Those of you who have watched my Q&A video know that my background is in photosynthesis
research, so I want to convince you that Photosynthesis is fascinating!
So I’ve come up with some quick nuggets of information that you probably don't know about
photosynthesis, so indulge me for just a few minutes and I’ll try and convince you.
Photosynthesis is the process that plants and other photosynthetic organisms such as
algae and cyanobacteria use to produce sugar from water, carbon dioxide and light.
It is responsible for almost all life on Earth, if you follow a food chain back far enough
you will almost always find a photosynthetic organism of some form as the primary producer
of the system.
Even creatures that don’t rely on photosynthesis for food (energy) can be reliant on the byproduct
of photosynthesis, oxygen.
The first steps of photosynthesis involves the production of this oxygen.
Water is split into oxygen and hydrogen by the enzyme Photosystem II.
Yes in photosynthesis, two comes before one because that is the order these enzymes were
discovered.
The splitting of water however is no small task, it is the most thermodynamically challenging
reaction in all of biology.
To put this into perspective Water can be split with heat, for example at 2200 °C about
three percent of all water molecules will disassociate into oxygen and hydrogen.
But in leaves, with the help of photosystem II, this can occur at anything above zero.
The energetics involved here do present some problems; water splitting results in the production
of reactive oxygen species that damage the proteins in the cell.
The core protein of Photosystem II, known as D1, has a half-life in a growing plant
of just 2 hours meaning the plant must constantly replace its photosynthetic machinery.
Oxygen itself was not all good news.
When oxygenic photosynthesis first evolved in cyanobacteria it caused massive climate
change and the mass extinction of life forms that couldn’t deal with this new aerobic
atmosphere.
This is known as the Great Oxygenation Event, and there is some debate about the exact timeline,
not too surprising since this happened 2.3 billion years ago.
But to simplify, the oxygen produced by early cyanobacteria was initially absorbed by minerals
such as iron in the earth’s crust.
But after this “mass rusting” these sinks were overpowered and oxygen levels in the
atmosphere slowly rose from almost zero to todays 21%.
Cyanobacteria had polluted the atmosphere with corrosive and reactive oxygen, resulting
in a mass extinction of countless species of anaerobic bacteria that were unable to
survive in this new oxygen rich atmosphere as well as oxidizing atmospheric methane,
a powerful greenhouse gas, plunging the planet into the longest snowball Earth period in
the geological timeline and lasting between 300 and 400 million years.
But it also opened the door for the evolution to aerobic respiration allowing much more
energy to be accessed by life; that in turn allowed for more complex life to evolve.
You know, like cat’s for staring in YouTube videos, and humans for making YouTube videos.
You probably know that plants are green because of the pigment chlorophyll, this is the pigment
that plants use to absorb light; the energy from this light is used in photosynthesis
to produce the sugars that the plant needs.
You might even know that plants make use two types of this pigment; Imaginatively named
chlorophyll a and chlorophyll b.
These chlorophylls absorb slightly different wavelengths of light from each other.
These are the only two chlorophylls found in plants, but, zoom into the microbial world
and there are chlorophylls out there?
The most recently discovered is chlorophyll f.
This was found in stromatolites from Shark Bay in Western Australia, the function of
this chlorophyll f in photosynthesis is unknown, as is its ecological distribution; but has
been revealed to be the most red-shifted chlorophyll yet discovered absorbing light in the infrared
range.
And its chemical structure is very similar to chlorophyll a; this has led to speculation
that chlorophyll f could be introduced into crop plants to allow the plants access to
a larger range of the light spectrum; potentially increasing the crop yield.
However adding chlorophyll to plants isn’t the only idea for increasing crop yields.
In fact doing the opposite and taking away chlorophyll can, paradoxically, also increase
yields.
The plants leaves are very, very good at absorbing the wavelengths of light that can be used
for photosynthesis.
However only a fraction of the energy absorbed is used; the remaining energy
is lost, mostly as heat in a process called non-photochemical quenching.
But a little is list as light through chlorophyll fluorescence
But with leaves so efficient at absorbing light, often the upper leaves absorb so much
light that there is not enough light left for the leaves further down in the canopy
to preform photosynthesis at maximum efficiency.
In a natural forest there is an advantage to denying a competitive plant these resources,
but in a monoculture crop we are less interested in the individual’s performance and more
concerned with the performance of the crop in total and the amount of delicious things
we can get; so reducing the chlorophyll content in the crops leaves can allow more leaves
to be working at peak efficiency, resulting in an overall higher yield.
So there you go, some quick nuggets in information that you now know about photosynthesis.
Let me know in the comments below if I convinced you that photosynthesis is fascinating!