Respiration and Photosynthesis Cycle

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Respiration and Photosynthesis Cycle
Respiration and Photosynthesis Cycle

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Respiration and Photosynthesis Cycle

Cellular respiration and photosynthesis form a critical cycle of energy and matter that supports the continued existence of life on earth. Describe the stages of cellular respiration and photosynthesis and their interaction and interdependence including raw materials, products, and amount of ATP or glucose produced during each phase. How is each linked to specific organelles within the eukaryotic cell. What has been the importance and significance of these processes and their cyclic interaction to the evolution and diversity of life?

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Quality of initial posting. 80
Connections and higher order thinking. 40
Reference to supporting readings. 40
Language and Grammar. 40
Total: 200

Respiration and Photosynthesis Cycle Sample Answer

Respiration and Photosynthesis Cycle

This paper will discuss the biochemical pathways that make cellular respiration and photosynthesis. In brief, the paper will explore the interaction and interdependence of glycolysis, the citric acid cycle, and oxidative phosphorylation as well as the light and dark reactions of photosynthesis. An emphasis will be put on the importance and significance of each pathway to life (Berg, Tymoczko, & Stryer, 2012; Voet & Voet, 2011).

Respiration and Photosynthesis Cycle for a Cellular Respiration

Respiration involves a series of catabolic reactions that convert nutrients into ATP energy. There are two types of respiration: anaerobic and aerobic respiration. Anaerobic respiration involves the glycolytic pathway and the tricarboxylic acid (TCA) cycle while aerobic respiration is also called oxidative phosphorylation. Glucose is the primary energy-yielding molecule, but amino acids and fatty acids are also oxidised to release energy (Berg et al., 2012).


The glycolytic pathway involves the breakdown of glucose to pyruvate with the concomitant release of energy. It occurs in the cytosol and is present in both prokaryotic and eukaryotic cells (Berg et al., 2012; Nelson & Cox, 2013). Once glucose enters the cell through specific transporters, it is phosphorylated to form glucose 6-phosphate in an ATP-requiring reaction catalysed by hexokinase. Glucose 6-phosphate has a negative charge and cannot diffuse out of the cell. The next step involves the isomerization of glucose 6-phosphate to form fructose 6-phosphate. An enzyme called phosphoglucose isomerase catalyses this reaction. A second ATP-dependent phosphorylation reaction follows the isomerization step where fructose 6 phosphate is phosphorylated to fructose 1,6-bisphosphate (F-1,6-BP) by an enzyme called phosphofructokinase (PFK). Next, F-1,6-BP is split into two 3-carbon units: glyceraldehyde 3 phosphate (GAP) and dihydroxyacetone phosphate (DHAP). An enzyme called aldolase catalyses this reaction. While GAP is on the direct pathway of glycolysis, DHAP is not. Thus, DHAP is isomerised to GAP in a rapid and reversible reaction catalysed by triose phosphate isomerase (Berg et al., 2012).

In the preceding step, GAP is converted to 1, 3-bisphospoglycerate (1,3-BPG) in an NAD+ -dependent reaction catalysed by glyceraldehyde 3-phosphate dehydrogenase. NAD+ is oxidised to NADH and hydrogen ion and a second phosphoryl group is added to GAP. In the next step, phosphoglycerate kinase catalyses the transfer of the phosphoryl group from 1,3-BPG to ADP to form 3-phospoglycerate and ATP. The formation of ATP in this manner is called substrate level phosphorylation. In the next steps, 3-phosphoglycerate is converted to 2-phosphoglycerate by an enzyme called phosphoglycerate mutase, and then to PEP (phosphoenolpyruvate) by a hydrolytic enzyme called enolase. In the last step, the phosphoryl group of PEP is transferred to ADP to form pyruvate and ATP. This reaction is catalysed by pyruvate kinase. Succinctly, glycolysis yields a net of two ATP and two NADH molecules. The NADH can be used in the electron transport chain to make more ATP as will be discussed later. The pyruvate formed serves as the raw material for the next respiratory step (Berg et al., 2012).

