Comprehensive AQA A-Level Biology Notes for Year 2

Course Introduction

You're diving into Year 2 A-Level Biology with AQA - this is where the real magic happens! This year builds on everything you've learned, taking you deeper into the fascinating world of how organisms capture, transfer, and use energy.

The content covers major biological processes that keep life ticking. You'll explore how plants capture sunlight and convert it to chemical energy, how all living things release that energy through respiration, and how energy flows through entire ecosystems.

Understanding these concepts isn't just about passing exams - it's about grasping the fundamental principles that govern all life on Earth. Every breath you take, every meal you eat, every movement you make involves these processes working together in perfect harmony.

Top Tip: Think of this year as connecting the dots - you'll see how molecular processes link to whole organism functions and ecosystem patterns.

Energy Transfer - Photosynthesis

Ever wondered how plants basically eat sunlight? Photosynthesis is nature's ultimate solar panel system, and it's happening in billions of chloroplasts right now. These tiny green powerhouses have a clever double-membrane design that maximises light capture.

Inside chloroplasts, you'll find thylakoid membranes packed with photosynthetic pigments attached to proteins called photosystems. Think of these as biological solar panels - Photosystem I works best with 700nm wavelength light, whilst the stroma (the fluid bit) handles the chemical assembly line work.

The whole process splits into two main stages. The light-dependent reaction happens in the thylakoids where water gets split (photolysis), electrons get excited through photoionisation, and energy is captured as ATP through chemiosmotic theory. Meanwhile, the Calvin cycle in the stroma fixes carbon dioxide using the enzyme RUBISCO, turning it into glucose through carbon fixation, reduction, and regeneration steps.

Leaves are perfectly designed for this job - thin for short diffusion distances, transparent cuticles, loads of chloroplasts in the upper mesophyll, and stomata that open and close based on light intensity. It's biological engineering at its finest.

Remember: Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration - Releasing Energy

Right, so plants make glucose through photosynthesis - but how do all living things actually get energy from it? Enter aerobic respiration, the process that breaks down glucose to release ATP (the cell's energy currency). It's like a controlled explosion that happens in three main stages.

Glycolysis kicks things off in the cytoplasm, splitting 6-carbon glucose into two 3-carbon pyruvate molecules. No oxygen needed here - it's completely anaerobic and gives you a net gain of 2 ATP plus 2 reduced NAD. Think of it as the appetiser before the main course.

Next up is the link reaction, which connects glycolysis to the main event. Pyruvate gets oxidised to form acetyl coenzyme A, producing reduced NAD and carbon dioxide in the process. This happens in the mitochondrial matrix where oxygen starts becoming important.

The Krebs cycle is where the real action happens. This series of oxidation-reduction reactions in the mitochondrial matrix breaks down the 2-carbon acetyl groups completely, regenerating the 4-carbon molecule that keeps the cycle spinning. You get loads of reduced FAD and reduced NAD, which are the real prizes - these power the final stage called oxidative phosphorylation.

Key Point: The Krebs cycle doesn't just make energy - it provides building blocks for fatty acids, amino acids, and even chlorophyll!

Energy and Ecosystems

Think of ecosystems as nature's economy where energy is the currency that flows from organism to organism. Producers (mainly plants) are like energy factories, capturing sunlight and converting it into chemical energy that powers entire food webs. Consumers then get their energy by munching on other organisms rather than making their own.

Biomass is basically the total mass of living material in an area, and scientists measure it using calorimetry - literally burning samples to see how much energy they contain. The key equation here is NPP = GPP - R, where net primary production equals gross primary production minus respiratory losses.

Energy transfer between trophic levels isn't very efficient - usually only about 10% makes it from one level to the next. That's why farmers use clever tricks to maximise energy transfer: battery farming reduces movement energy losses, heated enclosures stop animals wasting energy staying warm, and pesticides simplify food webs by removing competition.

