How is glucose converted into starch, glycogen, and cellulose in metabolism?

SouravNovember 12, 2024

Answered step-by-step

Glucose is converted into starch, glycogen, and cellulose through distinct metabolic pathways that involve specific enzymes and regulatory mechanisms. Here’s an overview of how these conversions occur:

1. Conversion of Glucose to Starch

• Starting Point: The process begins with glucose, which is derived from photosynthesis in plants.
• Key Enzymes:
  • ADP-Glucose Pyrophosphorylase (AGPase): This enzyme catalyzes the conversion of glucose-1-phosphate and ATP to ADP-glucose and inorganic pyrophosphate (PPi), initiating starch synthesis.
  • Starch Synthase (SS): This enzyme elongates the glucose chains using ADP-glucose as a glucosyl donor. It adds glucose units to the non-reducing ends of the growing starch polymer.
  • Starch Branching Enzyme (SBE): This enzyme introduces branches into the starch molecule by transferring segments of the glucose chain to create α(1→6) linkages, resulting in a branched structure composed of amylopectin and amylose.
• Final Structure: Starch is primarily composed of amylopectin (branched) and amylose (unbranched), both made up of α-glucose subunits linked by α(1→4) and α(1→6) glycosidic bonds .

2. Conversion of Glucose to Glycogen

• Starting Point: Glycogen synthesis occurs primarily in liver and muscle cells when there is an excess of glucose.
• Key Enzymes:
  • Glycogenin: This initiates glycogen synthesis by forming short chains of glucose that serve as a primer for further elongation.
  • Glycogen Synthase: This enzyme adds UDP-glucose to the growing glycogen chain, forming α(1→4) glycosidic bonds. It is responsible for the majority of glycogen synthesis.
  • Branching Enzyme: Similar to starch synthesis, this enzyme creates branches in glycogen by forming α(1→6) linkages after every 10 glucose units.
• Final Structure: Glycogen is a highly branched polymer of α-glucose, allowing for rapid mobilization during energy demands.

3. Conversion of Glucose to Cellulose

• Starting Point: Cellulose synthesis occurs in plant cells, primarily in the cell wall formation.
• Key Enzymes:
  • Cellulose Synthase: This enzyme catalyzes the polymerization of β-glucose units into long chains linked by β(1→4) glycosidic bonds. Unlike starch and glycogen, cellulose consists of β-glucose subunits, which results in a linear and rigid structure.
• Final Structure: Cellulose forms long, straight chains that can hydrogen bond with adjacent chains, creating strong fibers that provide structural support to plant cell walls