AQA A-Level Biology: Homeostasis, Response and Organisms in Their Environment

Topic 6 of the AQA A-Level Biology specification -- Organisms Respond to Changes in Their Internal and External Environments -- is one of the most demanding sections of the course. It brings together physiology, biochemistry, and cell biology, and it features heavily on Paper 2 and the synoptic Paper 3. Alongside this, the ecology content on populations, succession, and nutrient cycles rounds out the material you need for a thorough understanding of how organisms interact with each other and their surroundings.

This guide works through the core content systematically, highlighting the areas that examiners target most frequently and the details that students commonly overlook.

Where This Content Appears in the Exam

AQA A-Level Biology (specification 7402) is assessed across three papers:

• Paper 1 covers Topics 1--4. It is 2 hours long, worth 91 marks, and accounts for 35% of the A-Level.
• Paper 2 covers Topics 5--8. It is 2 hours long, worth 91 marks, and accounts for 35% of the A-Level.
• Paper 3 can draw on any content from Topics 1--8. It is 2 hours long, worth 78 marks, and accounts for 30% of the A-Level.

The content in this guide falls primarily within Paper 2, but the synoptic nature of Paper 3 means that questions on homeostasis and ecology can appear there as well -- often linked to topics such as biological molecules, cell biology, or energy transfers.

Stimuli, Receptors and Effectors

All organisms must detect and respond to changes in their environment. A stimulus is a detectable change in the internal or external environment. Receptors are specialised cells or proteins that detect specific stimuli, while effectors -- muscles and glands -- carry out responses.

The key distinction at A-Level is between nervous and hormonal coordination. Nervous responses are fast, short-lived, and localised. Hormonal responses are slower, longer-lasting, and more widespread. Many physiological processes, such as blood glucose regulation, involve both systems working together.

The Nervous System

Structure of a Neuron

You need to know the structure and function of three types of neuron: sensory neurons (which carry impulses from receptors to the central nervous system), relay neurons (which connect sensory and motor neurons within the CNS), and motor neurons (which carry impulses from the CNS to effectors).

All neurons share certain features -- a cell body containing the nucleus, dendrons that carry impulses towards the cell body, and axons that carry impulses away from it. Motor neurons have long axons, while sensory neurons have long dendrons. Many neurons are surrounded by a myelin sheath, formed by Schwann cells wrapping around the axon. The gaps between adjacent Schwann cells are called nodes of Ranvier.

Resting Potential and Action Potential

At rest, a neuron maintains a resting potential of approximately -70 mV across its membrane. This is established by the sodium-potassium pump, which actively transports three sodium ions out of the cell for every two potassium ions pumped in, and by the greater permeability of the membrane to potassium ions, which leak out through potassium ion channels.

When a stimulus reaches threshold, voltage-gated sodium ion channels open, causing a rapid influx of sodium ions. This is depolarisation, and the membrane potential rises to around +40 mV. The sodium channels then close, and voltage-gated potassium channels open, allowing potassium ions to flow out. This is repolarisation. The membrane temporarily becomes more negative than the resting potential -- a phase called hyperpolarisation -- before the sodium-potassium pump restores the resting potential.

The all-or-nothing principle states that a neuron either fires a full action potential or does not fire at all. There is no variation in the size of the action potential. The intensity of a stimulus is communicated instead by the frequency of action potentials.

Factors Affecting the Speed of Conduction

Several factors influence how quickly an action potential travels along an axon:

• Myelination -- myelin acts as an electrical insulator. In myelinated neurons, the action potential jumps from one node of Ranvier to the next, a process called saltatory conduction. This is significantly faster than continuous conduction in unmyelinated neurons.
• Axon diameter -- wider axons have less resistance to the flow of ions, so impulses travel faster.
• Temperature -- higher temperatures increase the rate of ion diffusion and enzyme activity, increasing conduction speed up to a point. Beyond an optimum temperature, proteins begin to denature.

