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|Learning objectives |Understanding | | |( |( |( |Revised | |To know that organisms increase their chance of survival by responding to changes in their | | | | | |environment | | | | | |To know that taxes and kineses are simple responses that can maintain a mobile organism in a | | | | | |favourable environment | | | | | |To know that tropisms are responses to directional stimuli than can maintain the roots and | | | | | |shoots of flowering plants in a favourable environment | | | | | |To know that in flowering plants specific growth factors diffuse from growing regions to other | | | | | |tissues and regulate growth in response to directional stimuli | | | | | |To know the role of indoleacetic acid (IAA) in controlling tropisms in flowering plants | | | | |
Response to stimuli
Reference Chapter 9.1 page 142-193 in A2 text book.
Key concepts you must understand
Organisms respond to changes in their external environment and to changes in their internal environment.
When an organism responds to a stimulus (a change in the external or internal environment of an organism) there are a number of processes that are common to all, no matter how simple or complex. There is always: • a receptor — a structure that detects the stimulus • an effector — a structure, such as a muscle, that produces the response • some kind of linking system or coordinating system — this receives information from the receptor and passes information to the effector • a response — the action that results from the stimulus
Many examples of behaviour have evolved to increase the chances of survival of an organism. These include the following:
• Plant shoots (stems) grow towards the region of most intense light; this exposes the leaves to the maximum amount of light and increases the rate of photosynthesis. • Woodlice move more quickly in the light than in the shade; this increases their chances of moving out of the light and into the shade (where it is also likely to be more humid, so the animals will not dehydrate as quickly). • Reflex actions in mammals are often protective — for example: – withdrawal reflexes move parts of the body away from potentially damaging stimuli, such as heat or sharp objects – when a bright light is shone into the eye, the iris reflex prevents too much light from entering the eye and damaging the retina
Other reflex actions are corrective rather than protective and include: • reducing the heart rate if blood pressure becomes too high • increasing the breathing rate if the partial pressure of oxygen in the blood becomes too low or the partial pressure of carbon dioxide becomes too high
We have learned that those organisms with a selective advantage survive and are able to reproduce and pass on their alleles to the next generation....
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