Photosynthesis MiraCosta Biology


Uploaded by MeerdesIrrtums on 17.11.2011

Transcript:
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.