Photosynthesis
Ayush Noori | EduSTEM Advanced Biology
Light-Dependent Reactions:
- Photons of light excite electrons of chlorophyll (α and β) and carotenoids in the antenna of Photosystem II, which eventually excite P680 in the reaction center by resonance energy transfer. The excited electron is accepted by pheophytin, and then travels down an electron transport chain (of plastoquinone, cytochrome b6-f, and plastocyanin) generating a proton gradient across the thylakoid membrane which is used to power the synthesis of ATP by ATP synthase. Once again, photons excite P700 of Photosystem I. P700 accepts the excited electron from the previous electron transport chain and donates its electron to phylloquinone and iron-sulfur complexes, which subsequently transfer the electron to ferrodoxin, and then to ferrodoxin-NADP+ reductase (FNR), which generates NADPH. The ATP and NADPH generated are used to power the generation of G3P in the Calvin cycle.
Calvin Cycle:
- The carboxylation of ribulose bisphosphate (RuBP) by CO2 is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly termed RuBisCO, to ultimately yield one molecule of glyceraldehyde 3-phosphate (G3P) for every three CO2 molecules which enter the cycle (after phosphorylation and reduction of 3-phosphoglycerate by chemical energy from the light-independent reactions – ATP and NADPH, respectively). Two G3P molecules are used to generate a single glucose molecule via the gluconeogenic pathway, which can then serve a variety of functions in the cell, including starch storage, cellulose production, intracellular transport as sucrose.
EXTENSION This extension explores the process of photosynthesis and related plant biology.
Kirkham MB. Principles of Soil and Plant Water Relations. 2nd edition. Elsevier; 2014. Section 24.1, Definition pf Stomata and Their Distribution. doi: 10.1016/C2013-0-12871-1
Pores in the epidermis of the plant called stomata regulate the exchange of carbon dioxide between the environment and the intercellular spaces; the opening of stomata facilitates the diffusion of CO2 and oxygen into and out of the substomatal and mesophyllic intercellular spaces. Stomata are bordered by a pair of specialized guard cells which regulate the opening and closing of the stomatal pores.
Kinoshita T, Shimazaki K. Biochemical evidence for the requirement of 14-3-3 protein binding in activation of the guard-cell plasma membrane H±ATPase by blue light. Plant Cell Physiol. 2002 Nov; 43 (11): 1359-65. https://www.ncbi.nlm.nih.gov/pubmed/12461136
Not only does photosynthesis increase when more light is available because PSI and PSII are more frequently excited – the stomata of plants also open upon simulation by light. Blue light specifically induces phosphorylation of serine and threonine residues of H±ATPase in the plasma membrane of guard cells by phototropins, light-dependent kinases. The activity of H±ATPase generates a negative potential across the membrane and thus directs diffusion of K+ into the cell, which in turn decreases the water potential. The lower water potential causes osmosis into the cell, and the turgidity of the guard cells causes stomata to open.
Horner HT. Peperomia leaf cell wall interface between the multiple hypodermis and crystal-containing photosynthetic layer displays unusual pit fields. Ann Bot. 2012 Jun; 109(7): 1307–1316. Published online 2012 Apr 25. doi: 10.1093/aob/mcs074 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3359927/
Some plants such as Peperomia obtusifolia contain spherical aggregates called druses of various crystals in their palisade cells. The druses, in the central vacuole, are surrounded by chloroplasts and are accompanied by specialized pit fields in the hypodermis. Research suggests that plants use the crystals in druses to focus light (which enters from the pit fields) to the chloroplasts, especially in low-light environments. Please see Figure 4.
“Diagram of a Peperomia obtusifolia palisade parenchyma cell showing the interface of the common wall with the hypodermis containing pit fields, and a protoplast with a large central vacuole containing a multifaceted druse surrounded by chloroplasts with large grana oriented perpendicular to the druse. Visible light waves are shown filtering through hypodermal pit fields, striking druse facets and dispersed to surrounding chloroplasts.”
