Q: What does CORE IDEAS, Topic 3 - Energy and Equilibrium cover?
A: Connect photosynthesis, respiration, and cell signalling into a coherent energy story for H2 Biology so you can explain molecular workflows, analyse data, and model regulation in the 2026 examinations.
Why this unit is a Paper 3 favourite
Integrative thinking: Expect essays that bridge photosynthesis and respiration or tie signalling to metabolism.
Quantitative skills: Limiting factor experiments, respirometry, and chemiosmosis questions appear in Papers 2 and 4.
Systems focus: You must show how energy capture, conversion, and utilisation maintain cellular equilibrium.
Syllabus overview
Chloroplast and mitochondrion structure recognition
Light-dependent and light-independent reactions
Limiting factors of photosynthesis
Glycolysis, link reaction, Krebs cycle, oxidative phosphorylation
Aerobic vs anaerobic pathways
Chemiosmosis as a unifying mechanism
Cell signalling: ligand reception, transduction cascades, responses, insulin/glucagon control
Concept 1: Photosynthetic energy capture
Chloroplast architecture: Label thylakoid stacks (grana), intergranal lamellae, stroma, DNA, ribosomes. Relate structure to function-thylakoid membranes embed photosystems and ATP synthase; stroma hosts Calvin cycle enzymes.
Light-dependent reactions: Photons excite chlorophyll in photosystem II (P680); water photolysis supplies electrons and releases OX2
. Electrons travel through an electron transport chain, pumping protons into thylakoid lumen. Photosystem I (P700) re-excites electrons for NADP
+
reduction. Proton gradient drives ATP synthesis as protons flow through ATP synthase.
Light-independent reactions (Calvin cycle): Three phases-
Investigate how temperature, light intensity, and COX2 concentration limit photosynthesis. Plot rate vs factor; identify plateaus or optima. For Paper 4, justify controlled variables (same leaf age, constant light wavelength).
Worked data example
Given OX2 output data at varying light intensities, determine which curve was recorded at 10∘C vs 25∘C and explain enzyme kinetics and stomatal behaviour.
Concept 2: Cellular respiration pathways
Glycolysis (cytosol): Glucose to pyruvate with a net gain of 2 ATP (substrate-level phosphorylation) and 2 NADH. Emphasise energy investment vs payoff phases.
Link reaction (mitochondrial matrix): Pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA, releasing COX2 and producing NADH.
Krebs cycle (matrix): Acetyl-CoA joins oxaloacetate; through a series of decarboxylations and dehydrogenations, the cycle yields 3 NADH, 1 FADH2, 1 ATP (or GTP), and regenerates oxaloacetate per turn.
Oxidative phosphorylation (inner mitochondrial membrane): Electron carriers donate to the ETC (complexes I–IV). Energy released pumps protons into intermembrane space, establishing electrochemical gradient. ATP synthase synthesises ATP as protons return; oxygen acts as terminal electron acceptor, forming water.
Respiratory control: Chemiosmotic gradient drives ATP yield; uncouplers dissipate gradient, reducing ATP production and releasing heat (brown adipose tissue example).
ATP accounting drill
Explain why ATP yield per glucose varies (NADH shuttle differences, proton leak). Estimate theoretical yields: 2 ATP (glycolysis) + 2 via substrate-level in Krebs + ~26 via oxidative phosphorylation = ~30 ATP.
Concept 3: Anaerobic pathways
Yeast fermentation: Pyruvate to ethanol; regenerates NAD+ for glycolysis. Discuss industrial applications, redox balance.
Mammalian muscle: Pyruvate to lactate; clarifies fatigue and oxygen debt. Correlate with higher blood lactate during intense exercise questions.
Explain why ATP yield is limited (only glycolytic ATP). Discuss Cori cycle contributions to homeostasis.
Concept 4: Chemiosmosis as the unifying theme
Describe proton motive force Δp=Δψ−F2.303RTΔpH. Even if explicit calculation is rare, referencing equation shows conceptual depth. Highlight parallels:
Thylakoid lumen accumulation vs matrix usage.
Inner mitochondrial membrane gradient vs intermembrane space.
ATP synthase rotary mechanism in both organelles.
Connect to inhibitors (oligomycin blocking ATP synthase) and experimental evidence (Racker and Stoeckenius reconstituted vesicles).
Concept 5: Cell signalling and metabolic regulation
Tie in feedback control, cross-talk with sympathetic signals, and clinical context (Type II diabetes involves receptor/signalling defects).
Practice prompt
Explain how adrenaline binding to a GPCR in liver cells rapidly increases blood glucose, detailing second messengers and target enzymes.
Practical and data skills
Paper 4 photosynthesis lab: Use hydrogencarbonate indicator to monitor COX2 uptake, discuss calibration and colourimetry.
Respirometry: Set up a simple respirometer with soda lime, calculate rate using rate=Δt×massΔvolume. Account for temperature using a control respirometer.
Data interpretation: Recognise compensation point, Q10 (temperature coefficient) calculations, and graph plateau analyses.
Exam strategy reminders
Paper 2: Practise short answers on photophosphorylation, oxidative phosphorylation comparisons, and cell signalling diagrams.
Paper 3: Build essay outlines linking photosynthesis and respiration or addressing “Describe how cell signalling coordinates responses to changes in blood glucose concentration.”
Paper 4: Rehearse MMO/PDO/ACE evidence for respirometry, including calibration, repeats, and error analysis.
