AP Syllabus focus:
‘Organisms exchange information in response to internal changes and external cues, leading to changes in behavior.’
Organisms constantly gather and share information to match their behavior to changing conditions. These responses integrate internal state (physiology) with external cues, improving survival and reproductive opportunities in variable environments.
Core idea: information → processing → behavior
Information exchange can occur within an organism (among cells/tissues) or between organisms. In both cases, a stimulus is detected, interpreted, and translated into a behavioral response that can be immediate or long-term.
Stimulus: Any internal change or external cue that can be detected by receptors and that triggers a response.
Two sources of information
Internal changes: hunger, dehydration, injury, infection, hormonal state, temperature, blood glucose, osmolarity.
External cues: light levels, temperature shifts, predator presence, food availability, toxins, social context (presence of mates, rivals, group members).
From cue detection to action
Sensory detection and receptor specificity
Receptors detect particular kinds of cues and convert them into biological signals.
Labeled diagram comparing rod and cone photoreceptors and their position within retinal tissue. It highlights how receptor structure (outer segment packed with photopigment vs. inner segment with organelles) supports stimulus transduction, illustrating receptor specificity for light cues. Source
Sensory receptors (e.g., chemoreceptors, mechanoreceptors, photoreceptors) initiate neural signals when activated.
Cell-surface receptors in tissues bind chemical messengers (e.g., hormones), changing cell activity and potentially altering behavior.
Cue: A piece of information from the environment or the body that can be detected and used to guide a behavioral response.
Integration by nervous and endocrine systems
The nervous system supports rapid responses (milliseconds to seconds), often via reflexes or fast decision-making circuits.
The endocrine system supports slower, longer-lasting modulation (minutes to days), changing motivation, activity level, or responsiveness to stimuli.
A key feature is integration: the same external cue can produce different behaviors depending on internal state. For example, food odor may trigger strong searching behavior when energy stores are low but little response when satiated.
Information exchange between organisms
Many behaviors depend on information produced by other organisms, which can alter receiver behavior.
Alarm cues can trigger freezing, fleeing, or aggregation.
Territorial cues can reduce direct conflict by shaping spacing and movement.
Social cues can coordinate group timing (e.g., synchronizing activity or migration departure).
These exchanges often function as signals when they evolved specifically to change another organism’s behavior, but organisms can also respond to incidental cues (e.g., rustling indicating a predator).
Signal: Information produced by one organism that affects the behavior of another organism and that has been shaped by natural selection for communication.
Behavioral outputs commonly shaped by cues
Orientation and movement
Kinesis: change in activity level in response to stimulus intensity (no specific direction).
Taxis: directed movement toward or away from a stimulus source (e.g., moving toward light or away from a chemical).
Chemotaxis demonstration showing motile bacteria accumulating near the opening of a capillary containing an attractant (e.g., glucose/amino acids). The figure visualizes taxis as directional movement guided by a stimulus gradient, contrasting with nondirectional changes in activity seen in kinesis. Source
Foraging and risk management
Trade-offs between resource acquisition and predator avoidance can shift dynamically with cues (predator odor, cover availability) and internal condition (energy reserves).
Timing and rhythms
Organisms use environmental cues (especially light and temperature) to align behavior with predictable cycles, such as daily activity timing. These cue-driven adjustments can coordinate feeding, sheltering, and social interactions.
Key takeaways for AP Biology
Information exchange can be triggered by internal changes and external cues, and it often changes behavior in ways that improve survival.
Responses depend on detection, integration, and output, frequently involving both neural and hormonal control.
The same cue can produce different behaviors depending on internal state, showing that behavior is context-dependent and regulated.
FAQ
Differences in internal state (e.g., hunger, reproductive status, stress hormone levels) can shift decision thresholds.
Genetic variation and prior experience can also tune sensitivity of receptors and neural circuits.
A chemical binds a receptor, initiating a signalling cascade that changes ion flow or gene expression.
This can modify neuron firing patterns or hormone release, ultimately altering motor output.
Sensory adaptation can reduce receptor responsiveness during persistent stimulation.
Neural filtering can prioritise novel or rapidly changing cues over constant ones.
Clock networks can weight cue timing and duration, responding most strongly during sensitive phases.
Brief or mistimed cues may have little effect due to gating within clock circuitry.
They test whether the producer’s trait increases in frequency due to effects on receivers.
Experiments may alter the trait and measure consistent receiver responses and fitness consequences.
Practice Questions
Describe how internal changes and external cues can each lead to a behavioural response in an organism. (2 marks)
Internal change described as a stimulus that alters behaviour (1)
External cue described as a stimulus from the environment that alters behaviour (1)
Explain how information exchange can lead to changes in behaviour, including detection of a cue, processing of information, and the resulting behavioural response. Include one example of context dependence (internal state affecting response). (6 marks)
Cue/stimulus detected by receptors/sensory system (1)
Transduction to neural and/or hormonal signalling (1)
Integration/processing by nervous system and/or endocrine pathways (1)
Output: behaviour changes (movement, foraging, avoidance, timing, etc.) (1)
Information exchange between organisms can modify receiver behaviour (1)
Context dependence stated: same cue produces different response depending on internal state (1)
