Photosynthesis
In 1779, a scientist observed that in the
presence of light plants give off bubbles
from their green parts while, in the shade,
the bubbles eventually stop.
He identified the gas as oxygen.
He also observed that, in the dark, plants
give off carbon dioxide.
This man was Jan Ingenhousz and this is how
he became the first man to discover the process
of photosynthesis.
You may already know that ‘photos’ is
a Greek word meaning ‘light’ and ‘synthesis’
is another Greek word meaning, ‘putting
together.’
However, what is photosynthesis and why is
it important?
This is the process of making glucose, which
is the energy source for most cell.
It makes this from sunlight energy, water
and carbon dioxide.
Only plant cells can do this, and the special
organelle in the plant cells that can do through
this process is called a chloroplast.
Photosynthesis goes through two stages.
Stage 1: light-dependent reaction (this reaction
depends on the presence of light; it cannot
happen in the dark):
This starts when photons from sunlight strike
the leaf, excite the chlorophyll (the light
absorbing pigment that drives photosynthesis)
and activate electrons.
During this process, water is split into oxygen
and hydrogen ions.
This is where the oxygen generated during
photosynthesis comes from.
The activated electrons then go through a
series of electron carriers, also referred
to as the electron transport chain, and in
the process leads to the accumulation of the
hydrogen ions inside the thylakoid membrane.
The thylakoid membranes are the disc-like
structures in the chloroplast that contain
chlorophyll, generating a proton gradient.
As the protons go down their concentration
gradient through the ATP synthase, ADP and
P come together to form ATP.
The Adenosine tri phosphate or the ATP is
an energy molecule that cells exchange like
currency to release energy.
The electrons going through the electron transport
chain eventually end up combining with NADP+
to form NADPH, another energy-rich molecule
that will be used later.
Thus, the result of the light reaction is
the production of oxygen (released from the
leaves), and ATP and NADPH, which will be
needed for the later steps.
The main difference between ATP and NADPH
is the amount of energy molecules they release.
Stage 2: Light-independent or dark reaction
(this reaction can proceed in the absence
of light, but calling it a dark reaction might
be misleading as it doesn’t happen only
in the dark—it can just as well happen in
the light).
This reaction uses the energy from light reaction
to convert carbon dioxide into glucose.
This might sound simple, but in fact the conversion
of carbon dioxide to glucose proceeds through
a series of reactions that start with 3-ribulose
bisphosphate (RuBP) and eventually end up
with the same molecule, producing glucose
in the process.
Because these series of reactions start and
end with the same molecule, they are referred
to as a cycle, specifically the Calvin cycle.
The enzyme rubisco (RuBP carboxylase) is a
very important component of this cycle.
The overall, final photosynthesis reaction
looks like this:
6CO2 + 6H2O (+ light energy) → C6H12O6 +
6O2
Comment below to let us know if you would
like us to cover the Calvin cycle in an upcoming
lecture.
As long as a plant remains green, it can benefit
our environment in many ways and also transfers
the energy in us when we eat it.
