As mentioned earlier, the ultimate source of most of the chemical energy in cells is the sun. Because this source is directly available only to the cells of photosynthesizers, most organisms on earth are either directly or indirectly dependent on photosynthesis, except for a few chemoautotrophs that derive their energy and nutrients solely from inorganic substrates. The other major products of photosynthesis are organic carbon compounds, which are produced from carbon dioxide through a process called carbon fixation. On land, green plants are the primary photosynthesizers; and in aquatic eco systems, where 80% to 90% of all photosynthesis occurs, this role is filled by algae, cyanobacteria, and green sulfur, purple sulfur, and purple nonsulfur bacteria.
The summary equation for the main reactants and products of photosynthesis in aerobic organisms is
The anatomy of photosynthetic cells is adapted to trapping sunlight, and their physiology effectively uses this solar energy to produce high-energy glucose from low-energy CO2 and water. Photosynthetic organisms achieve this remarkable feat through a series of reactions involving light, pigment, CO2, and water, which is used as a source for electrons.
Photosynthesis proceeds in two phases: the light-dependent reactions, which proceed only in the presence of light waves, and the light-independent reactions, which can operate without direct exposure to light, but are still reliant on the energy molecules made in the light-dependent reactions (figure 1).
Fig1. Overview of photosynthesis. The general reactions of photosynthesis are into two phases called light dependent reactions and light-independent reactions. The light dependent reactions involve two major events: (1) the trapping of light with a photosensitive pigment, such as chlorophyll, and converting the light energy into chemical energy in the form of ATP and NADPH; and (2) the splitting of water molecules with the release of oxygen gas. The light-independent reactions utilize ATP and NADPH produced during the light-dependent reactions to fix CO2 into organic compounds such as glucose.
Solar energy is delivered in discrete energy packets called photons (also called quanta) that travel as waves. The wave lengths of light operating in photosynthesis occur in the visible spectrum between 400 nanometers (violet) and 700 nanometers (red) and above. As this light strikes photosynthetic pigments, some wavelengths are absorbed, some pass through, and some are reflected. The activity that has the greatest impact on photosynthesis is the absorbance of light by photosynthetic pigments. These include the chlorophylls, which are green; carotenoids, which are yellow, orange, or red; and phycobilins, which are red or blue-green.3 By far the most important of these pigments are the chlorophylls, which contain a photocenter that consists of a magnesium atom held in the center of a complex ringed molecule called a porphyrin (process figure 2, part 3). As we will see, the chlorophyll molecule harvests the energy of photons and converts it to electron (chemical) energy. Accessory photosynthetic pigments such as carotenes trap light energy and shuttle it to chlorophyll, thereby functioning like antennae. These light- dependent reactions are responsible for photophosphorylation, the channeling of energy extracted from light to make high- energy bonds of ATP. This sets the scene for the light-independent reactions, which use both ATP and NADPH for synthesis. During this phase, carbon atoms from CO2 are fixed to the carbon backbones of organic molecules.
Process Figure 2 The location and participants in the light-dependent reactions of a chloroplast. (1) A cell of the algae Chlamydomonas with a cutaway view of a single chloroplast to show the internal structure of stroma and grana. (2) A granum consists of a stack of hollow membrane-bound discs called thylakoids, which house (3) the chlorophyll molecules, arranged in arrays (antennae) for capturing light. (4) The chlorophylls exist in separate systems called PS II and PS I, embedded in the thylakoid membrane. Other players in the process are an electron transport chain, an oxygen-evolving complex, a final electron acceptor (NADP+), and ATP synthase. The text covers the main mechanisms of this process.
The detailed biochemistry of photosynthesis is beyond the scope of this text, but we will provide an overview of the general process as it occurs in green plants, algae, and cyanobacteria. It is worth noting that many of the basic activities are biochemically similar to certain pathways of aerobic metabolism.
Light-Dependent Reactions
The systems that carry the photosynthetic pigments are also the sites for the light-dependent reactions. They occur in the thylakoid membranes of compartments called grana (singular, granum) in chloroplasts (process figure 2, part 1) and in thylakoid layers of the cell membranes of cyanobacteria. These systems exist as two separate complexes called photosystem I (PS700) and photosystem II (PS680)4 (process figure 2, part 4).
Both systems contain chlorophyll, but their chlorophylls are sensitive to different wavelengths: PS I absorbs longer wavelengths (above 680 nm) and PS II absorbs shorter wavelengths (680 nm and below), which efficiently covers the range of light to which phototrophs are exposed. Both photosystems are stationed in antennae— clusters of about 300 molecules of chlorophyll located in the thylakoid membranes. Antennae absorb light and convey it to reaction centers, where the light-dependent reactions take place (process figure 2, part 4).
The chloroplast is composed of separate compartments similar to mitochondria. One compartment—the stroma—is the region surrounding the grana and thylakoids, and the other one is the space inside the thylakoids called the lumen (process figure 2, part 4).
