Topic 1: Homeostasis
Describe endocrine control of the internal environment, including the role of hormones, target cells, the hypothalamus and pituitary gland, and the regulation of blood glucose by insulin and glucagon
A focused answer to the QCE Biology Unit 2 dot point on endocrine control. Defines hormones, distinguishes steroid and peptide signalling at target cells, lays out the hypothalamic-pituitary axis and traces blood glucose regulation by insulin and glucagon as a negative feedback loop.
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What this dot point is asking
QCAA expects you to describe how the endocrine system controls the internal environment: what a hormone is, how it acts on a target cell, the role of the hypothalamus and pituitary, and how insulin and glucagon regulate blood glucose. Pancreatic islet diagrams and glucose-time graphs are common stimulus.
The answer
The endocrine system is the slower, longer-acting partner to the nervous system. Hormones are signalling molecules released into the blood and carried to target cells anywhere in the body.
Hormones and target cells
A hormone is a chemical messenger released by an endocrine gland directly into the bloodstream. The blood carries it to all parts of the body, but it acts only on target cells that express the matching receptor.
Two broad chemical classes:
- Steroid hormones (cholesterol-derived; oestrogen, testosterone, cortisol, aldosterone). Lipid-soluble. Diffuse through the plasma membrane and bind intracellular receptors that act as transcription factors. Slow but long-lasting effects (hours to days).
- Peptide and amine hormones (made of amino acids; insulin, glucagon, ADH, growth hormone, adrenaline). Water-soluble. Bind cell-surface receptors and trigger second-messenger cascades inside the cell. Fast but short-lived effects (seconds to minutes).
The specificity of hormonal signalling comes from receptor expression: only cells with the matching receptor respond to a given hormone, even though every cell is bathed in the same blood.
The hypothalamus and pituitary gland
The hypothalamus is the integrator that links the nervous and endocrine systems. It sits at the base of the brain, attached to the pituitary gland.
Posterior pituitary. Stores and releases two hormones made by neurosecretory cells in the hypothalamus:
- ADH (antidiuretic hormone). Increases water reabsorption by the kidney collecting duct (see osmoregulation and excretion).
- Oxytocin. Triggers uterine contractions during labour and milk ejection during breastfeeding.
Anterior pituitary. Produces its own hormones in response to releasing or inhibiting hormones from the hypothalamus, carried in a portal blood system.
- Growth hormone (GH). Promotes growth of bone and soft tissues.
- Thyroid stimulating hormone (TSH). Stimulates thyroid hormone release.
- Adrenocorticotrophic hormone (ACTH). Stimulates cortisol release from the adrenal cortex.
- Follicle stimulating hormone (FSH) and luteinising hormone (LH). Regulate reproduction.
- Prolactin. Stimulates milk production.
Each anterior pituitary axis is regulated by negative feedback: rising hormone levels from the target gland inhibit further release from the hypothalamus and pituitary.
Major endocrine glands and their hormones
| Gland | Hormone | Main effect |
|---|---|---|
| Hypothalamus | Releasing hormones, ADH, oxytocin | Controls pituitary and posterior pituitary release |
| Anterior pituitary | GH, TSH, ACTH, FSH, LH, prolactin | Master regulator of other glands |
| Thyroid | Thyroxine (T4, T3) | Raises metabolic rate |
| Parathyroid | PTH | Raises blood calcium |
| Adrenal cortex | Aldosterone, cortisol | Salt balance, stress response |
| Adrenal medulla | Adrenaline | Fight or flight |
| Pancreas (islets) | Insulin (beta), glucagon (alpha) | Blood glucose regulation |
| Ovaries | Oestrogen, progesterone | Female reproductive cycle |
| Testes | Testosterone | Male reproductive function |
Blood glucose regulation
Blood glucose is held around 4 to 6 mmol per L by two antagonistic hormones from the pancreatic islets.
Rising glucose (after a meal).
- Stimulus: blood glucose rises above the set point.
