Photosynthesis is a biochemical process in which plants, green algae, and some bacteria use the energy of light to combine water and carbon dioxide into oxygen and energy. It nourishes nearly all living things directly or indirectly, making it vital to life on Earth.
What is photosynthesis?
Plants, green algae, and some bacteria are autotrophs, which means that they they sustain themselves without eating other organisms or substances derived from other organisms. Autotrophs produce their raw organic molecules from carbon dioxide (CO2) and other inorganic raw materials derived from the environment.
Photosynthesis is the process by which these autotrophs (such as plants, green algae and some bacteria) use the energy contained in sunlight to split carbon dioxide and water, and recombine them into oxygen gas and a sugar called glucose. Glucose (C6H12O6) is a simple molecule that is used by autotrophs for many things, such as fuel to make energy, and building material to make cellulose (which is used by plants to make their rigid cell walls).
The overall chemical reaction of photosynthesis is:
- 12H2O + 6CO2 + light → C6H12O6 (glucose) + 6O2 + 6H2O
In simple English, water plus carbon dioxide plus light energy yields sugar plus oxygen plus water. This is the exact opposite of the process of respiration in animals: oxygen plus sugar yields carbon dioxide plus water plus energy. Even though this reaction may seem simple, the details of photosynthesis are actually very complex.
The sites of photosynthesis in plants
The green color of plants is from chlorophyll, a green pigment that absorbs the light energy that drives the process of photosynthesis.
Chlorophyll is contained in millions of tiny structures called chloroplasts, which are small organelles (parts of cells) found in the cells of plants and green algae. All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants.
Chloroplasts are found mainly in the cells of the mesophyll, which is the tissue on the interior of a leaf. There are about half a million chloroplasts per square millimeter of leaf.
Some autotrophs can appear colors other than green if they use primary photosynthetic pigments other than chlorophyll. For example, halobacteria use red bacteriorhodopsin instead of chlorophyll, and purple sulfur bacteria use carotenoids and bacteriochlorophylls, which combine to make the bacteria appear purple, red, brown, or orange.
Photosynthesis and its environment
Plants are autotrophs, which means that they they sustain themselves without eating other organisms or substances derived from other organisms. Autotrophs produce their raw organic molecules from CO2 and other inorganic raw materials derived from the environment, and are the ultimate sources of organic compounds for all nonautotrophic organisms (such as animals). For this reason, biologists refer to plants and other autotrophs as the producers of the biosphere (the global ecosystem).
The carbon cycle
Photosynthetic plants play an important role in the carbon cycle. Plants absorb carbon dioxide, which is produced by animals and many human activities. Plants convert the carbon, together with water, into glucose that can be used for energy. Around the world, plants are able to draw about 10% of the carbon dioxide out of the air and convert it into glucose. Much of this glucose is held onto by the plant, for energy storage for eventual use. This means that plants account for a substantial part of the global carbon budget.
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 Adenosine triphosphate (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 H2S will diffuse into the cell only slowly. A mobile cell benefits from a continuously higher concentration, increasing not only the access to H2S 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.
Photosynthesis at the molecular level
Although some of the steps of photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800's: in the presence of light, the green parts of plants produce glucose (C6H12O6) and oxygen (O2) from carbon dioxide (CO2) and water (H2O).
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 organisms are bacteria which use hydrogen sulfide instead of water in their photosynthetic pathway.
- 12H2S + 6CO2 + light → C6H12O6 + 6H2O + 12S
Van Neil's proposed that during the process of photosynthesis, hydrogen is extracted from water and incorporated into glucose, as summarized below (the path of specific molecules of oxygen or sulphur are indicated by the colors red and blue):
- Plants: CO2 + 2H2O → CH2O + H2O + O2
- Sulphur bacteria: CO2 + 2H2S → CH2O + H2O + 2S
The equation for photosynthesis may appear simple at this stage, but it is actually a summary of a very complex process. Actually, photosynthesis is not a single process, but is in fact the combination of two seperate cooperating processes, each with its own multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle.
