The dark reactions

The pathway by which, in the stroma of the chloroplast, the NADPH2 and ATP produced in the light reactions are used to convert CO2 to carbohydrate was elucidated largely by the work of Calvin and Benson, and is outlined in Fig. 8.18. The cycle is somewhat involved and needs

Fig. 8.18 The photosynthetic CO2 fixation cycle. P = phosphate group; Pi = inorganic phosphate. The enzymes involved in each step are: (1) Rubisco; (2) 3-phosphoglyceric acid kinase; (3) glyceraldehyde-3-phosphate dehydrogenase; (4) triose phosphate isomerase; (5) fructose-diphosphate aldolase; (6) fructose 1,6 diphosphatase; (7) transketolase; (8) aldolase; (9) sedoheptu-lose 1,7 diphosphatase; (10) transketolase; (11) xylulose-5-phosphate epimerase; (12) ribose-5-phosphate isomerase; (13) ribulose-5-phosphate kinase.

Fig. 8.18 The photosynthetic CO2 fixation cycle. P = phosphate group; Pi = inorganic phosphate. The enzymes involved in each step are: (1) Rubisco; (2) 3-phosphoglyceric acid kinase; (3) glyceraldehyde-3-phosphate dehydrogenase; (4) triose phosphate isomerase; (5) fructose-diphosphate aldolase; (6) fructose 1,6 diphosphatase; (7) transketolase; (8) aldolase; (9) sedoheptu-lose 1,7 diphosphatase; (10) transketolase; (11) xylulose-5-phosphate epimerase; (12) ribose-5-phosphate isomerase; (13) ribulose-5-phosphate kinase.

several turns to produce one molecule of hexose. To understand the overall effect of the dark reactions let us consider the fate of six molecules of CO2. These react with six molecules of ribulose bisphosphate (C5) to give 12 molecules of phosphoglyceric acid (C3). The phosphoglyceric acid is, with the help of ATP and NADPH2, reduced to triose phosphate. For simplicity we can regard these as being converted to six molecules of hexose phosphate (C6)- One of these molecules of hexose can be removed to form starch or other reserve carbohydrate, leaving five hexose phosphate molecules. These are now rearranged to form six pentose phosphate (C5) molecules which are, with the help of ATP converted to six molecules of ribulose bisphosphate. Thus the cycle is completed with six molecules of CO2 being converted to one of hexose. The overall process in terms of carbon atoms may be summarized

6CO2

5C6^

Reserve Carbohydrate

Some higher plants, known as C4 plants, possess a variant of this cycle in which CO2 is first fixed in the form of a C4 acid, such as malic acid. The C4 acid is then translocated to another tissue where CO2 is liberated from it again and converted to carbohydrate by the normal cycle. It seems very unlikely that this C4 pathway occurs in any alga, and it does not appear to exist in submerged aquatic higher plants, so we may disregard it as a contributor to aquatic photosynthesis within the water column. Some aquatic higher plants do, however, have a simplified form of C4 pathway in which a substantial proportion of the CO2 fixation is indeed initially into C4 acids, from which the CO2 is subsequently liberated and used for photosynthesis, but within the same cel1.149,681,1111 The effect of this is to increase the CO2 supply to the CO2-fixing enzyme, ribulose bisphosphate carboxylase (commonly referred to as Rubisco), this being a significant limiting factor in aquatic photosynthesis, as we shall discuss in more detail in a later chapter (see ยง11.3). In cyanobacteria, Rubisco occurs together with carbonic anhydrase in icosahedral crystalline bodies, ~0.1 mm in diameter, referred to as carboxysomes, and it seems likely that this is where CO2 fixation occurs. In eukaryotic algae most of the Rubisco is localized in the pyrenoid.958,384

In bright sunlight, photosynthesis produces carbohydrates faster than they can be used in respiration or growth. Accordingly the plant must store the fixed carbon in some form that it can utilize later, most commonly as grains of polysaccharide. The chemistry of algal storage products was reviewed by Craigie (1974). In higher plants and green algae, fixed carbon is accumulated inside the chloroplast as starch grains in the stroma, or forming a shell around the pyrenoid in those algae that have them. In algal phyla other than the Chlorophyta, the photosynthetic end products accumulate outside the chloroplast. Where a pyrenoid is present the polysaccharide grains are usually found in the cytoplasm but in close contact with the pyrenoid region of the chloroplast.

The Floridean starch of the Rhodophyta is an a-D-(1^4)-linked glu-can with a-D-(1^6)-branch points, and can thus be regarded as an amylopectin, like the major component of higher plant starch. Crypto-phytes accumulate a starch of the higher plant type, containing both amylose and amylopectin. Members of the Pyrrophyta accumulate grains of a storage polysaccharide with staining properties similar to starch. Brown algae (Phaeophyta) accumulate the polysaccharide laminaran, a b-D-(1!3)-linked glucan with 16 to 31 residues per molecule and a mannitol residue at the reducing end of each chain: some of the laminaran molecules are unbranched, some have two to three b-D-(l^6) branch points per molecule. In addition, these algae accumulate large amounts of free mannitol. In the Euglenophyta, the photosynthetic storage product accumulates as granules of paramylon, a b-D-(1^3) glucan, with 50 to 150 residues per molecule. The photosynthetic storage product of the Heterokontophyta and Haptophyta is chrysolaminaran (also known as leucosin), a b-D-(l^3)-linked glucan with possibly two 1^6 branch points per 34-residue molecule. Diatoms also contain polysaccharides of the chrysolaminaran type. The storage carbohydrate of the cyanophytes and prochlorophytes is an a-1,4-glucan, similar to starch.

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