Hormones, Homeostasis and Tropic Responses
Biology· 14.3, 14.4, 14.5· 25 min read
1. Core: Homeostasis and Hormone Fundamentals★★☆☆☆⏱ 7 min
Homeostasis
The maintenance of a constant internal environment in the body, including temperature, water balance and blood glucose concentration.
Homeostasis ensures cells can function optimally by keeping conditions within narrow, safe ranges. The body uses two coordination systems to regulate these conditions: the nervous system and the hormonal (endocrine) system.
Hormone
A chemical messenger secreted by an endocrine gland, transported in the blood plasma to target organs where it produces an effect, then broken down by the liver.
Feature | Nervous System | Hormonal System |
|---|---|---|
Type of signal | Electrical impulses | Chemical messengers |
Transmission path | Nerve cells (neurones) | Bloodstream |
Speed of response | Very fast | Slower |
Duration of response | Short-lived | Long-lasting |
State two differences between the nervous system and the hormonal system. (2 marks)
- 1
- Identify a clear comparative feature: Speed of response. Correct answer: Nervous responses are faster than hormonal responses. (1 mark)
- 2
- Identify a second comparative feature: Duration of effect. Correct answer: Nervous responses are shorter-lived than hormonal responses. (1 mark)
You also need to know four specific endocrine glands and the hormone each one secretes:
Endocrine gland | Hormone secreted |
|---|---|
Adrenal glands | Adrenaline |
Pancreas | Insulin |
Testes | Testosterone |
Ovaries | Oestrogen |
Adrenaline
The hormone secreted by the adrenal glands in 'fight or flight' situations (such as fear or excitement). Its effects are limited to: increased breathing rate, increased heart rate and increased pupil diameter.
A student is about to give a presentation and feels their heart beating faster and their breathing quicken. Name the hormone responsible and the gland that secretes it, and state one other effect it has.
- 1
The hormone is adrenaline, secreted by the adrenal glands in a 'fight or flight' situation.
- 2
As well as increasing heart rate and breathing rate, adrenaline increases the pupil diameter.
Exam tip:
Always use direct comparative language when asked for differences between coordination systems, not separate descriptions of each.
2. Core: Tropic Responses in Plants★★★☆☆⏱ 8 min
Tropism
A directional growth response of a plant to an external, directional stimulus, controlled by the hormone auxin.
Phototropism: Growth response to light. Shoots show positive phototropism (grow towards light), roots show negative phototropism (grow away from light).
Geotropism (Gravitropism): Growth response to gravity. Shoots show negative geotropism (grow against gravity), roots show positive geotropism (grow towards gravity).
Auxin is produced at the tips of growing shoots and roots. It moves away from light and accumulates on the lower side of shoots and roots when they are placed horizontally. In shoots, high auxin concentration increases growth, so the shaded/lower side grows faster, bending the shoot towards light/away from gravity. In roots, high auxin concentration inhibits growth, so the lower side grows slower, bending the root downwards.
Explain why a plant shoot placed next to a window will grow towards the light. (3 marks)
- 1
- Auxin is produced at the tip of the shoot and moves away from the light source, accumulating on the shaded side of the shoot. (1 mark)
- 2
- High auxin concentration increases growth in shoot cells, so the shaded side of the shoot grows faster than the lit side. (1 mark)
- 3
- The unequal growth causes the shoot to bend towards the light source. (1 mark)
Exam tip:
When explaining tropisms, always mention auxin distribution, unequal growth, and direction of growth to get all available marks.
3. Extended Only: Negative Feedback Mechanisms★★★★☆Extended only⏱ 10 min
Negative Feedback
A control mechanism that detects a change in internal conditions, triggers a response to reverse the change, and returns conditions to their normal set point. All homeostatic mechanisms in the 0610 syllabus use negative feedback.
A negative feedback loop has three core components: a receptor that detects the change, a coordination centre (brain or gland) that processes the information, and an effector that produces the response to reverse the change. Once conditions return to normal, the effector stops producing the response.
Describe how negative feedback controls blood glucose concentration after a person eats a meal high in sugar. (4 marks)
- 1
- Receptors in the pancreas detect a rise in blood glucose concentration above normal levels. (1 mark)
- 2
- The pancreas secretes the hormone insulin into the bloodstream. (1 mark)
- 3
- Insulin causes liver and muscle cells to take up excess glucose from the blood and store it as glycogen. (1 mark)
- 4
- Blood glucose concentration falls back to normal levels, and the pancreas stops secreting insulin. (1 mark)
Exam tip:
For Extended negative feedback questions, you must link each step of the loop explicitly to show how conditions are returned to normal.
