4.8.1Additional lessons
Homeostasis: Maintaining the Internal Environment — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 129 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Variables Kept Steady
Homeostasis is the maintenance of the internal environment within tolerable limits.
Core body temperature
- Core body temperature It is kept near 37 °C so that enzymes work efficiently and cell function is reliable.
Blood pH
- Blood pH It is held close to 7.4 because most enzymes and proteins are sensitive to changes in hydrogen ion concentration.
Blood glucose
- Blood glucose It is regulated so cells have a steady energy supply and tissues are not damaged by very high or low levels.
Water and solutes
- Water and solutes Osmotic concentration of blood is controlled so that body cells do not swell or shrink.
Part 2 of 5Negative Feedback Loop
Because a response removes the stimulus that caused it, the variable fluctuates only slightly around the set point.
1
A receptor detects a change in the monitored variable, such as a rise in temperature.
2
It sends the information, as nerve impulses or hormones, to a control centre such as the hypothalamus or pancreas.
3
The control centre compares the value with the set point.
4
Effectors such as muscles or glands respond in a way that reduces the change.
5
The variable returns towards the set point, the stimulus declines, and the response is switched off.
Part 3 of 5Communication Systems
Communication Systems
Nervous system
- Nervous system Sends fast electrical impulses, giving rapid, short-lived responses such as vasoconstriction.
Endocrine system
- Endocrine system Hormones travel in blood to target cells, giving slower and longer-lasting responses such as insulin action.
Coordination
- Coordination Many homeostatic loops use both systems, with sensory information processed by the brain or glands.
Set point
- Set point The target value of a variable, for example 37 °C, which the body returns to after any deviation.
Part 4 of 5Negative and Positive Feedback
Negative feedback
- Negative feedback The response reduces the change, producing stability; it is the basis of most homeostasis.
Positive feedback
- Positive feedback The response increases the change, driving a process towards completion, as in oxytocin during childbirth.
Why different
- Why different Positive feedback does not maintain a set point and is usually switched off by an external event.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| Effect on change | Reverses it | Amplifies it |
| Result | Stability | Rapid change |
| Example | Body temperature | Contractions in labour |
Part 5 of 5Exam Checklist & Tips
4.8.1 – Key Points for the Exam
- Homeostasis Maintenance of the internal environment within narrow limits.
- Set point The target value of a regulated variable.
- Receptor A cell or structure that detects a stimulus.
- Effector A muscle or gland that carries out a response.
- Negative feedback A response that opposes the change that triggered it.
Common Mistakes
- Saying that homeostasis keeps conditions exactly constant; they fluctuate around the set point.
- Describing negative feedback as only a response to a rise;
- it also opposes a fall, as when shivering raises a low temperature.
- Forgetting the receptor and control steps in an answer on feedback.
- Confusing hormones from the endocrine system with the nerve impulses of the nervous system.
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