Responses

Taxes and Kineses

Organisms must respond to changes in their environment (stimuli) so that they can increase their chances of survival by avoiding predation and abiotic stress, or by increasing access to essential resources. Animals and mobile organisms including some bacteria can physically move away from unfavourable or dangerous environments. They can also move towards more favourable conditions. Plants cannot physically move themselves, but they can change how they grow in order to try and improve their environmental conditions. 

Organisms must also respond to changes in their internal environment in order to maintain stable internal conditions which are optimised for them to carry out chemical reactions (homeostasis). 

Types of Response

There are two types of simple (automatic) response that can occur in mobile organisms to keep them in a favourable environment: Taxes and Kineses. These movements are not choices made by the organism they are automatic responses to stimuli. The prefix before these indicates the type of stimulus. E.g photo = light

Taxis	Kinesis
A tactic response is directional. The direction of the stimulus determines the direction of the movement in response. If an organism moves towards the stimulus it is a positive response, if it moves away it is a negative response.	A kinetic response is random (non-directional). The intensity of the stimulus affects the speed of the movement in response.
Examples:	Examples:
Phototaxis in woodlice – woodlice show negative phototaxis which means that they will move away from a light source. This behaviour may be helpful because woodlice require a moist environment, and a sunny, light, spot is more likely to be warm and dry.	Orthokinesis in woodlice – When an organism is in a favourable environment it slows down, and when in an unfavourable environment, it speeds up. Overall, this leads to an organism spending more time in the favourable environments and less in the unfavourable. This can be seen with woodlice and humidity.
Chemotaxis in bacteria – It is the movement of an organism in response to a chemical stimulus. This is important for bacteria to find food (e.g. positive chemotaxis for glucose) by swimming toward the highest concentration of food molecules, or to flee from poisons (e.g. negative chemotaxis for phenol).	Klinokinesis in flatworms – When an organism is in an unfavourable environment the frequency or rate of turning increases with stimulus intensity. For example, the flatworm Dendrocoelum lacteum turns more frequently in response to increasing light intensity thus ensuring that it spends more time in dark areas which are more likely to be moist and easier to hide from predators.
Chemotaxis in phagocytes – Phagocytes move towards pathogens as they are attracted to chemicals released by the pathogen such as histamines. They show positive chemotaxis by moving down the concentration gradient towards the pathogen. They are then able to squeeze through holes in the capillary endothelium into the infected tissues to begin engulfing pathogens.	Chemokinesis in cells – It is a movement response of organisms to chemicals that cause the cell to make some kind of change in their migratory/swimming behaviour. Changes involve an increase or decrease of speed, alterations of amplitude or frequency of movements, or direction of migration. However, in contrast to chemotaxis, chemokinesis is random.
Rheotaxis in fish – positive rheotaxis keeps trout facing the current in rivers as this is the direction from which the most food will come it also increases the rate of flow of oxygenated water over their gills. These both lower the amount of energy the fish needs to use.	Photokinesis in photosynthetic prokaryotes –It is when organisms change the speed of their movement in response to changes in light intensity. The change in speed is unrelated to the direction of the light. This may help organisms avoid predation (not staying in the light too long), or in single celled photosynthetic organisms, (e.g Euglena) an increase in light intensity results in increased photosynthesis which enables an increase in metabolic activity.

Investigating Animal Responses- Required practical 10: Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze. Apparatus and technique to be assessed: 

ATh - Safely and ethically use organisms to measure: plant or animal responses or physiological functions.

It is important that living organisms are treated with respect and nothing is done that could harm them or cause them to suffer. Using soft and plastic implements like plastic spoons and soft paintbrushes to move and manipulate the organisms prevents them from being injured. Animals should be returned promptly to their natural environment or a suitable holding tank after being tested. This supports ethical approaches in field work where animals are returned to their habitat after observations have been made.

Hazards/Risks:

Take sensible hygiene precautions after handling organisms and the soil in which organisms have been kept. If using drying agent take precautions as they can irritate eyes and skin. Anhydrous calcium chloride can heat up with water. 

