C6H12O6 + 6O2 -> 6CO2 + 6H2O. Yields 2755 kJ/mole of glucose. The reverse of this reaction – combing carbon dioxide and water to make sugar – is known as photosynthesis. Photosynthesis is the process responsible for storing all the energy we extract from fossil fuels, crops, and all of our food.
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Answer:C6H12O6 + 6O2 -> 6CO2 + 6H2O
Yields 2755 kJ/mole of glucose
The reverse of this reaction – combing carbon dioxide and water to make sugar –
is known as photosynthesis. Photosynthesis is the process responsible for storing all the
energy we extract from fossil fuels, crops, and all of our food. We will also see that it is
part of a globally important cycle affected by our consumption of fossil fuels.
I. Photosynthesis How is photosynthesis able to run the reaction above in the reverse
direction? Somehow it must come up with 2755 kJ of energy to make each mole of
glucose. Where does that energy come from? The short answer: photons of sunlight. The
long answer: When the pigment chlorophyll inside the chloroplasts of a photosynthetic
organism (phytoplankton, trees, other plants) absorbs sunlight, it becomes energetically
‘excited’ and grabs the hydrogen atoms away from a water molecule, leaving the oxygen
atoms to escape as O2 gas. This is called ‘splitting water.’
The hydrogen atoms are then split into their component protons and electrons.
The electrons are used to reduce carbon dioxide, in a series of many steps requiring more
absorption of sunlight by chlorophyll, to glucose. When carbon dioxide receives those
electrons, the extra negative charge attracts protons from elsewhere, creating hydrogen
atoms attached to the carbon atom. This process is called reduction. When those reduced
carbon dioxide molecules are combined together in a larger molecule, the result is
glucose. This ‘combing together’ of small molecules requires an input of energy, which is
provided by the ATP molecules made by the protons diffusing through the membrane of
the chloroplast. The ATP molecule is simply a molecule that biology uses to store energy
for later use. In this case, the mechanical energy created by the protons diffusing across
the membrane turns a sort of molecular turbine that smashes together its precursors,
forming ATP.
II. The carbon cycle Where does photosynthesis occur? It occurs in the phytoplankton
of the ocean and the trees and other plants on land. On land, the carbon dioxide consumed
by plants to make organic matter (known as ‘fixing carbon’) is stored for fairly long
periods of time until the plant is harvested, eaten, or otherwise dead. Trees, for example,
can live for hundreds of years and store lots of fixed carbon for long periods of time. In
the ocean, in contrast, carbon is fixed by fast-growing, quickly eaten phytoplankton. The
word ‘phytoplankton’ means any organism that undergoes photosynthesis in the water
and includes many species of algae and bacteria.
Because the carbon fixed by phytoplankton is ‘turned over’ so quickly (a term
meaning the phytoplankton get eaten often), the fixed carbon is released into the
environment creating an intricate web of carbon transfer in the ocean. The phytoplankton
are grazed (eaten) by various species of bacteria and zooplankton (insects and
crustaceans). These grazers are in turn eaten by larger animals, which are eaten by larger
animals, which are eaten by big fish, which are eaten by bigger fish, which are eaten by
humans. The key concept here is that all of these organisms are continuously pooping
throughout their lives before they get eaten. If the poop is heavy enough, it sinks, and the
technical term for this process is known as the “biological carbon pump.” The feces of
animals eating phytoplankton transports the carbon contained in the phytoplankton down
to the seafloor.
As the fecal pellets (the technical term for poop) fall, the carbon in the poop is
consumed by bacteria with oxygen – that is, the bacteria oxidize the carbon in poop with
O2 gas dissolved in the ocean. Almost the entire ocean contains abundant quantities of
dissolved oxygen, but in some places the local concentration of oxygen in small
microenvironments (for example, around a fecal pellet) can become quite low. Also,
when the fecal pellet falls all the way down to the sediment on the seafloor, oxygen can
become depleted quite quickly. When this happens, the bacteria oxidize the carbon with
other oxidizers than oxygen like nitrate, sulfate, and hydrogen, which become abundant
when oxygen is absent. The byproducts of these reactions include ammonia, sulfide, and
methane.