Photosynthesis is a biochemical process in which plants, algae, and some bacteria harness the energy of light 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.
Photosynthesis in plants
Plants are autotrophs, which means they are able to synthesize food directly from inorganic compounds, instead of eating other organisms or relying on material derived from them. Most notably, they use carbon dioxide gas and water to produce sugars and oxygen gas. The energy for these processes comes from photosynthesis. For instance, the over-all equation for the production of glucose is:
- 12H2O + 6CO2 + light → C6H12O6 (glucose) + 6O2 + 6H2O
The glucose is variously used to form other organic compounds, such as the building material cellulose, or it may be used as a fuel. This takes place through respiration, found in both animals and plants. In general outline, this is the opposite of the above: glucose and other compounds react with oxygen to produce carbon dioxide, water, and chemical energy. However, both processes actually take place through a complex sequence of steps, and are very different in detail.
Fallen leaf. Areas where green chlorophyll has been destroyed now appear yellow
Plants capture light using the pigment chlorophyll, which gives them their green color. This is contained in organelles (compartments within the cells) called chloroplasts. Although all green parts of a plant have chloroplasts, most of the energy is produced 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 absorption of light and evaporation of water. The transparent, colourless epidermis layer allows light to pass through to the 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 in the same way. Light is absorbed by chlorophyll, although various accessory pigments give them a wide variety of colors, 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 developed from them. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen.
At the molecular level
Photosynthesis produces more energy for certain wavelengths of light. In plants, there are two photosystems involved, which are most active at 700 and 680 nm. However, other wavelengths are also peaks in the action spectrum for photosynthesis.
Photosynthesis begins when light ionizes a chlorophyll molecule, releasing two electrons, which are transferred along an electron transport chain, similar to that involved in respiration. Their energy is used for photophosphorylation, which produces adenosine triphosphate (ATP), the main "energy currency" in cells.
In photosystem I, the electrons are returned to the chlorophyll. In photosystem II, they are used to drive the reaction
- NADP+ + H+ + 2e− → NADPH
The NADPH is the main reducing agent in cells, providing a source of energetic electrons to other reactions. This leaves chlorophyll with a deficit of electrons, which must be obtained from some other reducing agent. In plants and algae this role is played by water, resulting in the production of oxygen:
- 2 H2O → O2 + 4 H+ + 4 e−
Note that oxygen is only produced from the water, and not from the carbon dioxide that plants use. This was first proposed by C. B. van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, they use reducing agents such as sulfide or hydrogen, so no oxygen is 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 ATP and NADPH produced by photosynthesis drive a variety of other biochemical processes. In plants, the most notable is the Calvin cycle, by which carbon dioxide is converted into ribulose (and thence into other sugars, such as PGAL, which is then converted into glucose and sometimes proteins). These are called light-independent reactions, or dark reactions, since they can occur in the absence of light.
However, when oxygen is abundant in the chloroplast in comparison to carbon dioxide due to some reason, such as competition in a dense overgrowth or closure of stomata, photorespiration can occur. This is a counter-productive pathway, and instead works against sugar production.
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 off in the mid-1600s when he, by carfully measuring the mass of the soil in the pot of a plant and the mass of the plant as it grew, discovered that, with the soil mass changed very little, the mass of the growing plant must come from the water, the only substance he added to the potted plant.
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 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.
A first experiment to prove that the oxygen developed during the photosynthesis of green plants came from water, was performed by Robin 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 Noble Prize winning scientist, Rudolf Marcus , was able to discover the function and significance of the electron transport chain.
Factors affecting photosynthesis
- Light intensity and wavelength
- Carbon dioxide concentration
Metabolic pathways involved in photosynthesis: