Photosynthesis

Photosynthesis is a biochemical process by which the energy of light is converted into chemical energy in plants, algae, and certain bacteria. It is a process in which practically all life on earth ultimately depends on.

Table of contents

1 Overview 2 The production of oxygen 3 Light-dependent reaction 4 The Calvin cycle 5 The evolution of photosynthesis 6 The Discovery of Photosynthesis 7 Overview of Light and Dark Reactions 8 The Chloroplast 9 Related articles 10 External links

Overview

Most plants, unlike animals, do not get food by eating other organisms (there are exceptions: carnivorous plants such as the Venus Flytrap get some or most of their nutrients but not energy from predation). They make their own food, usually in the form of glucose, from the inorganic compounds carbon dioxide and water. Carbon dioxide is taken in through the leaves, and water is taken in mainly through the roots. Sunlight acts as the energy needed to run the reaction that yields glucose as the product the plant needs and oxygen as a waste product that is released into the environment. In green plants and algae, the pigment molecules that initially absorb the light energy are chlorophyll, with accessory pigments such as carotenoids, phycobilin, or phycoerythrin. Some halobacteria use other primary photosynthetic pigments than chlorophyll, notably bacteriorhodopsin. It may be noted that the typical colors of photosynthetic organisms (green, brown, golden, or red) result from the light that is not absorbed by the pigment molecules, but instead is reflected before meeting the eye. The typical overall chemical reaction of photosynthesis is:

12H₂O + 6CO₂ + light → C₆H₁₂O₆ (glucose) + 6O₂ + 6H₂O

In simple English, this is water plus carbon dioxide plus light (energy) yields sugar plus oxygen plus water. In animals, this is exactly reversed in the process of respiration (which plants also use, to release the energy stored in photosynthesis): oxygen plus sugar yields carbon dioxide plus water plus energy. However, it is important to note that this chemical equation is highly simplified; in reality photosynthesis employs a very complex mechanism for the adsorption and conversion of light into chemical energy, using chemical pathways with many important intermediates. Photosynthesis has two distinct stages, called the light reaction and carbon fixation (often called the dark reaction as it is not dependent on light, but the term is confusing as it has nothing to do with the dark), which typically occurs via the Calvin cycle. Primary production is the amount of carbon fixed by plants per unit area over time via photosynthesis.

The production of oxygen

It is interesting to note that the oxygen released during photosynthesis is not in fact derived from the carbon dioxide, but rather from the water molecules which are consumed in the reaction. This was first proposed in the 1930s by C. B. van Neil of Stanford University, while investigating photosynthetic bacteria, many of which do not release oxygen. One significant group of such organism are bacteria which use hydrogen sulfide instead of water in their photosynthetic pathway:

12H₂S + 6CO₂ + light → C₆H₁₂O₆ + 6H₂O + 12S

Some of these produce globules of sulfur as a waste product instead of oxygen, while others further oxidize it, producing sulfates. In general, photosynthesis requires a source of hydrogen with which to reduce carbon dioxide into carbohydrates. Van Neil's proposal was confirmed 20 years later by using the ¹⁸O isotope of oxygen as a tracer label to follow the fate of oxygen atoms during photosynthesis. Oxygen is not only a waste product of photosynthesis, it can even harm the photosynthetic process. This is because RubisCO, the primary CO₂-fixing enzyme in most plants, also "fixes" oxygen, but this does not lead to useful sugar production. Rather, it results in the loss of both CO₂ and nitrogen (in the form of ammonia, NH₃) from the plant, in a process known as photorespiration. While some evidence indicates that photorespiration can help protect plants from damage due to very high light intensities, it is generally considered a wasteful process, in which as much as 50% of the plant's fixed carbon can be lost to the atmosphere. Some plants have evolved strategies to minimize photorespiration; these plants are grouped into C4 plants and CAM plants.

Light-dependent reaction

The "light reactions" are the first processes of photosynthesis. In them, light is absorbed by molecules of the green pigment chlorophyll. The light is used to "charge" an electron, which is transported via an electron transport system to a molecule of NADP⁺, which turns into the hydrogen carrier NADPH (used later on in the Calvin cycle). In the meantime, a molecule of water is split. The oxygen is released into the atmosphere, while the hydrogen ions (which are merely protons after being split from oxygen) diffuse through Transmembrane ATPase. This energy is harnessed to synthesize a molecule of ATP.

