Main article
The fixation of carbon dioxide is a light-independent process in which carbon dioxide combines with a five-carbon sugar, ribulose bisphosphate (RuBP), to give two molecules of a three-carbon compound, glycerate 3-phosphate (GP). This compound is also sometimes known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehye 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate (a three-carbon sugar). This is the point at which carbohydrates are produced during photosynthesis. Some of the triose phosphates condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetylcoenzyme A to make amino acids and lipids. Others go on to regenerate RuBP so the process can continue (see 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. This was a partially accurate hypothesis - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signalling 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 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, 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.
Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things.
Cornelius 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 Robert Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like 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.
Samuel Ruben and Martin Camen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.
Melvin Calvin and his partner Benson were able to puzzle out each stage in the dark or light-independent phase of photosynthesis, known as the Calvin Cycle.
A Nobel Prize winning scientist, Rudolph A. Marcus, was able to discover the function and significance of the electron transport chain.
Bioenergetics of Photosynthesis
This section is a stub. You can help by [ adding to it].
Photosynthesis is a physiological phenomenon that coverts solar energy into photochemical energy. This physiological phenomenon may be described thermodynamically in terms of changes in energy, entropy and free energy. The energetics of photosynthesis, driven by light, causes a change in entropy that in turn yields a usable source of energy for the plant.
The following equation summarizes the products and reactants of photosynthesis in the typical green photosynthesizing plant:
CO2 + H2O → O2 + (CH2O) + 112 kcal/mol CO2
On earth, there are two sources of free energy: light energy from the sun, and terrestrial sources, including volcanoes, hot springs and radioactivity of certain elements. The biochemical value of electromagnetic radiation has led plants to use the free energy from the sun in particular. Visible light, which is used specifically by green plants to photosynthesize, may result in the formation of electronically excited states of certain substances called pigments (Gregory). For example, Chl a is a pigment which acts as a catalyst, converting solar energy into photochemical energy that is necessary for photosynthesis (Govindjee).
With the presence of solar energy, the plant has a usable source of energy, which is termed the free energy (F) of the system. However, thermal energy is not completely interconvertible, which means that the character of the solar energy may lead to the limited convertibility of it into forms that may be used by the plant. This relates back to the work of Josiah Willard Gibbs: the change in free energy (ΔF) is related to both the change in entropy (ΔS) and the change in enthalpy (ΔH) of the system (Rabinowitch).
Gibbs energy equation: ΔF = ΔH – TΔS
Past experiments have shown that the total energy produced by photosynthesis is 112 kcal/mol. However in the experiment, the free energy due to light was 120 kcal/mol. An overall loss of 8 kcal/mol was due to entropy, as described by Gibbs equation (Gonindjee). In other words, since the usable energy of the system is related directly to the entropy and temperature of the system, a smaller amount of thermal energy is available for conversion into usable forms of energy (including mechanical and chemical) when entropy is great (Rabinowitch). This concept relates back to the second law of thermodynamics in that an increase in entropy is needed to convert light energy into energy suitable for the plant.
Overall, in conjunction with the oxidation-reduction reaction nature of the photosynthesis equation, and the interrelationships between entropy and enthalpy, energy in a usable form will be produced by the photosynthesizing green plant.
References
Govindjee. Bioenergetics of Photosynthesis. New York: Academic Press, 1975.
Gregory, R.P.F. Biochemistry of Photosynthesis. Belfast: Universities Press, 1971.
Rabinowitch, Eugene and Govindjee. Photosynthesis. New York: John Wiley & Sons, Inc., 1969.
Factors affecting photosynthesis
There are three main factors affecting photosynthesis and several corollary factors. The three main are:
- Light irradiance and wavelength
- Carbon dioxide concentration
- Temperature
Light Intensity (Irradiance), Wavelength and Temperature
In the early 1900s F.F. Blackman investigated the effects of light intensity (irradiance) and temperature on the rate of photosynthesis. At constant temperature the rate of photosynthesis varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of photosynthesis reaches a plateau. The effect on the rate of photosynthesis of varying the temperature at constant irradiance can be seen in image to the left. At high irradiance the rate of photosynthesis increases as the temperature is increased over a limited range. At low irradiance, increasing the temperature has little effect on the rate of photosynthesis. These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of photosynthesis, so there must be two sets of reactions in the full process of photosynthesis. These are of course the light-dependent 'photochemical' stage and the light-independent, temperature-dependent stage. Secondly, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria which reside several metres underwater cannot recieve the correct wavelengths required to cause photoinduced charge seperation in conventional photosynthetic pigments. To combat this problem a series of proteins with different flourescent pigments surround the reaction centre. This unit is called a phycobilisome.
Carbon Dioxide
This section is a stub. You can help by [ adding to it].
An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor. Carbon dioxide helps increase the rate of photosynthesis. This is because rubisco, the enzyme fixing the carbon dioxide in the light-dependent reactions, has affinity for both carbon dioxide and oxygen. Thus, an increase in the concentration of carbon dioxide increases the probability of rubisco fixing carbon dioxide instead of oxygen. This allows the plant to be more productive since the fixation of oxygen requires photorespiration to remove glycolate a product of rubisco's oxygenase activity. Photorespiration is bad for a plant since it actually releases carbon dioxide and uses energy in the process.
Corollary Factors
This section is a stub. You can help by [ adding to it].
- Amount of water
- Leaf morphology
- Leaf nitrogen content
- Molecular Carriers such as NADP and FAD
In Detail
Metabolic pathways involved in photosynthesis:
- Light-dependent reaction
- Light-independent reaction
See also
- Artificial photosynthesis
- Calvin Cycle
External links
- Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology
- Overall examination of Photosynthesis at an intermediate level
- Overall Energetics of Photosynthesisaf:Fotosintese
Search Term: "Photosynthesis"
Categories: Section stubs | Biochemistry | Botany | Photosynthesis | Metabolism | Agronomy