For consumers, the energy equation is N = I - $F + R$, where net production equals ingested food minus faeces/urine losses and respiratory losses. This explains why there are always more plants than herbivores, and more herbivores than carnivores - the energy just isn't there to support massive populations at higher levels.

Farming Insight: Reducing animal movement and controlling temperature can dramatically increase the efficiency of energy transfer in agricultural systems.

Nutrient Cycles

Plants might photosynthesise brilliantly, but they still need essential nutrients like nitrogen and phosphorus to build proteins and DNA. The nitrogen cycle is like nature's recycling system, constantly moving nitrogen between the atmosphere, soil, and living organisms through four key processes.

Nitrogen fixation converts atmospheric nitrogen gas into ammonia that plants can actually use - thanks to specialised bacteria that have mastered this tricky chemistry. Nitrification then converts ammonia into nitrites and finally nitrates (the form plants prefer). When organisms die, ammonification breaks down their proteins back to ammonia, whilst denitrification completes the circle by converting nitrates back to nitrogen gas.

The phosphorus cycle works differently since phosphorus doesn't hang out in the atmosphere much. Instead, it cycles between rocks, soil, water, and living organisms through erosion, absorption, and decomposition processes.

Farmers often add fertilisers to boost these nutrient levels - either organic ones (like manure) or artificial ones (mined and processed). But there's a catch: leaching can wash nitrates into waterways, leading to eutrophication where algae bloom, use up oxygen, and kill fish.

Different bacteria play crucial roles: mycorrhizae help plants absorb nutrients, nitrogen-fixing bacteria provide ammonia, and denitrifying bacteria complete the nitrogen cycle under anaerobic conditions.

Environmental Note: Excess fertiliser use can lead to eutrophication - rapid plant growth that depletes oxygen and kills aquatic life.

Response to Stimuli

Living things need to sense and respond to their environment to survive - it's basically biological common sense. Taxis and kinesis are simple movement responses that help organisms find favourable conditions and avoid nasty ones.

Taxis involves moving your whole body towards good stuff (positive taxis) or away from bad stuff (negative taxis) - like bacteria swimming towards food or away from toxins. Kinesis is more about changing how fast you move and turn - if you hit unfavourable conditions, you increase your turning rate to get back to the good zone quickly.

Plants can't walk around, but they're not helpless. Tropisms are growth responses where plants bend towards or away from stimuli. The star player here is IAA (indoleacetic acid), an auxin that controls cell elongation. Light hitting one side of a shoot causes IAA to move to the darker side, making those cells grow faster and bending the shoot towards the light - that's positive phototropism.

Your nervous system coordinates responses through a pathway: stimulus → receptor → sensory neurone → coordinator → motor neurone → effector → response. The Pacinian corpuscle is a brilliant example - it's a pressure receptor with stretch-mediated sodium channels that only open when deformed, creating a generator potential.

Plant Trick: IAA moves away from light in shoots but towards gravity in roots, explaining why shoots grow up and roots grow down!

Vision and Heart Control

Your eyes contain two types of photoreceptors that work completely differently. Rod cells are the night vision specialists - they contain rhodopsin pigment and show retinal convergence where multiple rods connect to one bipolar cell. This gives brilliant sensitivity in low light but poor visual acuity (you can't distinguish fine details).

Cone cells are the colour vision experts. You've got three types (red, blue, green sensitive) containing iodopsin pigment that needs brighter light to function. Each cone connects to its own bipolar cell, giving excellent visual acuity but poor low-light performance. They're concentrated in the fovea for sharp central vision.

Your heart is a self-contained electrical system controlled by the sino-atrial node (the natural pacemaker) in the right atrium. It generates electrical impulses that spread across the atria, then pause at the atrioventricular node before travelling down the Bundle of His and Purkyne fibres to coordinate ventricular contraction.