Respiration and Photosynthesis Cycle for The Fates of Pyruvate

Pyruvate, the end-product of glycolysis has three fates. Yeasts and other microorganisms convert pyruvate to ethanol in a two-step reaction. In the first step, pyruvate is converted to acetaldehyde and carbon dioxide by pyruvate decarboxylase. In the second step, acetaldehyde is reduced to ethanol in an NADH-dependent reaction catalysed by alcohol dehydrogenase. The conversion of pyruvate to ethanol is referred to as alcoholic fermentation (Berg et al., 2012). Some other microorganisms and cells of higher organisms limited in oxygen can reduce pyruvate to form lactate in an NADH requiring reaction catalysed by lactate dehydrogenase. This is called lactic acid fermentation. In most cells, pyruvate is shuttled into the mitochondrial matrix where it is oxidatively decarboxylated to acetyl-CoA with the concomitant generation of carbon dioxide and NADH. Acetyl CoA joins the TCA cycle while NADH can be used to synthesise ATP in the electron transport chain (ETC) (Berg et al., 2012; Voet & Voet, 2011).

Respiration and Photosynthesis Cycle and The Citric Acid Cycle

The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA) cycle involves a series of cyclic ten reactions that result in the oxidation of acetyl-CoA to two molecules of carbon dioxide. The reactions of the TCA cycle occur in the mitochondrial matrix. In the first reaction of the TCA cycle, acetyl-CoA is condensed to a four-carbon molecule called oxaloacetate to form citrate. This reaction is catalysed by citrate synthase. Citrate is then isomerised to form isocitrate by an enzyme called aconitase. Isocitrate then undergoes two successive oxidation decarboxylation reactions. First, isocitrate is converted to alpha-ketoglutarate by isocitrate dehydrogenase and then to succinyl-CoA by an alpha-ketoglutarate dehydrogenase. These two reactions result to formation of NADH and liberation of carbon dioxide. In the next step, succinyl-CoA synthetase catalyses the cleavage of succinyl-CoA to succinate. GTP is formed in this reaction and can easily be converted into ATP. Succinate is oxidised by a FAD-dependent dehydrogenase to form fumarate and FADH2. Fumarate is hydrolysed to malate by fumarase. In the last step, malate is oxidised by malate dehydrogenase to form oxaloacetate and NADH. In summary, the TCA cycle degrades acetyl-CoA to carbon dioxide with concomitant formation of energy storage molecules: one GTP, three NADH, and one FADH2 per cycle. The reducing equivalents are used to synthesise more ATP in the ETC. The cycle also yields precursors for the biosynthesis of heme, nucleotides, amino acids, and cholesterol (Berg et al., 2012; Nelson & Cox, 2013; Voet & Voet, 2011).

Respiration and Photosynthesis Cycle and Oxidative Phosphorylation

In oxidative phosphorylation, FADH2 and NADH are used to reduce molecular oxygen to water. These reductions involve electron transfers that occur in a set of inner mitochondrial membrane proteins called the electron transport chain (ETC) or simply the respiratory chain. The ETC consists of four large complexes: NADH-Q oxidoreductase, succinate-Q reductase, Q-cytochrome c oxidoreductase and cytochrome c oxidase, which are also called complex I, II, III, and IV respectively. Q refers to coenzyme Q, also called ubiquinone. Electrons donated by NADH flow through NADH-Q oxidoreductase to coenzyme Q with the concomitant pumping of four protons out of the mitochondrial matrix to the cytosolic side of the inner mitochondrial membrane. Similarly, succinate Q reductase accepts electrons from FADH2 and relays them to coenzyme Q. However, the flow of electrons through complex II does not result to pumping of protons out of the matrix. As a result, fewer ATP molecules are formed by the oxidation of FADH2 as compared to NADH. Electrons then flow from reduced coenzyme Q (ubiquinol) to cytochrome c through complex III. This leads to the pumping of four protons out of the matrix. In the last step of the ETC, cytochrome c oxidase receives the electrons and reduces molecular oxygen to two water molecules. The four hydrogen ions used to form water come exclusively from the matrix, and this contributes to the proton gradient. In addition, cytochrome c oxidase also pumps four protons out of the matrix in the course of each reaction cycle (Berg et al., 2012; Nelson & Cox, 2013; Voet & Voet, 2011).

The respiratory chain is coupled to the synthesis of ATP. The pumping of protons out of the matrix into the cytosolic side of the inner mitochondrial membrane creates a proton gradient that drives the phosphorylation of ADP by-ATP synthase complex to form ATP. Each molecule of NADH yields 2.5 ATP molecules, while FADH2 yields 1.5ATP molecules (Berg et al., 2012).

