Photosynthesis and cellular respiration are two of the most important biochemical pathways in IB Biology. They are often taught together because they represent opposite but complementary processes in the cycling of energy and matter. Understanding how they differ allows students to recognize how organisms obtain energy, maintain life processes, and contribute to global nutrient cycles.
The Core Difference: Anabolic vs Catabolic
The most fundamental difference between the two processes is:
- Photosynthesis is anabolic — it builds large molecules (glucose) from smaller ones (CO₂ and H₂O).
- Cellular respiration is catabolic — it breaks down glucose into CO₂ and H₂O to release ATP.
This connection reflects how energy flows through ecosystems: photosynthesis stores energy in glucose, and respiration releases that stored energy for cellular processes.
Opposite Overall Equations
The overall equations show clearly how the two processes reverse each other:
Photosynthesis
6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
(Requires light energy)
Aerobic Cellular Respiration
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP
The respiration equation was explored in detail in your previous article and matches the depth required in the IB Biology syllabus. Understanding these equations is essential for both SL and HL, with HL requiring more mechanistic detail.
Energy Flow: Input vs Output
Another major difference lies in the direction of energy transformation:
- Photosynthesis stores energy by converting sunlight into chemical energy in glucose.
- Cellular respiration releases energy by breaking glucose to produce ATP.
Photosynthesis therefore requires energy input, while respiration generates energy output. This distinction becomes central in questions comparing autotrophs and heterotrophs.
Organelle Location
- Photosynthesis occurs in chloroplasts.
- Cellular respiration occurs in mitochondria.
Chloroplasts contain thylakoids and chlorophyll for absorbing light energy, while mitochondria contain cristae and matrix enzymes for ATP production. Students often link these structural differences to functional explanations in Paper 2 answers.
Gas Exchange Differences
Because of their opposing processes:
- Photosynthesis uses CO₂ and produces O₂.
- Respiration uses O₂ and produces CO₂.
Many Paper 1 and Paper 3 questions use this pattern in graphical data. For example, light intensity experiments often require identifying when photosynthetic CO₂ uptake exceeds respiratory CO₂ output.
Interdependence of the Two Processes
These processes are opposite but deeply interdependent:
- Plants carry out both photosynthesis and respiration.
- Autotrophs produce oxygen and glucose that heterotrophs depend on.
- Heterotrophs produce CO₂ and water used by plants in photosynthesis.
Photosynthesis & Respiration in the IA
Many Internal Assessments explore:
- CO₂ production in respiration
- Oxygen release in photosynthesis
- Light intensity or temperature effects on either process
FAQ
Do plants respire only at night?
No. Plants respire all the time, but photosynthesis only occurs in light.
Which produces more ATP—photosynthesis or respiration?
Respiration produces ATP directly; photosynthesis stores energy in glucose instead.
Do all organisms use cellular respiration?
Yes. Nearly all organisms respire, including plants.
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