Co-ordination and Response
Biology· 2.80–2.95B· 25 min read
1. Core Foundations of Coordination and Homeostasis★★☆☆☆⏱ 4 min
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Homeostasis
The maintenance of a constant internal environment within narrow limits, despite changes in external or internal conditions. Key examples include regulation of body water content and core body temperature.
All organisms respond to environmental changes to survive. A coordinated response requires three core components: a stimulus (change in environment), a receptor (structure that detects the stimulus), and an effector (muscle or gland that carries out the response).
A dog smells food and begins to salivate. Identify the stimulus, receptor and effector in this response.
- 1
- Stimulus: Chemical molecules from the food in the air.
- 2
- Receptor: Smell receptors in the dog's nose.
- 3
- Effector: Salivary glands in the dog's mouth that produce saliva.
Exam tip:
Always explicitly name all three components of a coordinated response for 2-3 mark short answer questions to gain full marks.
2. Plant Coordination: Tropisms and Auxin Action★★☆☆☆⏱ 4 min
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Tropism
A directional growth response of a plant to a directional stimulus, classified as positive (growth towards the stimulus) or negative (growth away from the stimulus).
Shoots (stems): Positively phototropic (grow towards light), negatively geotropic (grow away from gravity)
Roots: Positively geotropic (grow towards gravity), negatively phototropic (grow away from light)
Auxin is a growth hormone produced at the tip of plant shoots. When a shoot is exposed to light from one side (unilateral light), auxin moves to the shaded side of the shoot. Higher auxin concentration on the shaded side causes cells there to elongate faster than cells on the lit side, so the shoot bends towards the light source.
Explain why a seedling planted in a pot in a dark cupboard with a single small hole for light will grow towards the hole after 5 days.
- 1
- Light only enters through the hole, creating unilateral light exposure for the seedling's shoot tip.
- 2
- Auxin produced at the shoot tip moves to the side of the shoot furthest from the light (shaded side).
- 3
- Higher auxin levels on the shaded side stimulate faster cell elongation on this side than the lit side.
- 4
- The faster growth of the shaded side causes the shoot to bend towards the light source (the hole).
Exam tip:
Never refer to plant responses as 'nervous' or 'muscular' – plants only use chemical (hormonal) coordination, so these terms will not be credited.
3. Human Nervous System and Reflex Arcs★★★☆☆⏱ 5 min
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Central Nervous System (CNS)
Consists of the brain and spinal cord, linked to sense organs by peripheral nerves, that coordinates all body responses.
Feature | Nervous System | Hormonal System |
|---|---|---|
Type of signal | Electrical impulses along neurones | Chemical messengers in blood |
Speed of response | Very fast | Slower |
Duration of response | Short-lived | Long-lived |
Target | Precise, specific effectors | Widespread, multiple target organs |
Synapses are gaps between two neurones. When an electrical impulse reaches the end of the first neurone, a neurotransmitter chemical is released. This diffuses across the synapse gap, binds to receptors on the next neurone, and triggers a new electrical impulse in the second neurone.
Reflexes are rapid, automatic responses that do not require conscious input from the brain, protecting the body from harm. The standard reflex arc pathway required for exam answers is: stimulus → receptor → sensory neurone → relay neurone (in spinal cord/CNS) → motor neurone → effector → response.
A child touches a sharp needle and immediately pulls their hand away. Describe the full reflex arc for this response in the correct order.
- 1
- Stimulus: Sharp point of the needle piercing the skin.
- 2
- Receptor: Pain receptors in the skin of the child's finger.
- 3
- Sensory neurone carries electrical impulse from receptor to the spinal cord (CNS).
- 4
- Relay neurone in the spinal cord passes the impulse to the motor neurone (no conscious brain processing needed).
- 5
- Motor neurone carries the impulse to the effector (arm muscles responsible for pulling the hand away).
- 6
- Response: Muscle contracts, pulling the finger away from the needle to avoid further injury.
Exam tip:
Always list reflex arc steps in the exact sequence given in the specification to gain full marks – mixing up neurone types will lose points.
4. Structure and Function of the Human Eye★★★☆☆⏱ 4 min
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The eye is a sense organ that detects light stimuli. It carries out two key regulatory processes: accommodation (focusing on near/distant objects) and the pupil reflex (adjusting light entry to protect the retina).
