The Basics of Photosynthesis

Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems. Photosynthesis is carried out by many different organisms, ranging from plants to bacteria (Figure 1). The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans. All these organisms convert CO2 (carbon dioxide) to organic material by reducing this gas to carbohydrates in a rather complex set of reactions. Electrons for this reduction reaction ultimately come from water, which is then converted to oxygen and protons. Energy for this process is provided by light, which is absorbed by pigments (primarily chlorophylls and carotenoids). Chlorophylls absorb blue and red light and carotenoids absorb blue-green light (Figure 2), but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.

Other photosynthetic organisms, such as cyanobacteria (formerly known as blue-green algae) and red algae, have additional pigments called phycobilins that are red or blue and that absorb the colors of visible light that are not effectively absorbed by chlorophyll and carotenoids. Yet other organisms, such as the purple and green bacteria (which, by the way, look fairly brown under many growth conditions), contain bacteriochlorophyll that absorbs in the infrared, in addition to in the blue part of the spectrum. These bacteria do not evolve oxygen, but perform photosynthesis under anaerobic (oxygen-less) conditions. These bacteria efficiently use infrared light for photosynthesis. Infrared is light with wavelengths above 700 nm that cannot be seen by the human eye; some bacterial species can use infrared light with wavelengths of up to 1000 nm. However, most pigments are not very effective in absorbing ultraviolet light (<400 nm), which also cannot be seen by the human eye.

Light with wavelengths below 330 nm becomes increasingly damaging to cells, but virtually all light at these short wavelengths is filtered out by the atmosphere (most prominently the ozone layer) before reaching the earth. Even though most plants are capable of producing compounds that absorb ultraviolet light, an increased exposure to light around 300 nm has detrimental effects on plant productivity.

Reaction Centers and Antennae

Photosynthetic pigments come in a huge variety: there are many different types of (bacterio) chlorophyll, carotenoids, and phycobilins, differing from each other in their precise chemical structure. Pigments generally are bound to proteins, which provide the pigment molecules with the appropriate orientation and positioning with respect to each other. Light energy is absorbed by individual pigments, but is not used immediately by these pigments for energy conversion. Instead, the light energy is transferred to chlorophylls that are in a special protein environment where the actual energy conversion event occurs: the light energy is used to transfer an electron to a neighboring pigment. Pigments and protein involved with this actual primary electron transfer event together are called the reaction center. A large number of pigment molecules (100-5000), collectively referred to as antenna, “harvest” light and transfer the light energy to the same reaction center. The purpose is to maintain a high rate of electron transfer in the reaction center, even at lower light intensities.

Many antenna pigments transfer their light energy to a single reaction center by having this energy “hop” to another antenna pigment, and yet to another, etc., until the energy is “trapped” in the reaction center. Each step of this energy transfer must be very efficient to avoid a large loss in the overall transfer process, and the association of the various pigments with proteins ensures that transfer efficiencies are high by having appropriate pigments close to each other, and by having an appropriate molecular geometry of the pigments with respect to each other. An exception to the rule of protein-bound pigments are green bacteria with very large antenna systems: a large part of these antenna systems consists of a “bag” (named chlorosome) of up to several thousand bacteriochlorophyll molecules that interact with each other and that are not in direct contact with protein.

In many systems the size of the photosynthetic antenna is flexible, and photosynthetic organisms growing at low light (in the shade, for example) generally will have a larger number of antenna pigments per reaction center than those growing at higher light intensity. However, at high light intensities (for example, in full sunlight) the amount of light that is absorbed by plants exceeds the capacity of electron transfer initiated by reaction centers. Plants have developed means to convert some of the absorbed light energy to heat rather than to use the absorbed light necessarily for photosynthesis. However, in particular the first part of photosynthetic electron transfer in plants is rather sensitive to overly high rates of electron transfer, and part of the photosynthetic electron transport chain may be shut down when the light intensity is too high; this phenomenon is known as photo inhibition.

Photosynthetic Electron Transfer

The initial electron transfer (charge separation) reaction in the photosynthetic reaction center sets into motion a long series of redox (reduction-oxidation) reactions, passing the electron along a chain of cofactors and filling up the “electron hole” on the chlorophyll, much like in a bucket brigade. All photosynthetic organisms that produce oxygen have two types of reaction centers, named photosystem II and photosystem I (PS II and PS I, for short), both of which are pigment/protein complexes that are located in specialized membranes called thylakoids. In eukaryotes (plants and algae), these thylakoids are located in chloroplasts (organelles in plant cells) and often are found in membrane stacks (grana) (Figures 3 and 4). Prokaryotes (bacteria) do not have chloroplasts or other organelles, and photosynthetic pigment-protein complexes either are in the membrane around the cytoplasm or in invaginations thereof (as is found, for example, in purple bacteria), or are in thylakoid membranes that form much more complex structures within the cell (as is the case for most cyanobacteria).

