Question

The first step of glycolysis is: (a) Breakdown of glucose (b) Phosphorylation of glucose (c) Conversion of glucose into fructose (d) Dehydrogenation of glucose

Short answer

The first step of glycolysis is (b) Phosphorylation of glucose.

Step-by-step solution

Understand Glycolysis

To solve this question, one must understand the glycolysis process. Glycolysis is the breakdown of glucose and forms part of cellular respiration. It is a series of chemical reactions used by all aerobic organisms to release stored energy.

Identify the first step of glycolysis

The first step of glycolysis involves the phosphorylation of glucose, which is catalyzed by the enzyme hexokinase. This step consumes ATP, but it serves to keep glucose concentration low, allowing continuous transport of glucose into the cell.

Key concepts

Phosphorylation of Glucose

Phosphorylation of glucose is the initial step in the glycolysis pathway. This process modifies glucose by adding a phosphate group to it. When glucose enters a cell, the enzyme hexokinase helps transfer a phosphate group from ATP (adenosine triphosphate) to the glucose molecule. Once a phosphate group is added, glucose becomes glucose-6-phosphate.
This transformation is significant for several reasons:

• Glucose-6-phosphate is more likely to stay inside the cell, as it does not easily cross the cell membrane.
• The phosphorylation of glucose helps in maintaining a concentration gradient, allowing more glucose to enter the cell.
• This reaction also ensures that glycolysis can proceed in a sequence of steps essential for energy extraction from glucose.

Without the phosphorylation step, cells might struggle to process glucose effectively and sustain energy needs.

Cellular Respiration

Cellular respiration is a comprehensive process that involves breaking down nutrients to produce energy. Glycolysis, which includes the phosphorylation of glucose, is just one part of this complex system. It occurs in the cytoplasm and does not require oxygen, making it an anaerobic step in cellular respiration. After glycolysis, the products are usually further processed in the mitochondria through aerobic pathways.
The three main stages of cellular respiration are:

• Glycolysis: Breaking down glucose into pyruvate, with a net production of 2 ATP molecules per glucose molecule.
• Krebs Cycle: Occurring within the mitochondria, this cycle processes the products of glycolysis to release CO₂ and more energy.
• Electron Transport Chain: This is where most ATP is generated. Electrons are transferred through proteins to release energy, which is used to convert ADP to ATP.

Through these stages, cellular respiration efficiently extracts energy from nutrients, critical to cellular functions and survival.

Hexokinase

Hexokinase is a pivotal enzyme that plays a crucial role in the phosphorylation of glucose. It facilitates the very first step of glycolysis. When glucose enters a cell from the bloodstream, hexokinase catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate.
This enzyme is highly effective due to:

• Its ability to act quickly and with high specificity to glucose, ensuring a rapid phosphorylation reaction.
• Regulation features that adjust its activity based on the energy needs of the cell, preventing waste of resources.
• Localizing action within the cytoplasm, which suits the cellular environment for performing the glycolytic process.

Without hexokinase, the conversion of glucose to glucose-6-phosphate would not occur efficiently, impacting the entire glycolytic process and subsequent cellular respiration stages.