An ECG shows this electrical activity: P wave (atrial contraction), QRS complex (ventricular contraction), and T wave (ventricular relaxation). Baroreceptors detect blood pressure changes whilst chemoreceptors monitor oxygen, carbon dioxide, and pH levels, feeding back to control heart rate through the autonomic nervous system.

Heart Math: Cardiac output = stroke volume × heart rate - this equation appears frequently in exams!

Nervous System Control

Your autonomic nervous system runs on autopilot, controlling everything you don't consciously think about. The sympathetic nervous system uses noradrenaline to speed things up during stress or exercise (fight-or-flight response), whilst the parasympathetic nervous system uses acetylcholine to slow things down during rest and digestion.

Motor neurones are perfectly designed communication cables. The cell body contains loads of rough ER for making neurotransmitters, whilst the long axon carries signals and dendrites receive them. Schwann cells wrap around the axon creating a myelin sheath - this lipid-rich insulation speeds up transmission and provides protection.

The nodes of Ranvier are gaps in the myelin where the actual electrical action happens. This creates saltatory conduction where electrical signals literally jump from node to node, massively increasing transmission speed compared to non-myelinated neurones.

You've got three main neurone types: sensory neurones carry signals from receptors to the CNS, motor neurones carry signals from CNS to effectors, and intermediate/relay neurones connect the other two. The direction is always the same: dendrites → cell body → axon.

Speed Factor: Myelinated axons conduct impulses much faster than unmyelinated ones of the same diameter - it's all about that insulation!

Nerve Impulses

Nerve impulses are basically controlled electrical explosions racing along your neurones. At rest, your axon membrane sits at -65mV (the resting potential) thanks to sodium-potassium pumps that constantly move 3 sodium ions out and 2 potassium ions in.

The axon membrane contains voltage-gated ion channels that open and close based on electrical changes. When a strong enough stimulus hits, sodium channels open and sodium ions flood in down their electrochemical gradient, causing depolarisation. This triggers more channels to open in a positive feedback loop.

The action potential reaches +40mV before sodium channels close and potassium channels open, starting repolarisation. So much potassium leaves that the membrane becomes even more negative than normal (hyperpolarisation) before the sodium-potassium pumps restore the resting potential.

Here's the clever bit: each action potential triggers the next one along the axon, creating a self-propagating wave that maintains its strength over long distances. In myelinated neurones, the action potential jumps from node to node (saltatory conduction), which is much faster than the continuous conduction in non-myelinated axons.

The beauty of this system is that it's all-or-nothing - either you get a full action potential or you don't. This digital system prevents signal degradation over long distances.

Key Concept: Action potentials are self-propagating and maintain constant amplitude - they don't get weaker as they travel along the axon.

Action Potential Propagation

The final piece of the nerve impulse puzzle is understanding exactly how these electrical signals maintain their strength and travel in one direction. Once an action potential starts, it creates a domino effect that's impossible to stop.

At +40mV, sodium channels slam shut and potassium channels open wide, pumping positive charge out of the axon. This repolarisation actually overshoots, creating hyperpolarisation where the inside becomes more negative than the normal resting potential of -65mV.

The refractory period is crucial - during this time, the axon can't fire another action potential. This ensures impulses travel in one direction only and prevents the signal from going backwards. It's like a biological one-way valve built into the system.

Myelinated axons are the Formula 1 cars of the nervous system. The fatty myelin sheath acts as electrical insulation, forcing the action potential to jump between nodes of Ranvier. This saltatory conduction is incredibly efficient - signals can travel over 100 metres per second compared to just 2 metres per second in unmyelinated fibres.

The action potential maintains exactly the same size and shape as it travels - there's no signal degradation over distance. This is completely different from electrical cables where signals get weaker, and it's why your brain can control your toes just as precisely as your fingers.

Speed Demon: Saltatory conduction in myelinated neurones can be 50 times faster than conduction in unmyelinated neurones of the same diameter!