Laterre R, Pottier M, Remacle C, Boutry M. Photosynthetic Trichomes Contain a Specific Rubisco with a Modified pH-Dependent Activity. Plant Physiol. 2017 Apr; 173(4): 2110–2120. Published online 2017 Mar 1. doi: 10.1104/pp.17.00062. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5373067/
Trichomes are hair-like appendages on the epidermis of cells and usually serve defensive purposes against UV light, pathogens, or herbivores. Some trichomes are capable of photosynthesis.
Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 20.1, The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water. https://www.ncbi.nlm.nih.gov/books/NBK22344/#A2799
Below is an excerpt from Biochemistry 5th edition by Berg et al. which provides an in depth look at the reactions of the Calvin cycle.
20.1.3. Hexose Phosphates Are Made from Phosphoglycerate, and Ribulose 1,5-bisphosphate Is Regenerated
The 3-phosphoglycerate product of rubisco is next converted into three forms of hexose phosphate: glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate. Recall that these isomers are readily interconvertible (Sections 16.1.2 and 16.1.11). The steps in this conversion (Figure 20.9) are like those of the gluconeogenic pathway (Section 16.3.1), except that glyceraldehyde 3-phosphate dehydrogenase in chloroplasts, which generates glyceraldehyde 3-phosphate (GAP), is specific for NADPH rather than NADH. Alternatively, the glyceraldehyde 3-phosphate can be transported to the cytosol for glucose synthesis. These reactions and that catalyzed by rubisco bring CO2 to the level of a hexose, converting CO2 into a chemical fuel at the expense of NADPH and ATP generated from the light reactions.
Figure 20.9
Hexose Phosphate Formation. 3-Phosphoglycerate is converted into fructose 6-phosphate in a pathway parallel to that of glyconeogenesis.
The third phase of the Calvin cycle is the regeneration of ribulose 1,5-bisphosphate, the acceptor of CO2 in the first step. The problem is to construct a five-carbon sugar from six-carbon and three-carbon sugars. A transketolase and an aldolase play the major role in the rearrangement of the carbon atoms. The transketolase , which we will see again in the pentose phosphate pathway (Section 20.2.3), requires the coenzyme thiamine pyrophosphate (TPP) to transfer a two-carbon unit (CO-CH2OH) from a ketose to an aldose.
We will consider the mechanism of transketolase when we meet it again in the pentose phosphate pathway (Section 20.3.2). Aldolase , which we have already encountered in glycolysis (Section 16.1.3), catalyzes an aldol condensation between dihydroxyacetone phosphate and an aldehyde. This enzyme is highly specific for dihydroxyacetone phosphate, but it accepts a wide variety of aldehydes.
With these enzymes, the construction of the five-carbon sugar proceeds as shown in Figure 20.10.
Figure 20.10
Formation of Five-Carbon Sugars. First, transketolase converts a six-carbon sugar and a three-carbon sugar into a four-carbon sugar and a five-carbon sugar. Then, aldolase combines the four-carbon product and a three-carbon sugar to form a seven-carbon (more…)
Finally, ribose-5-phosphate is converted into ribulose 5-phosphate by phosphopentose isomerase while xylulose 5-phosphate is converted into ribulose 5-phosphate by phosphopentose epimerase . Ribulose 5-phosphate is converted into ribulose 1,5-bisphosphate through the action of phosphoribulose kinase (Figure 20.11). The sum of these reactions is
Figure 20.11
Regeneration of Ribulose 1,5-Bisphosphate. Both ribose 5-phosphate and xylulose 5-phosphate are converted into ribulose 5-phosphate, which is then phosphorylated to complete the regeneration of ribulose 1,5-bisphosphate.
This series of reactions completes the Calvin cycle (Figure 20.12). The sum of all the reactions results in the generation of a hexose and the regeneration of the starting compound, ribulose 5-phosphate. In essence, ribulose 1,5-bisphosphate acts catalytically, similarly to oxaloacetate in the citric acid cycle.
Figure 20.12
Calvin Cycle. The diagram shows the reactions necessary with the correct stoichiometry to convert three molecules of CO2 into one molecule of DHAP. The cycle is not as simple as presented in Figure 20.1; rather, it entails many reactions that lead ultimately (more…)
Figure 20.9 Hexose Phosphate Formation
3-Phosphoglycerate is converted into fructose 6-phosphate in a pathway parallel to that of glyconeogenesis.
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Ayush Noori
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