- Receptor and control centre: beta cells of the pancreatic islets detect the rise.
- Effector: insulin is released into the blood. Liver, skeletal muscle and adipose tissue respond.
- Response: insulin binds cell-surface receptors and triggers insertion of GLUT4 glucose transporters into the plasma membrane. Glucose enters cells and is stored as glycogen in liver and muscle (glycogenesis) or converted to fat in adipose tissue. Blood glucose falls back to the set point.
Falling glucose (between meals or during exercise).
- Stimulus: blood glucose falls below the set point.
- Receptor and control centre: alpha cells of the pancreatic islets detect the fall.
- Effector: glucagon is released. The liver is the main target.
- Response: liver glycogen is broken down to glucose (glycogenolysis); amino acids and lactate are converted to glucose (gluconeogenesis). Glucose is released into the blood. Blood glucose rises back to the set point.
Diabetes mellitus. Failure of the insulin pathway.
- Type 1. Autoimmune destruction of beta cells; insulin is absent. Managed by insulin injection.
- Type 2. Cells become resistant to insulin; beta cell function eventually declines. Managed by diet, exercise and oral drugs (sometimes insulin).
In both forms, blood glucose runs high (hyperglycaemia), spills over into urine and damages blood vessels and nerves long-term.
Endocrine vs nervous control
| Feature | Nervous | Endocrine |
|---|---|---|
| Signal | Action potentials and neurotransmitters | Hormones in blood |
| Speed | Milliseconds | Seconds to hours |
| Duration | Brief | Sustained |
| Target | Specific (synapse) | All cells with matching receptor |
| Examples | Reflexes, voluntary movement | Growth, metabolism, reproduction |
The two systems are integrated through the hypothalamus.
Cross-link to Year 12 assessment
The endocrine system underpins QCAA Unit 2 questions on disease (autoimmune diabetes is a classic crossover with immunity), and the hypothalamus-pituitary control structure is foreshadowed in Unit 4 reproductive genetics. EA Paper 1 short-response questions on insulin and glucagon are extremely common.
Examples in context
Example 1. Brisbane Diabetes Centre insulin pumps. A type 1 diabetic in clinic at the Mater Brisbane Diabetes Centre cannot secrete insulin from pancreatic beta cells. Without insulin, GLUT4 receptors fail to translocate to muscle and fat cell membranes, blood glucose stays above 11 mmol/L after meals, and glucagon-driven hepatic gluconeogenesis is unopposed. A continuous subcutaneous insulin infusion pump delivers a small basal dose (around 0.6 units per hour) plus mealtime boluses, mimicking pancreatic feedback. The case integrates the hypothalamus-pituitary axis (which is bypassed here), pancreatic islet hormones, target-cell receptor signalling and the role of negative feedback in maintaining blood glucose between 4 and 7 mmol/L.
Example 2. Australia Zoo crocodile breeding and prolactin. At Australia Zoo's research facility, female saltwater crocodiles undergo seasonal egg production triggered by photoperiod and warming wet-season temperatures. The hypothalamus releases gonadotropin-releasing hormone (GnRH), the anterior pituitary releases follicle-stimulating hormone (FSH) and luteinising hormone (LH), and the ovary responds by producing oestrogen, which travels through the bloodstream to target receptors in the oviduct. Prolactin from the pituitary then drives maternal nest-guarding behaviour. The same hypothalamus-pituitary-target-organ axis operates in humans for reproduction, demonstrating that endocrine signalling is conserved across vertebrates.
Try this
Q1. Describe the role of insulin and glucagon in regulating blood glucose, naming the source cells and the principal target organ for each hormone. [4 marks]
- Cue. Insulin from beta cells lowers glucose via liver and muscle uptake. Glucagon from alpha cells raises glucose via liver glycogenolysis.
Q2. A patient's blood test shows glucose 14 mmol/L, insulin near zero, and ketone bodies elevated. Identify the most likely condition and explain why ketones accumulate. [3 marks]
- Cue. Type 1 diabetes. Without insulin, cells switch to fatty acid oxidation, producing ketones.
