Survival and response (A-level only)
AQA
A-Level
A stimulus is a detectable change in the internal or external environment; a response is the change in behaviour or physiology that follows. Responding raises the chance of surviving long enough to reproduce. The general pathway involves a stimulus, receptor, coordinator, effector and response. Receptors are specific to one form of energy, such as a Pacinian corpuscle detecting pressure. Coordinators include the nervous system, hormonal systems, or plant growth factors. Effectors vary: animals use muscles or glands, whereas plants use elongating cells. Responses range from reflex withdrawals to taxes and kineses that keep invertebrates in favourable conditions.
24
Objectives
15
Exam Tips
25
Pitfalls
31
Key Terms
28
Mark Points
Subtopics in this area
Organisms increase their chance of survival by responding to changes in their environment.
In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.
The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.
Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment.
The protective effect of a simple reflex, exemplified by a three- neurone simple reflex. Details of spinal cord and dorsal and ventral roots are not required.
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.
Survival and response (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Sequence an unfamiliar animal response into stimulus, receptor, coordinator, effector and response.
- Explain how a faster response increases an organism's chance of surviving.
- Identify the different types of effectors in animals and plants.
- Give one example each of a response coordinated by nerves, by hormones and by a plant growth factor.
- State where IAA is produced in a flowering plant and where it exerts its effect.
- Explain how lateral redistribution of IAA converts a directional stimulus into unequal growth.
- Contrast coordination by plant growth factors with hormonal coordination in a mammal.
- Predict the effect on growth of removing the tip of a shoot, and justify the prediction.
- Explain positive phototropism in a shoot as a sequence from unilateral light to unequal elongation.
- Explain negative phototropism in roots, detailing how IAA accumulation on the shaded side inhibits elongation.
- Explain gravitropism in roots and shoots, detailing how high IAA concentration on the lower side affects elongation differently.
- Contrast the effect of high IAA concentrations on root cells and on shoot cells.
- Distinguish a taxis from a kinesis by the presence or absence of direction in the response.
- Name a taxis correctly using the stimulus and its sign for a given example.
- Explain how a kinesis keeps an organism in a favourable environment without directed movement.
- Interpret choice chamber data to decide whether an organism is showing a taxis or a kinesis.
- Sequence a three-neurone reflex arc from receptor to effector, naming each neurone.
- Explain why a reflex response is faster than a voluntary one, using the number of synapses.
- Describe the protective outcome of a named reflex, such as withdrawing a hand from a hot object.
- Identify which structures in a reflex pathway lie outside the specification and need not be described.
- Design a choice chamber investigation, stating the independent, dependent and controlled variables.
- Explain why a settling period is needed before the distribution of the animals is recorded.
- Apply a chi-squared test to choice chamber counts and state what the result shows.
- Evaluate a choice chamber method by identifying an uncontrolled variable and its effect on the results.
Marking Points
Key points examiners look for in your answers
- Defining a stimulus as a detectable change in the internal or external environment
- Identifying the general pathway: stimulus, receptor, coordinator, effector, response
- Linking a specific response to an increased chance of survival
- Identifying the correct effector for a given response, such as a muscle in animals or elongating cells in plants
- one mark for growth factors being produced in the growing regions, the meristems of shoot and root tips
- one mark for their movement to the zone of elongation just behind the tip
- one mark for lateral redistribution in response to a directional stimulus such as light or gravity
- one mark for unequal distribution causing unequal elongation and therefore bending
- Explaining that unilateral light or gravity causes the lateral redistribution of IAA to the shaded or lower side.
- Stating that a high concentration of IAA promotes cell elongation in shoots, causing upward or light-directed bending.
- Stating that a high concentration of IAA inhibits cell elongation in roots, causing downward bending in positive gravitropism.
- Explaining negative phototropism in roots by stating IAA accumulates on the shaded side, inhibiting elongation so the lit side grows faster.
- Explaining that gravity is sensed by cells, for example via statoliths, and triggers the active transport of IAA to the lower side, rather than gravity directly pulling IAA molecules.
- Deducing the direction of bending by identifying which side of the organ elongates more.
- Taxis and kinesis maintain mobile organisms in favourable environments to reduce predation or water loss.
- Taxis is a directional response involving movement towards or away from a stimulus.
- Kinesis involves changing the rate of movement or turning in response to a non-directional stimulus.
- Organisms turn more frequently or move faster in unfavourable conditions to return to favourable areas.
