Responding to the Environment

What you'll learn

This guide covers how organisms detect and respond to changes in their environment — essential for survival. You'll master the nervous and hormonal systems in humans, temperature regulation, blood glucose control, and how plants respond to stimuli. These concepts are fundamental to WJEC GCSE Biology and frequently appear in Paper 1 and Paper 2 questions.

Key terms and definitions

Homeostasis — the maintenance of a constant internal environment despite external changes

Stimulus — a change in the environment that is detected by receptors

Receptor — a cell or organ that detects stimuli (light, sound, temperature, chemicals)

Effector — a muscle or gland that carries out a response to a stimulus

Neurone — a specialised nerve cell that transmits electrical impulses rapidly around the body

Synapse — the junction between two neurones where chemical transmission occurs

Hormone — a chemical messenger produced by endocrine glands and transported in the blood plasma

Tropism — a plant growth response to a directional stimulus

Core concepts

The nervous system structure and function

The nervous system enables rapid responses to stimuli through electrical impulses. It consists of two main parts:

Central Nervous System (CNS)

• Brain and spinal cord
• Coordinates all responses
• Processes information from receptors

Peripheral Nervous System

• All nerves outside the CNS
• Carries information to and from the CNS
• Contains sensory and motor neurones

Types of neurones:

Sensory neurones

• Carry impulses from receptors to the CNS
• Long dendrites, cell body in middle, short axon

Relay neurones

• Found in the CNS
• Connect sensory and motor neurones
• Many short dendrites and axons

Motor neurones

• Carry impulses from CNS to effectors
• Short dendrites, cell body at one end, long axon

The reflex arc

A reflex action is a rapid, automatic response that doesn't involve conscious thought. This protects the body from harm.

Pathway:

1. Stimulus detected by receptor
2. Impulse travels along sensory neurone to CNS
3. Impulse passes across synapse to relay neurone
4. Relay neurone connects to motor neurone via another synapse
5. Motor neurone carries impulse to effector
6. Effector produces response

Examples include:

• Pupil reflex (bright light → pupil constricts)
• Knee-jerk reflex
• Withdrawing hand from hot surface

Synapse structure and function

Synapses ensure impulses travel in one direction only:

1. Electrical impulse arrives at end of first neurone
2. Triggers release of chemical neurotransmitters into synaptic cleft (gap)
3. Neurotransmitters diffuse across gap (approximately 20 nanometres)
4. Chemicals bind to receptors on second neurone membrane
5. This triggers a new electrical impulse in the second neurone
6. Enzymes break down neurotransmitters to prevent continuous stimulation

Hormonal control systems

The endocrine system uses hormones for slower, longer-lasting responses than the nervous system.

Key endocrine glands:

Gland	Hormone	Function
Pituitary	Many hormones	"Master gland" controls other glands
Thyroid	Thyroxine	Controls metabolic rate
Pancreas	Insulin and glucagon	Control blood glucose
Adrenal	Adrenaline	"Fight or flight" response
Ovaries	Oestrogen, progesterone	Female reproductive hormones
Testes	Testosterone	Male reproductive hormone

Comparison of nervous and hormonal systems:

Nervous system:

• Very rapid response
• Short-lived effects
• Impulses travel along neurones
• Acts on specific locations

Hormonal system:

• Slower response
• Long-lasting effects
• Hormones travel in blood
• Can affect multiple target organs

Homeostasis and temperature regulation

Homeostasis maintains optimal conditions for enzyme action and cellular function. Key controlled factors include:

• Body temperature (37°C)
• Blood glucose concentration
• Water levels

Temperature control mechanisms

The thermoregulatory centre in the brain monitors blood temperature. Skin temperature receptors provide additional information.

When body temperature rises:

• Vasodilation — blood vessels near skin surface dilate (widen)
• More blood flows near surface, increasing heat loss by radiation
• Sweat glands produce more sweat
• Evaporation of sweat removes heat energy from skin
• Hair muscles relax so hairs lie flat

When body temperature falls:

• Vasoconstriction — blood vessels near skin surface constrict (narrow)
• Less blood flows near surface, reducing heat loss
• Sweat production decreases
• Shivering — rapid muscle contractions generate heat through respiration
• Hair muscles contract making hairs stand up, trapping insulating air layer

Blood glucose regulation

Blood glucose must remain relatively constant (approximately 90 mg per 100 cm³) to provide cells with glucose for respiration.

The pancreas monitors and controls blood glucose concentration:

When blood glucose is too high (after eating):

1. Pancreas detects high glucose
2. Releases hormone insulin into blood
3. Insulin travels to liver and muscle cells
4. Causes glucose to move from blood into cells
5. Liver converts glucose to glycogen for storage
6. Blood glucose concentration decreases

When blood glucose is too low (during exercise/between meals):

1. Pancreas detects low glucose
2. Releases hormone glucagon into blood
3. Glucagon travels to liver cells
4. Causes liver to convert glycogen back to glucose
5. Glucose released into blood
6. Blood glucose concentration increases

This is an example of negative feedback — when a level rises too high, mechanisms reduce it; when it falls too low, mechanisms increase it. This maintains equilibrium.

