In 1779 Jan Ingenhousz showed that green plants produce oxygen in light — a simple experiment that separated how plants make food from how animals release energy.
That insight still matters. From field-scale crop yields to the design of microbial factories, knowing how organisms capture and use energy shapes ecology, agriculture, bioengineering and human health.
Photosynthesis and cellular respiration are complementary but distinct processes: one stores solar energy in chemical bonds, the other releases that energy as ATP; understanding eight clear differences between them reveals how life powers itself at every scale.
This piece lays out eight differences grouped into four practical categories: core roles, energy carriers and pathways, inputs/outputs and global impact, and cellular location and organism distribution. The goal: clear contrasts you can use in class, lab notes, or policy briefs.
For readers comparing the differences between photosynthesis and cellular respiration, expect concrete numbers, real examples, and actionable context rather than abstract theory.
Core biological roles
This category describes each process’s basic purpose and how they steer carbon and energy through cells and ecosystems. They are complementary: one builds biomass and the other harvests energy from it.
1. Purpose: energy capture versus energy release
Photosynthesis captures sunlight and converts CO2 into carbohydrates; cellular respiration breaks those carbohydrates down to free usable energy as ATP.
On a global scale, net primary production from photosynthesis is on the order of ≈120 billion tonnes of carbon per year (≈120 Gt C/yr), creating the material base for food webs and many industries.
Plants store that chemical energy as starch or sucrose in leaves, stems and roots—think sugarcane or potato fields waiting for harvest. Animals and humans then oxidize those sugars; for example, a runner covering 5 km depends on muscle cells converting glucose into ATP to power contraction.
These roles link ecosystems: producers lock up solar energy, consumers and decomposers return it as work and heat.
2. Direction of energy flow: light to chemical versus chemical to usable work
Energy flows in opposite directions. Photosynthesis turns photons into high-energy C–C and C–H bonds. Respiration converts chemical bond energy into ATP and heat.
Think of photosynthesis like solar panels wiring sunlight into stored fuel, and respiration like a power plant burning that fuel to run machines.
Biochemically, photosynthesis drives a light-driven electron flow (the Z-scheme) to reduce CO2. Respiration funnels electrons through an electron transport chain that ultimately reduces O2 to water. Aerobic respiration yields roughly 30–32 ATP per glucose in eukaryotic cells, whereas photosynthesis stores energy without immediately exporting large amounts of ATP to the whole organism.
That difference explains why biofuel crops matter: they store sunlight as biomass you can harvest and metabolize later.
Energy carriers and biochemical pathways
Mechanisms differ at the molecular level. Which carriers and cycles are used affects physiology, disease, and engineering strategies.
3. Primary energy carriers: NADPH and ATP in photosynthesis versus ATP and NADH in respiration
Photosynthesis produces NADPH and ATP to drive CO2 fixation via the Calvin cycle. That NADPH is reducing power used to build sugars and other biomass components.
Respiration generates NADH and FADH2 (plus a small direct ATP from glycolysis). Those reduced carriers feed the mitochondrial electron transport chain to make most ATP by oxidative phosphorylation.
Quantitatively, glycolysis gives 2 ATP directly per glucose while oxidative phosphorylation supplies the bulk—totaling roughly 30–32 ATP in aerobic cells. Metabolic engineers often boost NADPH availability to increase microbial yields of pharmaceuticals, fatty acids, or other bioproducts.
Human muscle cells switch strategies depending on demand: sprinting relies more on fast glycolysis (and its 2 ATP), endurance work taps full aerobic respiration for higher yield.
4. Electron transport and final electron acceptors
One stark contrast is the terminal electron acceptor. Oxygenic photosynthesis splits water at Photosystem II and releases O2; some photosynthetic bacteria instead oxidize H2S and do not produce oxygen.
