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This lesson brings together the concepts of photosynthesis and respiration into a unified understanding of energy transfer in living systems. It covers the role of ATP, the coupling of metabolic reactions, and the integration of anabolic and catabolic pathways as required by the Edexcel A-Level Biology (9BI0) specification.
Adenosine triphosphate (ATP) is the immediate energy source for virtually all cellular processes. It is often described as the "energy currency" of the cell.
ATP consists of:
When the terminal phosphate bond is broken by the enzyme ATPase (or ATP hydrolase), energy is released:
ATP + H₂O → ADP + Pᵢ + energy (~30.5 kJ mol⁻¹)
This is an exergonic (energy-releasing) reaction.
ATP is synthesised from ADP and inorganic phosphate (Pᵢ) in a condensation reaction:
ADP + Pᵢ + energy → ATP + H₂O
This is an endergonic (energy-requiring) reaction.
| Method of ATP synthesis | Location | Mechanism |
|---|---|---|
| Substrate-level phosphorylation | Cytoplasm (glycolysis), mitochondrial matrix (Krebs cycle) | Direct transfer of phosphate from substrate to ADP |
| Oxidative phosphorylation | Inner mitochondrial membrane | Chemiosmosis driven by ETC |
| Photophosphorylation | Thylakoid membrane | Chemiosmosis driven by light-dependent reactions |
Exam Tip: ATP is not a long-term energy store. A human uses approximately 40–75 kg of ATP per day, but the body contains only about 50 g at any time. ATP is rapidly recycled — the ATP/ADP cycle turns over approximately every minute.
| Property | Explanation |
|---|---|
| Releases manageable amounts of energy | The hydrolysis of one phosphate bond releases ~30.5 kJ mol⁻¹ — enough for cellular reactions without excessive heat |
| Small, soluble molecule | Easily transported within the cell to where energy is needed |
| Rapidly recycled | The ATP ⇌ ADP + Pᵢ cycle is fast and efficient |
| Universal | Used by all living organisms in all kingdoms |
| Cannot pass through the cell membrane | Ensures that energy produced in a cell stays within that cell |
| Couples exergonic and endergonic reactions | ATP hydrolysis can be directly linked to energy-requiring processes (phosphorylation of substrates) |
In metabolism, exergonic reactions (which release energy) are coupled to endergonic reactions (which require energy) through ATP.
This means the energy from food is not transferred directly to cellular processes — instead, it passes through ATP as an intermediary.
In many cases, ATP transfers its terminal phosphate group directly to a substrate, forming a phosphorylated intermediate. This makes the substrate more reactive and lowers the activation energy of subsequent reactions.
Example: In glycolysis, glucose is phosphorylated to glucose 6-phosphate by hexokinase using ATP. The phosphorylation activates the glucose molecule for further reactions.
Metabolism is the sum of all chemical reactions occurring in an organism. It is divided into:
| Type | Definition | Examples | Energy |
|---|---|---|---|
| Anabolism | Building up complex molecules from simpler ones | Protein synthesis, DNA replication, glycogen synthesis, photosynthesis (Calvin cycle) | Requires energy (endergonic) |
| Catabolism | Breaking down complex molecules into simpler ones | Glycolysis, Krebs cycle, digestion, hydrolysis of glycogen | Releases energy (exergonic) |
Catabolic reactions provide the energy and raw materials for anabolic reactions:
Photosynthesis and respiration are complementary processes at the ecosystem level:
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Type | Anabolic (builds glucose) | Catabolic (breaks down glucose) |
| Energy transformation | Light → chemical (glucose) | Chemical (glucose) → ATP |
| Inputs | CO₂ + H₂O + light | Glucose + O₂ |
| Outputs | Glucose + O₂ | CO₂ + H₂O + ATP |
| Organisms | Photoautotrophs (plants, algae, cyanobacteria) | All living organisms |
| Location | Chloroplasts | Cytoplasm + mitochondria |
Exam Tip: Do not state that photosynthesis is the "reverse" of respiration. While the overall equations are superficially opposite, they involve completely different pathways, enzymes, and cellular locations.
