When chlorophyll is exposed to light, it is bombarded by photons. When an individual
chlorophyll molecule absorbs a photon, the energy of that photon pushes the outermost
electron—a negatively charged subatomic particle—in the chlorophyll molecule to
a new, larger orbit. When this happens, the molecule is said to be in an
"excited," or energized, state. The molecule has acquired the energy of the
photon. This is only the very beginning of the
process. The chlorophyll molecule doesn’t remain for long in this excited
state—only for about one thousand-millionth of a second! But this is enough time to
trigger a series of reactions in which this outer electron is transferred from the
chlorophyll molecule to other molecules in a process called electron transfer. The
chlorophyll is, in essence, "harvesting" the sun's energy, transforming it from
radiant energy into chemical energy.
But what about the original chlorophyll molecule, now
lacking an outer electron? Almost immediately upon losing the electron, the molecule gains
another one—donated by water, one of the raw materials that must be present for
photosynthesis to take place. The effect is that water is split into its components,
hydrogen and oxygen. The oxygen, now in a gaseous form, escapes through the stomata. This
completes the first phase, called the light
reaction.
The chemical energy acquired during the light reaction
fuels the second phase of photosynthesis. This phase is called the dark reaction, since light is not required for
the process to proceed. In the dark reaction, carbon and oxygen from the carbon dioxide
molecule, along with the hydrogen left over from the light reaction, combine (in a series
of complex reactions) to form glucose, a simple sugar.
The plant may then take these simple sugars and combine
them into long chains, forming the large molecules of starch and cellulose. Plants store
food in the form of starch; cellulose is incorporated into the structure of cell walls.
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Pumping iron
The chlorophyll molecule is relatively large and shaped something like a lollipop.
At the top of the lollipop is a ring consisting of carbon, hydrogen, and nitrogen atoms
surrounding a central magnesium atom.
Remarkably, if you could somehow remove that
magnesium atom and replace it with an iron atom, you’d end up with what is
essentially a molecule of hemoglobin! Hemoglobin is a molecule contained in animal blood
that is responsible for transporting oxygen throughout the body. Nature really is amazing,
isn’t it?
Fill 'er up!
Not only do we rely on plants for food and oxygen, we also depend on them for
energy. Most of our civilization’s energy and fuel requirements are filled by plants.
Heating with wood is an obvious example. But did you know that oil, coal, and natural gas
are all mined from ancient deposits of fossilized plants? |
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