In aerobic respiration the terminal acceptor is O2, reduced to H2O in the mitochondrial ETC (complexes I–IV). The photosynthetic Z-scheme (PSII → PSI) and the mitochondrial chain use similar physics but different components and directionality.
Microbes add variety: anaerobic respiration uses acceptors like nitrate or sulfate, and fermentation avoids external acceptors altogether, producing alternative end products such as ethanol or lactate.
Inputs, outputs and gas exchange
At the level of molecules and gases the two processes are mirror images, with measurable impacts from a single cell to the whole planet.
5. Reactants and products: CO2 and water versus CO2, water and oxygen exchange
Write the simplified reactions as mirror images. Photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2. Aerobic respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.
Biologically, that means plants take up CO2 and release O2 while animals and many microbes consume O2 and release CO2. Atmospheric oxygen is about 21% by volume, setting the backdrop for aerobic life.
In closed indoor environments, a healthy number of houseplants can slightly alter CO2 and O2 concentrations, though ventilation dominates air chemistry in most modern buildings.
6. Quantitative scales and ecological impact
Scaling up: global net primary production is roughly 120 Gt C/yr, with marine phytoplankton and terrestrial plants each contributing large fractions—roughly half of the total each on that order of magnitude.
At human scales, an average adult exhales about 1 kilogram of CO2 per day. Energetically, aerobic respiration yields about 30–32 ATP per glucose versus just 2 ATP per glucose from fermentation, which explains why oxygen availability alters activity and growth.
These numbers matter for climate models, crop breeding and ecosystem energy budgets; policy and engineering decisions rest on quantifying rates and yields, not just directions.
Cellular location and organismal distribution
Where each process occurs and who performs it determines ecological roles and biotechnological applications.
7. Cellular location: chloroplasts versus mitochondria
Oxygenic photosynthesis takes place in chloroplasts: light reactions on thylakoid membranes and the Calvin cycle in the stroma. Glycolysis happens in the cytosol; the Krebs cycle and oxidative phosphorylation occur in mitochondria (matrix and inner membrane).
Compartmentalization lets cells separate anabolic from catabolic pathways, improving regulation. Prokaryotes accomplish these tasks across plasma membranes or thylakoid-like membranes instead of organelles.
Compare cell types: leaf mesophyll cells pack chloroplasts for light capture, while active muscle fibers contain hundreds of mitochondria to meet ATP demand.
8. Organismal distribution: who does what and why it matters
Oxygenic photosynthesis is done by plants, algae and cyanobacteria. Aerobic respiration is widespread among eukaryotes and many bacteria. Some microbes use mixed strategies—photoheterotrophs or bacteria performing anoxygenic photosynthesis.
Prochlorococcus, a marine picoplankter, is an example of an abundant photosynthesizer that drives large-scale ocean productivity. Yeast offers a familiar example of anaerobic metabolism: fermentation used in baking and brewing produces ethanol and CO2 rather than lots of ATP.
These distributions guide choices in biotechnology: algal strains for biofuels, cyanobacteria for carbon capture, and mitochondria-targeted therapeutics in medicine.
Summary
- Photosynthesis captures sunlight and builds sugars; respiration unlocks that stored energy as ATP and heat.
- The processes use different carriers and pathways: NADPH and the Calvin cycle in chloroplasts versus NADH/FADH2, the Krebs cycle and mitochondrial ETC for ATP production.
- Stoichiometry and gas exchange are complementary (6 CO2 + 6 H2O ↔ C6H12O6 + 6 O2), and photosynthesis at ~120 Gt C/yr underpins global food and carbon cycles.
- Organismal placement matters: chloroplast-based photosynthesis in plants and cyanobacteria versus mitochondrial respiration in animals and fungi—choices that influence agriculture, climate and biotech strategies.
- Remember Jan Ingenhousz (1779): that simple experiment set the stage for understanding why ~30–32 ATP per glucose in aerobic cells matters so much compared with the 2 ATP of fermentation.