The intermediates of photosynthesis and respiration serve as precursors for the synthesis of many other biological molecules. This illustrates how metabolic pathways are interconnected.
| Intermediate | Used to synthesise |
|---|---|
| Triose phosphate (G3P) | Glycerol (for lipids), glucose (via gluconeogenesis) |
| Pyruvate | Alanine and other amino acids (by transamination) |
| Acetyl CoA | Fatty acids (by lipogenesis), cholesterol |
| Krebs cycle intermediates (α-ketoglutarate, oxaloacetate) | Amino acids (glutamate, aspartate) by transamination |
| Glucose 6-phosphate | Glycogen (glycogenesis), pentose sugars (pentose phosphate pathway for nucleotides) |
| Intermediate | Used to synthesise |
|---|---|
| G3P (triose phosphate) | Glucose, sucrose, starch, cellulose, amino acids (with nitrogen), lipids, nucleotides |
| GP | Can be redirected to amino acid synthesis in some conditions |
Coenzymes play essential roles in transferring chemical groups between metabolic reactions:
| Coenzyme | Function | Key pathway |
|---|---|---|
| NAD⁺ / Reduced NAD | Hydrogen carrier (transfers H⁺ + e⁻) | Glycolysis, link reaction, Krebs cycle → ETC |
| FAD / Reduced FAD | Hydrogen carrier | Krebs cycle → ETC |
| NADP⁺ / Reduced NADP | Hydrogen carrier | Light-dependent reactions → Calvin cycle |
| Coenzyme A (CoA) | Carries acetyl groups | Link reaction → Krebs cycle |
| ATP / ADP | Energy carrier (phosphoryl group transfer) | All metabolic pathways |
Exam Tip: Note that NAD⁺ and NADP⁺ are different coenzymes used in different pathways. NAD⁺ is used in respiration; NADP⁺ is used in photosynthesis. Do not confuse them in your answers.
The energy content of different nutrients can be measured using a calorimeter (burning the food and measuring the temperature rise of water).
| Nutrient | Energy value (kJ g⁻¹) | Explanation |
|---|---|---|
| Carbohydrate | ~16 | Moderate number of C–H bonds |
| Protein | ~17 | Similar to carbohydrates; nitrogen is not fully oxidised |
| Lipid | ~39 | Many C–H bonds; highly reduced; yield more energy per gram |
| Law | Application to biology |
|---|---|
| First law (energy cannot be created or destroyed) | Energy is converted between forms (light → chemical → kinetic → heat) but the total remains constant |
| Second law (entropy tends to increase) | No energy conversion is 100% efficient; some energy is always lost as heat. Living organisms maintain low entropy locally by continuously using energy. |
The efficiency of ATP production from glucose can be calculated:
Efficiency = (energy stored in ATP / total energy in glucose) × 100%
Exam Tip: The efficiency of aerobic respiration (~34%) is much higher than many human-made engines. In exam calculations, show your working clearly and remember that the rest of the energy is dissipated as heat, not "lost" — energy is conserved (first law of thermodynamics).
Metabolic pathways are regulated to ensure that the cell produces the right amounts of each molecule at the right time:
| Mechanism | Example |
|---|---|
| Allosteric regulation | PFK is inhibited by ATP and activated by AMP |
| End-product inhibition | Excess product inhibits an early enzyme in the pathway (negative feedback) |
| Substrate availability | Pathways run faster when substrates are abundant |
| Compartmentalisation | Separating pathways in different organelles prevents unwanted interactions (e.g. glycolysis in cytoplasm, Krebs cycle in matrix) |
| Gene expression | Enzymes are only synthesised when needed (induction and repression) |
| Term | Definition |
|---|---|
| Metabolism | The sum of all chemical reactions in an organism |
| Anabolism | Metabolic reactions that build complex molecules from simpler ones, requiring energy |
| Catabolism | Metabolic reactions that break down complex molecules, releasing energy |
