Photosynthesis

Overview

Photosynthesis uses light energy and carbon dioxide to make starch.

A commonly used but slightly simplified equation for photosynthesis is:

6 CO_2(gas) + 12 H₂O_(liquid) + C₆H₁₂O₆(aqueous) + 6 O_2(gas) + 6 H₂O_(liquid)

carbon dioxide + water + light energy → glucose + oxygen + water

When written as a word equation the light energy appears above the arrow as it is required for photosynthesis but it is not actually a reactant. Here the glucose is shown as a product, although the actual processes in plants produce disaccarides.

The equation is often presented in introductory chemistry texts in an even more simplified form as:^[2]

6 CO_2(gas) + 6 H₂O_(liquid) + photons → C₆H₁₂O_6(aqueous) + 6 O_2(gas)

Photosynthesis occurs in two stages. In the first phase, light-dependent reactions or photosynthetic reactions (also called the Light reactions) capture the energy of light and use it to make high-energy molecules. During the second phase, the light-independent reactions (also called the carbohydrates.

In the oxygen gas.

In the Calvin-Benson cycle releases three-carbon sugars, which are later combined to form sucrose and starch.

Photosynthesis may simply be defined as the conversion of light energy into temperature.

In plants

Most plants are photoautotrophs, which means that they are able to inorganic compounds using light energy - for example from the sun, instead of eating other organisms or relying on nutrients derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds.

6 CO₂ + 12 H₂O → C₆H₁₂O₆ + 6 O₂ + 6 H₂O

The energy for photosynthesis ultimately comes from absorbed NADPH, which are used for synthetic reactions in photoautotrophs. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:

2 H₂O + 2 NADP⁺ + 2 ADP + 2 P_i + light → 2 NADPH + 2 H⁺ + 2 ATP + O₂ ^[3]

Most notably, plants use the chemical energy to fix light-independent reactions. The overall equation for carbon fixation (sometimes referred to as carbon reduction) in green plants is:

3 CO₂ + 9 ATP + 6 NADPH + 6 H⁺ → C₃H₆O₃-phosphate + 9 ADP + 8 P_i + 6 NADP⁺ + 3 H₂O ^[3]

To be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material cellular respiration. The latter occurs not only in plants but also in animals when the energy from plants gets passed through a food chain. Organisms dependent on photosynthetic and chemosynthetic organisms are called heterotrophs. In general outline, cellular respiration is the opposite of photosynthesis: Glucose and other compounds are oxidised to produce carbon dioxide, water, and chemical energy. However, both processes take place through a different sequence of chemical reactions and in different cellular compartments.

Plants absorb light primarily using the palisade mesophyll cells where most of the photosynthesis takes place.

Plants use up to 90% of the light that strikes them, whereas commercial solar panels use less than 30%. This is achieved by groups of chlorophyll molecules spending a long time in a superposition of states.^[4]

In algae and bacteria

Algae come in multiple forms from multicellular organisms like kelp, to microscopic, xanthophylls in green algae and phycoerythrin in red algae (rhodophytes), resulting in a wide variety of colors. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. For example, in coral reefs, there is a mutualistic relationship between zooxanthellae and the coral polyps.^[5]

Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. sulfur as waste.

Evolution

  The ability to convert light energy to chemical energy confers a significant evolutionary advantage to living organisms. Early photosynthetic systems, such as those from green and purple sulfur and green and purple non-sulfur bacteria, are thought to have been anoxygenic, using various molecules as reduced at that time.^{[citation needed]}

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.^[6]

photosynthetic reaction center.

Origin of chloroplasts

In plants the process of photosynthesis occurs in organelles called ribosomes, and similar proteins in the photosynthetic reaction center.

The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis or gene fusion) by early eukaryotic cells to form the first plant cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, whereas plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex animals, including humans, whose cells depend upon mitochondria as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modern Rickettsia bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells.

This contention is supported by the finding that the marine molluscs Elysia viridis and Elysia chlorotica seem to maintain a symbiotic relationship with chloroplasts from algae with similar RDA structures that they encounter. However, they do not transfer these chloroplasts to the next generations.

Cyanobacteria and the evolution of photosynthesis

The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant nitrogen fixation. Green algae joined blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251-65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.^[7]

Molecular production

 

Light to chemical energy

Main article: Light-dependent reaction

The light energy is converted to chemical energy using the light-independent reactions.

Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the phycobilins for blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.

Z scheme

    In plants, electron acceptors. The energy created by the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is used to reduce the co-enzyme NADP, which has functions in the light-independent reaction. The cyclic reaction is similar to that of the non-cyclic, but differs in the form that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns back to photosystem I, from where it was emitted, hence the name cyclic reaction.

Water photolysis

Main articles: Oxygen evolution

The NADPH is the main cellular respiration, including photosynthetic organisms.