All chlorophyll in oxygenic organisms is located in thylakoids, and is associated with PS II, PS I, or with antenna proteins feeding energy into these photosystems. PS II is the complex where water splitting and oxygen evolution occurs. Upon oxidation of the reaction center chlorophyll in PS II, an electron is pulled from a nearby amino acid (tyrosine) which is part of the surrounding protein, which in turn gets an electron from the water-splitting complex. From the PS II reaction center, electrons flow to free electron carrying molecules (plastoquinone) in the thylakoid membrane, and from there to another membrane-protein complex, the cytochrome b6f complex. The other photosystem, PS I, also catalyzes light-induced charge separation in a fashion basically similar to PS II: light is harvested by an antenna, and light energy is transferred to a reaction center chlorophyll, where light-induced charge separation is initiated. However, in PS I electrons are transferred eventually to NADP (nicotinamid adenosine dinucleotide phosphate), the reduced form of which can be used for carbon fixation. The oxidized reaction center chlorophyll eventually receives another electron from the cytochrome b6f complex. Therefore, electron transfer through PS II and PS I results in water oxidation (producing oxygen) and NADP reduction, with the energy for this process provided by light (2 quanta for each electron transported through the whole chain). A schematic overview of these processes is provided in Figure 6.

Carbon Fixation

Electron flow from water to NADP requires light and is coupled to generation of a proton gradient across the thylakoid membrane. This proton gradient is used for synthesis of ATP (adenosine triphosphate), a high-energy molecule. ATP and reduced NADP that resulted from the light reactions are used for CO2 fixation in a process that is independent of light. CO2 fixation involves a number of reactions that is referred to as the Calvin-Benson cycle.

The initial CO2 fixation reaction involves the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which can react with either oxygen (leading to a process named photorespiration and not resulting in carbon fixation) or with CO2. The probability with which RuBisCO reacts with oxygen vs. with CO2 depends on the relative concentrations of the two compounds at the site of the reaction. In all organisms CO2 is by far the preferred substrate, but as the CO2 concentration is very much lower than the oxygen concentration, photorespiration does occur at significant levels.

To boost the local CO2 concentration and to minimize the oxygen tension, some plants (referred to as C4 plants) have set aside some cells within a leaf (named bundle-sheath cells) to be involved primarily in CO2 fixation, and others (named mesophyll cells) to specialize in the light reactions: ATP, CO2 and reduced NADP in mesophyll cells is used for synthesis of 4-carbon organic acids (such as malate), which are transported to bundle sheath cells.

Here the organic acids are converted releasing CO2 and reduced NADP, which are used for carbon fixation. The resulting 3-carbon acid is returned to the mesophyll cells. The bundle sheath cells generally do not have PS II activity, in order to minimize the local oxygen concentration. However, they retain PS I, presumably to aid in ATP synthesis. Even though C4 plants have reduced amounts of photorespiration, the amount of ATP they need per amount of CO2 fixed is a little higher than in other plants, and therefore their total production rate is similar to that of plants with higher rates of photorespiration.

Some plants living in desert climates, such as cacti, keep their stomates closed during the day to minimize evaporation (stomates are openings in the leaf surface to enhance gas exchange). These plants take up CO2 during the night when the stomates are open, and temporarily bind the CO2 to organic acids in the leaf. During the day the CO2 is released from the acids and used for photosynthesis. Plants using this mechanism of CO2 fixation are called CAM plants.

Increasing CO2 levels

The amount of overall CO2 fixation in plants growing under optimal conditions is limited primarily by the amount of CO2 available. Therefore, the increase of CO2 in the atmosphere will lead to somewhat higher rates of plant growth in environments where the CO2 concentration limits growth rates. This is usually the case in an agricultural setting, where nutrients and water availability are not limiting. However, also in natural conditions, where limitations other than the CO2 concentration will generally limit plant productivity, plant productivity has been found to often increase upon increasing the CO2 concentration.

Photosynthesis and respiration

Virtually all oxygen in the atmosphere is thought to have been generated through the process of photosynthesis. Obviously, all respiring organisms (including plants) utilize this oxygen and produce CO2. Thus, photosynthesis and respiration are interlinked, with each process depending on the products of the other. The global amount of photosynthesis is on the order of a trillion kg of dry organic matter produced per day, and respiratory processes convert about the same amount of organic matter to CO2. A large part (probably the majority) of photosynthetic productivity occurs in open oceans, mostly by oxygenic prokaryotes. Without photosynthesis, the oxygen in the atmosphere would be depleted within several thousand years. It should be emphasized that plants respire just like any other higher organism, and that during the day this respiration is masked by a higher rate of photosynthesis.