An in-depth informative booklet on Survival and Response in plants and animals including tropisms, the nervous system, receptors, muscle structure and contraction and much more. Also includes useful summary questions and the specification for this topic.
, Survival and response
Organisms increase their chances of survival by responding to changes in their external
environment. Animal and plants respond in different ways. Eg animals can move away from harmful
environments such as places that are too hot or too cold and plants cannot actually move
themselves, but they can change the way they grow in an attempt to find more favourable
environmental conditions.
Any change in the internal or external environment eg a change in temperature, light intensity
or pressure, is called a stimulus. A stimulus leads to a response. The ability to respond increases
the organisms chance of survival. Those that survive have a greater chance of reproducing and
passing their alleles to their offspring. There is always, therefore a selection pressure favouring
organisms with the most appropriate responses.
Tropisms: a directional growth response in which the direction of the response is determined by
the direction of the external stimulus.
Positive tropisms are growing towards the direction of the stimulus and negative are growing away
from the direction of the stimulus.
• Phototropism: shoots grow towards the light to increase the rate of photosynthesis
• Gravitropisms: shoots grow upwards and roots grow downwards towards the pull of
gravity.
• Chemotropism: pollen tubes grow towards chemicals given off by the ovary in a flower.
• Thigmotropism: shoots respond to touch by growing around other plants
• Hydrotropism: roots grow towards water
Plant growth regulators/plant growth factors (plant hormones) coordinate plant growth. They
are produced in small quantities and are transported to the target cells by active transport,
diffusion and in the phloem and xylem.
Remember – Plant growth regulators are a specific shape – bind to complementary shaped
receptors on target cell plasma membrane
Phototropism
A shoot will grow towards a light stimulus because cells in the side of the shoot away from the light
elongate more rapidly than cells in the side facing the light.
e.g. cereal coleoptiles: the shoot of grass plants are called coleoptiles (a leaf enclosed by a
sheath)
1. Cells in the tip of the shoot produce indoleacetic acid (IAA), this belongs to a group of
substances called auxins. This is then transported down the stem.
2. Light on one side of the stem causes IAA to move to the shaded side, so a high concentration
builds up on the shaded side.
3. IAA causes cell elongation and there are more IAA on the shaded side the cells elongate more
and the shoot bends towards the light
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,Role of IAA
Hydrogen ions are actively transported into the cell wall by an ATPase enzyme on the plasma
membrane. The resulting low pH activates wall loosening enzymes which break bonds within the
cellulose so that the wall becomes less rigid/more stretchy and can expand as water moves into
the cell by osmosis.
Gravitropism
The response of a horizontally growing root to gravity:
IAA also controls the bending of the roots in response to gravity. Whereas in shoots a high
concentration of IAA increases cell elongation in the roots it inhibits cell elongation.
1. Cells in the tip of the root produce IAA, which is then transported along the root. Initially it is
transported to all sides of the root.
2. Gravity influences the movement of IAA from the upper to lower side of the root.
3. A greater concentration of IAA builds up on the lower side of the root.
4. IAA inhibits cell elongation so the cells on this side elongate less and the root bends down.
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, Responses in animals
Simple responses
Taxis (plural taxes)
Taxis is a change in directional movement in response to a stimulus. Positive taxis is moving
towards a stimulus and negative taxis is moving away from the stimulus. E.g. maggots move away
from the light so they are less visible and less likely to desiccate. Single celled algae move
towards light as they are photosynthetic organism.
Kinesis (plural kineses)
Kinesis is a change in the rate of movement (speed) of an animal in response to a stimulus. E.g.
woodlice move more quickly at random in dry conditions than moist conditions. Increasing the rate
of movement in unfavourable conditions increases the chance of moving away from the
conditions.
A choice chamber is a container with different compartments, in which different environmental
conditions can be created. It can be used to investigate how animals, such as woodlice or
maggots, respond to conditions such as light intensity or humidity in the laboratory.
To investigate light – cover one half with black paper
To investigate humidity – place damp filter paper on one side and a desiccating (drying) agent such as
anhydrous calcium chloride (NB Irritant) on the other.
Assess the risks
Use at least 10 simple animals, handle carefully for ethical reasons and wash hands after.
Repeat after moving the maggots back to the centre.
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