Quantum mechanical effects

Through photosynthesis, sunlight energy is transferred to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. The transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat. However, only 43% of the total solar incident radiation can be used (only light in the range 400-700 nm), 20% of light is blocked by canopy, and plant respiration requires about 33% of the stored energy, which brings down the actual efficiency of photosynthesis to about 6.6%^[8].

A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley suggests that long-lived wavelike electronic quantum coherence plays an important part in this instantaneous transfer of energy by allowing the photosynthetic system to simultaneously try each potential energy pathway and choose the most efficient option. Results of the study are presented in the April 12, 2007 issue of the journal Nature.^[9]

Oxygen and photosynthesis

With respect to oxygen and photosynthesis, there are two important concepts.

• Plant and cyanobacterial (blue-green algae) cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.
• Oxygen is a product of the light-driven water-oxidation reaction catalyzed by photosystem II; it is not generated by the fixation of carbon dioxide. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide.

Bacterial variation

The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Van Niel in the 1930s, who studied photosynthetic bacteria. Aside from the hydrogen, so for most of these bacteria oxygen is not produced.

Others, such as the halophiles (an Archaea), produced so-called purple membranes where the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.

 

Carbon fixation

Main articles: Light-independent reaction

The fixation or reduction of carbon dioxide is a light-independent process in which metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

C₄, C₃ and CAM

  In hot and dry conditions, plants will close their stomata to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves causing photorespiration to occur. Some plants have evolved mechanisms to increase the CO₂ concentration in the leaves under these conditions.

3-phosphoglycerate directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C4 plants reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C₄ plants can produce more sugar than C₃ plants in conditions of strong light and high temperature. Many important crop plants are C₄ plants including maize, sorghum, sugarcane, and millet.

Xerophytes such as cacti and most succulents also can use PEP Carboxylase to capture carbon dioxide in a process called phosphoenolpyruvate to oxaloacetate, which is then reduced to malate). Nevertheless, C₄ plants capture the CO₂ in one type of cell tissue (mesophyll) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. They also have a different leaf anatomy than C₄ plants. They grab the CO₂ at night, when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate. C4 metabolism physically separates CO₂ fixation from the Calvin cycle, while CAM metabolism temporally separates CO₂ fixation from the Calvin cycle.

Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s.

Jan van Helmont began the research of the process in the mid-1600s when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself.

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 sunlight on the plant that could cause it to rescue a mouse in a matter of hours.

In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterwards, Nicolas-Théodore 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 by which photosynthesis is used to produce food (such as glucose) was outlined.

Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces carbon dioxide.

Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction is as follows:

2 H₂O + 2 A + (light, chloroplasts) → 2 AH₂ + O₂

where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved.

Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.

Calvin cycle, which inappropriately ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some even call it the Calvin-Benson-Bassham (or CBB) Cycle.

A Nobel Prize winning scientist, Rudolph A. Marcus, was able to discover the function and significance of the electron transport chain.

Factors

There are three main factors affecting photosynthesis and several corollary factors. The three main are:

• Light irradiance and wavelength
• concentration
• Temperature.

Light intensity (Irradiance), wavelength and temperature

In the early 1900s Frederick Frost Blackman along with Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation.

• At constant temperature, the rate of carbon assimilation varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of carbon assimilation reaches a plateau.
• At constant irradiance, the rate of carbon assimilation increases as the temperature is increased over a limited range. This effect is only seen at high irradiance levels. At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation.

These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by phycobilisome.

Carbon dioxide levels and photorespiration

As carbon dioxide concentrations rise, the rate at which sugars are made by the fix carbon dioxide. However, if the oxygen concentration is high, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not make sugar.

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

1. One product of oxygenase activity is phosphoglycolate (2 carbon) instead of Calvin-Benson cycle.
2. Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
3. Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.

A highly-simplified summary is:

2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH₃

The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.

See also

Notes

References

• Blankenship, R.E., 2002. Molecular Mechanisms of Photosynthesis. Blackwell Science.
• Campbell, N., & Reece, J., 2005. Biology 7th ed. San Francisco: Benjamin Cummings.
• Gregory, R.P.F., 1971. Biochemistry of Photosynthesis. Belfast: Universities Press.
• Govindjee, 1975. Bioenergetics of Photosynthesis. New York: Academic Press.
• Govindjee; Beatty, J.T., Gest, H. and Allen, J.F. (Eds.), 2005. Discoveries in Photosynthesis. Advances in Photosynthesis and Respiration, Volume 20, Springer.
• Rabinowitch, E. and Govindjee., 1969. Photosynthesis. New York: John Wiley & Sons, Inc.
• Stern, Kingsley R., Shelley Jansky, James E Bidlack, 2003. Introductory Plant Biology. McGraw Hill. ISBN 0-07-290941-2