📌Definition Table

Term	Definition
Homeostasis	Maintenance of stable internal conditions in response to external changes.
Negative feedback	Regulatory mechanism where a change triggers responses that counteract the initial stimulus, restoring balance.
Positive feedback	Mechanism where a change is reinforced or amplified, driving processes to completion.
Endocrine axis	A hierarchical signalling pathway where one gland controls another via tropic hormones (e.g., hypothalamus–pituitary–thyroid).
Receptor down-regulation	Decrease in receptor number in response to high hormone levels, reducing sensitivity.
Circadian rhythm	Biological cycle regulated by feedback loops of signalling molecules and clock genes.

📌Introduction

Chemical signalling systems must be tightly regulated to prevent under- or over-activation of pathways. Feedback mechanisms, both negative and positive, ensure appropriate hormone levels and cellular responses. These mechanisms underpin homeostasis, growth, reproduction, and stress responses, and their disruption can lead to disease.

📌 Negative Feedback in Hormonal Control

• Most common form of regulation in endocrine systems.
• Example: Blood glucose control
  • High glucose → insulin secretion → glucose uptake/storage → lowers glucose → reduces insulin secretion.
  • Low glucose → glucagon secretion → glycogen breakdown → raises glucose → reduces glucagon secretion.
• Example: Thyroid axis
  • Hypothalamus (TRH) → pituitary (TSH) → thyroid (thyroxine).
  • High thyroxine inhibits TRH and TSH release.

🧠 Examiner Tip: Always specify which hormone or gland is inhibited in negative feedback diagrams — vague answers often lose marks.

📌 Positive Feedback in Hormonal Control

• Less common but important for processes requiring completion.
• Example: Childbirth
  • Oxytocin release stimulates uterine contractions.
  • Contractions trigger more oxytocin release until delivery.
• Example: Lactation
  • Suckling stimulates prolactin and oxytocin release, sustaining milk production and ejection.

🧬 IA Tips & Guidance: Students could simulate feedback with data-logging software, e.g., glucose regulation models or heart rate feedback after exercise, linking physiology to regulation concepts.

📌 Endocrine Axes and Integration

• Multi-gland pathways provide fine control and amplification.
• Hypothalamus integrates nervous and endocrine signals.
• Pituitary releases tropic hormones that regulate other glands (thyroid, adrenal, gonads).
• Axes allow integration of multiple feedback loops.

🌐 EE Focus: An EE could examine disruptions in endocrine feedback, e.g., Cushing’s syndrome (excess cortisol), or model circadian rhythm regulation by melatonin feedback.

📌 Adaptation and Desensitization

• Chronic high hormone levels cause receptor down-regulation (e.g., insulin resistance in type 2 diabetes).
• Chronic low levels cause up-regulation, increasing sensitivity.
• Desensitization protects against overstimulation but contributes to disease when feedback fails.

❤️ CAS Link: Students could run health-awareness programs on diabetes or thyroid disorders, highlighting the role of feedback regulation in maintaining balance.

📌 Feedback in Biological Cycles

• Circadian rhythms controlled by feedback loops of clock genes and melatonin secretion.
• Menstrual cycle regulated by interplay of positive and negative feedback among FSH, LH, estrogen, and progesterone.

🔍 TOK Perspective: Feedback systems are often represented in simplified diagrams. TOK reflection: To what extent do simplified models capture the complexity of living systems, and when does simplification risk misrepresenting reality?

📝 Paper 2: Be ready to explain negative vs positive feedback with examples, outline endocrine axes, and analyze data on hormone levels over time. Graph-based questions often require identifying feedback patterns.