Photosynthesis
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| Photosynthesis |
What is Photosynthesis?
Photosynthesis, process by which green
plants and certain other organisms use the energy of light to convert carbon
dioxide and water into the simple sugar glucose. In so doing, photosynthesis
provides the basic energy source for virtually all organisms. An extremely
important byproduct of photosynthesis is oxygen, on which most organisms
depend.
Photosynthesis occurs
in green plants, seaweeds, algae, and certain bacteria. These organisms are
veritable sugar factories, producing millions of new glucose molecules per
second. Plants use much of this glucose, a carbohydrate, as an energy source to
build leaves, flowers, fruits, and seeds. They also convert glucose to
cellulose, the structural material used in their cell walls. Most plants
produce more glucose than they use, however, and they store it in the form of
starch and other carbohydrates in roots, stems, and leaves. The plants can then
draw on these reserves for extra energy or building materials. Each year,
photosynthesizing organisms produce about 170 billion metric tons of extra
carbohydrates, about 30 metric tons for every person on earth.
Photosynthesis has far-reaching
implications. Like plants, humans and other animals depend on glucose as an
energy source, but they are unable to produce it on their own and must rely
ultimately on the glucose produced by plants. Moreover, the oxygen humans and
other animals breathe is the oxygen released during photosynthesis. Humans are
also dependent on ancient products of photosynthesis, known as fossil fuels,
for supplying most of our modern industrial energy. These fossil fuels,
including natural gas, coal, and petroleum, are composed of a complex mix of
hydrocarbons, the remains of organisms that relied on photosynthesis millions
of years ago. Thus, virtually all life on earth, directly or indirectly,
depends on photosynthesis as a source of food, energy, and oxygen, making it one
of the most important biochemical processes known.
WHERE PHOTOSYNTHESIS OCCUR
Plant photosynthesis occurs
in leaves and green stems within specialized cell structures called
chloroplasts. One plant leaf is composed of tens of thousands of cells, and
each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped
structure, is divided by membranes into numerous disk-shaped compartments.
These disklike compartments, called thylakoids, are arranged vertically in the
chloroplast like a stack of plates or pancakes. A stack of thylakoids is called
a granum (plural, grana); the grana lie suspended in a fluid
known as stroma.
Embedded in the membranes
of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping
pigment required for photosynthesis. Additional light-trapping pigments,
enzymes (organic substances that speed up chemical reactions), and other
molecules needed for photosynthesis are also located within the thylakoid
membranes. The pigments and enzymes are arranged in two types of units,
Photosystem I and Photosystem II. Because a chloroplast may have dozens of
thylakoids, and each thylakoid may contain thousands of photosystems, each
chloroplast will contain millions of pigment molecules.
HOW PHOTOSYNTHESIS
WORKS
Photosynthesis is a very
complex process, and for the sake of convenience and ease of understanding,
plant biologists divide it into two stages. In the first stage, the
light-dependent reaction, the chloroplast traps light energy and converts it
into chemical energy contained in nicotinamide adenine dinucleotide phosphate
(NADPH) and adenosine triphosphate (ATP), two molecules used in the second
stage of photosynthesis. In the second stage, called the light-independent
reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms
that help form glucose, and ATP provides the energy for this and other
reactions used to synthesize glucose. These two stages reflect the literal
meaning of the term photosynthesis, to build with light.
LIGHT DEPENDENT
REACTION
Photosynthesis relies
on flows of energy and electrons initiated by light energy. Electrons are
minute particles that travel in a specific orbit around the nuclei of atoms and
carry a small electrical charge. Light energy causes the electrons in
chlorophyll and other light-trapping pigments to boost up and out of their
orbit; the electrons instantly fall back into place, releasing resonance
energy, or vibrating energy, as they go, all in millionths of a second. Chlorophyll
and the other pigments are clustered next to one another in the photosystems,
and the vibrating energy passes rapidly from one chlorophyll or pigment
molecule to the next, like the transfer of energy in billiard balls.
Light contains many colors,
each with a defined range of wavelengths measured in nanometers, or billionths
of a meter. Certain red and blue wavelengths of light are the most effective in
photosynthesis because they have exactly the right amount of energy to
energize, or excite, chlorophyll electrons and boost them out of their orbits
to a higher energy level. Other pigments, called accessory pigments, enhance
the light-absorption capacity of the leaf by capturing a broader spectrum of
blue and red wavelengths, along with yellow and orange wavelengths. None of the
photosynthetic pigments absorb green light; as a result, green wavelengths are
reflected, which is why plants appear green.