Q3. Refer to the hypothalamic-pituitary-thyroid axis. (a) Name the releasing hormone, pituitary hormone and target gland. (b) Explain how negative feedback limits thyroxine secretion. (c) Predict the effect of a tumour secreting TSH-like hormone on circulating thyroxine. [2+2+2 marks]
- Cue. (a) TRH, TSH, thyroid. (b) Thyroxine inhibits hypothalamus and pituitary. (c) Hyperthyroidism; feedback overridden.
Exam-style practice questions
Practice questions written in the style of QCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2023 QCAA style5 marksCompare the action of a steroid hormone (oestrogen) and a peptide hormone (insulin) at their target cells.Show worked answer →
A 5-mark answer needs both transport modes, receptor locations and mechanism, plus speed and duration.
Steroid hormone (oestrogen). Lipid-soluble; transported in the blood bound to a carrier protein. Diffuses straight through the plasma membrane of target cells and binds an intracellular receptor in the cytoplasm or nucleus. The hormone-receptor complex acts as a transcription factor, binding DNA and switching genes on or off. Response is slow (hours) but long-lasting.
Peptide hormone (insulin). Water-soluble; transported free in blood plasma. Cannot cross the plasma membrane; binds a receptor on the cell surface. Receptor activation triggers a second-messenger cascade (phosphorylation of intracellular proteins). Response is fast (seconds to minutes) but short-lived. For insulin, the effect includes inserting GLUT4 glucose transporters into the membrane of liver and muscle cells.
Markers reward the lipid-vs-water solubility framing, the receptor-location contrast and the speed-vs-duration trade-off.
2022 QCAA style4 marksDescribe how insulin and glucagon work as antagonistic hormones to regulate blood glucose concentration.Show worked answer →
A 4-mark answer needs both hormones, their cell of origin, their effect and the antagonistic relationship.
- Insulin
- Released by beta cells of the pancreatic islets when blood glucose rises above the set point (around 5 mmol per L). Insulin promotes glucose uptake into liver and muscle cells (GLUT4 insertion) and conversion to glycogen (glycogenesis). Blood glucose falls back to the set point.
- Glucagon
- Released by alpha cells of the pancreatic islets when blood glucose falls below the set point. Glucagon promotes the breakdown of glycogen back to glucose (glycogenolysis) and the production of glucose from amino acids and lactate (gluconeogenesis) in the liver. Glucose is released into the blood; blood glucose rises back to the set point.
- Antagonistic action
- The two hormones oppose each other's effect on liver and adipose tissue. Together they keep blood glucose oscillating in a narrow range. This is negative feedback through two parallel pathways.
Markers reward both hormones, their cells of origin and the explicit antagonistic-pair statement.
Related dot points
- Explain the concept of homeostasis and the role of negative feedback in maintaining a stable internal environment, including stimulus, receptor, control centre, effector and response
A focused answer to the QCE Biology Unit 2 dot point on homeostasis. Defines homeostasis around a set point, lays out the stimulus to receptor to control centre to effector to response pathway, contrasts negative and positive feedback and uses thermoregulation and blood glucose as worked examples.
- Describe nervous control, including the structure of a neuron, the generation of action potentials, synaptic transmission and the reflex arc
A focused answer to the QCE Biology Unit 2 dot point on nervous control. Describes the structure of a neuron (dendrites, soma, axon, myelin sheath), the three phases of an action potential, chemical synaptic transmission and the five components of a reflex arc.
- Describe osmoregulation and excretion in mammals, including the structure and function of the nephron and the role of ADH in regulating water balance
A focused answer to the QCE Biology Unit 2 dot point on osmoregulation. Walks through the four processes of the nephron (filtration, reabsorption, secretion, excretion), names each region (glomerulus, PCT, loop of Henle, DCT, collecting duct) and explains the role of ADH in adjusting urine concentration through negative feedback.