Every day this time of year the world gets more and more
green. Over night it seems like lawns spring up lushly, and the light green
fuzz that covers the deciduous forest asserts itself more strongly every day.
All of that green is a result of the growth of the photosynthetic structures of
plants, structures that are replacing the ones that died last fall in
preparation for the cold and dry winter season, or are growing anew from last
year’s seeds. The primary photosynthetic structure for land plants is the leaf,
a structure that has a lot of surface area relative to its volume. The surface
area is important, as the leaf’s main job is to absorb solar radiation, so
surface area equals absorption potential.
You all learned (I hope) in school that plants take carbon
dioxide and water and sunlight and make sugar and oxygen gas*. The energy from
the sunlight, electromagnetic radiation, gets transformed into chemical energy
as stored in the bonds between the carbons and the hydrogens in the glucose
molecule that is formed. Something we can’t store gets turned into something we
can. It sounds quite simple, and if you look at them chemical equation for this
reaction, it even looks quite simple. The reality though is far different, as
my biology students this spring learned. Photosynthesis is amazing, and
beautiful, and a process that virtually all life depends on, but it is not
simple.
Nor should it be. Plants execute a very tricky procedure in
fixing the carbon from carbon dioxide in to a glucose molecule. It isn’t easy
to transform and store energy. Photosynthesis starts with light and that light
energy gets transferred to electrons, electrons that are hanging out on
chlorophyll molecules in the chloroplasts of plant leaves. Chlorophyll is a
pigment that absorbs light, mostly red and blue light, because those
wavelengths are especially good at activating chlorophyll’s electrons. Green
light incidentally, is not good at all at exciting chlorophyll’s electrons, so
it isn’t absorbed, it is reflected instead, which is why plants look green. Those
electrons go through a series of steps in processes imaginatively called
photosystems I and II. The light bumps them up to a high energy state, and over
the course of photosystem I and II they bump back down, releasing that energy
along the way. The end results of photosystems I and II are a whole bunch of
temporary energy storage molecules called ATP, and a few electron acceptor
molecules. ATP is adenosine triphosphate and is the go to molecule for
temporary chemical energy in cells. The phosphate part is what makes it good
for holding energy, but is also what makes it only able to do it temporarily,
as the phosphate is very unstable. So ATP isn’t a viable long term storage
solution, but cells make ATP to then use the energy stored in ATP to do other cellular
business. Photosystems I and II are the light dependent parts of
photosynthesis, and where the water (a reactant in the reaction) is used and
the oxygen (a product of the reaction) gets formed. The water gets split, and is where the
electrons that get moved around ultimately come from, and the oxygen is a
byproduct of splitting the water. Notice, no where in this part of
photosynthesis have we talked about carbon or glucose. Carbon doesn’t have
anything to do with this part.
The other nearly entirely separate part of photosynthesis is
the light independent reaction—called the Calvin cycle. This is where all that
ATP gets used, and those electron acceptor molecules from the photosystems give
their electrons back up. Carbon dioxide gets incorporated into several
different intermediate molecules with names like PGA and G3P and RuBP, and
after 6 full turns of the Calvin cycle, you end up with enough carbons being
fixed to make one glucose molecule. That requires a lot of electrons , and even
more ATP. When you study the Calvin cycle you gain an appreciation for just how
much energy really goes into making glucose, the energy molecule of life. And note here, in the Calvin cycle, this is
where the carbon comes into play. The carbon dioxide is a reactant in the chemical
equation, and the glucose is the product linked to it. The water and oxygen gas
that appear right next to the carbon and glucose on paper aren’t really
connected to them at all in the plant cell, except for a few electrons.
And what that also means, if you think about it, is that the
material of glucose, the carbon that makes up the main backbone of the glucose
molecule comes only from the carbon dioxide the plant inhales from the atmosphere.
That means that plants make food out of air. The material portion of that food,
the actual atoms that make up the sugar we all eat, came out of thin air. And
that is something I can’t get over, every time I think about how amazing plants
are that is what I come back to, plants make food out of air (specifically the
carbon dioxide gas) and oxygen out of water. I only wish I could make that much
of a difference in the world.
*Here’s that reaction equation: 6 CO2 +
6 H2O → C6H12O6 + 6 O2
References:
Look at any college level biology book and you will find the basic mechanics of
photosynthesis. I used Freeman’s Biological Science 5th ed in my
class this past year https://www.pearsonhighered.com/product/Freeman-Biological-Science-5th-Edition/9780321743671.html
