# Hormones, homeostasis and reproduction

> IB Biology SL · Theme B: Form and Function
> Source: https://www.owlsprep.com/study/ib-biology-sl-u2-hormones-homeostasis-and-reproduction/

This sub-topic explores how hormones act as chemical messengers to maintain stable internal homeostasis and regulate human reproduction. You will learn hormone types, mechanisms of action, and negative feedback control tested frequently in IB exams.

**Prerequisites:** [Cell membrane structure](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-membrane-structure/); [Gene expression and protein synthesis](https://www.owlsprep.com/study/ib-biology-sl-u2-gene-expression-protein-synthesis/)

## Learning objectives

- Explain how hormones maintain homeostasis via negative feedback control
- Compare the mechanisms of action of steroid and protein hormones
- Outline the roles of key hormones in regulating the human menstrual cycle
- Distinguish between the two main types of diabetes mellitus as homeostatic failure

## Homeostasis and Negative Feedback

**Negative Feedback** — A regulatory mechanism where the product of a response reverses the original stimulus to keep a variable around a stable set point.

*Example:* Lowering high blood glucose concentration back to normal after a meal

All homeostatic variables (including body temperature, blood pH, and blood glucose) are maintained within a narrow tolerance range. Endocrine glands detect deviations from the set point and secrete hormones to trigger corrective responses. Once the set point is restored, hormone secretion stops.

**Worked example:** Explain how insulin regulates blood glucose concentration after a meal via negative feedback.

1. 1. After eating, digested carbohydrates raise blood glucose above the normal set point (~90 mg/100 mL).
2. 2. Receptor cells in the pancreas detect the elevated glucose concentration.
3. 3. Pancreatic beta cells secrete insulin into the bloodstream.
4. 4. Insulin triggers liver and muscle cells to take up glucose from the blood and store it as glycogen.
5. 5. Blood glucose concentration drops back to the normal set point.
6. 6. The pancreas stops insulin secretion, completing the negative feedback loop.

> **Exam tip:** Always explicitly state that negative feedback reverses the original stimulus when answering IB exam questions.

## Mechanisms of Hormone Action

Hormones are grouped into two classes by chemical structure, which determines their mechanism of action on target cells.

**Comparing methods**

The two main hormone classes have distinct mechanisms:

- **Steroid Hormones** — Lipid-soluble (derived from cholesterol), so can diffuse across the cell membrane. Bind to receptors inside the cytoplasm or nucleus, the hormone-receptor complex regulates gene transcription.
  - Pros: Long-lasting effects via altered gene expression
  - Cons: Slow response (minutes to hours)

- **Protein (Peptide) Hormones** — Water-soluble, cannot cross the hydrophobic core of the cell membrane. Bind to cell surface receptors, triggering an internal secondary messenger cascade that activates existing enzymes.
  - Pros: Fast response (seconds to minutes)
  - Cons: Short-lived effects, do not alter gene expression directly

**Worked example:** Explain why steroid and protein hormones have different receptor locations.

1. 1. Steroid hormones are lipid-soluble, which matches the hydrophobic interior of the phospholipid bilayer, so they diffuse freely into the cell.
2. 2. Once inside, they bind to internal receptors, and the complex enters the nucleus to regulate gene expression.
3. 3. Protein hormones are water-soluble and cannot cross the hydrophobic lipid core of the membrane, so they cannot enter the cell.
4. 4. They bind to receptors on the cell surface, triggering internal signaling cascades to produce a fast response.

> **Exam tip:** Comparison questions on hormone types are extremely common: always mention solubility, receptor location, and response speed.

## Hormonal Control of the Menstrual Cycle

**Menstrual Cycle** — Monthly hormone-regulated cycle in the ovary and uterus that prepares the body for pregnancy, with an average length of 28 days in humans.

*Notation:* Ovulation typically occurs at ~day 14 of the cycle

Four key hormones interact via feedback loops to regulate the cycle: follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone. These control follicle development, ovulation, and preparation of the uterine lining for embryo implantation.

**Worked example:** Outline hormone changes and events in the first 14 days of the menstrual cycle.

1. 1. Day 1 marks the start of menstruation (shedding of the uterine lining), and the anterior pituitary secretes FSH.
2. 2. FSH stimulates the growth of immature follicles in the ovary, each containing one egg.
3. 3. Growing follicles secrete increasing levels of estrogen, which stimulates thickening and repair of the uterine lining.
4. 4. Peak estrogen levels around day 12 trigger a positive feedback response, causing a surge in LH secretion from the pituitary.
5. 5. The LH surge triggers ovulation around day 14, releasing the mature egg into the fallopian tube.