Choice chamber with maggots or woodlice

Independent variables (stimuli) you can create in the chamber could be two or four conditions: 

• Light intensity – cover half the chamber with black paper and use a lamp on the other side. 

• Humidity – place wet paper towels or cotton wool on one side and use a drying agent e.g silica gel or anhydrous calcium chloride on the other side which absorbs moisture. 

Control Variables

• Number of animals in the chamber each time

• Where animals are placed into the chamber e.g middle every time

• Time allowed for animals to choose before recording numbers

Possible Predictions

When in unfavourable environments the organisms move faster and change direction more frequently to increase their chances of finding more suitable surroundings so after the allotted time more organisms will be found in the favourable environment than less favourable. 

Woodlice and maggots demonstrate negative phototaxis so they should move away from areas of higher light intensity into darker areas. This means that after the allotted time more organisms would be expected to be found in the dark/shaded side of the choice chamber.

Method:

1. Set up the choice chamber as a control with nothing but the muslin. 

2. Place 10 organisms through the central hole using a plastic spoon and/or soft paintbrush

3. Leave them for 10 minutes then record the number of organisms in each half/quadrant of the chamber. 

4. Set up the choice chamber to have four quadrants as follows: dark and dry, dark and damp, light and dry, light and damp. 

5. Place 10 organisms in the centre of the choice chamber using the spoon. 

6. Leave for 10 minutes then record how many woodlice are in each quadrant. Repeat and take a mean.

Data Analysis: Chi Squared Test

A statistical test (Chi squared) can be used to determine if the observed distribution of the organisms at the end of the time is different from the expected (which would be no difference in the number of organisms in each zone). This will tell you if your results were likely due to chance or not. If not (P >0.05) then something is causing them to behave this way. 

Example:

To find whether or not a chi-squared of 3.6 is significant it is compared to the numbers found in a chi-squared table at the 95% confidence level (p=0.05).

Because there are only 2 conditions the number of degrees of freedom is 2-1 = 1. 

If the calculated value for χ² is greater than or equal to the critical value (found from the table below), then we can reject the null hypothesis and because there is a less than 5% probability that the difference between the observed and expected values are due to chance.

However, if the calculated value for χ² is less than the critical value, then we accept the null hypothesis because there is a more than 5% probability that the difference between the observed and expected values are due to chance.

Critical value for 0.05 in the table is 3.84, 3.6 is less than 3.84 so we accept the null hypothesis because there is a more than 5% probability that the difference between the observed and expected values are due to chance. So there is no significant difference between the observed and expected number of woodlice in each environment here. 

REMEMBER: You won’t be asked to calculate χ² in the exam but you may be asked to look up critical values and explain results.

Plant Responses

Plants change the way they grow in response to changes in their environment in order to increase their chances of survival they show responses similar to taxes and kineses. Plant tactic responses are known as tropisms – they either result in growth towards (positive) or away (negative) from a directional stimulus. 

Examples (also include chemotropism (chemical stimulus) and hydrotropism (water stimulus) in roots): 

Phototropism The growth response of a plant to light. Shoots are positively phototropic and grow towards the light. They are able to sense the direction of light and grow towards it in order to maximise light absorption for photosynthesis. Roots are negatively phototropic and grow away from light. This makes sure the grow downwards into soil.		Shoots = positive, Roots = negative
Gravitropism (geotropism) The growth response of a plant to gravity. Shoots are negatively gravitropic and grow upwards. This ensures they are more likely to grow out of the ground where there will be more light.  Roots are positively gravitropic and grow downwards. This ensures they grow deeper into the ground in order to get more water and mineral ions.		Shoots = negative, Roots = positive
Thigmotropism Climbing plants have a sense of touch, when they sense an object their growth changes to curl around the object which is touching the plant. This allows plants to climb up above others to receive more light for photosynthesis. Specialised shoots called tendrils are positively thigmotropic. Roots tend to grow away from objects they touch so they are negatively thigmotropic.

Extra Info: Plants also have nastic responses which are similar to kineses as they are non-directional responses to stimuli. The rate or frequency of these responses increases as intensity of the stimulus increases. They can be growth responses or changes in turgor pressure to swell or shrink tissues. 

