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| Photosynthetic Light Reactions |
Photosynthetic light reactions involve the absorption of light energy by plant pigments and the conversion of light energy into adenosine triphosphate (ATP).
Photosynthesis is the process by which plants, algae, and certain types of bacteria use the energy of sunlight to manufacture organic molecules from carbon dioxide and water.
The process may be divided into two parts: the light reactions and the dark reactions. In the light reactions of photosynthesis, light energy coming from the sun or from an artificial light source is absorbed by pigments and used to boost electrons into higher energy levels so they can be used to do cellular work.
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In the dark reactions (also called the Calvin cycle), energy containing molecules from the light reactions are used to convert carbon dioxide into carbohydrates. All living organisms ultimately depend upon this process as their source of food.
In algae and higher plants, the light reactions of photosynthesis take place on thylakoid membranes located within chloroplasts. The surfaces of the thylakoids are covered with molecules of the green pigment chlorophyll as well as yellow carotenoid pigments.
Also located on the thylakoids, though fewer in number, are special structures called reaction centers. The process begins when a unit of light energy (referred to as a photon or quantum) strikes a pigment molecule and causes one of its electrons to be raised to a higher energy level, or an excited state.
Many chlorophyll and carotenoid molecules are located adjacent to one another on the thylakoids, and the energy of the excited electrons may be transferred from one to the next until it reaches a reaction center.
Overall, a very large number of pigment molecules are absorbing light, becoming excited, and passing the excitation energy to reaction centers. It is in the reaction centers that the central events of the photosynthetic light reactions take place.
Photosystem I
Higher plants contain two different types of reaction centers, referred to as photosystem I and photosystem II. (The numbers I and II have no functional significance; they simply reflect the order in which they were discovered.)
What the two types of reaction centers actually consist of are groups of special proteins complexed with several chlorophyll molecules and structured in a very specific arrangement within the thylakoid membrane. Also embedded in the thylakoid membrane are a series of proteins and other molecules that are capable of transporting electrons from photosystem II to photosystem I.
This process is referred to as the electron transport system. The two photosystems and the electron transport system work together to form the energy-containing molecules produced in the photosynthetic light reactions.
The process is best understood by looking first at what happens in photosystem I. As described above, light energy is absorbed by a pigment molecule and transferred to the reaction center, causing two photosystem I electrons to be raised to an excited state.
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The excited electrons are then passed from the reaction center to a primary electron acceptor. The primary electron acceptor passes the electrons to the first of a series of electron transport proteins in the inner thykaloid membrane, the last of these proteins being ferredoxin, an iron containing protein.
Ferredoxin passes the electrons to a coenzyme called nicotinamide adenine dinucleotide phosphate (NADP+), which then becomes reduced and, after joining with a free proton (H+), becomes NADPH. The important point is that some of the energy of the excited electrons is incorporated into the molecule of NADPH, thereby converting light energy into chemical energy.
The NADPH is not attached to the thylakoid membrane but remains in the stroma (the region inside both the outer membranes of the chloroplast and surrounding the thykaloids) of the chloroplast,where it will be used in the Calvin cycle.
Photosystem II
However, photosystem I, by itself, could not continue to function in the manner described above without having someway of replacing the electrons which are being removed. This is where photosystem II comes in. The basic operation of photosystem II is similar to photosystem I insofar as light energy is absorbed by pigment molecules and transferred to the reaction center.
However, the excited electron is not used to form NADPH; instead it enters the electron transport system and is passed from one molecule to the next until it eventually reaches photosystem I, where it replaces the electron previously lost in the formation of NADPH.
The electron transport system does much more than merely replace electrons in photosystem I. As an electron moves through the electron transport system, its energy is used to transfer protons (hydrogen ions) from the stroma to the thykaloid space (the region inside the thykaloids).
The thylakoids are not simply flat sheets of membrane but are folded in such a way as to form numerous saclike compartments within the chloroplast. The result is that a proton gradient is established across the thykaloid membrane.
Many electrons are carried through the electron transport system, resulting in the accumulation of a high concentration of protons within the thylakoid compartments.
This high concentration of protons on one side of the membrane and relatively low concentration on the other represents a source of potential energy, somewhat analogous to the energy contained in a body of water held back by a dam.
The protons are then allowed to move back across the membrane through special proteins called ATP synthase. ATP synthase is an enzyme capable of catalyzing the joining of adenosine diphosphate (ADP) with inorganic phosphate (Pi) in the stroma to form adenosine triphosphate (ATP).
The passage of protons through ATP synthase results in a change in shape in the protein, which brings about the reaction between ADP and Pi. The formation of ATP by this mechanism is called photophosphorylation. Once again, light energy has been transformed into chemical energy.
Electron Replacement
If electrons from photosystem II are used to replace those from photosystem I, then the electrons from photosystem II must be replaced as well. This problemis solved by the use of water as a source of electrons.
Photosystem II is capable of splitting apart molecules of water to extract electrons, using the electrons to replace the ones used by the electron transport system. Water is always available in a functioning photosynthetic plant cell, and this represents a virtually limitless source of electrons.
Water molecules contain hydrogen and oxygen, and when they are split apart in this manner, the protons left over from hydrogen simply go into solution, but the oxygen forms oxygen gas and is released into the atmosphere. Although the oxygen is really no more than a by-product as far as photosynthesis is concerned, it is of profound significance to all higher organisms.
The overall flow of electrons in the light reactions is from water to photosystem II, through the electron transport system where ATP is formed, to photosystem I and finally to NADPH. This arrangement was hypothesized by Robin Hill and Fay Bendall in 1960. Because of the way in which the process was diagrammed in their paper, it is often referred to as the Z scheme.
In energy terms, what has been accomplished is the conversion of light energy to chemical energy in the form of ATP and NADPH. These energy-rich molecules are essential to the Calvin cycle as energy for the conversion of carbon dioxide into carbohydrates. The process is known as noncyclic electron flow.
Alternatively, in cyclic electron flow, the electrons can travel within the electron transport system as described above but are diverted to an acceptor in the electron transport chain between photosystems I and II.
Passing through the chain back to photosystem I, the electrons enable the transport of protons across the thykaloid membrane, thus supplying power for the generation of ATP.
This process is called cyclic photophosphorylation, because it involves a cyclic flow of electrons. In this way, photosystem I can work independently of photosystem II. Apparently, this is the manner in which some bacteria carry out photosynthesis.
Bacterial Photosynthesis
Certain types of bacteria also carry on photosynthesis. These organisms do not contain chloroplasts, so the light-absorbing pigments and reaction centers are located on membranes spread throughout the cell.
In one group, the cyanobacteria, the photosynthetic process is similar to that described for algae and higher plants in that there are two photosystems, water serves as the primary electron donor, and oxygen is released.
The other types of photosynthetic bacteria, however, are more primitive. They have only one photosystem and use inorganic compounds such as hydrogen sulfide instead of water as a source of electrons.
The cyanobacteria possess chlorophyll, and the other photosynthetic bacteria contain a similar pigment known as bacterio chlorophyll. In addition, these organisms contain a variety of accessory pigments that also are involved in the photosynthetic light reactions.
