Cellular respiration, the reverse reaction

Photosynthesis makes glucose; the energy still has to be extracted from it. That is the job of cellular respiration, which operates in every living thing — plants included.

The balance, exactly reversed

   C₆H₁₂O₆  +  6 O₂   ->   6 CO₂  +  6 H₂O  +  ENERGY (ATP)

Compare with photosynthesis:

PHOTOSYNTHESIS                      RESPIRATION
6 CO₂ + 6 H₂O + light               C₆H₁₂O₆ + 6 O₂
      -> glucose + 6 O₂                   -> 6 CO₂ + 6 H₂O + ATP

STORES energy                       RELEASES energy
in CHLOROPLASTS                     in MITOCHONDRIA
autotrophs only                     ALL living organisms
only in the light                   constantly

The two reactions do not cancel out: they form a cycle, one filling the chemical energy reservoir the other empties.

ATP, the energy currency

The energy in glucose cannot be used as it is. The cell converts it into ATP, a molecule that releases energy by losing a phosphate group:

   ATP  ---->  ADP  +  Pi  +  usable energy
        <----
      (recharged by respiration)

ATP is a currency, not a reserve: a cell stores only a few seconds' worth. An adult recycles about their own body weight per day — not by making 70 kg of it, but by endlessly reusing the same molecules.

1 glucose  ->  about 30 ATP          (respiration, with O₂)
1 glucose  ->  2 ATP                 (fermentation, without O₂)

The ratio is fifteen to one. This is why complex life appeared only after the atmosphere was oxygenated: without oxygen, the energy yield rules out large organisms.

When oxygen runs short: fermentation

RESPIRATION (with O₂)              FERMENTATION (without O₂)
glucose fully broken down          incomplete breakdown
-> CO₂ + H₂O                       -> lactic acid (muscle)
                                   -> ethanol + CO₂ (yeast)
30 ATP                             2 ATP

During intense effort, oxygen supply can no longer keep up: muscles switch to lactic fermentation. The yield collapses, lactic acid accumulates, and the effort cannot be sustained for long.

Alcoholic fermentation by yeast, meanwhile, gave us bread, beer and wine — three millennia-old techniques exploiting a micro-organism's inability to respire in a closed vessel.

What circulates in the biosphere

        photosynthesis
   CO₂ -----------------> organic matter
    ^                            |
    |                            | respiration
    +----------------------------+

        O₂ travels the reverse path

Carbon and oxygen cycle between the mineral and living worlds. The one thing that does not cycle is energy: it enters as light and leaves as heat, in a one-way flow. An ecosystem recycles its matter, never its energy.

Summary

• Respiration: glucose + O₂ → CO₂ + H₂O + ATP, the reverse of photosynthesis.
• It occurs in the mitochondria, in all living things, constantly.
• ATP is the energy currency: continuously recycled, never stored for long.
• Respiration: 30 ATP per glucose; fermentation: only 2 ATP.
• Without oxygen: lactic fermentation (muscle) or alcoholic fermentation (yeast).
• Matter cycles; energy flows through, from light to heat.