4. Extended Only: Adrenaline and Metabolic Activity★★★☆☆Extended only⏱ 3 min
Beyond its immediate 'fight or flight' effects, Extended candidates must know the role of adrenaline in controlling metabolic activity:
Increases the blood glucose concentration, providing more glucose for respiration in muscle cells
Increases heart rate, delivering glucose and oxygen to the muscles more quickly
Explain how the metabolic effects of adrenaline help prepare the body for vigorous physical activity.
- 1
Adrenaline increases the blood glucose concentration, so more glucose is available to muscle cells for respiration.
- 2
Adrenaline also increases heart rate, so this glucose and oxygen are delivered to the muscles more quickly.
- 3
Together these prepare the muscles to release energy rapidly for vigorous activity.
Exam tip:
Keep adrenaline's metabolic role to the two syllabus points — raising blood glucose and raising heart rate — do not add other effects.
5. Common Pitfalls
Wrong move:
Describing nervous and hormonal systems separately instead of comparing them directly
Why:
Exam questions asking for differences require comparative points to award marks; separate unlinked descriptions get zero credit.
Correct move:
Use comparative language e.g. 'Nervous responses are faster than hormonal responses' instead of 'Nervous responses are fast. Hormonal responses are slow'.
Wrong move:
Stating that auxin is destroyed by light in phototropism explanations
Why:
The 0610 syllabus specifies auxin moves away from light, not destroyed by it; this common mistake loses marks for accuracy.
Correct move:
Write 'Auxin accumulates on the shaded side of the shoot' instead of 'Light breaks down auxin on the lit side'.
Wrong move:
Mixing up positive and negative tropism direction
Why:
Students often confuse growth direction relative to the stimulus, leading to lost marks for factual error.
Correct move:
Remember: positive tropism = growth towards the stimulus (associate 'positive' with gaining access to the stimulus), negative tropism = growth away from the stimulus.
Wrong move:
Only mentioning speed when comparing nervous and hormonal systems
Why:
Most comparison questions are worth 2 or more marks, so only giving one difference will limit your score.
Correct move:
Always list both speed and duration of response, plus transmission pathway if required, to get full marks for comparison questions.
Wrong move:
Describing positive feedback for homeostatic mechanisms (Extended tier)
Why:
All homeostatic mechanisms in the 0610 syllabus use negative feedback; mentioning positive feedback is a factual error.
Correct move:
Explicitly state that homeostasis relies on negative feedback to reverse changes and keep conditions within normal ranges.
6. Quick Reference Cheatsheet
Concept | Core Required Knowledge | Extended Required Knowledge |
|---|---|---|
Homeostasis | Define as maintenance of constant internal environment | Define + explain role of negative feedback in homeostatic regulation |
Hormones | Define as chemical messengers, compare 2 key differences with nervous system | Define + compare 3+ features of nervous and hormonal coordination |
Tropisms | Define phototropism/geotropism, describe auxin role in shoot growth towards light | Define + explain auxin distribution and effect on both shoot and root growth |
Negative Feedback | Not assessed for Core | Define, describe full feedback loop for blood glucose and temperature regulation |
Endocrine glands & adrenaline | Glands/hormones: adrenal→adrenaline, pancreas→insulin, testes→testosterone, ovaries→oestrogen; adrenaline in 'fight or flight' increases breathing rate, heart rate and pupil diameter | Adrenaline also controls metabolic activity: increases blood glucose concentration and heart rate |
7. Frequently Asked
What is the difference between positive and negative tropism?
Positive tropism is growth towards a stimulus, while negative tropism is growth away from a stimulus. For example, plant shoots show positive phototropism (grow towards light) while roots show negative phototropism (grow away from light).
Do I need to learn other plant hormones besides auxin for Core tier?
No, for Core tier you only need to know the role of auxin in phototropism and geotropism. Other plant hormones are not part of the 0610 Core syllabus.
Going deeper
- syllabusCIE IGCSE Biology 0610 2026-2028 SyllabusRefer to section 14 for full topic requirements
- practical_guideRequired Practical: Investigating TropismsCovers experimental design for phototropism and geotropism investigations
What's Next
Now you have mastered hormones, homeostasis and tropic responses for CIE IGCSE Biology 0610, you can build on this foundational knowledge to tackle related higher-weight exam topics. First, study specific homeostatic mechanisms including body temperature and blood glucose regulation, which are common extended response question topics for both Core and Extended tiers. Next, revise the structure and function of the nervous system to solidify your understanding of the two human coordination systems, a frequent compare-and-contrast exam question. Finally, practice structured Unit 10 past paper questions to refine your answer structure and avoid the common pitfalls listed in this guide before your mock or final exams.