SAC 2 Quick Reference
Cellular Respiration & Photosynthesis
SAC 2 Master Guide — Cellular Respiration & Photosynthesis
Student: Alea Coutinho | School: Haileybury College | SAC Date: 13 May 2026
Generated: 2025-07-14 | Covers: Unit 3 AOS2 — KK4, KK5, KK6
Priority Snapshot
| Topic | Marks Weighting | Difficulty | #1 Exam Trap |
|---|---|---|---|
| KK6: Cellular Respiration (Glycolysis, Krebs, ETC) | High — typically 35–45% of SAC 2 marks | ★★★★☆ | Confusing location of stages — e.g., writing Krebs occurs on cristae instead of matrix |
| KK5: Photosynthesis (Light-dependent & Calvin Cycle) | High — typically 30–40% | ★★★☆☆ | Writing that chlorophyll is found in the stroma, or that O₂ is produced in the Calvin Cycle |
| KK4: Enzyme Regulation, Cofactors & Inhibition | Medium — typically 20–30% | ★★★★★ | Calling NAD⁺ an enzyme rather than a coenzyme; confusing competitive vs non-competitive inhibition |
| Anaerobic Pathways | Low–Medium — 10–15% | ★★☆☆☆ | Stating animals produce ethanol in anaerobic fermentation (they produce lactate) |
| Integration / Comparison | Medium — often 1–2 extended response marks | ★★★☆☆ | Treating photosynthesis and respiration as completely separate — forgetting shared molecules (ATP, NADH) |
KK4: Biochemical Pathway Regulation & Enzymes
Enzyme Structure, Active Site, Substrate Specificity
Enzymes are biological catalysts — proteins that speed up chemical reactions without being consumed. Every reaction in both photosynthesis and cellular respiration is catalysed by a specific enzyme.
- Enzymes have a specific three-dimensional region called the active site — the precise location where substrates bind.
- The shape of the active site is complementary to the shape of its specific substrate (the molecule the enzyme acts upon).
- This is described by the induced fit model: binding of the substrate causes a slight conformational change in the active site, improving the enzyme-substrate interaction and lowering activation energy.
- The result is formation of an enzyme-substrate complex, followed by product release, and the enzyme is then available to catalyse the next reaction.
Substrate specificity — each enzyme will only bind to its complementary substrate(s). In cellular respiration, for example, the enzyme hexokinase specifically phosphorylates glucose during glycolysis.
> VCAA trap: Do NOT say enzymes are "used up" or "destroyed." Enzymes are regenerated and reused.
> VCAA trap: The study design states "details of biochemical pathway mechanisms are not required" for photosynthesis and respiration — but you MUST know the names of key enzymes like Rubisco and ATP synthase.
Cofactors & Coenzymes (NAD⁺, FAD, NADP⁺)
Cofactors are non-protein molecules required for enzyme activity. A subset of cofactors called coenzymes are organic molecules that temporarily bind to enzymes and assist in transferring chemical groups between reactions.
The Haileybury workbook uses the "taxi" analogy 🏫: coenzymes are like taxis — they pick up passengers (electrons and hydrogen ions / protons) and deliver them to their destination (the electron transport chain / ETC).
Coenzyme "Taxi" Table
| Coenzyme (Unloaded) | Loaded Form | Passengers Carried | Where Loaded (produced) | Where Unloaded (used) | Pathway |
|---|---|---|---|---|---|
| NAD⁺ (nicotinamide adenine dinucleotide) | NADH | 2 electrons + 1 H⁺ (proton) | Glycolysis (cytosol); Krebs Cycle (matrix) | Electron Transport Chain (inner mitochondrial membrane / cristae) | Cellular Respiration |
| FAD (flavin adenine dinucleotide) | FADH₂ | 2 electrons + 2 H⁺ | Krebs Cycle (matrix) only | Electron Transport Chain (inner mitochondrial membrane / cristae) | Cellular Respiration |
| NADP⁺ (nicotinamide adenine dinucleotide phosphate) | NADPH | 2 electrons + 1 H⁺ | Light-dependent stage (thylakoid membrane) | Calvin Cycle / Light-independent stage (stroma) | Photosynthesis |
> VCAA trap: NAD⁺ and NADP⁺ are different molecules with different roles in different pathways. NAD⁺ is used in cellular respiration; NADP⁺ is used in photosynthesis. Do not interchange them.
> VCAA trap: FAD is only loaded in the Krebs Cycle — it is NOT involved in glycolysis.
