📌Definition Table

Term	Definition
Hormone	A chemical messenger secreted by endocrine glands, transported in blood to target organs, where it alters physiology.
Steroid hormone	Lipid-soluble hormone derived from cholesterol, able to diffuse through membranes and bind intracellular receptors.
Peptide hormone	Hormone made of amino acids; water-soluble and binds membrane receptors, activating signal cascades.
Amino acid derivative	Small hormones derived from tyrosine or tryptophan (e.g., epinephrine, melatonin).
Endocrine signalling	Long-distance communication where hormones are secreted into the bloodstream.
Target cell	Cell possessing specific receptors for a hormone, enabling response.

📌Introduction

Hormones are essential regulators of growth, metabolism, reproduction, and homeostasis. Unlike nervous signalling, which is rapid and localized, hormonal communication is slower but longer-lasting and systemic. Hormones differ in chemical structure, solubility, and mechanisms of action, which determine how they interact with receptors and affect gene expression or enzyme activity.

📌 Hormone Types

• Steroid hormones (e.g., cortisol, estrogen, testosterone):
  • Lipid-soluble, pass through membranes.
  • Bind intracellular receptors in cytoplasm or nucleus.
  • Hormone–receptor complex acts as transcription factor, altering gene expression.
• Peptide hormones (e.g., insulin, glucagon, ADH):
  • Water-soluble, cannot cross membrane.
  • Bind receptors on cell surface.
  • Trigger signal transduction cascades (second messengers like cAMP).
• Amino acid derivatives (e.g., adrenaline, thyroxine):
  • Adrenaline behaves like a peptide hormone.
  • Thyroxine behaves like a steroid (lipid-soluble).

🧠 Examiner Tip: Always specify whether a hormone acts via intracellular receptors (steroid/thyroxine) or membrane receptors (peptide/adrenaline). This distinction is commonly tested.

📌 Mechanisms of Hormone Action

• Steroid pathway
  • Hormone diffuses into cell → binds receptor → moves into nucleus.
  • Directly regulates transcription and protein synthesis.
  • Longer onset, longer duration.
• Peptide pathway
  • Hormone binds to receptor → activates G-protein/kinase cascade.
  • Generates second messengers (cAMP, IP₃, Ca²⁺).
  • Leads to enzyme activation or channel regulation.
  • Rapid, short-lived effects.

🧬 IA Tips & Guidance: A practical extension could be measuring glucose levels in blood samples before and after food intake to illustrate insulin/glucagon action. Graphing hormone effects strengthens data analysis skills.

📌 Integration with Endocrine System

• Hormones often act in antagonistic pairs (insulin vs glucagon).
• Pituitary gland releases “tropic” hormones controlling other glands.
• Hormone action is highly specific due to receptor–ligand binding.

🌐 EE Focus: An EE could explore molecular differences in hormone signalling, such as comparing steroid hormone receptor binding vs peptide hormone cascades, linking biochemistry to physiology.

📌 Hormones and Cellular Adaptation

• Steroid hormones reprogram transcription, producing long-term structural/functional changes.
• Peptide hormones regulate immediate metabolic activity.
• Cells adapt by altering receptor number (up- or down-regulation).

❤️ CAS Link: Students could develop awareness campaigns on lifestyle diseases (e.g., diabetes), showing how hormones regulate blood sugar and why balance is crucial for health.

📌 Integration with Physiology

• Nervous and endocrine systems interact (hypothalamus controls pituitary).
• Hormonal cascades allow small signals to have amplified systemic effects.

🔍 TOK Perspective: Hormone action involves invisible molecules inferred by effects on physiology. TOK reflection: How do scientists gain knowledge about unobservable processes, and to what extent can indirect measurements be trusted as evidence?

📝 Paper 2: Expect questions contrasting peptide vs steroid mechanisms, examples of each hormone type, and diagrams showing pathways. Data questions may involve blood hormone concentrations in feedback systems.