3.6.1.1. survival and response 12/11/2016

Survival and response

Prior knowledge from GCSE

What is the function of the nervous system?

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What is a reflex action?

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What are the three types of neurones involved in a reflex action?

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Fill in the gaps:

stimulus / / / / response

Plants respond to ………………………. ……………………………… and ……………………………….

Their shoots grow towards ………………………………. and against the force of ………………………………. Their roots grow towards ……………………………. and in the direction of the force of ………………………………

Growth towards light is ……………………………… ……………………………………

Growth in the direction of gravity is …………………………………. ……………………………..

These responses are controlled by a …………………………… called ……………………………..

They are caused by an …………………………….. distribution of this hormone.

Stimulus and response

Use chapter 14.1 to help you to write definitions for the following words:

Stimulus……………………………………………………………………………………………………………………

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Receptor.…………………………………………………………………………………………………………………

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Coordinator..……………………………………………………………………………………………………………

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Effector……………………………………………………………………………………………………………………

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Response…………………………………………………………………………………………………………………

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How does the ability to respond to stimuli increase the chances of survival for an organism? Give some examples:

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Taxes

A taxis (plural taxes) is a directional response to a directional stimulus.

Taxes are common in invertebrates, and even motile bacteria and protoctists show taxes.

The taxis response can be positive (towards the stimulus) or negative (away from the stimulus).

Common stimuli include:

• light (phototaxis)

• gravity (geotaxis)

• chemicals (chemotaxis)

• movement (rheotaxis)

Kineses

A kinesis (plural kineses) is a response to a changing stimulus by changing the amount of activity.

Sometimes the speed of movement varies with the intensity of the stimulus (orthokinesis) and sometimes the rate of turning depends on the intensity of the stimulus (klinokinesis).

The response is not directional, so kinesis is a suitable response when the stimulus isn’t particularly localised. The end result is to keep the animal in a favourable environment. For example woodlice, who breathe using gills, use kinesis to stay in

damp environments, where they won’t dry out.

See page 327 for more details on this response.

Are these examples taxis or kinesis?

Maggots moving away from light to avoid dessication ……………………………………

Earthworms burrow down underground………………………………………

Flatworms turn more frequently in response to increasing light……………………..

Male moths will fly towards a pheromone released by female moths……………………

Salmon swim upstream to reach spawning grounds…………………………

Lampreys are more active in high light intensity………………………………….

Plant Responses

Plants don’t have nervous or endocrine systems, but they can sense and respond to stimuli. For example plants can grow towards the light, their roots can grow towards water and they can flower at certain times of the year. Many of these responses are directional growth responses, called tropisms.

Tropisms can be ……………………………… (growing towards the stimulus) or …………………………….. (growing away from the stimulus) and occur in response to a variety of stimuli:

Name of tropism / Stimulus / Examples
Phototropism / Young shoots (……………..)
Roots (…………………)
Gravity
Hydrotropism
Chemicals / Pollen tube
Thigmotropism / Touch vines, bindweed

Plant growth factors

Plant responses involve hormone-like substances called plant growth factors.

An example of a plant growth factor is IAA (indoleacetic acid) which belongs to a group of substances called auxins, and controls plant cell elongation.

Phototropism in flowering plants

The following description of the response of shoots of flowering plants to light comes from page 328 of your textbook. Illustrate each point with an appropriate diagram.

1)  Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
2)  The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
3)  Light causes the movement of IAA from the light side to the shaded side of the shoot.
4)  A greater concentration of IAA builds up on the shaded side of the shoot than on the light side.
5)  As IAA causes elongation of shoot cells and there is a greater concentation of IAA on the shaded side of the shoot, the cells on this side elongate more.
6)  The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light.

Gravitropism in flowering plants

The following description is from page 329 of your textbook. Illustrate each statement with an appropriate diagram, as before:

1)  Cells in the tip of the root produce IAA, which is then transported along the root.
2)  The IAA is initially transported to all sides of the root.
3)  Gravity influences the movement of IAA from the upper side to the lower side of the root.
4)  A greater concentration of IAA builds up on the lower side of the root than on the upper side.
5)  As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side.
6)  The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.
Further reading – exam questions on this topic often refer to the classic experiments carried out by Charles Darwin and others into the role of IAA in tropisms. Work through the Application section in your textbook which starts on page 331.


Reflex arc

Use page 334 to help you write definitions for the following:

CNS…………....……………………………………………………………………………………………………………

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PNS…….……………………………………………………………………………………………………………………

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Sensory neurones……………………………………………………………………………………………………

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Motor neurones…………………………………………………………………………………………………………

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Intermediate neurones……………………………………………………………………………………………..

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Voluntary nervous system…………………………………………………………………………………………

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Autonomic nervous system………………………………………………………………………………………

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The three types of neurones (sensory, intermediate and motor) are arranged in circuits and networks, the simplest of which is the reflex arc.

In a simple reflex arc, such as the knee jerk, a stimulus is detected by a receptor cell, which synapses with a sensory neurone.

The sensory neurone carries the impulse from the site of the stimulus to the central nervous system (the brain or spinal cord), where it synapses with an intermediate neurone.

The intermediate neurone synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting.

A reflex arc can also be represented by a simple flow diagram:

Why are reflex arcs important? Summarise the points on page 336 here:

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Further reading and questions:
See chapters 14.1, 14.2 and 14.3
Complete summary questions.

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