Unit 1 · Biological Bases of Behavior
The Complete AP Psychology Guide · AP PsychologyUnit 1 · Biological Bases of Behavior
- The Complete AP Psychology Guide
- AP Psychology
- 5 sections
Unit 1: Biological Bases of Behavior accounts for 15% to 25% of the AP Psychology exam score in Section I (multiple choice), and its mechanisms also appear in Section II free-response questions. A pounding heart can be read three ways on this exam: as a nervous-system division doing its job, as a chemical message crossing a synapse, or as one data point in a sleep study.
- How AP Psychology assesses this 15–25% of the Section I score · mechanisms recur in Section II FRQs · 75 MCQs in 90 min
- Key skills interaction of heredity and environment, overview of the nervous system, sensation, sleep
- How to study for Unit 1 By the end of this page you can route any biological scenario to the right division, structure, cell, or chemical — and read the trace or record that measures it.
- The organizing decision Which level of the biological system — division, structure, cell, or chemical — does the scenario actually describe, and what evidence would show that level at work?
Unit 1: Biological Bases of Behavior accounts for 15% to 25% of the AP Psychology exam score in Section I (multiple choice), and its mechanisms also appear in Section II free-response questions.
Weighting is the College Board CED range for the current exam. Higher-weight units repay proportionally more review time; use this share to size your effort before drilling the mechanisms below. See the AP Psychology exam guide & review for all five units.
A pounding heart can be read three ways on this exam: as a nervous-system division doing its job, as a chemical message crossing a synapse, or as one data point in a sleep study. Unit 1 is where behavior gets a physical address.
By the end of this page you can route any biological scenario to the right division, structure, cell, or chemical — and read the trace or record that measures it.
The decision that organizes this unit
Ask where the signal lives before naming it
Which level of the biological system — division, structure, cell, or chemical — does the scenario actually describe, and what evidence would show that level at work?
Most Unit 1 traps put the right word at the wrong level. Fix the level first and the terms sort themselves.
Mechanism route and repair branches
Step 1 · Map the divisions before the scenario moves
Route it: central (brain plus cord) or peripheral; somatic for willed movement or autonomic for self-running regulation; then the mobilizing or restoring arm. Reflex arcs complete at the cord.
- Diagnostic cue: Reaching for a cup is routed through the autonomic branch, or a racing heart through the somatic branch.
- Wrong branch: The peripheral branches are assigned by how dramatic the behavior feels, not by whether it is willed.
- Repair: Ask first: skeletal muscle under voluntary command, or organs and glands on their own? Willed is somatic, automatic is autonomic — only then pick an arm.
Step 2 · Follow one neuron to its firing decision
Graded inputs sum toward threshold; past it, the action potential runs all-or-none. A refractory pause caps the rate; intensity is carried by firing frequency, never spike height.
Step 3 · Cross the synapse chemically
Release, receptor binding, then reuptake or breakdown. Excitatory messengers push the next cell toward firing, inhibitory ones pull away, and drugs mimic, block, or prolong a messenger's work.
Step 4 · Read sensation through its thresholds
Each sense transduces energy into signal. Detection has two floors — absolute for whether anything is there, difference for whether it changed — and constant stimulation fades through adaptation.
- Diagnostic cue: Noticing that a backpack got heavier is answered with the absolute threshold.
- Wrong branch: Detecting that a stimulus exists is merged with detecting that it changed.
- Repair: Two floors, two jobs: absolute threshold for the faintest detectable stimulus, difference threshold for the smallest detectable change — a floor that grows with the size of the standard.
Step 5 · Put behavior on its two biological clocks
The circadian rhythm schedules the night; the within-night cycle steps through NREM and REM. Deprivation is repaid with REM rebound; only five listed disorders are examined.
- Diagnostic cue: A shifted bedtime after a long flight is explained by the stages of one night's sleep.
- Wrong branch: The 24-hour circadian rhythm and the within-night NREM-REM cycle are treated as one clock.
- Repair: Separate the scales: the circadian rhythm schedules the night; the sleep cycle runs inside it. Jet lag lives on the first clock, stage architecture on the second.
Representation lab: the mechanism on the page
All-or-none firing on a membrane-potential trace
Read the trace against the dashed threshold line before naming anything. The first input depolarizes the membrane but never crosses threshold, so no action potential occurs. The second and third inputs both cross threshold, and this is the load-bearing observation: the third input is plainly stronger, yet its spike peaks at the same height as the second. A neuron does not fire harder for a harder push; the action potential runs at full amplitude, and stimulus intensity is carried by how often the neuron fires, not by taller spikes. The brief dip after each spike is the refractory period, when another full response cannot immediately follow. The trace shows one neuron's firing decision; it does not display graded synaptic inputs summing at the dendrites.