| ATP | Adenosine triphosphate; the universal energy currency of cells |
| Coenzyme | A non-protein organic molecule required by an enzyme to function; often acts as a carrier |
| Phosphorylation | The addition of a phosphate group to a molecule, often from ATP |
| Coupling | Linking an exergonic reaction to an endergonic reaction via ATP |
This material sits in Edexcel 9BI0 Topic 5 (On the Wild Side — Photosynthesis, Energy and Ecosystems) as the synoptic capstone of the topic — the lesson that integrates everything that has come before into a single, coherent map of cellular energy flow. Catabolic synoptic ties: lesson 4 (glycolysis) — the cytoplasmic gateway that all hexoses pass through; lesson 5 (link reaction and Krebs cycle) — the central hub where carbohydrate, lipid and amino-acid carbons converge as acetyl-CoA; lesson 6 (oxidative phosphorylation and chemiosmosis) — the ATP-yielding endpoint; lesson 7 (anaerobic respiration and respiratory substrates) — the fermentation fallback and the C/H/O ratios that set energy yield. Anabolic counterpart: lessons 1–3 (photosynthesis) — the source of the reduced carbon skeletons that respiration consumes, with the Calvin cycle producing triose phosphate that flows directly into glycolysis-in-reverse (gluconeogenesis-style biosynthesis) or storage as starch and sucrose. Wider synoptic ties: Topic 1 (biological molecules) — carbohydrates, lipids and proteins as alternative respiratory substrates with characteristic RQ values; Topic 7 (exercise physiology and VO2max) — whole-body integration of all metabolic pathways during increasing workload; Topic 8 (insulin and glucagon) — hormonal coordination of fed-versus-fasted metabolic states. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks): Palmitate (a C16 saturated fatty acid) is fully oxidised in skeletal muscle. β-oxidation removes acetyl-CoA two carbons at a time, producing reduced NAD and reduced FAD at each cycle. Activation of palmitate to palmitoyl-CoA in the cytoplasm consumes 2 ATP equivalents.
(a) State how many cycles of β-oxidation a single palmitate molecule undergoes, and how much acetyl-CoA, reduced NAD and reduced FAD this generates. (3)
(b) Calculate the net ATP yield per palmitate molecule, taking the activation cost into account and assuming oxidative phosphorylation yields ~2.5 ATP per reduced NAD and ~1.5 ATP per reduced FAD. (4)
(c) Compare this with the ~30–32 ATP per glucose and account for the difference. (1)
Solution with mark scheme:
(a) M1 (AO1) — Palmitate has 16 carbons; 2 carbons are removed per cycle; the final cycle splits a 4-carbon intermediate into 2 acetyl-CoA in one step. 7 cycles of β-oxidation are required to fully degrade palmitate.
M1 (AO2) — Each cycle yields 1 acetyl-CoA + 1 reduced NAD + 1 reduced FAD; the seventh cycle yields 2 acetyl-CoA. Total acetyl-CoA = 8; reduced NAD from β-oxidation = 7; reduced FAD from β-oxidation = 7.
A1 (AO2) — The 8 acetyl-CoA enter the Krebs cycle (8 turns), each producing 3 reduced NAD + 1 reduced FAD + 1 ATP by substrate-level phosphorylation. From Krebs: 24 reduced NAD + 8 reduced FAD + 8 ATP.
(b) M1 (AO2) — Grand-total reduced NAD = 7 (β-ox) + 24 (Krebs) = 31; grand-total reduced FAD = 7 (β-ox) + 8 (Krebs) = 15; substrate-level ATP from Krebs = 8.
M1 (AO2) — ATP from oxidative phosphorylation = (31 × 2.5) + (15 × 1.5) = 77.5 + 22.5 = 100 ATP.
M1 (AO2) — Gross ATP = 100 (ox-phos) + 8 (substrate-level) = 108 ATP.
A1 (AO3) — Net ATP = 108 − 2 (activation cost of palmitate → palmitoyl-CoA) = ~106 ATP per palmitate.
(c) A1 (AO3) — Palmitate yields ~3.5× more ATP per molecule than glucose (~30–32). The reason is chemical: fatty-acid carbons are far more reduced than carbohydrate carbons (more C–H bonds, fewer C–O bonds), so complete oxidation generates more reduced coenzymes per carbon and per molecule. (Total: 8 marks; M7 A1.)