Respiration and Photosynthesis Cycle

Photosynthesis is the use of light energy to combine water and carbon dioxide to form sugars and molecular oxygen. The process occurs in the chloroplast of green plants. The process involves two stages: the light reactions and the Calvin cycle (Berg et al., 2012; Voet, 2012).

The Light Reactions of Photosynthesis

The light phase, also called hill reaction or the photochemical reaction occur in the present of light, which is then transformed into chemical energy. Chlorophyll absorbs light energy and converts it to ATP and NADPH with the evolution of oxygen. The light harvesting complexes (LHC) of the light phase are called photosystem I (PSI) and II (PSII) and are located in the thylakoid membrane of the chloroplast. Prokaryotes have PSI only while eukaryotes have both. PSII occurs first and is named so because it was discovered after PSI (Berg et al., 2012).

The excitation molecule of PSII is called P680 named so because it maximally absorbs at 680nm. Electrons generated during photolysis of water by a complex called manganese centre reduce oxidized P680. P680 absorbs light energy, gets excited and rapidly transfers its electrons to a nearby molecule called pheophytin (pheo). Pheo transfers the electrons to a tightly bound plastoquinone (QA) and then to an exchangeable plastoquinone (QB). Upon uptake of two electrons, QB is reduced to plastoquinol (QH2). Therefore, two protons from the stroma are used to reduce each molecule of plastoquinone and four protons released during water photolysis are liberated into the thylakoid lumen. This contributes to the generation of a proton gradient characterized by more protons in the thylakoid lumen then the stroma. QH2 transfers the electrons to plastocyanin (Pc) and the two hydrogen ions from QH2 are released into the lumen which further contributes to the proton gradient (Berg et al., 2012; Voet, 2012; Voet & Voet, 2011).

The last stage of the light reactions is catalysed by PSI. The primary light absorbing molecule of PSI is called P700 because it absorbs maximally at 700nm. P700 absorbs light energy, gets excited and donates its electrons first to acceptor quinolone molecule (A0) and then to A1. From here, the electrons are transferred to a ferrodoxin, which reduces NADP+ to NADPH in a reaction catalysed by a flavoprotein called ferrodoxin- NADP+ reductase. The uptake of hydrogen ions by NADP+ further contributes to the proton motive force across the thylakoid lumen. This proton gradient is used to drive ATP synthesis by the ATP synthase of the chloroplast. In summary, the light reactions result in the formation of NADPH and ATP, which are utilised in the Calvin cycle to make sugars.

Respiration and Photosynthesis Cycle and The Calvin Cycle

These are light independent reactions of photosynthesis and occur in the stroma of the chloroplast. The first step of the Calvin cycle is the fixation of carbon dioxide by condensing it to ribulose 1,5-bisphosphate to form an unstable compound that is rapidly hydrolysed to two 3-phosphoglycerate molecules. This reaction is catalysed by the most important enzyme on the planet called ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco). 3-phosphoglycerate is converted to 1,3-BPG in an ATP-requiring reaction catalyzed by phosphoglycerate kinase. 1,3-BPG is then reduced to the GAP in a reaction that utilises an NADPH-dependent dehydrogenase. GAP is combined with xylulose 5-phosphate to regenerate ribulose 1,5-bisphosphate. Alternatively, GAP is used as a precursor for the synthesis of glucose 6-phosphate and fructose 6-phosphate, which are the building blocks of carbohydrates. Thus, the ATP and NADPH formed in the light reactions are utilised in the Calvin cycle to make sugars.

Respiration and Photosynthesis Cycle Conclusion

Cellular respiration and photosynthesis are the most important metabolic pathways in the cell. Cellular respiration involves glycolysis, the Krebs cycle, and the electron transport chain and results in the formation of ATP energy. Photosynthesis has two phases; the light phase and the Calvin cycle. The light reactions produce ATP and NADPH, which are used in the Calvin cycle to make sugars.

Respiration and Photosynthesis Cycle References

Berg, J. M., Tymoczko, J. L., & Stryer, L. (2012). Biochemistry (7 ed.): W. H. Freeman and Company.

Nelson, D. L., & Cox, M. M. (2013). Lehninger Principles of Biochemistry (6 ed.): W. H. Freeman and Company.

Voet, D. (2012). Fundamentals of Biochemistry: Life at the molecular level: Wiley.

Voet, D., & Voet, J. G. (2011). Biochemistry (4th ed.): Wiley.

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