> When NADH and FADH₂ deliver electrons to the ETC, the coenzymes are recycled back to their unloaded forms (NAD⁺ and FAD), allowing the earlier stages to continue.
Enzyme Inhibition (Competitive / Non-Competitive)
Enzyme inhibition is a key mechanism for regulating the rate of biochemical pathways.
Competitive Inhibition
- A competitive inhibitor has a shape complementary to the active site of the enzyme.
- It competes directly with the substrate for binding at the active site.
- The inhibitor does NOT change the shape of the active site.
- Effect: the rate of reaction decreases because fewer enzyme-substrate complexes form.
- Can be overcome by increasing substrate concentration — the substrate can "outcompete" the inhibitor.
Non-Competitive Inhibition
- A non-competitive inhibitor binds to a site on the enzyme that is NOT the active site — called the allosteric site.
- Binding causes a conformational change (shape change) in the enzyme, distorting the active site so the substrate can no longer bind effectively.
- Effect: rate of reaction decreases; the maximum reaction rate (Vmax) is reduced.
- Cannot be overcome by increasing substrate concentration.
| Feature | Competitive Inhibition | Non-Competitive Inhibition |
|---|---|---|
| Binding site | Active site | Allosteric (non-active) site |
| Effect on active site shape | None | Distorts / changes shape |
| Substrate overcomes it? | Yes (at high [S]) | No |
| Effect on Vmax | Unchanged (at high [S]) | Reduced |
> VCAA trap: A common error is stating that non-competitive inhibitors bind to the active site — they bind to the allosteric site, which is a different site on the enzyme.
Allosteric Regulation & Feedback Inhibition
Allosteric regulation is a type of enzyme regulation in which a molecule (an allosteric effector) binds to a site other than the active site (the allosteric site), causing a conformational change that either activates or inhibits the enzyme.
Feedback inhibition (also called end-product inhibition) is the most important example of allosteric regulation in biochemical pathways:
- As a metabolic pathway proceeds, the final product accumulates.
- When the concentration of the end product is high enough, it acts as an allosteric inhibitor and binds to the first enzyme in the pathway, switching off the pathway.
- When the product is consumed, inhibition is relieved and the pathway resumes.
- This is a self-regulating mechanism that prevents overproduction of products and conserves resources.
> VCAA trap: Feedback inhibition is an example of non-competitive / allosteric inhibition — the end product does NOT bind to the active site.
> Mark-scheme point: To gain full marks, state both (1) that the end product binds to the allosteric site of the first enzyme AND (2) this causes a conformational change that reduces enzyme activity.
KK5: Photosynthesis
Chloroplast Structure & Location of Reactions
Photosynthesis is the biochemical process in which light energy is used to convert the inorganic compounds carbon dioxide (CO₂) and water (H₂O) into the organic compound glucose (C₆H₁₂O₆). It is an anabolic (endergonic) process.
Overall equation:
> 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (requires light energy and chlorophyll)
> Include light/radiant energy and chlorophyll above and below the arrow — they are conditions/catalysts, NOT reactants or products. Placing them incorrectly costs marks.
Chloroplast Structure
The chloroplast is the organelle where all photosynthesis occurs. Key structures:
| Structure | Description | Function / Role in Photosynthesis |
|---|---|---|
| Outer membrane | Smooth phospholipid bilayer | Encloses the chloroplast |
| Inner membrane | Phospholipid bilayer inside outer | Boundary of the interior compartment |
| Thylakoid membrane | Disc-shaped, interconnected membrane-bound compartments | Embedded with chlorophyll; site of light-dependent stage |
| Grana (sing. granum) | Stacks of thylakoids (like stacks of pancakes) | Maximise surface area for light capture |
| Stroma | Gel-like fluid filling the chloroplast, surrounding the grana | Contains enzymes for the light-independent stage (Calvin Cycle) |
> Memory tip (from Jacaranda): Grana comes before Stroma alphabetically → Stage 1 (light-dependent) occurs in grana first; Stage 2 (light-independent) occurs in stroma second. Stroma = Solution = Second stage.
> VCAA trap: Chlorophyll is embedded in the thylakoid membranes, NOT in the stroma. Stroma contains the enzymes for the Calvin Cycle.
> VCAA trap: Not all plant cells contain chloroplasts — they are found primarily in mesophyll cells (palisade and spongy mesophyll) of leaves.
Light-Dependent Reactions (Thylakoid Membrane)
Location: Thylakoid membranes (within the grana) of the chloroplast.
