What is the Difference Between Cytosolic and Chloroplastic Glycolysis

The key difference between cytosolic and chloroplastic glycolysis is that cytosolic glycolysis is a linear pathway, while chloroplastic glycolysis is a cyclic pathway.

Glycolysis occurs in the cytosol, chloroplast, and plastid of photosynthetic and non-photosynthetic plants, depending on energy requirements and precursors. These reactions occur in different compartments catalyzed by separate enzyme isoforms. Cytosolic glycolysis takes place in the cytosol of a plant cell, while chloroplastic glycolysis takes place in the chloroplast or plastid of a plant cell.

CONTENTS

1. Overview and Key Difference
2. What is Cytosolic Glycolysis
3. What is Chloroplastic Glycolysis
4. Similarities – Cytosolic and Chloroplastic Glycolysis
5. Cytosolic vs Chloroplastic Glycolysis in Tabular Form
6. Summary – Cytosolic vs Chloroplastic Glycolysis

What is Cytosolic Glycolysis?

Cytosolic glycolysis is a complex network that consists of alternative enzymatic reactions. Two alternate cytosolic reactions enhance the yield of ATP through the use of pyrophosphate in place of ATP. The cytosolic glycolysis pathway provides essential metabolic flexibility, which facilitates plant development and acclimation to environmental stress conditions. This process takes place in the cytosol and has two main phases: the energy-requiring phase and the energy-releasing phase.

Initially, a phosphate group is added to glucose in the cytoplasm by the enzyme hexokinase and forms glucose-6-phosphate. This molecule is isomerized into fructose-6-phosphate by phosphoglucomutase. ATP molecule transfers a phosphate group to fructose-6-phosphate and converts it into fructose-1,6-bisphosphate by phosphofructokinase. Enzyme aldolase converts fructose-1,6-bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, which are isomers. Triose-phosphate isomerase converts dihydroxyacetone phosphate into glyceraldehyde 3-phosphate. This step undergoes two reactions.

In the first reaction, glyceraldehyde 3-phosphate dehydrogenase transfers one hydrogen molecule from glyceraldehyde phosphate to NAD to form NADH + H⁺. In the second reaction, glyceraldehyde 3-phosphate dehydrogenase adds a phosphate to the oxidized glyceraldehyde phosphate to form 1,3-bisphosphoglycerate. The phosphate transfers from 1,3-bisphosphoglycerate to ADP to form ATP by phosphoglycerate kinase. The phosphate of both the phosphoglycerate molecules is relocated by phosphoglyceromutase to yield 2-phosphoglycerate. Enolase removes a water molecule from 2-phosphoglycerate to form phosphoenolpyruvate. A phosphate from phosphoenolpyruvate (PEP) is transferred to ADP to form pyruvate and ATP by pyruvate kinase. The end products are pyruvate and ATP.

What is Chloroplastic Glycolysis?

Chloroplastic glycolysis is a central metabolic pathway that produces ATP in the dark and generates precursors for the synthesis of primary metabolites. This process is also referred to as plastidial glycolysis. Plastidial glycolytic enzymes modulate the metabolism of carbon and nitrogen in plants. This usually takes place in autotrophic and heterotrophic cells differently, with the requirement of glycolytic energy and precursors.

Plastidial/chloroplastic phosphoglycerate kinase (PGK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have different isoforms of the same enzyme catalyzing the same reactions but in opposite directions of chloroplastic glycolysis and in Calvin cycle. Plastidial glycolytic GAPDH converts glyceraldehyde-2-phosphate (GAP) into 1,3-bisphosphoglycerate, which is then again converted into 3-phosphoglycerate. In contrast, the assimilation of carbon dioxide during photosynthesis leads to the production of 3-phosphoglycerate (3-PGA), which is then converted to triose phosphates by a sequence of reactions of photosynthetic isoforms of PGK ad GAPDH. The 3-PGA production through chloroplastic glycolysis during the daytime is a wasteful process. Therefore, the glycolysis process occurs in the dark when the Calvin cycle is not operating. Moreover, chloroplast has an incomplete glycolytic system where PGA is not further metabolized but exported into the cytoplasm.

What are the Similarities Between Cytosolic and Chloroplastic Glycolysis?

• Cytosolic and chloroplast glycolysis are two metabolic pathways.
• They both occur in plants.
• Moreover, both glycolytic processes produce ATP.
• Both processes take place in the presence of oxygen.
• These pathways are catalyzed by enzymes.

What is the Difference Between Cytosolic and Chloroplastic Glycolysis?

Cytosolic glycolysis is a linear pathway, while chloroplastic glycolysis is a cyclic pathway. Thus, this is the key difference between cytosolic and chloroplastic glycolysis. Cytosolic glycolysis takes place in the cytosol of a cell, while chloroplastic glycolysis takes place in the chloroplast or plastid. Moreover, cytosolic glycolysis occurs throughout the day, while chloroplastic glycolysis mainly occurs in the dark.

The following table summarizes the difference between cytosolic and chloroplastic glycolysis.

Summary – Cytosolic vs Chloroplastic Glycolysis

Glycolysis in plants occurs in the cytosol and chloroplast. Cytosolic glycolysis is a linear pathway, while chloroplastic glycolysis is a cyclic pathway. Cytosolic glycolysis is a complex network that consists of alternative enzymatic reactions. This consists of two main phases; the energy-requiring phase and the energy-releasing phase. Chloroplastic glycolysis is a central metabolic pathway that produces ATP in the dark and generates precursors for the synthesis of primary metabolites. Enzymes of cytosolic glycolysis break down a glucose molecule through the traditional glycolytic pathways, while enzymes of chloroplastic glycolysis take part in the Calvin cycle to transform ambient carbon dioxide into glucose. So, this summarizes the difference between cytosolic and chloroplastic glycolysis.

Reference:

1. “Glycolysis IV (Plant Cytosol).” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.
2. WC, Plaxton. “The Organization and Regulation of Plant Glycolysis.” Annual Review of Plant Physiology and Plant Molecular Biology, U.S. National Library of Medicine.

Image Courtesy:

1. “Glycolysis metabolic pathway 3 annotated” By Thomas Shafee – Own work (CC BY 4.0) via Commons Wikimedia
2. “Fpls-02-00050-g005” By Fabio Facchinelli, Andreas P. M. Weber – doi:10.3389/fpls.2011.00050 (CC BY 3.0) via Commons Wikimedia