Photosynthesis and cellular respiration can feel like twins that refuse to wear matching outfits. You revise one, feel confident, then an exam question flips the arrows, swaps the organelle, and suddenly your brain insists mitochondria have chlorophyll. In IB Biology, the fastest way to stop that panic is to anchor both processes to one idea: photosynthesis stores energy in glucose; cellular respiration releases that energy as ATP.
Quick comparison checklist for IB Biology
-
Purpose: store energy (photosynthesis) vs release energy (respiration)
-
Type of pathway: anabolic vs catabolic
-
Overall equation: reversed reactants/products
-
Location: chloroplast vs mitochondrion
-
Gas exchange: CO₂ in/O₂ out vs O₂ in/CO₂ out
-
When it happens in plants: photosynthesis in light; respiration all the time
The core difference: building vs breaking
In IB Biology, examiners love the language of metabolism.
Photosynthesis is anabolic: it builds glucose from smaller molecules (CO₂ and H₂O). You’re taking low-energy ingredients and, using light, assembling a higher-energy product.
Cellular respiration is catabolic: it breaks glucose down to CO₂ and H₂O, transferring energy into ATP so the cell can actually use it.
If you want a clean syllabus-aligned refresher, use RevisionDojo’s topic pages for C1.3 Photosynthesis and C1.2 Cell respiration.
Overall equations: same atoms, opposite direction
Memorising the balanced equations helps you catch “trap” multiple-choice options.
Photosynthesis
6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
(Requires light energy)
Aerobic cellular respiration
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP
A good practice habit in IB Biology is to write both equations side by side and circle what changes: CO₂ and H₂O switch sides, O₂ flips role, and ATP appears only as an output of respiration.
For equation-focused exam wording, this article is a strong companion: What Is the Correct General Equation for Cellular Respiration?
Energy story: sunlight to glucose, glucose to ATP
Think of photosynthesis as energy capture: light energy is converted into chemical energy stored in glucose.
Think of respiration as energy release: energy in glucose bonds is transferred into ATP through a controlled series of reactions.
To make this stick for IB Biology, zoom in on one high-yield detail: respiration uses oxygen as the terminal electron acceptor in aerobic conditions, which helps explain why it yields far more ATP than anaerobic pathways.
Where they happen: organelles matter in exam marks
Photosynthesis occurs in chloroplasts, particularly across the thylakoid membranes (light-dependent reactions) and in the stroma (Calvin cycle).
Cellular respiration mainly occurs in mitochondria (Krebs cycle in the matrix; electron transport chain on the inner membrane), with glycolysis in the cytoplasm.
If you need organelle context, RevisionDojo’s endosymbiosis post is a surprisingly exam-useful bridge: How Mitochondria and Chloroplasts Provide Evidence for Endosymbiosis.
Gas exchange patterns (and the classic plant misconception)
Photosynthesis uses CO₂ and produces O₂.
Cellular respiration uses O₂ and produces CO₂.
The misconception that “plants respire only at night” costs marks every year in IB Biology. Plants respire continuously; it’s just that in bright light, photosynthesis can outpace respiration, so the net gas exchange looks like CO₂ uptake and O₂ release.
How to revise this fast on RevisionDojo
When time is tight, use targeted practice rather than rereading.
-
Drill photosynthesis with the C1.3 Photosynthesis Questionbank
-
Drill respiration with the C1.2 Cell Respiration Questionbank
-
Lock in definitions using Flashcards for C1.2.4 Cell respiration
-
Use notes for precision wording, like Notes for 2.8 Cell respiration and Notes for C1.3.2 Conversion of carbon dioxide to glucose
Closing: make it exam-proof
In IB Biology, photosynthesis and cellular respiration aren’t just two topics -- they’re one loop told from opposite ends. When you can explain “store energy in glucose” vs “release energy as ATP,” the equations, organelles, and gas exchange fall into place.
To make that understanding automatic, use RevisionDojo as your daily system: build confidence with the Questionbank, tighten explanations with Study Notes, test recall with Flashcards, and clear up confusion with AI Chat and Grading tools. Then finish with Predicted Papers and Mock Exams so your timing and wording hold up when it matters most in IB Biology.