Synaptic Transmission

Synapses are the junctions between neurons. At a cholinergic synapse, the neurotransmitter is acetylcholine (ACh). When an action potential arrives at the presynaptic knob, calcium ion channels open and calcium ions diffuse in. This triggers synaptic vesicles containing ACh to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft by exocytosis. ACh binds to specific receptors on the postsynaptic membrane, causing sodium ion channels to open and generating a new action potential in the postsynaptic neuron.

The enzyme acetylcholinesterase rapidly breaks down ACh into choline and ethanoic acid (acetyl), preventing continuous stimulation. The products are reabsorbed into the presynaptic knob and used to resynthesise ACh, using energy from mitochondria.

Summation

A single release of neurotransmitter may not be sufficient to reach the threshold in the postsynaptic neuron. Temporal summation occurs when repeated impulses arrive at the same presynaptic knob in quick succession, building up enough neurotransmitter to reach threshold. Spatial summation occurs when impulses arrive simultaneously from multiple presynaptic neurons, and the combined effect of neurotransmitter from several synapses reaches threshold.

Inhibition

Some synapses are inhibitory. An inhibitory neurotransmitter causes chloride ion channels to open on the postsynaptic membrane, or potassium channels to open, making the inside of the postsynaptic neuron more negative (hyperpolarised). This makes it harder for excitatory neurotransmitters to reach threshold, effectively reducing or preventing a response.

Receptors: Pacinian Corpuscle and the Retina

The Pacinian Corpuscle

The Pacinian corpuscle detects changes in pressure. It consists of a sensory neuron ending surrounded by layers of connective tissue (lamellae). When pressure is applied, the lamellae are deformed, which stretches the membrane of the sensory neuron ending. This opens stretch-mediated sodium ion channels, sodium ions flow in, and if threshold is reached, an action potential is generated. This is an example of a generator potential.

Rods and Cones

The retina contains two types of photoreceptor: rods and cones.

• Rods are highly sensitive to light and function well in low-light conditions. Many rods share a single bipolar cell (retinal convergence), which increases sensitivity but decreases visual acuity. The pigment in rods is rhodopsin, which breaks down in light (bleaching) to generate a nerve impulse.
• Cones are less sensitive and require higher light intensities to function. Each cone typically connects to its own bipolar cell, giving high visual acuity but lower sensitivity. There are three types of cone, each sensitive to a different wavelength of light (red, green, or blue), which together enable colour vision. The pigment in cones is iodopsin.

Muscle Contraction

Structure of Skeletal Muscle

Skeletal muscle is made up of bundles of muscle fibres. Each fibre contains many myofibrils, which are made up of repeating units called sarcomeres -- the functional units of contraction.

Within a sarcomere, there are two key protein filaments:

• Actin (thin filaments) -- with binding sites that are normally blocked by the protein tropomyosin. The protein troponin is attached to tropomyosin.
• Myosin (thick filaments) -- with globular heads that can form cross-bridges with actin.

The arrangement of these filaments produces the banding pattern visible under a microscope: the I band (actin only, light), the A band (myosin, with or without overlapping actin, dark), and the H zone (myosin only, in the centre of the A band). The Z line marks the boundary of each sarcomere.

The Sliding Filament Theory

Muscle contraction occurs when actin filaments slide over myosin filaments, shortening the sarcomere. The process is as follows:

1. A nerve impulse arrives at the neuromuscular junction, causing calcium ions to be released from the sarcoplasmic reticulum.
2. Calcium ions bind to troponin, causing it to change shape and move tropomyosin away from the binding sites on actin.
3. Myosin heads bind to the exposed sites on actin, forming cross-bridges.
4. The myosin heads pivot, pulling the actin filaments towards the centre of the sarcomere (the power stroke). ADP and inorganic phosphate are released.
5. A new molecule of ATP binds to the myosin head, causing it to detach from actin.
6. ATP is hydrolysed by ATPase on the myosin head, re-cocking it for another cycle.