Key Events:
- Chlorophyll (embedded in thylakoid membranes) absorbs light/radiant energy.
- Electrons in chlorophyll become "excited" (move to higher energy level).
- Photolysis of water: H₂O molecules are split using light energy, producing:
- Electrons (e⁻) — replace lost electrons in chlorophyll
- Hydrogen ions / protons (H⁺)
- Oxygen (O₂) — released as a waste product (or byproduct)
- High-energy electrons pass down a chain of electron acceptors (electron transport chain in the thylakoid membrane), releasing energy.
- This energy pumps H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient.
- H⁺ ions flow back through ATP synthase (down the gradient) → drives production of ATP from ADP + Pᵢ (photophosphorylation).
- NADP⁺ is loaded with electrons and H⁺ ions → forms NADPH: NADP⁺ + H⁺ + 2e⁻ → NADPH
Inputs & Outputs — Light-Dependent Stage
| Inputs | Outputs |
|---|---|
| Light / radiant energy | O₂ (released as waste — from photolysis of water) |
| Water (H₂O) | ATP (via ATP synthase / photophosphorylation) |
| NADP⁺ (unloaded coenzyme) | NADPH (loaded coenzyme) |
| ADP + Pᵢ | — |
> VCAA trap: Oxygen is produced in the light-dependent stage (from splitting of water), NOT in the Calvin Cycle.
> VCAA trap: The question from the 2005 VCAA exam confirmed oxygen is the "waste product" of the light-dependent phase — this is a classic exam question type.
Calvin Cycle / Light-Independent Reactions (Stroma)
Location: Stroma of the chloroplast.
The Calvin Cycle (also called the light-independent stage or carbon fixation) does NOT directly require light, but it DOES require the products of the light-dependent stage (ATP and NADPH). It cannot proceed without them.
Key Events:
- Carbon fixation: CO₂ from the atmosphere is attached to a 5-carbon acceptor molecule called RuBP (ribulose bisphosphate).
- This reaction is catalysed by the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) — the most important enzyme in the Calvin Cycle.
- The product is an unstable 6-carbon compound that immediately splits into two molecules of PGA (phosphoglycerate / 3-carbon).
- ATP and NADPH (from the light-dependent stage) are used to convert PGA → PGAL (phosphoglyceraldehyde / G3P — a 3-carbon sugar).
- Some PGAL molecules are used to regenerate RuBP (to keep the cycle going); others exit the cycle to form glucose and other organic molecules.
- NADP⁺ and ADP + Pᵢ are released and returned to the light-dependent stage to be reloaded.
Inputs & Outputs — Light-Independent Stage (Calvin Cycle)
| Inputs | Outputs |
|---|---|
| CO₂ | Glucose (C₆H₁₂O₆) — and other organic molecules |
| ATP (from light-dependent stage) | ADP + Pᵢ (returned to light-dependent stage) |
| NADPH (from light-dependent stage) | NADP⁺ (returned to light-dependent stage) |
| RuBP (recycled within cycle) | — |
Role of Rubisco
- Rubisco catalyses the first step of the Calvin Cycle: the fixation of CO₂ to RuBP.
- It is the primary enzyme responsible for converting inorganic carbon (CO₂) into organic molecules.
- Rubisco can also react with O₂ instead of CO₂ — this is called photorespiration, which is wasteful because it does NOT produce sugar.
- C₃, C₄ and CAM plant adaptations aim to maximise Rubisco's efficiency with CO₂ and minimise photorespiration.
> VCAA trap: The Calvin Cycle is called "light-independent" — NOT "dark reactions." It can occur in the light; it simply does not directly use light.
Limiting Factors & Graphs
The rate of photosynthesis is affected by:
| Factor | How it limits photosynthesis | Effect of increasing factor |
|---|---|---|
| Light availability | Light provides energy for light-dependent stage; without it, NADPH and ATP cannot be produced | Rate increases up to a saturation point, then plateaus (another factor becomes limiting) |
| CO₂ concentration | CO₂ is a substrate for Rubisco and the Calvin Cycle; without it, glucose cannot be produced | Rate increases up to a saturation point |
| Water availability | Water is split in the light-dependent stage (photolysis) to provide electrons and H⁺; water stress causes stomata to close, limiting CO₂ entry | Decreased water → decreased photosynthesis. A wilting plant closes stomata → less CO₂ → decreased rate |
| Temperature | Enzymes (including Rubisco, ATP synthase) have an optimal temperature; too high = denaturation; too low = reduced kinetic energy | Rate increases up to optimum (~25–30°C for most plants), then decreases sharply |
> VCAA trap (Q3 from VCAA Workbook): When a plant wilts, the rate of photosynthesis decreases because stomata close → less CO₂ enters (answer D). The enzymes are NOT denatured (answer A). The plant recovered — denaturation is irreversible.
