Definition:

Photosynthesis 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 in archaea.

HOW OLD IS PHOTOSYNTHESIS?

The first 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 cyanobacteria.

The purple 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.

In plants, 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.

The basic equation for Photosynthesis is shown below.

This equation is for oxygenic (oxygen evolving) photosynthesis since it uses water as an electron donor and produces oxygen gas.

WHERE DOES PHOTOSYNTHESIS OCCUR?

Photosynthesis 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.

The cyanobacteria 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
		A 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?

Plants absorb 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 and xanthophylls.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 complex.

THE CHLOROPHYLL MOLECULE   Chlorophyll 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. Besides 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 complex.

PHOTOSYNTHESIS HAS TWO MAIN REACTIONS

Photosynthesis has two main reactions. Light-dependent reactions - which need light to work - and light-independent reactions - which do not need light to work. Light-dependent reaction Light 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 The 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 of chloroplasts. Light-independent reaction During 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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