that GA directly causes stem growth, researchers needed to know how GA is…

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A2 Biology Revision Notes Responding to the Environment


Brewing

To make beer you need malt
When barley seeds germinate, the seed produces amylase enzymes that break down stored
starch into maltose
Usually the genes for amylase production are switched on by naturally occurring gibberellins
Adding gibberellin can speed up the…

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A2 Biology Revision Notes Responding to the Environment


Restricting ethene's effects can also be useful. Storing fruits in low temperature with little
oxygen and high carbon dioxide concentrations prevent ethene synthesis and so prevents
fruit ripening. This means fruit can be stored for longer.

The brain




Structure of the brain…

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A2 Biology Revision Notes Responding to the Environment


o Joints

Medulla oblongata

Controls action of smooth muscle in the gut wall, controls breathing rate and heart rate
It is effectively in control of the autonomic nervous system
Regulatory centres for a number of vital processes are found in the medulla…

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A2 Biology Revision Notes Responding to the Environment


Sensory and motor systems of the peripheral nervous system

Sensory neurones carry impulses from the many receptors, in and around the body, to the
CNS
Motor neurones carry impulses to the CNS to the effector organs
Many neurones are bundled together and…

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A2 Biology Revision Notes Responding to the Environment


Action of muscles

Muscles produce a force when they contract. The movement of any bone at a joint requires the
coordinated action of at least two muscles.

Movement at the elbow joint



The biceps and triceps muscles act
antagonistically to move the…

Page 10

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A2 Biology Revision Notes Responding to the Environment


The motor unit

Some muscular movements require a stronger contraction than others. The brain controls the
strength of contraction as many motor neurones stimulate a single muscle. Each one branched to
neuromuscular junctions, causing the contraction of a cluster of muscle cells…

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