This cycle repeats as long as calcium ions and ATP are available. During contraction, the I band and H zone shorten, but the A band stays the same length.

Fast and Slow Twitch Muscle Fibres

• Slow twitch (Type I) fibres contract slowly but are resistant to fatigue. They have many mitochondria, a rich blood supply, and a high concentration of myoglobin. They rely on aerobic respiration and are suited to endurance activities.
• Fast twitch (Type II) fibres contract rapidly and powerfully but fatigue quickly. They have fewer mitochondria, less myoglobin, and rely more on anaerobic respiration. They store more glycogen and are suited to short bursts of intense activity.

Homeostasis

Homeostasis is the maintenance of a constant internal environment within restricted limits. It is essential because enzyme activity, membrane permeability, and other cellular processes are sensitive to changes in temperature, pH, water potential, and blood glucose concentration.

Homeostasis relies on a negative feedback mechanism: a change in a variable is detected by a receptor, which sends information to a coordination centre, which activates an effector to reverse the change. Once the variable returns to its set point, the corrective mechanism is reduced.

Positive feedback amplifies a change away from the set point. It is less common but occurs in specific situations, such as the release of oxytocin during labour and the rapid depolarisation phase of an action potential.

Control of Blood Glucose

Blood glucose concentration is regulated by the hormones insulin and glucagon, both secreted by the islets of Langerhans in the pancreas.

• When blood glucose rises (for example, after a meal), beta cells detect the change and secrete insulin. Insulin stimulates cells to take up glucose, increases the rate of respiration, and promotes the conversion of glucose to glycogen in the liver and muscles -- a process called glycogenesis.
• When blood glucose falls, alpha cells detect the change and secrete glucagon. Glucagon stimulates the liver to break down glycogen into glucose (glycogenolysis) and to produce glucose from non-carbohydrate sources such as amino acids and glycerol (gluconeogenesis).

The Second Messenger Model

Glucagon and adrenaline are water-soluble hormones that cannot cross the cell membrane. Instead, they bind to specific receptors on the surface of liver cells, activating the enzyme adenylyl cyclase inside the cell. Adenylyl cyclase converts ATP to cyclic AMP (cAMP), which acts as a second messenger. cAMP activates a cascade of enzyme reactions inside the cell, ultimately activating the enzymes that catalyse glycogenolysis.

Diabetes

• Type 1 diabetes is an autoimmune condition in which the immune system destroys the beta cells of the pancreas. The body can no longer produce insulin. It is managed with insulin injections and careful monitoring of blood glucose.
• Type 2 diabetes occurs when the body's cells become less responsive to insulin (insulin resistance), or the beta cells produce insufficient insulin. It is strongly associated with obesity, inactivity, and genetic factors. It is usually managed through diet, exercise, and sometimes medication.

Control of Blood Water Potential

The Kidney

The kidney plays a central role in osmoregulation -- the control of blood water potential. Each kidney contains around one million nephrons, the functional units of the kidney.

Ultrafiltration occurs in the Bowman's capsule. Blood enters the glomerulus at high pressure, and small molecules (water, glucose, amino acids, urea, ions) are forced out of the blood and into the Bowman's capsule. Large molecules such as proteins and blood cells remain in the blood. The basement membrane of the capillary wall acts as the filter.

Selective reabsorption occurs in the proximal convoluted tubule. All glucose and amino acids, along with most water and ions, are reabsorbed back into the blood by active transport and co-transport. The cells of the proximal convoluted tubule are adapted for this -- they have microvilli (to increase surface area), many mitochondria (to provide ATP for active transport), and numerous co-transporter proteins in their membranes.

The Loop of Henle and the Countercurrent Multiplier

The loop of Henle creates a gradient of water potential in the medulla of the kidney, which is essential for the reabsorption of water from the collecting duct.

• The descending limb is permeable to water but not to ions. Water moves out by osmosis into the surrounding medulla, which has a low (more negative) water potential.
• The ascending limb is impermeable to water but actively pumps sodium and chloride ions out into the medulla, lowering the water potential of the medulla further.