Type 1 diabetes

• Pancreas produces insufficient or no insulin
• Blood glucose can rise to dangerously high levels
• Treatment: regular insulin injections, careful diet monitoring, exercise
• Usually develops in childhood/early adulthood
• Caused by destruction of insulin-producing cells (autoimmune response)

Type 2 diabetes

• Body cells stop responding properly to insulin (insulin resistance)
• Blood glucose regulation fails
• Risk factors: obesity, poor diet, lack of exercise, genetic predisposition
• Treatment: lifestyle changes (diet and exercise), medication if needed
• More common in older adults but increasingly affecting younger people

Plant responses to stimuli

Plants respond to environmental changes through growth responses called tropisms. These are slower than animal responses but don't require a nervous system.

Types of tropism:

Phototropism — growth response to light direction

• Shoots show positive phototropism (grow towards light)
• Maximises photosynthesis
• Roots show negative phototropism (grow away from light)

Gravitropism (geotropism) — growth response to gravity

• Roots show positive gravitropism (grow downwards)
• Anchors plant and reaches water
• Shoots show negative gravitropism (grow upwards)

Role of auxin

Auxin is a plant hormone controlling growth responses:

Shoot phototropism mechanism:

1. Auxin produced in shoot tip
2. Light causes auxin to accumulate on shaded side
3. Higher auxin concentration on shaded side causes cells to elongate more
4. Shoot bends towards light

Root gravitropism mechanism:

1. Auxin produced in root tip
2. Gravity causes auxin to accumulate on lower side
3. In roots, high auxin concentration inhibits cell elongation
4. Upper side grows more, root bends downwards

Practical investigations

You may need to describe investigations into plant responses:

• Using cress seedlings in different light conditions
• Observing root/shoot growth in different orientations
• Controlling variables (temperature, water, seed type)
• Measuring angle of curvature or growth rate

Worked examples

Example 1: Reflex arc pathway (3 marks)

Question: Describe the pathway of a reflex arc when a person touches a hot object.

Mark scheme answer:

• Stimulus (heat) detected by temperature receptors in skin [1 mark]
• Impulse travels along sensory neurone to CNS / spinal cord, then via relay neurone to motor neurone [1 mark]
• Motor neurone carries impulse to effector / muscle which contracts to move hand away [1 mark]

Example 2: Blood glucose control (4 marks)

Question: Explain how the body responds when blood glucose concentration becomes too high after a meal.

Mark scheme answer:

• Pancreas detects high blood glucose [1 mark]
• Pancreas releases insulin into the blood [1 mark]
• Insulin causes liver/muscle cells to take up glucose from blood [1 mark]
• Liver converts glucose to glycogen for storage / glucose removed from blood [1 mark]

Example 3: Plant tropisms (5 marks)

Question: A seedling is placed horizontally. Explain how the shoot responds to grow upwards.

Mark scheme answer:

• Shoot shows negative gravitropism / grows against gravity [1 mark]
• Auxin produced in shoot tip [1 mark]
• Auxin/hormone accumulates on lower side due to gravity [1 mark]
• Cells on lower side elongate more than upper side [1 mark]
• Causing shoot to bend/grow upwards [1 mark]

Common mistakes and how to avoid them

• Confusing nervous and hormonal responses — Remember nervous system is electrical impulses (fast, short-lived), hormonal system uses chemicals in blood (slower, long-lasting). Don't say hormones travel along neurones.

• Incorrect synapse direction — Synapses only allow transmission in one direction (chemical released from one neurone only). Don't describe impulses travelling backwards.

• Mixing up insulin and glucagon — Insulin decreases blood glucose (think: insulin = in to cells). Glucagon increases blood glucose (think: glucagon = glucose gone from storage).

• Forgetting negative feedback — Homeostasis works through negative feedback loops. When a factor increases, the body responds to decrease it, and vice versa. Don't describe responses that would make the problem worse.

• Vague plant responses — Be specific: state which part of the plant (shoot/root), the type of tropism (positive/negative, photo/gravi), and that auxin causes unequal growth, not movement.

• Confusing vasodilation and vasoconstriction — Vasodilation widens blood vessels when hot (more heat loss). Vasoconstriction narrows vessels when cold (less heat loss). Don't say blood vessels move closer/further from skin.

Exam technique for "Responding to the Environment"

• Use sequence correctly — For reflex arcs and control systems, marks depend on the correct order. Use numbered points or sequence words (first, then, next, finally) to show the pathway clearly.

• Name specific structures — Don't write "nerve" when you mean neurone. Specify sensory/motor/relay neurone. Name the pancreas, liver, or specific glands. Generic terms lose marks.

• Match detail to marks — A 1-mark question needs one clear point. A 4-mark "explain" question needs 4 distinct points showing mechanism/cause-effect. Don't write paragraphs for 1-mark answers or single sentences for extended responses.

• Distinguish "describe" and "explain" — Describe = state what happens. Explain = state what happens AND why/how it happens. "Explain" questions always require more depth and causal links.

Quick revision summary

Organisms respond to environmental changes for survival. The nervous system uses electrical impulses and neurones for rapid responses via reflex arcs. The hormonal system uses chemical messengers transported in blood for slower, sustained responses. Homeostasis maintains constant internal conditions through negative feedback — key examples include temperature regulation and blood glucose control by insulin and glucagon. Plants respond to light and gravity through tropisms controlled by auxin hormone causing unequal growth.