Photosynthesis vs Respiration: Key Differences

Photosynthesis and aerobic respiration are almost exact opposites. Put their equations side by side:

• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy is taken in)
• Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy is released)

The products of one are the reactants of the other. Photosynthesis is endothermic: it takes in energy from light and stores it as chemical energy in glucose. Respiration is exothermic: it releases that stored energy so cells can use it for building molecules, moving and, in animals such as mammals, keeping warm. If you learn one equation, you can write the other by reading it backwards.

A very common mistake is to think that plants photosynthesise while animals respire. In fact respiration goes on all the time in every living cell, in plants as well as animals, by day and by night. Most of it happens in the mitochondria.

Photosynthesis only happens in cells with chloroplasts, and only when there is light. So during the day a green leaf is doing both at once. It is easy to miss the respiration because, in bright light, photosynthesis runs much faster and hides it.

You can see the balance shift with hydrogencarbonate indicator, which is red at normal carbon dioxide levels, turns yellow when carbon dioxide rises and purple when it falls. Put a leaf in a sealed tube with the indicator:

Conditions	What the leaf is doing	Indicator
Dark	Respiration only, so CO₂ builds up	Turns yellow
Dim light	Photosynthesis equals respiration	Stays red
Bright light	Photosynthesis faster than respiration, so CO₂ is used up	Turns purple

The point in dim light where the two rates are exactly equal is called the compensation point. At this light level there is no overall gain or loss of gases.

A plant can only grow if, over a whole day, it makes more glucose by photosynthesis than it uses up in respiration. A seedling kept in deep shade, below its compensation point for most of the day, slowly uses up its stores.

The same balance works on a global scale. In the Northern Hemisphere, where most of the land is, carbon dioxide in the air falls in spring and summer when plants are growing fast and rises again in winter when growth stops and dead leaves decay.

In everyday speech, 'respiration' and 'breathing' mean the same thing, but in biology they do not. Breathing (ventilation) is moving air in and out of the lungs. Respiration is a chemical reaction inside cells. Plants do not breathe at all: gases simply diffuse in and out of their leaves through the stomata.

When oxygen is short, for example in waterlogged soil around roots, plant cells can also respire anaerobically, turning glucose into ethanol and carbon dioxide. This releases much less energy than aerobic respiration.