Response to stimuli

Taxes, Kineses and Tropisms

Pages 142 – 143

Syllabus 3.5.1

Survival and response

• Organisms increase their chance of survival by responding to changes in the environment. e.g. moving towards food / light or away from predators

• Those that survive have a greater chance of raising offspring and passing their alleles to the next generation.

Taxes and Kineses

• Taxes and kineses are simple forms of behaviour that rely almost entirely

on simple reflex actions

• Taxes and kineses involve the locomotion of whole organisms or cells in response to a

specific external stimulus

• The adaptive value of taxes and kineses is their role in helping to maintain organisms

in the most favourable environment for their survival

Taxis• A taxis or tactic response is the

movement of a whole organism or cell in response to the directiondirection of a

stimulus• Certain organisms may move directly towards or away from a specific stimulus; they may show positive (towards) taxis or

negative (away from) taxis

• Movement of an organism directly towards a light source is described as positive

phototaxis; other taxes include chemotaxis, hydrotaxis and aerotaxis

Chlamydomonas is a motile, photosynthetic protoctistan thatswims directly towards regions of optimum light intensity;

this species displays positive phototaxis

Scent-tracking in humans and maggots. (A) Tracking of a trail of chocolate essential oil (faint white line) by a human subject in an open 10 m × 10 m arena (corners marked by blue squares). The track (red line) represents 5 min 34 s. Image courtesy of J. Porter. (B) Chemotaxis of a Drosophila larva across an agarose surface towards 50 μl isoamyl acetate (banana odour) in the cap of a centrifuge tube. The track (red line) represents 20.7 cm covered over 3 min 50 s. Image courtesy of M. Louis.

Positive Chemotaxis

Male, silkworm moths display positive chemotaxis when they fly directly towards the chemical bombykol, secreted from the

abdominal regions of the female moth; the feathery antennae of the males contain millions of bombykol chemoreceptors

Male, silkworm moths display positive chemotaxis when they fly directly towards the chemical bombykol, secreted from the

abdominal regions of the female moth; the feathery antennae of the males contain millions of bombykol chemoreceptors

Dictyostelium chemotaxis to pipette with cAMP in it

This water flea swims directly towards low-intensity light and towards oxygen-rich environments; it displays positive

phototaxis and positive aerotaxis

Kinesis

A kinetic response is a non-directional response in which the rate of movement is

related to the intensity of the stimulus,but not its direction

It occurs where the stimulus is diffuse and from no defined direction

There are two aspects to kinesis:

• 1: the rate of movement changes in response to a stimulus

• 2: the rate of turning changes in response to a stimulus

Click in image for a video clip and explanation of ‘Kinesis’

Woodlice display kinetic behaviour when they encounter atmospheres of different humidity; in

drier environments they increase their rate of movement and decrease their rate of turning

Example of a tracing from a low humidity

chamber

Example of a tracing from a high humidity

chamber

Tropisms in plantsTropisms in plants

• Plants respond to a range of different stimuli that are all around them in their immediate environment. Their responses occur by means of growth.

• A tropism is a response to a directional stimulus.

Phototropism is a response to light.

Hydrotropism is a response to water.

Geotropism is a response to gravity.

Thigmotropism is a response to touch. ( Mimosa)( tendrils)

Positive or Negative Tropism?

Positive or Negative Tropism?

If the response is towards the stimulus the response is described as a positive tropic response and if the growth is away from the stimulus the response is described as a negative tropism.

For example, shoots are positively phototropic and roots are negatively phototropic.

However, shoots are negatively geotropic and roots are positively geotropic.

A Plant grown out-of-doors grows straight up.

B Plant grown in the dark develops a long, thin, spindly stem and yellow leaves. This condition is known as ‘etiolation’.

C Plant illuminated from one side curves towards the light.

Light

Light Dark

A B C

Demonstrating PhototropismDemonstrating PhototropismPhototropism can be easily demonstrated in broad beans by setting up three different conditions...

Using a Clinostat to Demonstrate PhototropismUsing a Clinostat to Demonstrate Phototropism

A A plant illuminated from only one side curves towards the light.

Light

B A plant illuminated from one side while on a rotating clinostat is evenly illuminated, and so grows straight up.

Clinostat

Light

The significance of the direction of the light can also be easily demonstrated in any plant by setting up two different conditions...

These seedlings have been grown on a clinostat that is rotating once every 15 seconds. Although auxin still "tries" to concentrate on the bottom of the stem because the stem is rotating it is distributed all around the stem so the stems grow horizontally. So this experiment nullifies the effect of gravity.

To do

• Answer summary questions page 143

• Exam style Question 1 page 153

• Complete 2 worksheets on taxis and kinesis

• Woodlice are invertebrates belonging to the phylum Arthropoda, the class

Crustacea and the order Isopoda

• Woodlice are terrestrial species that lack many of the water-conserving adaptations found among

other members of their phylum

• Woodlice display a relatively high surface area to volume ratio and lack a waxy, waterproof cuticle

covering their exoskeleton – their primitive respiratory surfaces (a pair of ‘pleopod lungs’) open to the atmosphere through a permanently

open pore• Woodlice display patterns of behaviour aimed at

limiting water loss and promoting their survivalin a range of terrestrial habitats

Students investigated the behaviour of woodlice when given a choice between dry

and humid conditions

Twenty woodlice were introduced into a choice

chamber that provided them with a choice between dry and humid atmospheres

The distribution of woodlice was recorded after a period of

three minutes and the investigation was repeated to

include six trials

The choice chamber consisted of two halves, with distilled water introduced into one side and calcium chloride

(a drying agent) into the other

distilled water calcium chloride

In this investigation, tactic responses are being observed as the woodlice demonstrate their preference for a particular

environment by movement towards or away from a stimulus (level of humidity)

The results of this investigation (six trials) areshown in the following slide

distilled water calcium chloride

State a Null and Alternative Hypothesis for this investigation and analyse the results statistically using the Chi-square test

Use these results to determine the total number of woodlice, found in each environment, after

the three minute period

Table of critical values of chi square at different levels of P

Degrees of Freedom Probability, p

  0.99 0.95 0.05 0.01 0.001

1 0 0.004 3.84 6.64 10.83

2 0.02 0.103 5.99 9.21 13.82

3 0.115 0.352 7.82 11.35 16.27

4 0.297 0.711 9.49 13.28 18.47

5 0.554 1.145 11.07 15.09 20.52

6 0.872 1.635 12.59 16.81 22.46

7 1.239 2.167 14.07 18.48 24.32

8 1.646 2.733 15.51 20.09 26.13

9 2.088 3.325 16.92 21.67 27.88

10 2.558 3.94 18.31 23.21 29.59