- one mark for the pathway receptor, sensory neurone, relay neurone, motor neurone, effector in the correct order
- one mark for the response being rapid, automatic and involuntary
- one mark for the pathway not involving conscious regions of the brain, so no decision is needed
- one mark for few synapses giving a short response time, since synapses are the slowest step
- one mark for the protective outcome, such as withdrawal from the stimulus limiting tissue damage
- identifying the independent variable as the environmental condition and the dependent variable as the number or position of animals
- allowing a settling period and then recording at set time intervals
- naming a controlled variable, such as temperature, number of animals, species or time of day
- repeating the investigation and calculating a mean, or rotating the apparatus to control the direction of light
- selecting a suitable statistical test, such as chi-squared for comparing observed with expected categorical distributions in a choice chamber, or a t-test for continuous maze time data
Examiner Tips
Expert advice for maximising your marks
- 💡Use the general pathway (stimulus, receptor, coordinator, effector, response) to structure answers about unfamiliar animal responses.
- 💡Always explicitly state how a specific response provides a survival advantage, such as avoiding predators or finding food.
- 💡Name the regions precisely: produced at the tip, acting in the zone of elongation behind it.
- 💡For any tropism, state where IAA accumulates, then what those cells do, then the direction of bending.
- 💡Use the term growth factor rather than hormone when writing about plants.
- 💡Always specify that it is a high concentration of IAA that inhibits root elongation, as roots respond differently to different doses.
- 💡Structure tropism answers by stating where IAA accumulates, its specific effect on elongation in that organ, and the resulting direction of bending.
- 💡Specify the stimulus and the sign, for example negative phototaxis or positive chemotaxis.
- 💡If data show rate of turning or speed, identify it as a kinesis; if showing direction, it is a taxis.
- 💡Write the five-part pathway as a labelled sequence; it earns marks even in a suggest question.
- 💡When asked why a reflex is fast, the mark is for few synapses and no conscious brain involvement, not simply that the neurones are fast.
- 💡Do not revise the dorsal and ventral roots, because the specification says they are not required.
- 💡Name the independent, dependent and at least two controlled variables explicitly when describing the method.
- 💡Chi-squared is the test for counts of animals in categories, and the null hypothesis is an equal distribution between compartments.
- 💡Consider adding a sentence on the ethical treatment of the animals, ensuring they are returned to their natural habitat unharmed.
Common Mistakes
Pitfalls to avoid in your exam answers
- Writing that organisms respond 'in order to survive', which implies intent rather than natural selection; instead, state that responses increase survival chances.
- Omitting the coordinator, making it appear as though the stimulus acts directly on the effector.
- Treating the receptor and the effector as the same structure, rather than distinct parts of a pathway.
- calling IAA a hormone produced by a gland, when plants have neither glands nor a circulatory system
- saying the tip bends, when it is the cells in the zone of elongation behind the tip that elongate
- claiming the growth factor is destroyed by light rather than redistributed away from it
- assuming the same concentration has the same effect in every tissue
- describing the response as growth towards a stimulus without naming cell elongation
- Assuming high IAA promotes elongation in roots as it does in shoots. Correction: remember that high IAA inhibits root cell elongation.
- Stating that cells divide more on one side. Correction: the correct mechanism is differential cell elongation, not mitosis.
- Describing IAA as being destroyed by light. Correction: IAA is redistributed to the shaded side rather than being broken down.
- Saying gravity pulls IAA to the lower side. Correction: gravity is sensed by cells, for example via statoliths, and this triggers the active transport of IAA to the lower side.
- Confusing a tropism (plant growth) with a taxis (movement in mobile organisms, including algae).
- Describing a kinesis as movement away from unfavourable conditions, which incorrectly implies direction.
- Naming a taxis without specifying the direction (e.g., omitting 'positive' or 'negative').
- including the brain as a stage in the reflex arc, when the reflex does not require conscious processing
- listing only two neurones, so the relay neurone is missing from a three-neurone arc
- calling the effector a reaction rather than naming a muscle or a gland
- saying impulses travel in both directions across a synapse, when neurotransmitter is released only from the presynaptic neurone
- spending answer space on the dorsal and ventral roots, which the specification explicitly excludes
- counting the animals immediately after adding them, which records where they were placed rather than their response
- changing two conditions at once, for example humidity and light, so the cause of the distribution is unknown
- using different numbers of animals in different runs, so the counts cannot be compared
- ignoring an external light source or a warm bench acting on the chamber from one side
- concluding that a small difference in numbers is meaningful without a statistical test