In an earlier video, we described how photosynthesis—the
metabolism that has been generating molecular
oxygen for the past 2.5 billion years—was
an evolutionary innovation that completely
changed the face of life on earth.
Before the existence of these “green cells”
there would have been no oxygen and all life
would have been anaerobic.
With the introduction of oxygen, cells would
have been challenged to persist in the face
of oxygen’s destructive potential, and it
also created an opportunity for cells to evolve
aerobic metabolism, using oxygen’s destructive
potential to derive a lot more ATP energy
from their food.
Oxygen’s presence also provided an important
impetus for the evolutionary origin of eukaryotic
cells, and there was a second major change
marked by a dramatic increase in the size
and complexity of multicellular life that
occurred when the earth’s oxygen level increased
around the start of the Cambrian period—giving
rise to that fascinating time in life’s
history known as the Cambrian Explosion, just
a bit over 500 million years ago.
Oxygen also played a direct role in the first
invasion of dry land by the earliest land
plants around 450 million years ago.
In order for life to leave the seas, there
had to be a sufficient ozone layer to block
ultraviolet radiation from the sun.
Ozone is O3, and an ozone layer could not
have formed until there was an adequate amount
of O2 in the atmosphere.
The importance of oxygen in shaping life over
its long history is undisputed (except, I
suppose, by creationists, ha ha).
Photosynthesis is responsible for nearly all
of that oxygen, and thus it deserves our consideration—as
well as our thanks.
But when it comes to photosynthesis oxygen
is not even half of the story.
Oxygen is really more like a photosynthetic
waste product.
There is actually no benefit that plants get
from the oxygen that they produce—except
for the fact that plants also use oxygen metabolizing
aerobically with their mitochondria in the
same way that our cells do.
The full version of the photosynthesis story
here gets a little bit complicated, and I’ll
try to keep things on the basic side.
The main point of –the real reason plants
and other green cells carry out photosynthesis—is
all about taking carbon dioxide, which plants
get out of the air, and converting the CO2
into organic compounds like glucose.
Glucose is actually the main molecule that
photosynthesis produces so we’ll go with
it—you know already that its chemical formula
is C6H12O6.
If you’re going to make one glucose molecule,
you would need six carbon atoms.
Each CO2 contains one carbon, and so you’ll
need six of them.
You’ll also need to add 12 Hydrogen atoms,
and note that there are none already there
in the 6CO2.
In photosynthesis, those 12 hydrogens will
be coming from 6 molecules of water—good
ol’ H2O.
Now oxygen.
If you put together the six oxygen atoms in
6H2O and the twelve that you have in 6CO2,
you have a total of 18 oxygen atoms, right?
6 from the water and 12 from the 6CO2s-- but
you only need 6 oxygen atoms for the glucose
you’re making, so there are leftover 12
oxygen atoms and these are released in the
form of six molecules of O2—molecular oxygen.
It’s really pretty easy if you think of
it in terms of balancing the books.
But then thinking about it some more, you’ll
realize that it isn’t so easy at all—glucose
and oxygen represent a much higher level of
chemical energy potential relative to the
CO2 and water that we start out with.
We learned that from our previous lessons
on aerobic metabolism of glucose.
Starting with CO2 and water, we can’t get
from point A to point B without the addition
of lots of energy to power the rearrangement
of covalent bonds needed in order to convert
carbon dioxide and water into glucose and
oxygen.
Where can a photosynthetic organism get such
energy?
Before you answer that with a highly conditioned
response of “ATP,” allow me to ask the
question again—where can a PHOTOsynthetic
organism get such energy?
Well, you should know that plants need LIGHT
to grow, and so you might have already figured
out that light is important to the plants,
and light is, in fact, the source of energy
used by plants to carry out the conversion
of CO2 into glucose.
An inquisitive scientist like you should also
want to know HOW light is used to drive this
form of chemical work.
It’s not that hard to understand, but you’ll
need to get your mind around a couple of basic
concepts.
First, in order to use light energy, you need
to capture the light, and here’s where the
plant’s green pigments like chlorophyll
come in.
Visible light from the sun is “full spectrum”
and contains all the colors of the rainbow—red
orange yellow green blue indigo violet.
Now this is going to sound backwards because
plants are green, but plants appear green
because their chlorophyll molecules are absorbing
the red, orange, violet and blue colors of
light.
The green is not absorbed and therefore it’s
allowed to pass through the chlorophyll, where
it illuminates the rest of the plant tissue
with green light which is then reflected back
to our eyes.
Basically chlorophyll is a selective filter
that allows just green light to pass through.
But if chlorophyll is letting only green light
pass through, that means it’s absorbing
the other kinds of light (or as we say “wavelengths”),
which also means that the chlorophyll is taking
on the energy of the red, orange, violet and
blue light that it is absorbing.
The second concept is that when a molecule
“absorbs energy” the effect on the molecule
is a change to the electron arrangement in
the molecule’s atoms.
How do you know if a molecule is “less energetic”
or “more energetic”?
Well, just look at where its electrons are.
After chlorophyll absorbs light energy it
is changed into its “excited state,” which
means that a couple of chlorophyll’s electrons
have “jumped ” into new, more energetic
positions, and this is what allows chlorophyll
to rearrange the covalent bonds of CO2 and
water.
Ultimately it’s light energy from the sun
that is responsible for the conversion of
CO2 into all of the organic matter present
on earth.
Ultimately, it’s photosynthesis that makes
this conversion happen.
The existence of life as we know it would
not be possible except by the grace of the
sun and the green cells of photosynthesis.
