KHAEMENES ACADEMY · BIOLOGY 10 · WEEK 14 · DAY 01

Homeostasis, Variables & Negative Feedback

Model homeostasis as dynamic regulation of biological variables within workable ranges rather than as a perfectly fixed internal state.

Safety and evidence boundary

This week uses fictional organisms, classroom-safe models, and non-diagnostic datasets. Do not collect personal medical information, change medication, perform extreme-temperature exposure, deliberately alter blood glucose, restrict food or water, or use physiological measurements to diagnose health conditions.

Explicit science instruction

Homeostasis is the regulation of internal conditions so that cells and organisms can continue functioning as external and internal conditions change. Regulated variables may include temperature, ion concentrations, water balance, pH, gas levels, or nutrient availability. The target is usually a range, not one exact number.

A negative-feedback loop contains a regulated variable, a change or disturbance, sensing or detection, integration of information, an effector response, and a return influence that reduces the original deviation. “Negative” describes the direction of feedback, not whether the result is good or bad.

Vocabulary: homeostasis; regulated variable; set range; disturbance; sensor/receptor; integrator; effector; negative feedback; response; dynamic equilibrium.

Worked example with reasoning

A fictional aquatic organism regulates internal salt concentration near 10–12 arbitrary units. After entering slightly saltier water, internal concentration rises to 13. Sensors detect the deviation; regulatory cells increase ion export and reduce uptake. Concentration falls toward the working range. The response opposes the original increase, so this is negative feedback.

  1. Variable: internal salt concentration.
  2. Disturbance: external salinity increased.
  3. Detected deviation: internal concentration rose above the working range.
  4. Effector response: export increased and uptake decreased.
  5. Feedback direction: the response reduces the deviation.

Guided question

A plant loses water faster during a dry, windy afternoon. Which parts of a feedback model would you need to identify before claiming the plant is maintaining water balance?

Independent work

  1. Choose a fictional regulated variable.
  2. Draw a six-part negative-feedback loop.
  3. Label where information is detected and where action occurs.
  4. Explain why the target should be shown as a workable range.
  5. Name one delay or limitation in your model.

Misconception check

Homeostasis does not mean conditions never change. Healthy regulation often includes oscillation, delay, daily rhythms, and different operating ranges under different conditions.

Formative understanding check

Why is “the organism keeps the variable constant” usually a weaker explanation than “the organism regulates the variable within a dynamic range”?

Show the worked response

Biological variables fluctuate. Regulation reduces harmful deviations and supports function, but sensing, response delays, changing activity, and environmental conditions prevent exact constancy.

Open-book formative practice only; not an official mastery attempt or grade.

Mentor evidence conference

Ask Archaemenes to test whether your feedback model identifies the variable, sensor, integrating logic, effector, direction of response, and limitation without giving protected assessment answers.

Open Archaemenes for this lesson

Teacher guidance

Foundation: use a labeled loop template and supplied terms. Core: require a complete causal loop with variable, sensor, integrator, effector, and response direction. Extended: add time delay, competing demands, and a second interacting regulated variable.

Preserve learning evidence

Keep the model, calculation or graph, correction note, and evidence-based explanation in the matching Week 14 evidence field.

Return to monday Evidence Workspace