Accommodation for near objects: Ciliary muscles contract → suspensory ligaments slacken → lens becomes thicker/more convex, increasing light refraction to focus on the retina.
Accommodation for distant objects: Ciliary muscles relax → suspensory ligaments become taut → lens becomes thinner/flatter, reducing light refraction.
Pupil reflex in bright light: Circular iris muscles contract, radial iris muscles relax → pupil constricts, reducing light entry to avoid retinal damage.
Pupil reflex in dim light: Radial iris muscles contract, circular iris muscles relax → pupil dilates, increasing light entry to improve vision.
A driver is looking at their speedometer (close to their face) then looks up to view a road sign 100m ahead. Explain how the eye adjusts to focus on the distant road sign.
- 1
- Ciliary muscles in the eye relax.
- 2
- Relaxation of ciliary muscles pulls the suspensory ligaments taut.
- 3
- Taut suspensory ligaments pull the lens into a thinner, flatter shape.
- 4
- The flatter lens refracts light less, so parallel light rays from the distant road sign are focused correctly onto the retina.
Exam tip:
Do not write that ciliary muscles 'tighten' for distant objects – they relax, which is a common exam mistake that loses marks.
5. Skin Function in Temperature Homeostasis★★★☆☆⏱ 4 min
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The skin plays a key role in maintaining core body temperature at ~37°C, the optimum temperature for enzyme function. Three core mechanisms control heat gain and loss: vasodilation/vasoconstriction, sweating, and shivering.
When core temperature is too high: 1. Vasodilation: Arterioles supplying skin capillaries widen, increasing blood flow to the skin surface, so more heat is lost via radiation. 2. Sweat glands produce more sweat; evaporation of sweat from the skin surface transfers heat away from the body.
When core temperature is too low: 1. Vasoconstriction: Arterioles supplying skin capillaries narrow, reducing blood flow to the skin surface, so less heat is lost. 2. Sweat production stops. 3. Shivering: Uncontrolled muscle contractions release heat from respiration to warm the body.
Explain why a runner appears flushed and is sweating heavily after a 5km race on a warm day.
- 1
- Respiration for muscle contraction releases excess heat, raising the runner's core body temperature.
- 2
- Temperature receptors detect this change and trigger vasodilation of skin arterioles.
- 3
- Increased blood flow to the skin surface gives the flushed appearance, and more heat is lost to the environment via radiation.
- 4
- Sweat production increases, and evaporation of sweat from the skin cools the body down further.
Exam tip:
Never state that blood vessels 'move up to the skin surface' when hot – only refer to vasodilation/vasoconstriction of arterioles to gain marks.
6. Hormonal Coordination in Humans (Core + Biology Only)★★★☆☆core / Paper 2 only⏱ 4 min
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Hormone
A chemical messenger produced by an endocrine gland, secreted directly into the bloodstream, that acts on a target organ to produce a specific response.
Hormone | Source Gland | Key Role |
|---|---|---|
Adrenaline | Adrenal glands | Fight or flight response: raises heart rate, breathing rate and blood glucose levels to prepare the body for activity |
Insulin | Pancreas | Lowers blood glucose levels by stimulating the liver to convert excess glucose into glycogen for storage |
Testosterone | Testes (males) | Controls development of male secondary sexual characteristics during puberty |
Oestrogen | Ovaries (females) | Controls development of female secondary sexual characteristics, and regulates the menstrual cycle |
Progesterone | Ovaries (females) | Maintains the lining of the uterus during pregnancy |
Hormone | Source Gland | Key Role |
|---|---|---|
ADH | Pituitary gland | Controls water reabsorption in the kidney tubules to regulate body water content |
FSH | Pituitary gland | Stimulates egg maturation in the ovaries, and oestrogen production |
LH | Pituitary gland | Triggers ovulation (release of a mature egg from the ovary) |
Explain the effect of adrenaline release when a person is startled by a loud noise.
- 1
- Adrenaline is released from the adrenal glands into the bloodstream.
- 2
- Heart rate and breathing rate increase, delivering more oxygen and glucose to muscle cells.