Photosynthesis begins when light strikes
Photosystem I pigments and excites their electrons. The energy passes rapidly
from molecule to molecule until it reaches a special chlorophyll molecule
called P700, so named because it absorbs light in the red region of the
spectrum at wavelengths of 700 nanometers.
Until this point, only energy
has moved from molecule to molecule; now electrons themselves transfer between
molecules. P700 uses the energy of the excited electrons to boost its own
electrons to an energy level that enables an adjoining electron acceptor
molecule to capture them. The electrons are then passed down a chain of carrier
molecules, called an electron transport chain. The electrons are passed from
one carrier molecule to another in a downhill direction, like individuals in a
bucket brigade passing water from the top of a hill to the bottom. Each
electron carrier is at a lower energy level than the one before it, and the
result is that electrons release energy as they move down the chain. At the end
of the electron transport chain lies the molecule nicotine adenine dinucleotide
(NADP+). Using the energy released by the flow of electrons, two
electrons from the electron transport chain combine with a hydrogen ion and
NADP+ to form NADPH.
When P700 transfers its
electrons to the electron acceptor, it becomes deficient in electrons. Before
it can function again, it must be replenished with new electrons. Photosystem
II accomplishes this task. As in Photosystem I, light energy activates
electrons of the Photosystem II pigments. These pigments transfer the energy of
their excited electrons to a special Photosystem II chlorophyll molecule, P680,
that absorbs light best in the red region at 680 nanometers. Just as in
Photosystem I, energy is transferred among pigment molecules and is then
directed to the P680 chlorophyll, where the energy is used to transfer
electrons from P680 to its adjoining electron acceptor molecule.
From the Photosystem II
electron acceptor, the electrons are passed through a different electron
transport chain. As they pass along the cascade of electron carrier molecules,
the electrons give up some of their energy to fuel the production of ATP,
formed by the addition of one phosphorus atom to adenosine diphosphate (ADP).
Eventually, the electron transport carrier molecules deliver the Photosystem II
electrons to Photosystem I, which uses them to maintain the flow of electrons
to P700, thus restoring its function.
P680 in Photosystem II is now
electron deficient because it has donated electrons to P700 in Photosystem I.
P680 electrons are replenished by the water that has been absorbed by the plant
roots and transported to the chloroplasts in the leaves. The movement of
electrons in Photosystems I and II and the action of an enzyme split the water
into oxygen, hydrogen ions, and electrons. The electrons from water flow to
Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions
may be used to produce NADPH at the end of the electron transport chain, and
the oxygen from the water diffuses out of the chloroplast and is released into
the atmosphere through pores in the leaf.
The transfer of electrons in a
step-by-step fashion in Photosystems I and II releases energy and heat slowly,
thus protecting the chloroplast and cell from a harmful temperature increase.
It also provides time for the plant to form NADPH and ATP. In the words of
American biochemist and Nobel laureate Albert Szent-Gyorgyi, “What drives life
is thus a little electric current, set up by the sunshine.”
THE LIGHT –
INDEPENDENT REACTION
The chemical energy required
for the light-independent reaction is supplied by the ATP and NADPH molecules
produced in the light-dependent reaction. The light-independent reaction is
cyclic, that is, it begins with a molecule that must be regenerated at the end
of the reaction in order for the process to continue. Termed the Calvin cycle
after the American chemist Melvin Calvin who discovered it, the
light-independent reactions use the electrons and hydrogen ions associated with
NADPH and the phosphorus associated with ATP to produce glucose. These
reactions occur in the stroma, the fluid in the chloroplast surrounding the
thylakoids, and each step is controlled by a different enzyme.
The light-independent reaction requires
the presence of carbon dioxide molecules, which enter the plant through pores
in the leaf, diffuse through the cell to the chloroplast, and disperse in the
stroma. The light-independent reaction begins in the stroma when these carbon
dioxide molecules link to sugar molecules called ribulose bisphosphate (RuBP)
in a process known as carbon fixation.
With the help of an enzyme,
six molecules of carbon dioxide bond to six molecules of RuBP to create six new
molecules. Several intermediate steps, which require ATP, NADPH, and additional
enzymes, rearrange the position of the carbon, hydrogen, and oxygen atoms in
these six molecules, and when the reactions are complete, one new molecule of
glucose has been constructed and five molecules of RuBP have been
reconstructed. This process occurs repeatedly in each chloroplast as long as
carbon dioxide, ATP, and NADPH are available. The thousands of glucose
molecules produced in this reaction are processed by the plant to produce
energy in the process known as aerobic respiration, used as structural
materials, or stored. The regenerated RuBP is used to start the Calvin cycle
all over again.
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