Overview of the Light-Dependent Reactions and Photophosphorylation
The thylakoids are specially situated to bring together all components of the light-dependent reactions. Most of these components are embedded in the thylakoid membranes and are accessible from both the lumen and the stroma. They include chlorophyll molecules and their photosystems (PS II and PS I)5; an oxygen evolving complex; electron transport systems—consisting of a mobile electron carrier plastoquinone (PQ); cytochrome bf com plex; plastocyanin (PC), a second mobile electron carrier; ferredoxin (Fd) with nicotinamide adenine dinucleotide phosphate (NADP+) reductase (FNR); and ATP synthase. Several of the systems and their actions are very similar to those we previously examined in mitochondria.
The light-dependent pathway proceeds as follows:
PS II Reactions
1. When light strikes the magnesium in PS II, an electron becomes excited (is raised to a higher energy level) and is released by PS II to the first electron transport system.
2. An oxygen-evolving complex associated with PS II splits two water molecules into 4 electrons, 4 protons (H+), and oxygen. Because it occurs in the presence of light, this step is termed photolysis, and it results in the first major product of the light-dependent reactions—the O2 gas required by all aerobic organisms, including even most photosynthesizers. In addition, the electrons released by photolysis return the PS II complex to a ground state so it is again ready to respond to light. Another important effect of photolysis is the release of protons into the lumen, which contributes to the chemiosmotic proton gradient so essential to ATP synthesis.
Electron Transport
1. An electron given off by PS II is picked up by the first com pound of the electron transport chain—PQ—which moves it to the cytochrome bf complex. From here, it is transported to plastocyanin and to the PS I complex.
2. An event coupled to electron transport is the delivery of pro tons (H+) from the stroma side to the lumen side by the actions of the cytochrome bf complex, which helps maintain the electrochemical gradient between the stroma and lumen.
PS I Reactions
1. When PS I is excited by light, it also releases an electron that will be transported through a different series of carriers. In addition, it picks up the electrons from PS II delivered by plastocyanin.
2. These electrons are shuttled to the final electron acceptors— ferredoxin and the FNR complex. The function of the FNR is to transfer 2 electrons (and 2 protons) to NADP+, converting it to NADPH. This is the second major product of the light dependent reactions, one which provides reducing power for the light-independent reactions.
ATP Synthase and Photophosphorylation ATP synthase complexes are distributed throughout the thylakoid membranes. They have the same structure as those of mitochondria and they, too, capture the free energy inherent in the proton motive force to synthesize ATP. As protons flow through the F0 portion of the synthase from the lumen to the stroma, the F1 component pulls in ADP and Pi, releasing ATP, the third major product of the light reactions. You may notice that the energy-associated molecules (NADPH and ATP) are released in the stroma, which situates them for immediate use in the light-independent reactions covered in the next section.
Light-Independent Reactions
The anabolic reactions of photosynthesis occur in the stroma of a chloroplast or the cytoplasm of cyanobacteria. These reactions use energy produced by the light phase to synthesize glucose by means of the Calvin cycle (figure 3).
Fig3. The Calvin cycle. This includes the main events of photosynthesis during which carbon is fixed into organic form using the energy (ATP and NADPH) released by the light reactions. The end product, glucose, can be stored as complex carbohydrates, or it can be used in various amphibolic pathways to produce other carbohydrate intermediates or amino acids. Some PGAL is used in the regeneration of RuBP, which uses additional ATP made during the light-dependent reactions.
The cycle begins at the point where CO2 is combined with a 5-carbon acceptor molecule with two terminal phosphates called ribulose-1,5-bisphosphate (RuBP). This first critical step in carbon fixation generates a 6-carbon intermediate compound that immediately splits into two 3-carbon molecules of 3-phosphoglyceric acid (PGA). The subsequent steps use ATP and NADPH generated by the photosystems to form high-energy intermediates. First, 2 ATPs are expended to add a second phosphate to 3-PGA, producing two molecules of 1,3-bisphosphoglyceric acid (BPG). This is followed by NADPH contributing its electrons to BPG with the removal of one high-energy phosphate. These events give rise to two glyceraldehyde-3-phosphate (G3P). This molecule and its isomer dihydroxyacetone phosphate (DHAP) are key compounds in hexose synthesis leading to fructose and glucose. Some of the G3P is conveyed to the remainder of the cycle, where it participates in the regeneration of RuBP. This involves numerous steps not depicted in figure 3.
You may notice that this pathway is very similar to glycolysis, except that it runs in reverse.
Other Mechanisms of Photosynthesis
The oxygenic, or oxygen-releasing, photosynthesis described in the previous section occurs in plants, algae, and cyanobacteria and is the dominant type on the earth. Other photosynthesizers such as green and purple sulfur bacteria possess bacteriochlorophyll, which is more versatile in capturing light. They have only a cyclic photosystem I, which routes the electrons from the photocenter to the electron carriers and back to the photosystem again. This pathway generates a relatively small amount of ATP, and it may not produce NADPH. As photolithotrophs, these bacteria use H2, H2S, or elemental sulfur rather than H2O as a source of electrons and reducing power. As a consequence, they are anoxygenic (non–oxygen- producing), and many are strict anaerobes.