- 3
- Blood glucose levels rise, providing more energy for muscle contraction to support a fight or flight response.
Exam tip:
Do not mention glucagon when answering questions on insulin for this specification – it is out of scope and will not be credited.
7. Common Pitfalls
Wrong move:
Mixing up tropism directions (stating roots are positively phototropic)
Why:
Students confuse shoot and root responses to light and gravity stimuli
Correct move:
Memorise: Shoots = +phototropic, -geotropic; Roots = +geotropic, -phototropic
Wrong move:
Stating ciliary muscles contract to focus on distant objects
Why:
Students mix up muscle action for accommodation of near vs distant objects
Correct move:
Remember: Near objects = ciliary muscles contract; Distant objects = ciliary muscles relax
Wrong move:
Writing that blood vessels move up to the skin surface when hot
Why:
Incorrect description of heat loss mechanism that is not accepted in mark schemes
Correct move:
Only refer to vasodilation of skin arterioles, increasing blood flow to the skin surface
Wrong move:
Reordering reflex arc steps (e.g. putting motor neurone before sensory neurone)
Why:
Students forget the exact sequence required by the specification mark scheme
Correct move:
Memorise the exact sequence: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response
Wrong move:
Comparing nervous and hormonal systems without pairing contrasts
Why:
Mark schemes require direct, paired comparisons to award full marks for comparison questions
Correct move:
Pair each contrast point: Nervous = electrical impulses / hormonal = chemical messengers; Nervous = fast / hormonal = slow, etc.
Wrong move:
Mentioning glucagon when explaining insulin function
Why:
Glucagon is explicitly out of scope for this specification point
Correct move:
Only discuss insulin's role in lowering blood glucose by converting glucose to glycogen in the liver
8. Quick Reference Cheatsheet
Topic | Key Exam Facts |
|---|---|
Homeostasis | Maintenance of constant internal environment: e.g. body temp, water content |
Tropisms | Shoots: +phototropic, -geotropic; Roots: +geotropic, -phototropic |
Auxin | Accumulates on shaded side of shoot → faster elongation → bends to light |
Nervous vs Hormonal | Nervous: electrical, fast, short, precise; Hormonal: chemical, slow, long, widespread |
Reflex Arc Order | Stimulus → Receptor → Sensory neurone → Relay neurone → Motor neurone → Effector → Response |
Accommodation (Near) | Ciliary muscles contract → suspensory ligaments slack → lens thick |
Accommodation (Distant) | Ciliary muscles relax → suspensory ligaments taut → lens thin |
Pupil Reflex | Bright light: pupil constricts; Dim light: pupil dilates |
Temp Regulation | Hot: vasodilation, more sweat; Cold: vasoconstriction, no sweat, shivering |
Core Hormones | Adrenaline (fight/flight), Insulin (lower blood glucose), Testosterone (male characteristics), Oestrogen (female characteristics), Progesterone (uterus lining) |
Biology Only Hormones | ADH (water reabsorption), FSH (egg maturation), LH (ovulation) |
9. Frequently Asked
What is the difference between nervous and hormonal control?
Nervous control uses electrical impulses transmitted via neurones, is fast, short-lived and targets specific effectors. Hormonal control uses chemical messengers in the blood, is slower, longer-lasting and has widespread effects on multiple target organs.
How does auxin cause positive phototropism in shoots?
Auxin is produced at the shoot tip. When exposed to unilateral light, auxin moves to the shaded side of the shoot, stimulating faster cell elongation on that side, so the shoot bends towards the light source.
What is the correct order of a reflex arc?
The standard sequence required for full marks is: stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → response.
Going deeper
What's Next
Now that you have mastered coordination and response, you are ready to move on to related topics in the Edexcel IGCSE Biology specification. The concepts of homeostasis you learned here will underpin your study of osmoregulation and excretion, while hormonal control content will be expanded in the reproduction topic. For Biology-only students, you will explore the role of FSH and LH in the menstrual cycle in more detail in the organism reproduction unit. Practice past paper questions focused on reflex arc pathways, eye accommodation and temperature regulation to consolidate your knowledge, and make sure you can differentiate between nervous and hormonal control for 3-4 mark comparison questions.