This arrangement is called the countercurrent multiplier because the fluid in the ascending and descending limbs flows in opposite directions, which maintains a concentration gradient along the length of the loop and allows a progressively lower water potential to build up deep in the medulla.

ADH and Osmoregulation

Antidiuretic hormone (ADH) is secreted by the posterior pituitary gland in response to a decrease in blood water potential (detected by osmoreceptors in the hypothalamus).

ADH increases the permeability of the collecting duct walls to water by stimulating the insertion of aquaporin water channels into the cell membranes. More water is reabsorbed by osmosis from the collecting duct into the medulla and back into the blood, producing a small volume of concentrated urine.

When blood water potential is high, less ADH is released, fewer aquaporins are inserted, less water is reabsorbed, and a large volume of dilute urine is produced. This is a clear example of negative feedback.

Required Practical: Dilution Series and Colorimetric Assay

You are expected to be able to produce a dilution series of a glucose solution and use a colorimetric assay with Benedict's reagent to estimate the concentration of glucose in an unknown solution.

The method involves:

1. Preparing a serial dilution of a known glucose concentration to produce a range of standard solutions.
2. Adding an equal volume of Benedict's reagent to each standard solution and to the unknown solution.
3. Heating all tubes in a water bath at the same temperature for the same length of time.
4. Filtering or centrifuging the resulting solutions and measuring the absorbance or transmission of each using a colorimeter.
5. Plotting a calibration curve of absorbance against known glucose concentration.
6. Using the calibration curve to determine the glucose concentration of the unknown solution.

Key points for the exam: control variables carefully (volume of reagent, temperature, time), use a colorimeter rather than subjective colour judgement, and remember that Benedict's is a semi-quantitative test unless combined with colorimetry.

Populations and Ecology

Population Growth and Limiting Factors

A population is all the organisms of one species in a habitat at a given time. When a population colonises a new environment, it typically follows a characteristic growth curve:

• Lag phase -- slow initial growth as organisms acclimatise and numbers are small.
• Log (exponential) phase -- rapid growth as resources are abundant and there is little competition.
• Stationary phase -- growth levels off as the population reaches the carrying capacity of the environment, the maximum population size that can be sustained by available resources.

Limiting factors -- such as food availability, predation, disease, and space -- determine the carrying capacity. Predator-prey relationships show cyclical fluctuations: as prey numbers increase, predator numbers rise in response; increased predation then reduces prey numbers, which in turn causes predator numbers to fall.

Competition

• Intraspecific competition occurs between individuals of the same species. It is a key density-dependent factor that regulates population size. As population density increases, competition for resources intensifies, reducing the rate of reproduction and increasing mortality.
• Interspecific competition occurs between individuals of different species. When two species compete for the same niche, one will typically outcompete the other -- a process known as competitive exclusion. This can lead to niche separation, where species evolve to exploit slightly different resources.

Succession

Succession is the gradual change in community structure over time.

• Primary succession occurs on a previously uncolonised surface, such as bare rock or a new volcanic island. Pioneer species (such as lichens and mosses) are the first to colonise. They are adapted to harsh conditions and begin to break down rock and add organic matter, creating soil. Over time, the soil becomes deeper and richer, allowing more complex plant species to establish.
• Secondary succession occurs on land that has been previously colonised but has been disturbed -- for example, after a fire or the abandonment of farmland. Because soil is already present, secondary succession proceeds more rapidly than primary succession.

In both cases, succession leads to a climax community -- a stable, self-sustaining community that is in equilibrium with the prevailing environmental conditions. In the UK, the climax community is typically deciduous woodland.

Each stage of succession changes the abiotic conditions (light levels, soil depth, humidity), making the environment more suitable for new species and less suitable for the species already present. Biodiversity generally increases during succession, reaching a maximum at or near the climax community.

