is the process by which green plants and some other organisms
use sunlight to synthesize foods from carbon dioxide and water.
It is the driving force behind most of the life on earth. Photosynthesis
occurs in plants, algae, and many species of bacteria, but not
HOW OLD IS PHOTOSYNTHESIS?
photosynthetic organisms probably evolved about 3,500 million
years ago, early in the evolutionary history of life, when all
forms of life on Earth were microorganisms and the atmosphere
had much more carbon dioxide. They most likely used hydrogen
or hydrogen sulfide as sources of electrons, rather than water.
Cyanobacteria appeared later, around 3,000 million years ago,
and drastically changed the Earth when they began to oxygenate
the atmosphere, beginning about 2,400 million years ago. This
new atmosphere allowed the evolution of complex life such as
protists. Eventually, no later than a billion years ago, one
of these protists formed a symbiotic relationship with a cyanobacterium,
producing the ancestor of many plants and algae. The chloroplasts
in modern plants are the descendants of these ancient symbiotic
sulfur bacteria are a group of Proteobacteria capable of photosynthesis,
collectively referred to as purple bacteria. They are anaerobic
or microaerophilic, and are often found in hot springs or stagnant
water. Unlike plants, algae, and cyanobacteria, they do not
use water as their reducing agent, and so do not produce oxygen.
Instead they use hydrogen sulfide, which is oxidized to produce
granules of elemental sulfur. This in turn may be oxidized to
form sulfuric acid.
algae and cyanobacteria, photosynthesis releases oxygen. This
is called oxygenic photosynthesis. Although there are
some differences between oxygenic photosynthesis in plants,
algae and cyanobacteria, the overall process is quite similar
in these organisms.
equation for Photosynthesis is shown below.
+ 12H2O + sunlight ---> C6H12O6
can simply be stated as: six molecules of water plus six
molecules of carbon dioxide produce one molecule of glucose
plus six molecules of oxygen
is for oxygenic (oxygen evolving) photosynthesis since it uses
water as an electron donor and produces oxygen gas.
WHERE DOES PHOTOSYNTHESIS OCCUR?
occurs in many unrelated organisms from extremely small cyanobacteria
that measure only a few microns to giant sequoyah trees, big
enough to drive a truck through. In all cases a reaction center
protein complex is required. These proteins that gather light
for photosynthesis are embedded within cell membranes. The simplest
way these are arranged is in photosynthetic bacteria, where
these proteins are held within the plasma membrane. However,
this membrane may be tightly folded into cylindrical sheets
called thylakoids, or bunched up into round vesicles called
intracytoplasmic membranes. These structures can fill
most of the interior of a cell, giving the membrane a very large
surface area and therefore increasing the amount of light that
the bacteria can absorb.
are the most well-known photosynthetic prokaryotes, and it is
their form of oxygenic photosynthesis that has been co-opted
by the eukaryotes. In the cynobacteria, photosynthesis occurs
on special infoldings of the plasma membrane called thylakoids.
Around 2.0 - 2.2 billion years ago a single-celled eukaryote
ingested a cyanobacterium, and instead of digesting it, retained
it as an active intracellular symbiont. Over thousands of generations,
this "captured" cyanobacterium became reduced to a specialized
organelle, the chloroplast. All photosynthesis in green plants
and algae occurs in chloroplasts.
STRUCTURE OF A CHLOROPLAST
typical plant cell contains about 10 to 100 chloroplasts.
The chloroplast is enclosed by a membrane. This membrane
is composed of a phospholipid inner membrane, a phospholipid
outer membrane, and an intermembrane space between them.
Within the membrane is an aqueous fluid called the stroma.
The stroma contains stacks (grana) of thylakoids, which
are the site of photosynthesis. The thylakoids are flattened
disks, bounded by a membrane with a lumen or thylakoid
space within it. The site of photosynthesis is the thylakoid
membrane, which contains integral and peripheral membrane
protein complexes, including the pigments that absorb
light energy, which form the photosystems.
WHAT PIGMENTS ARE RESPONSIBLE FOR PHOTOSYNTHESIS?
light primarily using the pigment chlorophyll,
which is the reason that most plants have a green color. Besides
chlorophyll, plants also use pigments such as carotenes
pigments are embedded in plants and algae in special antenna-proteins.
In such proteins all the pigments are ordered to work well together.
Such a protein is also called a light-harvesting complex.
THE CHLOROPHYLL MOLECULE
is the green photosynthetic pigment present in chloroplasts
which provides the energy necessary for photosynthesis.
The intense green color of chlorophyll is due to its strong
absorbencies in the red and blue regions of the electromagnetic
spectrum, and because of these absorbencies the light
it reflects and transmits appears green. It is capable
of channeling the energy of sunlight into chemical energy
through the process of photosynthesis. In this process
the energy absorbed by chlorophyll transforms carbon dioxide
and water into carbohydrates and oxygen.
chlorophyll, plants also use pigments such as carotenes
and xanthophylls. Algae also use chlorophyll, but various
other pigments are present as phycocyanin, carotenes,
and xanthophylls in green algae, phycoerythrin in red
algae (rhodophytes) and fucoxanthin in brown algae and
diatoms resulting in a wide variety of colors. These pigments
are embedded in plants and algae in special antenna-proteins.
In such proteins all the pigments are ordered to work
well together. Such a protein is also called a light-harvesting
PHOTOSYNTHESIS HAS TWO MAIN REACTIONS
has two main reactions. Light-dependent reactions - which
need light to work - and light-independent reactions -
which do not need light to work.
energy from the sun is used to split water (photolysis)
which has been sucked in by plants by transpiration. The
sunlight hits chloroplasts in the plant, causing an enzyme
to break apart the water. Water, when broken, makes oxygen,
hydrogen, and electrons
hydrogen converts to NADPH which is then used in the light-independent
reactions. Oxygen diffuses out of the plant as a waste
product of photosynthesis and ATP is synthesised from
ADP and inorganic phosphate. This all happens in the grana
this reaction, sugars are built up using carbon dioxide
and the products of the light-dependent reactions (ATP
and NADPH) and various other chemicals found in the plant
in the Calvin Cycle. Therefore, the light-independent
reaction cannot happen without the light-dependent reaction.
Carbon dioxide diffuses into the plant and along with
chemicals in the stroma of the chloroplast and ATP and
NADPH, glucose is made and finally, transported around
the plant by translocation.
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