Graph interpretation — four rules:
- Rule 1: The factor on the x-axis is NOT the limiting factor at the plateau — it is now in excess. A different factor is now limiting.
- Rule 2: To identify the limiting factor, ask: what would you need to add to push the rate higher? (e.g. light-intensity graph plateaus → add CO₂ or raise temperature)
- Rule 3: Enzyme saturation looks identical to a limiting factor plateau — specify whether it is substrate (CO₂ → Rubisco saturation) or another variable.
- Rule 4: On a CO₂ concentration graph, plateau = Rubisco active sites all occupied (enzyme saturated) — not inhibited. "Excess CO₂ inhibits the enzyme" is a common wrong answer.
> Exam trap: MCQ asks which factor is limiting at the plateau of a light-intensity graph. Answer: CO₂ concentration (or temperature) — never light intensity (it's on the x-axis and is no longer limiting).
KK6: Cellular Respiration
Glycolysis (Cytosol)
Location: Cytosol (cytoplasm — the fluid outside organelles) of the cell.
Glycolysis = "splitting of sugar." This is the first stage of cellular respiration and occurs in all living cells (aerobic and anaerobic).
Key Events:
- One molecule of glucose (6-carbon) is split into two molecules of pyruvate (3-carbon each) through a 10-step enzyme-catalysed pathway.
- Investment phase: 2 ATP are consumed to activate glucose.
- Payoff phase: 4 ATP are produced.
- Net ATP yield = 2 ATP (substrate-level phosphorylation).
- NAD⁺ is loaded with electrons and H⁺ → 2 NADH are produced.
Inputs & Outputs — Glycolysis
| Inputs | Outputs |
|---|---|
| 1 glucose (C₆H₁₂O₆) | 2 pyruvate (C₃) |
| 2 ADP + 2Pᵢ | 2 ATP (net — substrate-level phosphorylation) |
| 2 NAD⁺ | 2 NADH |
> VCAA trap: Glycolysis produces a net of 2 ATP — not 4. Two are invested and four are produced, giving a net of 2.
> VCAA trap: Glycolysis does NOT require oxygen. It occurs in both aerobic and anaerobic conditions.
> If no oxygen is present → pyruvate stays in the cytosol → anaerobic fermentation.
> If oxygen is present → pyruvate moves into the mitochondrial matrix → Krebs Cycle.
Pyruvate Oxidation & Acetyl CoA (Mitochondrial Matrix)
Location: Mitochondrial matrix
Before entering the Krebs Cycle, pyruvate undergoes a linking reaction (pyruvate oxidation):
- Each pyruvate (3C) loses one carbon as CO₂ (released as waste).
- The remaining 2-carbon unit combines with Coenzyme A (CoA) to form Acetyl CoA (2C).
- NAD⁺ is loaded → produces 1 NADH per pyruvate (so 2 NADH per glucose).
> This step occurs in the mitochondrial matrix — NOT in the cytosol and NOT on the cristae.
Per glucose molecule (2 pyruvate → 2 Acetyl CoA):
- 2 CO₂ released
- 2 NADH produced
- 2 Acetyl CoA produced (each enters the Krebs Cycle)
Krebs Cycle (Mitochondrial Matrix)
Location: Mitochondrial matrix (the fluid inside the inner membrane of the mitochondrion).
Also called the Citric Acid Cycle (CAC). The cycle runs twice per glucose molecule (once per Acetyl CoA).
Key Events (per cycle — i.e., per Acetyl CoA):
- Acetyl CoA (2C) combines with oxaloacetate (4C) → citrate (6C).
- The cycle proceeds through 8 steps; carbon atoms are progressively removed as CO₂.
- Electrons and H⁺ are collected by NAD⁺ → NADH and FAD → FADH₂.
- Oxaloacetate is regenerated at the end of each cycle to accept the next Acetyl CoA.
- 1 ATP is produced per cycle via substrate-level phosphorylation.
