Survival & Response (AQA A-Level Biology): Revision Notes
Survival & Response
Organisms must detect and respond to changes in their environment to survive and reproduce. This ability to respond appropriately to stimuli gives organisms significant survival advantages and increases their chances of passing genes to future generations.
What are stimuli and responses?
A stimulus represents any detectable environmental change, whether internal or external, that triggers a reaction in living organisms. A response is the organism's reaction to this stimulus. This stimulus-response system is essential for survival because it allows organisms to detect beneficial conditions (like food sources) and avoid harmful situations (such as predators or extreme temperatures).
The ability to respond effectively to environmental changes creates selection pressure - organisms with better responses survive longer and reproduce more successfully. These individuals pass their beneficial alleles to offspring, leading to evolutionary advantages over time.
The response pathway
All biological responses follow a coordinated sequence:
Stimulus → Receptor → Coordinator → Effector → Response
Receptors detect specific types of stimuli and are highly specialised for particular environmental changes. A coordinator (such as the brain or hormone systems) processes the information and determines appropriate action. Effectors (muscles, glands, or other organs) carry out the response.
Coordination occurs through two main systems: the nervous system (rapid electrical signals) and hormones (slower chemical messengers). The nervous system provides quick responses through networks of receptors, coordinators, and effectors, while hormonal coordination offers longer-term responses throughout the organism.
Types of response
Taxes
A taxis describes directional movement responses where organisms move either towards or away from a stimulus source. The movement direction depends entirely on the stimulus direction, making this response highly predictable.
Taxes are classified based on movement direction:
- Positive taxis: movement towards the stimulus
- Negative taxis: movement away from the stimulus
The response type is named after the stimulus involved:
Phototaxis involves responses to light. Single-celled algae demonstrate positive phototaxis by moving towards light sources. This behaviour increases survival chances because these photosynthetic organisms require light energy to manufacture food through photosynthesis.
Chemotaxis describes responses to chemical gradients. Many bacteria exhibit positive chemotaxis towards glucose-rich areas, improving their access to this vital energy source.
Example: Negative Phototaxis in Earthworms
Earthworms show negative phototaxis, moving away from light into soil where they can:
- Conserve moisture in their bodies
- Locate food more easily in organic matter
- Avoid surface predators that hunt by sight
This response provides clear survival benefits in their underground habitat.
Kineses
A kinesis represents a non-directional response where organisms change their movement patterns rather than moving towards or away from stimuli. Instead of directional movement, organisms alter their speed and frequency of direction changes.
This response works through a simple mechanism: when organisms encounter unfavourable conditions, they increase both movement speed and turning frequency. This behaviour increases the likelihood of quickly leaving unfavourable areas. Conversely, in favourable conditions, organisms slow down and turn less frequently, helping them remain in beneficial environments.
Example: Woodlice Kinesis Response
Woodlice lose moisture rapidly in dry conditions. When moving from damp to dry areas, they:
Step 1: Detect unfavourable conditions - sense decreased humidity
Step 2: Accelerate movement - increase speed to leave dry area quickly
Step 3: Increase turning frequency - change direction more often to search randomly
Step 4: Detect favourable conditions - sense increased humidity
Step 5: Reduce activity - slow down and turn less to remain in damp area
This prevents dehydration and improves survival chances.
Kineses work particularly well when stimuli lack clear directional gradients - situations where humidity and temperature changes occur gradually rather than sharply.
Tropisms
A tropism describes growth responses in plants where specific plant parts grow towards or away from directional stimuli. Unlike animal movement responses, tropisms involve permanent growth changes rather than temporary position adjustments.
Tropisms are classified by stimulus type and growth direction:
- Positive tropism: growth towards the stimulus
- Negative tropism: growth away from the stimulus
Phototropism involves growth responses to light direction. Plant shoots typically show positive phototropism, growing towards light sources to optimise photosynthesis. Plant roots often demonstrate negative phototropism, growing away from light into darker soil regions.
Gravitropism describes growth responses to gravity. Shoots generally exhibit negative gravitropism, growing upwards against gravity to reach sunlight effectively. Roots show positive gravitropism, growing downwards with gravity to access water and mineral resources in soil.
These growth responses significantly improve plant survival by ensuring shoots reach optimal light conditions for photosynthesis while roots access essential water and nutrients below ground.
Key Points to Remember:
- Stimuli are environmental changes that trigger responses, providing survival advantages through natural selection
- The response pathway follows: stimulus → receptor → coordinator → effector → response
- Taxes are directional movements towards or away from stimuli (like algae moving towards light)
- Kineses involve changing movement speed and turning frequency rather than directional movement (like woodlice behaviour in dry conditions)
- Tropisms are plant growth responses to directional stimuli (like shoots growing towards light and roots growing towards gravity)