The Calvin cycle

The Calvin cycle is similar to the Krebs cycle in some regards. Carbon enters the Calvin cycle in the form of CO₂ and leaves in the form of a carbohydrate such as sugar, with the reaction being driven by ATP and NADPH. This ATP and NADPH is usually produced by the light reaction described above, but there is nothing inherent in the process which requires this to be the case; other sources of ATP and NADPH can be used, and in some cases are.

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The evolution of photosynthesis

Life is generally believed to have evolved on Earth between 3.5 and 4.5 billion years ago. The primordial atmosphere is thought to have consisted of mostly methane, carbon dioxide, water vapor, hydrogen sulfide, and ammonia. Fossil evidence shows that most life prior to the aerobic extinction event probably used hydrogen sulfide fixation to synthesize ATP. The original prokaryotic organisms were non-motile (couldn't move). Originally cells were dependent upon the environment to move them around to fresh sources of chemical energy. The next step saw the formation of primitive flagella, organelles that could cause the cell to move under its own power. Originally these flagella were more or less autonomic (on all the time). This increased the cell's access to fresh sources of hydrogen sulfide. A cell that sits in one spot will eventually reduce the surrounding concentration of hydrogen sulfide to the point of stasis, at which point HS will diffuse into the cell only slowly. A mobile cell benefits from a continuously higher concentration, increasing not only the access to HS but also the rate at which the cell absorbs it in general. Hydrogen sulfide is not the only resource needed for primitive life. The warm waters near the surface help to catalyze the reactions. Eventually photosensitive pigments evolved that allowed the flagella to move the cell towards the surface, and thus warmer regions. The region of the sun's spectrum that has the highest energy is in the yellow region; however, simple organic pigments have the largest bandwidth response in the red and infrared region. With infrared also being associated with heat, most likely the first photosensitive pigments responded to red and infrared light much as modern chlorophyll does. This would have given them a blue-green hue.

The Discovery of Photosynthesis

Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley's experiments. He discovered that it was the influence of sun and light on the plant that could cause it to rescue a mouse in a matter of hours. \nIn 1796, Jean Senebier, a French pastor, showed that CO₂ was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis. Soon afterwards, Theodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO₂, but also to the incorporation of water. Thus the basic reaction of photosynthesis was outlined: CO₂ + H₂O + light energy → (CH₂O)_n + O₂

Overview of Light and Dark Reactions

Early in the 20th Century, researchers took advantage of the use of isotopes to better understand the basic equation of photosynthesis. It was discovered that when carbon dioxide was labelled with a heavy isotope of oxygen, only the lighter isotope was emitted from the plant as oxygen gas. However, if the oxygen of the water was labelled, so was the oxygen gas emitted. This showed that the oxygen for photosynthesis was derived from the water.\nLight energy entering the plant splits the water into hydrogen and oxygen: H₂O + light energy → ½ O₂ + 2H⁺ + 2e^- These electrons travel through the mebrane much like the electrons in oxidative phosphorylation, using their energy to pump protons through the membrane. The proton gradient thus established can be used to synthesize ATP. More importantly, that same electron reduces NADP+ to NADPH. This molecule plays the same role in synthesis as does NAD+ in the respiratory pathway, as a carrier of reductive power. This store of power serves to reduce carbon dioxide to the more complex carbon structure of glucose, the building block of life. The reactions leading to the production of ATP and reduction of NADP+ are called the light reactions because they are initiated by the splitting of water by light energy. The reduction of carbon dioxide to glucose, using the NADPH produced by the light reactions, is governed by the dark reactions.

The Chloroplast

In plants and algae, photosynthesis takes place in chloroplasts. In many ways, these resemble the mitochondrion - both are surrounded by a double membrane, with reticulations filling their inner space to increase the surface area where reactions on bound proteins can take place, and have their own DNA. They are now considered reduced versions of endosymbiotic cyanobacteria. Most chloroplasts have three membranes: inner, outer, and thylakoid . It has three compartments: stroma, thylakoid space, and inter-membrane space. These compartments and the membranes that separate them serve to isolate different aspects of photosynthesis. Dark reactions take place in the stroma. Light reactions take place on the thylakoid membranes.

Related articles

\n*Rubisco\n*Chloroflexus_aurantiacus

External links

\n*Photosystem I: Molecule of the Month in the Protein Data Bank \n\n\n\n\n\n\n\n\nsimple:Photosynthesis\n\n\nzh-cn:光合作用/简\nzh-tw:光合作用 Category:Biochemistry\nCategory:Botany\nCategory:Photosynthesis