Photosynthesis

Photosynthesis is the biological conversion of light energy into chemical energy. It occurs in green plants and photosynthetic bacteria through a series of many biochemical reactions. In higher plants and algae, light absorption by chlorophyll catalyzes the synthesis of carbohydrate (C₆H₁₂O₆) and oxygen gas (O₂) from carbon dioxide gas (CO₂) and water (H₂O). Thus, the overall chemical equation for photosynthesis in higher plants is expressed as:

The overall equation in photosynthetic bacteria is similar, although not identical.

In the light reactions of photosynthesis, light energy excites photosynthetic pigments to higher energy levels and this energy is used to make two high energy compounds, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). ATP and NADPH do not appear in the overall equation for photosynthesis because they are consumed during the subsequent dark reactions in the synthesis of carbohydrates.

Non-cyclic energy transfer

Once light is absorbed by pigments in the chloroplast, its energy is transferred to one of two types of reaction centers, Photosystem-II (PS-II) or Photosystem-I (PS-I).

In non-cyclic energy transfer, light absorbed by PS-II splits a water molecule, producing oxygen and exciting chlorophyll to a higher energy level. Then, the excitation energy passes through a series of special electron carriers. Each electron carrier in the series is slightly lower in energy than the previous one. During electron transfer, the excitation energy is harnessed to synthesize ATP. This part of photosynthesis is referred to as non-cyclic photophosphorylation, where "photo-" refers to the light requirement and "-phosphorylation" refers to addition of a phosphate to ADP (adenosine diphosphate) to make ATP.

Finally, one of the electron carriers of PS-II transfers electrons to PS-I. When chlorophyll transfers its excitation energy to PS-I, it is excited to higher energy levels. PS-I harnesses this excitation energy to make NADPH, analogous to the way PS-II harnessed excitation energy to make ATP.

In the 1950s, the botanist Robert Emerson (1903-1959) demonstrated that the rate of photosynthesis was much higher under simultaneous illumination by shorter wavelength red light (near 680 nm) and long wavelength red light (near 700 nm). We now know this is because PS-II absorbs shorter wavelength red light (680 nm) whereas PS-I absorbs long wavelength red light (700 nm) and both must be photoactivated to make the ATP and NADPH needed by the dark reactions.

Cyclic energy transfer

ATP can also be made by a special series of light reactions referred to as cyclic photophosphorylation. This also occurs in the thylakoid membranes of the chloroplast. In cyclic photophosphorylation, the excitation energy from PS-I is transferred to a special electron carrier and this energy is harnessed to make ATP.

The relative rates of cyclic and non-cyclic photophosphorylation determine the ratio of ATP and NADPH which become available for the dark reactions. Photosynthetic plant cells regulate cyclic and non-cyclic energy transfer by phosphorylating (adding a phosphate) to the pigment-protein complexes associated with PS-I and PS-II.

The photosynthetic dark reactions consist of a series of many enzymatic reactions which make carbohydrates from carbon dioxide. The dark reactions do not require light directly, but they are dependent upon ATP and NADPH which are synthesized in the light reactions. Thus, the dark reactions indirectly depend on light and usually occur in the light. The dark reactions occur in the aqueous region of the chloroplasts, referred to as the stroma.

Calvin cycle

The main part of the dark reactions is often referred to as the Calvin cycle, in honor of their discoverer, the chemist Melvin Calvin. The Calvin cycle consists of 13 different biochemical reactions, each catalyzed by a specific enzyme. The Calvin cycle can be summarized as consisting of carboxylation, reduction, and regeneration. Its final product is starch, a complex carbohydrate.

In carboxylation, a molecule of carbon dioxide (with one carbon atom) is combined with a molecule of RuBP (ribulose bisphosphate, with five carbon atoms) to make two molecules of PGA (phosphoglycerate), each with three carbon atoms. This reaction is catalyzed by the enzyme RuBISCO (Ribulose bisphosphate carboxylase). RuBISCO accounts for about 20% of the total amount of protein in a plant leaf and is by far the most abundant enzyme on Earth.

In reduction, ATP and NADPH (made by the light reactions) supply energy for synthesis of high energy carbohydrates from the PGA made during carboxylation. Plants often store their chemical energy as carbohydrates because these are very stable and easily transported throughout the organism.

In regeneration, the carbohydrates made during reduction pass through a series of enzymatic reactions so that RuBP, the initial reactant in carboxylation, is regenerated. The regeneration of RuBP is the reason these reactions are considered a cycle. Once the Calvin cycle has gone around six times, six molecules of carbon dioxide have been fixed, and a molecule of glucose, a six-carbon carbohydrate, is produced.

The series of dark reactions described above is often referred to as C-3 photosynthesis because the first reaction product of carbon dioxide fixation is a 3-carbon molecule, PGA (phosphoglycerate).

C-4 photosynthesis

In the early 1960s, plant physiologists discovered that sugarcane and several other plants did not produce the three-carbon molecule, PGA, as the first reaction product of their dark reactions. Instead, these other plants combined carbon dioxide with PEP (phosphoenol pyruvate), a three-carbon molecule, to make OAA (oxaloacetate), a four-carbon molecule. After a series of additional enzymatic reactions, carbon dioxide is introduced to the Calvin cycle, which functions more or less as described above.

This variant of photosynthesis is referred to as C-4 photosynthesis because carbon dioxide is first fixed into a four-carbon molecule, OAA. C-4 photosynthesis occurs in many species of tropical grasses and in many important agricultural plants such as corn, sugarcane, rice, and sorghum.

Plants which have C-4 photosynthesis partition their C-4 metabolism and their Calvin cycle metabolism into different cells within their leaves. Their C-4 metabolism occurs in mesophyll cells, which constitute the main body of the leaf. The Calvin cycle occurs in specialized cells referred to as bundle sheath cells. Bundle sheath cells surround the vascular tissue (veins) which penetrate the main body of the leaf.

In at least 11 different genera of plants, some species have C-3 metabolism whereas other species have C-4 metabolism. Thus, plant physiologists believe that C-4 photosynthesis evolved independently many times in many different species. Recently, plant physiologists have found that some plant species are C-3/C-4 intermediates, in that they perform C-3 photosynthesis in some environments and C-4 photosynthesis in other environments. Study of these intermediates may help elucidate the evolution and physiological significance of C-4 photosynthesis.

Algae

There are many different groups of photosynthetic algae. Like higher plants, they all have chlorophyll-a as a photosynthetic pigment, two photosystems (PS-I and PSII), and the same overall chemical reactions for photosynthesis (equation 1). They differ from higher plants in having different complements of additional chlorophylls. The Chlorophyta and Euglenophyta have chlorophyll-a and chlorophyll-b. The Chrysophyta, Pyrrophyta, and Phaeophyta have chlorophyll-a and chlorophyll-c. The Rhodophyta have chlorophyll-a and chlorophyll-d. The different chlorophylls and other photosynthetic pigments allow algae to utilize different regions of the solar spectrum to drive photosynthesis.

This is a group of bacteria represented by a single genus, Prochloron. Like the Cyanobacteria, the Chloroxybacteria are prokaryotes. Like higher plants, Prochloron has chlorophyll-a, chlorophyll-b and carotenoids as photosynthetic pigments, two photosystems (PS-I and PS-II), and the same overall equation for photosynthesis (equation 1). In general, Prochloron is rather like a free-living chloroplast from a higher plant.