Examples include: flowers closing at night in response to temperature, the shutting of venus flytrap leaves in response to an insect landing on it, leaves bending toward the stem in plants that “sleep” at night. 

Plant Growth Factors

Plants do not have a nervous system so their responses to stimuli are controlled by growth factors which are hormone-like chemicals that speed up or decrease plant growth. These growth factors are produced in meristems at the tips of roots and shoots where growth (cell division) is happening, but they travel to where they are needed in other parts of the plant between cells by diffusion or active transport or in the phloem over long distances. 

There are different classes of growth factors including: auxins which promote cell elongation, gibberellins which promote seed germination and flowering, abscisic acid which inhibits seeds germination and causes closing of stomata and ethene which is a gas that promotes ripening of fruit. 

Auxin Example: IAA – Indoleacetic acid 

IAA is an auxin that is produced in shoots and roots of flowering plants but is transported to different parts of the plant to control tropisms. This means that distribution of IAA is uneven as different parts of the plant will be growing in different ways in response to directional stimuli. 

Phototropism in Roots - IAA moves to the more shaded side of roots. Increased concentration of IAA on the shaded side inhibits growth so the unshaded side elongates more, so the root bends away from the light.	Phototropism in Shoots - IAA moves to the more shaded part of shoots. Increased concentration of IAA on the shaded side promotes growth, causing those cells to elongate so the shoot bends towards the light.
Gravitropism in Roots – same IAA moves to lower side of roots and growth is inhibited	Gravitropism in Shoots – same IAA moves to lower side of shoots and cells elongate

 Investigating Plant Responses

Various experiments have been carried out over the years to demonstrate that the stimulus of light is detected by the shoot tip, which is where the growth factors are produced and that these are then transported down and across to the shaded side in order to make the shoots bend towards the light. This was discovered by doing the following treatments: 

• Removing the tip of the shoot – therefore removing the source of IAA

• Covering the tip of the shoot – no light can be detected by receptors in the shoot tip so IAA is distributed evenly. Shoots will grow straight up and fast in darkness or when they can’t detect light in order to increase their chances of finding light. 

• Cutting the tip but replacing it on top of a substance that allows diffusion. This demonstrates that the tip is required for IAA to be produced and the substance must be able to diffuse down in order to have uneven growth. When this is done with an insoluble barrier the shoot does not bend. 

• These experiments were then developed to look at how distribution of the IAA effects growth. Removing the tip and replacing it offset shows bending of shoot in the dark. In the light the IAA would move to the shaded side so we would expect it to bend towards the light, but there is no uneven distribution if there is no light.  

  Removing the tip allows IAA to diffuse into an agar block and this block can be replaced on its own to just be a source of IAA that can be controlled and to show that distribution of IAA is due to light sensing cells in the shoot tissue. 

In any experiments in the dark the plants must be provided with some nutrients in order to ensure growth continues as photosynthesis cannot occur. In agar experiments glucose can be added to the block in order to provide an energy source to make sure this does not affect growth. 

Controls should be used to make sure that it is only IAA causing observed effects. Making sure a negative control is used e.g tip removed and agar soaked in water added. 

Other effects of plant growth factors

Auxins also prevents lateral buds growing below the tip, this is known as apical dominance. It ensures the plant focuses energy on growing upwards towards the light in order to make sure it outcompetes other plants. The further away from the meristem tip the inhibition of auxins decreases as the concentration lowers. If the tip is removed then more lateral buds will grow as the source of auxins has been removed so the concentration drops and inhibitor is removed. 

Synthetic auxins

Synthetic auxins were discovered and have been used as herbicides (weed-killers) since the 1940’s. Auxins specifically target broadleaf plants, which make it effective for killing particular weeds with a low risk of killing non-target crops because most weeds have broader leaves than grass or wheat, the weedkiller is absorbed in larger quantities by the weeds. These are therefore often known as selective weedkillers. 

The selective weedkiller contains a synthetic auxin that causes the weeds to grow too quickly and die. The killing action of synthetic auxins is not caused by any single factor but rather by the disruption of several growth processes e.g cell wall elasticity, nucleic acid metabolism, protein synthesis, cell division and growth.