18 July 2020 | New Scientist | 41>evolution has had at least 2 billion years
to perfect it, we have to content ourselves
with the wonder that it is done at all, not that
it is done well. The maximum conversion
efficiency of solar energy to biomass in most
plants is a disappointing 4.6 per cent.
This is true for the C3 version of
photosynthesis, the metabolic process used
by almost 90 per cent of plants, including
wheat, rice and soya beans. The inefficiency
comes down to an enzyme called rubisco.
This piece of biochemical machinery picks
up CO 2 molecules and combines them with
another compound to form a molecule
containing three carbon atoms, as a first
step in the production of sugar. The trouble is
that 40 per cent of the time, rubisco slips and
picks up oxygen instead, wasting energy. The
problem gets worse when plants close their
leaf pores, or stomata, to prevent water loss.
Oxygen builds up inside the leaf and rubisco
is even more likely to mistakenly grab it.
None of this mattered when rubisco
evolved more than 3 billion years ago, when
Earth’s atmosphere was rich in CO 2 and
almost free of oxygen. But as oxygen has