Response to stimuli

Question 1

t is stimulus and response?

Answer 1

• a stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism
• the ability to respond to stimuli is a characteristic of life and increases the chances of survival for an organism
• for example, to be able to detect and move away from harmful stimuli, such as predators and extremes of temperature, or to detect and move towards a source of food clearly aid survival
• those organisms that survive have a greater chance of raising offspring and of passing their alleles their alleges to the next generation
• there is always therefore a selection pressure favouring organisms with more appropriate responses
• stimulus are detected by receptors
• receptors are specific to one type of stimulus
• a coordinator formulates a suitable response to a stimulus
• coordination may be at the molecular level or involve a large organ such as the brain
• a response is produced by an effector
• this response may be at the molecular level or involve the behaviour of a whole organism
• one means of communication in large, multicellular organisms occurs via chemicals called hormones, which is a relatively slow process found in both plants and animals
• in addition to hormonal communication, animals have another, more tepid, means of communication - the nervous system
• their nervous systems usually have many different receptors and control effectors
• each receptor and effect or is linked to a central coordinator of some type
• the coordinator acts like a switch board, connecting information from each receptor with the appropriate effector
• the sequence of events can therefore involve either chemical control or nerve cells

Question 2

What are taxes?

Answer 2

• a taxis is a simple response whose direction is determined by the direction of the stimulus
• as a result, a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one
• taxes are classified according to whether the movement is towards the stimulus which is a positive taxis or away from the stimulus which is a negative taxis and also by the nature of the stimulus

Question 3

What are examples of taxes?

Answer 3

• single-celled algae
• earthworms
• some species of bacteria

Question 4

How are single-celled algae taxes?

Answer 4

• single-celled algae will move towards light which is positive phototaxis
• this increases their chanced of survival since, being photosynthetic, they require light to manufacture their food

Question 5

How are earthworms taxes?

Answer 5

• earthworms will move away from light which is negative phototaxis
• this increases their chances of survival because it talked them into the soil, where they are better able to conserve water find food and avoid some predators

Question 6

How are some species of bacteria taxes?

Answer 6

• some species o bacteria will move towards a region where glucose is more highly concentrated which is called positive chemotaxis
• this increases their chances of survival because they use glucose as a source of food

Question 7

What is kineses?

Answer 7

• a kinesis is a form of response in which the organism does not move towards or away from a stimulus
• instead it changes the speed at which it moves and the rate at which it moves and the rate at which it changes direction
• if an organism crosses a sharp dividing line between a favourable and an unfavourable environment, its rate of turning increases
• this raises its chances of a quick return to a favourable environment
• however if it moves a considerable distance into an unfavourable environment its rate of turning may slowly decrease so that it moves in long straight lines before it turns often sharply
• this type of response tends to bring the organism into a new region with favourable conditions
• it is important when a stimulus is less directional
• humidity and temperature for example do not always produce a clear gradient from one extreme to another
• an example of kinesis occurs in woodlice
• woodlice lose water from their bodies in dry conditions
• when they move from a damp area into a dry one, they move more rapidly and change direction more often
• this increases their chance of moving back into the damp area
• once back in the damp area, they slow down and change direction less often
• this means they are more likely to stay within the damp area
• however if after some time spent changing direction rapidly they are in the damp area, their behaviour changes
• instead of moving through the dry area and into a new damp one
• in this way they spend more time in favourable damp conditions than in less favourable drier ones
• this prevents them drying out and so increases their chances of survival

Question 8

What is tropism?

Answer 8

• a tropism is the growth of part of a plant in response to a directional stimulus
• in almost all cases the plant part grows towards a positive response or away from negative responses of the stimulus
• again, the type of response is named after the stimulus

Question 9

What are examples of tropics?

Answer 9

• plant shoots grow towards light which is positive phototropism and towards gravity which is negative gravitropism so that their leaves are in the most favourable position to capture light for photosynthesis
• plant roots grow away from light which is negative phototropism and towards gravity which is positive gravitropism
• in both cases the response increases the probability that roots will grow into the soil, where they are better able to absorb water and mineral ions