The light reactions
- Main article: Light reaction
The light reactions, the first steps of photosynthesis, are two reactions that convert light into chemical energy that can be used to power the plant's life activities. In these reactions, light energy is converted into chemical energy in the form of two compounds: NADPH (a source of energized electrons) and ATP (the "energy currency" of cells). The NADPH is later used in the dark reactions.
The light reactions are more effective and produce more product at certain wavelengths of light, especially 680 and 700 nanometers. These wavelengths are the most active for the light-absorbing photosystems. However, otehr wavelengths are also peaks in the action spectrum of photosynthesis.
The light reactions produce no sugar: that is the job of the second stage of photosynthesis, the Calvin cycle.
When light strikes the chlorophyll of photosystem II, two electrons are driven off the porphyrin ring of the chlorophyll onto a primary electron acceptor. The electrons then move down an electron transport chain and into photosystem I. The energy from this chain is used to drive phosphoraltion of ADP (see below). In photosystem I, the electrons move onto another electron acceptor and down a second electron transport chain.
When the electrons reach the bottom of this chain, they and a hydrogen ion are bound to NADP+, converting it into NADPH. NADP+ functions as a temporary storage location for the electrons, and is converted back to NADP+ in the Calvin cycle (see below). This step of converting NADP+ to NADPH is called reduction, because the positive electric charge of NAPH+ is reduced from +1 to 0. This step can be summarized in the following two half-reactions:
- H2O + light → ½ O2 + 2H+ + 2e-
- 2NADP+ + 2H+ + 2e- → 2NADPH
Water is split during this process into O2 and H+. The electrons produced by this split are used to repair the porphyrin rings in photosystem II. The hydrogen is bound into NADP+. The oxygen is the valuable waste gas that plants produce.
The light reactions use the energy from the first electron transport chain to generate ATP (adenosine triphosphate), the chemical used by nearly all forms of life to supply energy for the activities of life. ATP is produced by adding a phosphate group (PO43-) to ADP (adenosine diphosphate), making ATP. This process is called photophosphorylation, meaning phosphorylation by light.
The dark reactions
- Main article: dark reaction
The dark reactions of photosynthesis do not need light to occur. They take the products of the light reactions and perform further chemical processes on them. The net goal of the dark reactions is the production of glucose, the sugar used by plants for energy. The dark reactions occur in two steps: carbon fixation and the Calvin cycle.
Carbon fixation in plants is the method by which carbon dioxide is first built into a sugar. Most plants are C3 plants, meaning they fix carbon dioxide first into a 3-carbon sugar, phosphoglyceraldehyde. This step occurs directly as the first step of the Calvin cycle, not as a separate carbon fixation reaction.
There are two other pathways by which the carbon fixation reactions can occur. The fixations occur before the Calvin cycle and the prducts are passed into that cycle. These pathways are favored in many desert plants, because they allow the plants to close some of their stomata, the pores that allow air in, and work with less CO2. By closing the stomata on sunny days, the plant can keep more of its moisture in, while still performing photosynthesis. Plants using these pathways fix carbon into a sugar then pass that sugar into the Calvin cycle where it is converted to glucose. C4 plants convert CO2 into oxaloacetate, a 4-carbon sugar. CAM plants perform the Calvin cycle at night, when they can open their pores to CO2 and not risk moisture loss. They pass a 4-carbon sugar malate into the Calvin cycle.
The Calvin cycle generally serves to convert CO2 and H2O into C6H12O6 (glucose). The overall reaction is driven by ATP and NADPH from the light reactions, but other sources can be used for these energy-laden chemicals. The product glucose is used by the cell to produce ATP through cellular respiration.
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. In 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
- Campbell, Neil A.; Reece, Jane B. (2002). Biology (6th ed.). Benjamin Cummings Press. ISBN 0-8053-6624-5.
- Solomon, E. P., Berg, E. L., Martin, D. W. (2004). Biology (7th ed.). Thomson Brooks/Cole. ISBN 0-534-49276-2.
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