Conservation and Ecosystem Management

Conservation involves managing ecosystems to maintain biodiversity. This can include maintaining habitats at a particular stage of succession (for example, through grazing, mowing, or controlled burning) to preserve species that would otherwise be outcompeted in a climax community. Effective conservation requires an understanding of the ecological processes at work and the needs of the species being protected.

Nutrient Cycles

The Nitrogen Cycle

Nitrogen is essential for the synthesis of amino acids, proteins, and nucleic acids. Although the atmosphere is approximately 78% nitrogen gas, most organisms cannot use it directly. The nitrogen cycle describes how nitrogen is converted between different forms:

• Nitrogen fixation -- nitrogen gas (N2) is converted to ammonia (NH3) by nitrogen-fixing bacteria such as Rhizobium, which live in root nodules of leguminous plants, and by free-living bacteria such as Azotobacter. Lightning can also fix small amounts of nitrogen.
• Ammonification -- decomposers (saprobionts) break down proteins and urea from dead organisms and animal waste, releasing ammonium ions (NH4+) into the soil.
• Nitrification -- nitrifying bacteria convert ammonium ions to nitrite ions (NO2-) and then to nitrate ions (NO3-). This is an aerobic process. Nitrosomonas converts ammonium to nitrite; Nitrobacter converts nitrite to nitrate. Plants absorb nitrate ions from the soil through their roots by active transport.
• Denitrification -- denitrifying bacteria (such as Pseudomonas) convert nitrate ions back to nitrogen gas, returning it to the atmosphere. This occurs under anaerobic conditions, such as in waterlogged soils.

The Phosphorus Cycle

Phosphorus is a key component of ATP, DNA, RNA, and phospholipids. Unlike nitrogen, phosphorus does not have a significant atmospheric phase. Phosphorus enters ecosystems primarily through the weathering of rocks, which releases phosphate ions into the soil. Plants absorb phosphate ions through their roots, and phosphorus passes through the food chain. When organisms die, decomposers release phosphate back into the soil. Phosphorus can also be lost from ecosystems through leaching into waterways, where it may accumulate in sediments.

Environmental Issues and Sustainability

Modern farming practices can disrupt nutrient cycles and reduce biodiversity. The excessive use of nitrogen and phosphorus-containing fertilisers can lead to eutrophication -- where nutrient run-off into waterways causes algal blooms, which block light, reduce photosynthesis, and lead to the death of aquatic organisms as oxygen levels fall due to increased decomposition.

Deforestation removes vegetation that would otherwise take up nutrients from the soil, increasing the rate of leaching and soil erosion. Monoculture farming reduces biodiversity and can deplete specific soil nutrients. Sustainable approaches -- such as crop rotation, the use of organic fertilisers, biological pest control, and maintaining hedgerows and field margins -- help to balance productivity with conservation of the environment.

Exam Strategy for This Topic

Homeostasis and response questions often demand precise, sequential descriptions. For example, a question on the action potential requires you to name the specific ion channels, state the direction of ion movement, and describe the changes in membrane potential at each stage. Vague references to "ions moving across the membrane" will not score marks.

For ecology questions, you must be able to explain processes in terms of named organisms and specific chemical conversions. The nitrogen cycle, in particular, frequently appears in extended-response questions -- practise writing out the full sequence with the names of the bacteria and the chemical changes at each stage.

Diagram-based questions on the sarcomere and the nephron are common. Make sure you can label these structures accurately and explain how their features relate to their functions.

Prepare with LearningBro

These topics require a combination of detailed factual recall and the ability to explain complex processes clearly. Practising with structured questions and receiving immediate feedback is one of the most effective ways to identify gaps in your knowledge and build confidence.

• AQA A-Level Biology: Homeostasis and Response
• AQA A-Level Biology: Organisms and Environment in Depth

Use these courses to work through the material systematically, test yourself on each sub-topic, and make sure you are fully prepared for Paper 2 and the synoptic challenges of Paper 3.