How Does Photosynthesis Power Life on Earth?

Plants turn sunlight into food via photosynthesis, which is pretty amazing -- and its evolution is fairly mysterious. Learn how researchers think it happened in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey,
brain Stuff, Lauren vogelbamb here, you probably don't appreciate plants enough.
It's okay, none of us do. Given that plants have
been the major player in the convoluted soap opera of
life that landed us on this planet, we should be
thanking our leafy friends every day for our existence. Honestly,

(00:24):
the whole story is so tangled and complicated we may
never know the truth about how our mean green ancestors
allowed everyone else to evolve. But one aspect of the
story certainly involves photosynthesis of plants ability to make its
own food out of sunlight. We spoke with Gregory Schmidt,
Professor emeritus in the Department of Plant Biology at the

(00:44):
University of Georgia. He said a great way to appreciate
photosynthesis is to compare Earth's atmosphere with that of our
sister planets, Mars and Venus. All three planets were most
likely similar when they formed and cooled, but the atmospheres
of both Venus and Mars have carbon dioxide, two point
seven percent nitrogen and zero point one three percent oxygen.

(01:07):
Earth's air is seventy seven percent nitrogen, twenty one percent oxygen,
and zero point four one percent carbon dioxide, although that
number is rising. That means there are eight hundred gigatons
of carbon dioxide in our atmosphere, but there's another ten
thousand gigatons missing or buried in the form of fossil, limestone, coal,
and oil. In other words, carbon has been smuggled out

(01:31):
of the atmosphere and into Earth's crust for billions of years,
which is the only reason this planet is at all
habitable by multi celled organisms. Schmidt said, So, how did
that dramatic atmospheric shift happen for Earth? There's only one answer,
and it's pretty simple. Photosynthesis, the most amazing factor in
Earth's evolution. Yes, friends, photosynthesis. A couple hundred million years

(01:54):
after the Earth was formed, life showed up, probably first
as some anaerobic bacteria, that is, bacteria that can't thrive
in the presence of oxygen. These single celled organisms lived
by slurping up the sulfur and hydrogen that came out
of hydrothermal vents. Now we've got everything from butterflies to giraffes.
But there were a few steps on the road between

(02:16):
the first bacteria and giraffes. Those ancient bacteria had to
figure out a means of finding new hydrothermal vents, which
led to the development of a thermal sensing pigment called
bacterial chlorophyll, which some bacteria still used to detect the
infrared waves otherwise known as heat. These bacteria were the
progenitors of descendants that could make chlorophyll, a pigment that's

(02:38):
able to capture shorter, more energetic light waves from the
sun and use them as a source of power. So,
in essence, these bacteria created a means to capture the
energy of sunlight. The next evolutionary lead necessitated working out
a means of stable energy storage, creating a sort of
sunlight battery that encouraged protons to accumulate on one side

(02:59):
of their internal membranes versus the other. The true wonder
of plant and algae evolution is the fact that at
some point these ancient chlorophyll producing bacteria started generating oxygen.
After all, billions of years ago, there was actually very
little oxygen in the atmosphere, and it was toxic to
a lot of early bacteria. It's still toxic to existing

(03:20):
anaerobic bacteria that's still thrive in the oxygen free places
on Earth. However, the new process of capturing and storing
sunlight required the participating bacteria to burn water. The process
that we call burning or combustion is basically just very
rapid oxidation, the ripping off of electrons from one atom

(03:41):
and the transfer of those electrons to another, which is
called reduction. Early photosynthetic bacteria developed a way to capture
photons or particles of light, and use their energy to
strip water of many of its protons and electrons to
use for energy production. The breakthrough of breakthroughs that happened
three billion years ago was when photosynthetic machinery was perfected

(04:02):
to the point that chlorophyll could split two water molecules
at the same time. These days, we call this a
photosystem to chlorophyll protein cluster. Cyanobacteria evolved once these photosynthetic
bacteria figured out how to burn water and store the
energy from that chemical reaction in photosynthesis, photosystem to water

(04:23):
burning can't really be sustained without the second stage photosystem, one,
which involves taking the electrons swiped off of the water
molecules in the first step and making use of them
before they decay. Photosystem one does this by sticking these
electrons on a chemical assembly line, so the organism is
able to retain that hard earned energy, which is then
used to convert carbon dioxide into sugar for the bacteria

(04:45):
to use as food. Once photosystems one and two were
sorted out, cyanobacteria took over the oceans, and because oxygen
was their waste product, it became plentiful in Earth's atmosphere.
As a result, many bacteria became aerobic. That is, they
required oxygen for their metabolic processes, or at the very
least they could tolerate it. About a billion years later,

(05:09):
Protozoa evolved as anaerobes scarfing up aerobic bacterial prey. What
researchers think happened is this, at some point in at
least one of these oxygen intolerant organisms, the oxygen tolerant
bacteria they ate weren't completely digested, but stayed within the
cell and ended up helping the oxygen intolerant anaerobic organism

(05:30):
cope with an aerobic environment. These two organisms stuck together,
and eventually the prey organism evolved into a cell organelle
called mitochondria. A similar scenario occurred with cyanobacteria around one
billion years ago. In this case, an anaerobic bacteria probably
gobbled up a photosynthetic bacteria, which ended up setting up

(05:52):
shop inside its host, resulting in a small membrane bound
organelle common to all plants. The chloroplast as, algae and
multicellular plants evolved and benefited from plentiful carbon dioxide and
increasing oxygen in Earth's atmosphere. Chloroplasts became the place where
photosynthesis of photosystem one, two, and even more complicated stuff

(06:13):
went down in each cell. Just like mitochondria, they have
their own DNA and spend their time busily harvesting light
for the plant, creating the entire foundation for life on Earth.
Today's episode was written by Joceline Shields and produced by Tyler.
Playing brain Stuff is a production of I Heart Radio's

(06:35):
How Stuff Works. For more in this and lots of
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