Photosynthesis
Photosynthesis is an important biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. It is also responsible for producing the oxygen that makes up a large portion of the Earth's atmosphere. Organisms that produce energy through photosynthesis are called phototrophs.
Plant photosynthesis
Plants are photoautotrophs, which means they are able to synthesize food directly from inorganic compounds using light energy, instead of eating other organisms or relying on material derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from chemical reactions.
The energy for photosynthesis ultimately comes from absorbed photons and involves a reducing agent, which is water in the case of plants, releasing oxygen as a waste product. The light energy is converted to chemical energy, in the form of ATP and NADPH, using the light-dependent reactions and is then available for carbon fixation. Most notably plants use the chemical energy to fix carbon dioxide into carbohydrates and other organic compounds through light-independent reactions. The overall equation for photosynthesis in green plants is:
- nCO2 + 2nH2O + light energy → (CH20)n + nO2 + nH2O
However, Hexose sugars and starch are the primary products, especially glucose, so the following equation is often used to represent photosynthesis:
- 6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O
The carbohydrates are variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis or as a fuel in cellular respiration. The later not only occurs in plants, but also in animals when the energy from plants get passed through a food chain. 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 actually take place through a different sequence of reactions and in different cellular compartments.
Plants capture light primarily using the pigment chlorophyll, which is responsible for the reason why most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenoids. Both chlorophyll and accessory pigments are contained in organelles (compartments within the cell) called chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, contain about half a million chloroplasts for every square millimetre of leaf. The surface of the leaf is uniformly coated with a water-resistant, waxy cuticle, that protects the leaf from excessive evaporation of water as well as decreasing the absorption of ultraviolet or blue light to reduce heating. The transparent, colourless epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.
Photosynthesis in algae and bacteria
Algae range from multicellular forms like kelp to microscopic, single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Light is absorbed by chlorophyll, although various accessory pigments to give them a wide variety of colours, located inside chloroplasts. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms.
Photosynthetic bacteria do not have chloroplasts. Instead, photosynthesis takes place directly within the cell. The cyanobacteria contain chlorophyll and oxygen, in the same way that chloroplasts do, in fact chloroplasts are now considered to have evolved from cyanobacteria by endosymbiosis. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen.
At the molecular level
The Light-dependent Reaction
The products of the light dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.
Z scheme
In plants, the light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to sythesize ATP and NADPH. The photons are captured in the antenna complexes of photosystem I and II by chlorophyll and accessory pigments (see diagram at right). When a chorophyll a molecule at a photosystems reaction center absorbs energy an electron is excited and transferred to an electron-acceptor molecule. These electrons are shuttled through an electron transport chain that initially functions to generate a chemiosmotic potential across the membrane, the so called Z-scheme shown in the diagram. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while NADPH is a product of the terminal redox reaction in the Z-scheme.
Splitting water
The NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by plastocyanin. However, since photosystem II includes the first steps of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. This role is played by water during a reaction known as photolysis and results in water being split to give electrons, oxygen and hydrogen ions. Photosystem II is the only known biological enzyme that carries out this oxidation of water. Initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the carrier molecule NADP+ to form NADPH. Oxygen is a waste product of photosynthesis but it has a vital role for all organisms that use it for cellular respiration.
Oxygen and photosynthesis
With respect to oxygen and photosynthesis, there are two important concepts.
- Plant and algal 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 photolysis reaction not the fixation of carbon dioxide during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, NOT carbon dioxide.
Bacterial variations
The second concept was first proposed by C. B. van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced.
Others, such as the halophiles (an Archeae) produced so called purple membranes where the bacterialrhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contain the purple membranes dropped as protons were pumping out of the membranes.
The Light-independent Reaction
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 PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to 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.
Factors affecting photosynthesis
There are three main factors affecting photosynthesis and several corollary factors. The three main are:
- Light intensity and wavelength
- Carbon dioxide concentration
- Temperature
Light Intensity and Temperature
In the early 1900s F.F. Blackman investigated the effects the effects of light intensity and temperature on the rate of photosynthesis. At constant temperature the rate of photosynthesis varies with light intensity, initially increasing as the light intensity increases. However at higher light intensities 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 light intensities can be seen in image to the left. At high light intensities the rate of photosynthesis increases as the temperature is increased over a limited range. At low light intensities, 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 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.
Carbon Dioxide
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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.
Corollary Factors
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In Detail
Metabolic pathways involved in photosynthesis: