Encyclopedia of Evolution.pdf - Online Reading Center

make food using carbon dioxide (CO 2) and the energy from
sunlight.
Autotrophic cells and organisms make their own food
from carbon dioxide, some of which they use themselves (see
respiration, evolution of) and some of which gets eaten
by heterotrophic cells or organisms that cannot make their
own food. The two main categories of autotrophic cells and
organisms are the chemotrophic cells and the photosynthetic
cells and organisms, which differ from one another in the
way that they obtain the energy that they need for making
small carbon dioxide molecules into large organic molecules.
• Chemotrophic bacteria obtain energy from inorganic chemical
reactions. Some inorganic reactions such as the oxidation
of ferrous into ferric iron (rusting) release energy, and
there are bacteria that can capture and use this energy. Chemotrophic
bacteria use many different inorganic sources of
energy, mostly minerals. The importance of bacteria in the
production of mineral deposits has only recently been recognized.
They do not require sunlight.
• Photosynthetic bacteria and eukaryotic cells obtain energy
from light, usually sunlight. They use pigments such as
chlorophyll to absorb the light energy.
Photosynthesis has two stages: the stage in which light
energy is captured, and the stage in which carbon dioxide is
fixed into carbohydrate.
The light capture phase. The light capture phase uses
light energy to create an electrical current which travels
through proteins that have been modified with metal atoms.
These reactions therefore require not only a source of light
but a source from which to obtain electrons.
In the most primitive forms of photosynthesis, found in
some bacteria (see bacteria, evolution of), the source of
the electrons can be hydrogen sulfide (H 2S), which becomes
elemental sulfur (S) after the electrons are removed; or hydrogen
gas (H 2), or organic molecules such as succinate or
malate. The bacteria that use hydrogen or organic molecules
can live as heterotrophs in darkness or as autotrophs when
light is available. All of these bacteria live in ponds in the
absence of oxygen. Because this means that they cannot be
near the surfaces of the ponds, they must be able to absorb
light very efficiently. Their pigment, bacteriochlorophyll, can
absorb infrared radiation that is past the red end of the visible
light spectrum. The light absorption system consists of a
single system of reactions.
In the more advanced form of photosynthesis, used by
cyanobacteria, the source of the electrons is water (H 2O),
which becomes elemental oxygen gas (O 2) after the electrons
are removed. Light energy is absorbed by chlorophyll molecules.
The light absorption system of the more advanced form
of photosynthesis consists of two cycles of reactions that
work in series. One of these cycles resembles the single cycle
of some of the primitive bacteria; the other cycle resembles
the single cycle of other primitive bacteria. The two-cycle
advanced system may have resulted from the genetic merger
of two different kinds of bacteria (see horizontal gene
transfer).
photosynthesis, evolution of
Some archaebacteria (such as Halobacterium salinum,
which lives in very salty water in the absence of oxygen) have
a pigment system that absorbs light and uses it to pump ions
across membranes. These bacteria are heterotrophic, but they
use light as an energy source during times when food molecules
are scarce. The pigment molecule is bacteriorhodopsin,
similar to one of the visual pigments found in the vertebrate
eye. Since the light energy is not used to make carbohydrates,
it is not considered a form of photosynthesis.
The bacteria with the primitive light absorption system do
not produce oxygen, and today grow in habitats in which the
water contains no oxygen (anaerobic conditions). This is consistent
with their origin in the Archaean era (see Precambrian
time) when the atmosphere of the Earth did not yet contain
oxygen. The bacteria with the more advanced light absorption
system produce oxygen. Most evolutionary scientists have concluded
that it was the oxygen produced by cyanobacteria that
filled the atmosphere with oxygen gas during the Proterozoic
era and created the aerobic conditions that now prevail upon
the Earth. Photosynthesis continues to put oxygen gas into the
atmosphere. Earth’s atmosphere is unique among known planets
in having an oxygen atmosphere, and no inorganic process
is known that could create an oxygen atmosphere on a planet.
Such an atmosphere is a clear indicator of photosynthesis and
therefore of life (see Gaia hypothesis).
The carbon dioxide fixation phase. The carbon dioxide
fixation stage involves a cycle of reactions called the Calvin
cycle, which produces carbohydrates such as sugar. All
advanced photosynthetic cells (and chemosynthetic cells as
well) use this or a similar set of reactions. The enzyme that
fixes the carbon dioxide, called rubisco, may be the most
abundant enzyme in the world.
Photosynthesis occurs in the chloroplasts of many protists
(see eukaryotes, evolution of) and almost all plants.
The set of light- and carbon-fixing reactions in chloroplasts is
nearly identical to the reactions in cyanobacteria. This is not
coincidence. Chloroplasts are the evolutionary descendants of
cyanobacteria that moved into and formed a mutualistic association
with primitive eukaryotic cells (see symbiogenesis).
Therefore it can be said that nearly all of the photosynthesis
in the world is conducted by cyanobacteria—either free-living
cyanobacteria in oceans, ponds, and rivers, or cyanobacteria
that have evolved into chloroplasts inside of the cells of
eukaryotic algae and plants.
Most plants have only the Calvin cycle as the carbon
dioxide fixation stage of photosynthesis. They are called C3 plants because the carbon dioxide is first made into a threecarbon
molecule. However, some plants (just over 10 percent
of plant species) have different forms of carbon fixation.
• C4 plants have two carbon fixation cycles that both occur
during the daytime. The first cycle fixes carbon dioxide
into an acid (which has four carbon atoms, hence the
name). The second cycle is the Calvin cycle. Therefore it
appears that a new carbon fixation cycle has been added
onto the Calvin cycle that was used by the ancestors of
the C4 plants. C4 photosynthesis has evolved as many as
31 times in separate lineages of plants and therefore is