Inputs & Outputs — Krebs Cycle (per glucose = ×2 cycles)
| Per cycle (per Acetyl CoA) | Per glucose (×2) | |
|---|---|---|
| Inputs | Acetyl CoA, NAD⁺, FAD, ADP + Pᵢ, H₂O | 2 Acetyl CoA, NAD⁺, FAD, ADP + Pᵢ |
| CO₂ released | 2 | 4 |
| NADH produced | 3 | 6 (+ 2 from linking reaction = 8 total from matrix) |
| FADH₂ produced | 1 | 2 |
| ATP produced | 1 | 2 (substrate-level phosphorylation) |
> 🏫 From the Haileybury workbook: per pyruvate molecule through the Krebs cycle (including linking reaction): 4 NADH + 1 FADH₂ produced. Per glucose: 8 NADH + 2 FADH₂ from Krebs + linking reaction.
> VCAA trap: The Krebs Cycle occurs in the matrix, NOT on the cristae. The ETC occurs on the cristae / inner mitochondrial membrane.
> VCAA trap: The CO₂ released in cellular respiration comes from the Krebs Cycle (and the linking reaction), NOT from glycolysis or the ETC.
Electron Transport Chain & Oxidative Phosphorylation (Inner Mitochondrial Membrane)
Location: Inner mitochondrial membrane (the folds are called cristae — singular: crista). The cristae greatly increase the surface area for ETC reactions.
Key Events:
- NADH and FADH₂ (from glycolysis, linking reaction, and Krebs Cycle) deliver their electrons and H⁺ to protein complexes embedded in the inner mitochondrial membrane (cristae).
- Electrons are passed down a chain of electron acceptors (the electron transport chain / ETC), releasing energy at each step.
- This energy is used to pump H⁺ ions (protons) from the matrix → into the intermembrane space (between inner and outer membranes), creating a proton gradient (high concentration in intermembrane space, low in matrix).
- H⁺ ions flow back into the matrix down the concentration gradient through the enzyme ATP synthase (also embedded in the inner membrane).
- The movement of H⁺ through ATP synthase drives the synthesis of ATP from ADP + Pᵢ — this is called oxidative phosphorylation (also called chemiosmosis).
- At the end of the chain, electrons + H⁺ combine with O₂ (the final electron acceptor) to form water (H₂O).
- ETC produces approximately 26–28 ATP per glucose (oxidative phosphorylation).
> VCAA trap: O₂ is the final electron acceptor — it forms water, not ATP. Never say "oxygen is used to make ATP."
> VCAA trap: ETC occurs on the inner mitochondrial membrane (cristae). The Krebs cycle occurs in the matrix. Do not mix these up.
Integration: Photosynthesis ↔ Cellular Respiration
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Overall type | Anabolic, endergonic | Catabolic, exergonic |
| Net equation | CO₂ + H₂O → glucose + O₂ | Glucose + O₂ → CO₂ + H₂O + ATP |
| Location | Chloroplasts (thylakoid + stroma) | Cytosol + mitochondria |
| Electron carriers | NADP⁺/NADPH | NAD⁺/NADH · FAD/FADH₂ |
| ATP role | Consumed (Calvin cycle) | Produced |
| O₂ role | Product (released from photolysis) | Final electron acceptor (→ H₂O) |
| CO₂ role | Substrate (fixed by Rubisco) | Waste product (released in Krebs) |
| When it occurs | Light-dependent stage: light only · Calvin cycle: continuous | Continuously (all organisms, 24/7) |
| Feature | Calvin Cycle (PS) | Krebs Cycle (CR) |
|---|---|---|
| Location | Stroma of chloroplast | Mitochondrial matrix |
| CO₂ role | Substrate — fixed into G3P by Rubisco | Product — released as waste |
| ATP role | Consumed | Produced (2 ATP per glucose) |
| NADPH/NADH | NADPH consumed | NADH produced (6 per glucose) |
Complementary link: O₂ from photosynthesis → final electron acceptor in CR. CO₂ from CR → substrate for Calvin cycle. Glucose from PS → substrate for glycolysis. These are not simply the reverse of each other — different compartments, enzymes, and coenzymes.
> Exam trap: "Photosynthesis and cellular respiration are the reverse of each other" — partially wrong. Net equations are inverse, but the mechanisms, compartments, coenzymes (NADPH vs NADH), and organisms differ significantly.
Practicals & Science Skills
This Quick Reference covers theory only (KK4/5/6). For practicals content — ET₅₀ method, the three experiments, Key Science Skills definitions, error types, variable templates, and model Q&As — use the dedicated sheet:
→ Practicals & Science Skills sheet