Question (6 marks): Following an overnight fast, blood glucose falls and glucagon rises. Explain how the metabolic state of the liver and skeletal muscle shifts from the fed state (high insulin) to the fasted state (high glucagon), with reference to glycogen handling, gluconeogenesis, lipolysis and ketogenesis.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Identifying that insulin promotes glycogen synthesis (glycogenesis), lipogenesis and glucose uptake into muscle and adipose tissue in the fed state |
| 2 | AO1.2 | Stating that glucagon (and rising adrenaline / epinephrine) promotes glycogenolysis in the liver, releasing glucose from glycogen stores back to the bloodstream |
| 3 | AO2.1 | Reasoning that once liver glycogen is depleted (~24 h fasting), gluconeogenesis synthesises new glucose from lactate, glycerol and gluconeogenic amino acids — running glycolysis essentially in reverse using PEP carboxykinase, fructose-1,6-bisphosphatase and glucose-6-phosphatase as the irreversible bypass enzymes |
| 4 | AO2.2 | Recognising that lipolysis in adipose tissue releases fatty acids; these travel to the liver and skeletal muscle for β-oxidation to acetyl-CoA, producing ATP without consuming glucose, sparing the limited glucose supply for glucose-dependent tissues (brain, red blood cells) |
| 5 | AO3.1 | Predicting that during prolonged fasting, hepatic acetyl-CoA exceeds Krebs cycle capacity (oxaloacetate diverted to gluconeogenesis), so liver mitochondria divert acetyl-CoA into ketogenesis, producing acetoacetate and β-hydroxybutyrate as alternative fuels for the brain |
| 6 | AO3.2 | Concluding that the fed-to-fasted switch is a coordinated, hormone-driven reorganisation of integrated metabolism — not a simple on/off of single pathways — preserving brain glucose supply through layered defences (glycogen → gluconeogenesis → ketogenesis) while skeletal muscle progressively shifts from glucose to fatty-acid oxidation |
Total: 6 marks (UMS-band-anchored at A; AO1 = 2, AO2 = 2, AO3 = 2).* Specimen question modelled on the Edexcel 9BI0 paper format. The structure mirrors Edexcel's preference for combining hormonal signalling (Topic 8) with integrated metabolism (Topic 5), demanding that candidates trace a physiological event through multiple coordinated pathways and recognise the layered redundancy that protects brain glucose supply.
Lessons 4–7 (catabolic pathways) all converge on Krebs. Glucose enters glycolysis and produces pyruvate (lesson 4); pyruvate enters the link reaction and Krebs cycle (lesson 5); reduced coenzymes feed the ETC (lesson 6); under O2 deficit the cell falls back on fermentation (lesson 7). This integrated lesson exposes the central design principle: acetyl-CoA is the metabolic crossroads — fatty acids enter as acetyl-CoA via β-oxidation, leucine and lysine enter as acetyl-CoA, ketone bodies are derived from acetyl-CoA. Whatever the substrate, the final shared pipeline is acetyl-CoA → Krebs → ETC → ATP.
Lessons 1–3 (photosynthesis) — the anabolic counterpart. The Calvin cycle (lesson 2) produces triose phosphate, which can be combined into hexose phosphates and onwards to sucrose and starch — the very substrates respiration consumes. The G3P / GP shared intermediate pool is the conceptual bridge. Photosynthesis and respiration are not simple reverses: they share intermediates but use different enzymes, locations and electron carriers (NADP+ in photosynthesis, NAD+ in respiration).
Topic 1 — biological molecules as alternative respiratory substrates. Carbohydrates, lipids and proteins all feed the central pathway but enter at different points. Carbohydrates → glucose → glycolysis (top); lipids → glycerol → glycolysis (mid) and fatty acids → β-oxidation → acetyl-CoA (Krebs entry); proteins → amino acids → various entry points after deamination — alanine and serine deaminate to pyruvate, glutamate to α-ketoglutarate, aspartate to oxaloacetate, leucine to acetyl-CoA. RQ values reflect the substrate mix: ~1.0 for carbohydrate, ~0.7 for lipid, ~0.8–0.9 for mixed protein.
Topic 7 — exercise physiology and VO2max. Whole-body metabolism integrates all of these pathways at varying intensities. Light exercise oxidises predominantly fatty acids (high RQ ~0.8); maximal exercise shifts towards glucose / glycogen (RQ approaches 1.0); prolonged endurance work depletes glycogen and forces a shift back to fatty acid oxidation. Skeletal muscle simultaneously oxidises a mixture of substrates — proportions shift with intensity, duration and fed/fasted state.
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