Photosynthesis vs Respiration

Photosynthesis and respiration are often described as opposites, and in their overall equations, they are exact mirror images. One builds sugar from carbon dioxide and water using light; the other breaks sugar back down to carbon dioxide and water, releasing energy. Together, they drive the flow of energy through living things and cycle carbon and oxygen between the living world and the air. But they’re not simply the same reactions run backwards, and understanding the differences clears up several common misconceptions.

The equations

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy absorbed)

Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released)

The reactants of one are the products of the other. The energy change is the same size with opposite signs: photosynthesis stores about 2,870 kJ per mole of glucose; respiration releases it. (For the details of each, see photosynthesis: the chemistry and cellular respiration.)

Side-by-side comparison

	Photosynthesis	Aerobic respiration
Purpose	Store energy in sugar	Release energy from sugar
Reactants	CO₂ + H₂O	Glucose + O₂
Products	Glucose + O₂	CO₂ + H₂O
Energy	Absorbed (light → chemical)	Released (chemical → ATP + heat)
ΔG per mole glucose	+2,870 kJ	−2,870 kJ
Type of redox	CO₂ reduced, H₂O oxidised	Glucose oxidised, O₂ reduced
Which organisms	Plants, algae, cyanobacteria	Almost all organisms, including plants
Where in eukaryotic cells	Chloroplasts	Cytoplasm and mitochondria
When	Only in light	All the time, day and night
Stages	Light-dependent reactions, Calvin cycle	Glycolysis, link reaction, Krebs cycle, oxidative phosphorylation
Electron carrier	NADP⁺/NADPH	NAD⁺/NADH and FAD/FADH₂
ATP made by	Photophosphorylation	Substrate-level and oxidative phosphorylation
Effect on atmosphere	Removes CO₂, adds O₂	Adds CO₂, removes O₂

The key similarities

It’s easy to focus on the differences, but the two processes share remarkable chemistry:

• Electron transport chains in membranes pass electrons along carriers, pumping protons across a membrane.
• ATP synthase, an almost identical rotary enzyme, uses the resulting proton gradient to make ATP. The chemiosmotic principle is the same in chloroplasts and mitochondria.
• Similar carriers: plastoquinone and ubiquinone, cytochromes containing iron, and copper proteins.
• Similar organelles: chloroplasts and mitochondria both have double membranes and their own DNA, and both are thought to descend from free-living bacteria engulfed by other cells long ago.
• Shared intermediates: steps in the Calvin cycle resemble steps in glycolysis run in reverse.

See the electron transport chain and the light-dependent reactions to compare the two chains in detail.

Misconception: “plants photosynthesise, animals respire”

Plants respire too, all the time, just like animals. They need ATP for growth, transport and building molecules, and they make it by respiration in their mitochondria. The glucose they break down comes from their own photosynthesis.

• In daylight, photosynthesis usually runs faster than respiration, so a plant takes in more CO₂ than it gives out and releases oxygen overall.
• At night, there’s no photosynthesis, so plants only respire: they take in oxygen and give out carbon dioxide.

The compensation point

At a certain light intensity, a plant’s rate of photosynthesis exactly equals its rate of respiration. There’s no net exchange of CO₂ or O₂ with the air. This is the compensation point.

• Below it (dim light, dawn, dusk), the plant is a net producer of CO₂.
• Above it, the plant is a net consumer of CO₂ and producer of O₂.

Shade-tolerant plants have low compensation points, so they can grow in dim forest light.

A classic experiment: hydrogencarbonate indicator

Hydrogencarbonate indicator changes colour with CO₂ concentration, because dissolved CO₂ makes the solution more acidic (see acid–base indicators):

• Yellow: more CO₂ than normal (net respiration).
• Orange-red: normal (atmospheric) CO₂.
• Purple: less CO₂ than normal (net photosynthesis).

Put pondweed or leaves in tubes of indicator under different conditions:

Condition	Result	Explanation
Bright light	Purple	Photosynthesis faster than respiration: CO₂ removed
Dim light	Orange-red	Near the compensation point
Darkness	Yellow	Respiration only: CO₂ added
No plant (control)	Orange-red	No change

How the two processes cycle matter and energy

• Carbon cycles: photosynthesis pulls CO₂ out of the air and builds it into sugars; respiration (by plants, animals, fungi and bacteria) returns it. Burning fossil fuels, formed from ancient photosynthesis, adds extra CO₂ faster than photosynthesis currently removes it.
• Oxygen cycles in the opposite direction: photosynthesis releases it (from water); respiration uses it (turning it back into water).
• Energy does not cycle. It flows: sunlight → chemical energy in sugar → ATP → work and heat. Energy lost as heat radiates away to space; ecosystems need a constant supply of new sunlight.

Why they aren’t simply the same reactions reversed

If the overall equations are mirror images, why doesn’t a plant just run respiration backwards to photosynthesise? The answer is thermodynamics. Every pathway in a cell includes some steps that are strongly downhill, releasing enough energy to make them effectively irreversible. Running such a step backwards would need a large input of energy at exactly that point. So cells build uphill pathways from different steps, powered by extra ATP or, in photosynthesis, by light, and they use different enzymes for the irreversible stages. Using separate pathways also lets cells control each direction independently, so that making and breaking sugar don’t cancel each other out in a pointless cycle.

The carriers differ for a similar reason. Photosynthesis uses NADPH to build molecules, while respiration uses NAD⁺ to break them down. Keeping two separate pools of electron carriers, differing only by one phosphate group, lets cells run building and breaking reactions side by side, each with its own supply of reducing power.

There’s also a difference in scale and timing. A leaf fixes CO₂ only while it’s lit, but it respires continuously, and so do roots, stems, flowers and every animal and microbe that eats plant material. Globally, the carbon taken up by photosynthesis each year is roughly balanced by the carbon released by respiration and decay. Human emissions from fossil fuels have upset that balance, which is why atmospheric CO₂ is rising.

Practice questions

1. Write balanced symbol equations for photosynthesis and aerobic respiration.
2. Give two ways in which photosynthesis and respiration are similar at the molecular level.
3. Explain why a plant gives out carbon dioxide at night.
4. What is the compensation point?
5. A tube of hydrogencarbonate indicator containing pondweed turns purple in bright light. Explain.

Answers:

1. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂; C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
2. Both use electron transport chains in membranes; both make ATP with ATP synthase driven by a proton gradient (also: both use similar carriers and double-membrane organelles).
3. Without light, there’s no photosynthesis, but respiration continues, releasing CO₂.
4. The light intensity at which the rate of photosynthesis equals the rate of respiration, so there’s no net gas exchange.
5. Photosynthesis is faster than respiration, so the pondweed removes CO₂ from the water, making it less acidic.

Key takeaways

• The overall equations of photosynthesis and respiration are mirror images, with equal and opposite energy changes (±2,870 kJ mol⁻¹).
• Photosynthesis stores light energy in sugar in chloroplasts; respiration releases it as ATP in the cytoplasm and mitochondria.
• Both use electron transport chains and ATP synthase driven by proton gradients.
• Plants respire all the time; at the compensation point, photosynthesis and respiration balance.
• Carbon and oxygen cycle; energy flows and is eventually lost as heat. For the energy molecule both processes make, see ATP.