> **Exam tip:** Learn the order of hormone surges: FSH → estrogen → LH surge → ovulation, do not mix up LH and progesterone roles.

## Application: Glucose Homeostasis and Diabetes

Failure of glucose homeostasis leads to diabetes mellitus, a common disorder characterized by chronically high blood glucose. There are two distinct types of diabetes with different causes and treatments.

| Characteristic | Type 1 Diabetes | Type 2 Diabetes |
| --- | --- | --- |
| Cause | Autoimmune destruction of pancreatic insulin-producing beta cells | Target cells become resistant to insulin |
| Insulin Production | None / very low | Normal (early stage) |
| Main Treatment | Insulin injections | Diet + exercise, medication |

**Worked example:** Distinguish between type 1 and type 2 diabetes mellitus.

1. 1. Type 1 diabetes is an autoimmune disorder that destroys insulin-producing beta cells in the pancreas, leading to no insulin production.
2. 2. It most often develops in childhood and requires lifelong insulin injections to regulate blood glucose.
3. 3. Type 2 diabetes develops when target body cells become resistant to insulin, so insulin cannot effectively trigger glucose uptake from the blood.
4. 4. It is strongly linked to lifestyle factors including obesity and inactivity, usually onsets in adulthood, and is managed first through diet and exercise.

## Common pitfalls

- **Wrong:** Confusing positive and negative feedback as the main mechanism for homeostasis.
  - Why it fails: Students often mix up the two types of feedback, forgetting that most homeostatic processes reverse change.
  - Correct: Remember homeostasis maintains a stable set point, so negative feedback (reverse change) is the norm; positive feedback is rare.
- **Wrong:** Claiming protein hormones enter the nucleus to alter gene expression.
  - Why it fails: Mixing up the mechanisms of steroid and protein hormones.
  - Correct: Only steroid hormones can cross the cell membrane and enter the nucleus; protein hormones bind to cell surface receptors.
- **Wrong:** Stating progesterone triggers ovulation in the menstrual cycle.
  - Why it fails: Students often mix up the roles of LH and progesterone after ovulation.
  - Correct: The LH surge triggers ovulation; progesterone maintains the thickened uterine lining after ovulation.
- **Wrong:** Claiming type 2 diabetes is caused by insufficient insulin production.
  - Why it fails: Common misconception from confusing the two types of diabetes.
  - Correct: Type 2 diabetes is caused by insulin resistance in target cells; insulin production is normal in the early stages.
- **Wrong:** Assuming hormones affect all cells they come into contact with.
  - Why it fails: Students forget the specificity of hormone signaling.
  - Correct: Only target cells with the specific receptor for a hormone will respond to the hormone signal.

## Cheatsheet

| Hormone / Type | Key Function | Receptor Location |
| --- | --- | --- |
| Steroid hormone | Regulate gene expression | Cytoplasm / nucleus |
| Protein hormone | Trigger fast enzyme response | Cell membrane surface |
| FSH | Stimulate follicle growth in ovary | N/A (pituitary hormone) |
| LH | Trigger ovulation | N/A (pituitary hormone) |
| Estrogen | Thicken uterine lining | Nucleus |
| Progesterone | Maintain uterine lining after ovulation | Nucleus |
| Insulin | Lower blood glucose concentration | Cell membrane surface |
| Glucagon | Raise blood glucose concentration | Cell membrane surface |

## What's next

Understanding hormone action and homeostasis is foundational for all further study of human physiology, and aligns with the IB Biology core theme of form matching function. This sub-topic is frequently assessed in both Paper 1 multiple choice and Paper 2 extended response questions, so mastering feedback loops and hormone mechanisms will boost your exam score significantly. The concepts of negative feedback you learn here will also support your understanding of other homeostatic processes like osmoregulation and thermoregulation. Explore the linked topics below to build on your knowledge.

- [Theme C: Interactions and Interdependencies](https://www.owlsprep.com/study/ib-biology-sl-u3-overview/)
- [Species and communities](https://www.owlsprep.com/study/ib-biology-sl-u3-species-and-communities/)
- [Ecosystems and carbon cycling](https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystems-and-carbon-cycling/)

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