Question

Why does the glucose need to be phosphorylated? Just to give it energy?

Answer 1

In short: To trap glucose inside the cell, to make ATP, and to facilitate enzyme binding.

Explanation:

Phosphorylation is very important in glycolysis for the following main reasons:

1) `color (blue)("To trap glucose")`
When insulin is released from the pancreas after a meal, it signals the tissues to uptake glucose. When glucose enters the cells via glucose transporters, there is a chance for them to leave the cell. If we are in need of energy, and the breakdown of glucose will provide that energy, we don't want glucose to leave the cell. That is why the glucose is phosphorylated by ATP to become glucose-6-phosphate, which now bears a charge. This disqualifies it from leaving through glucose transporters.

2) `color (green)("To make ATP")`
The phosphoryl groups attached to glycolytic intermediates will be donated to ADP to become ATP. The phosphoryl groups came from the 2 ATPs that were invested plus inorganic phosphate groups that are floating around in the cell.

3) `color (orange)("To facilitate enzyme binding+specificity")`
Glycolytic intermediates with phosphoryl groups attached bind to the active site of the enzyme leading to increase specificity of the catalyzed reactions. Magnesium, `Mg^(+2)`, a cofactor which is often required in many steps of glycolysis, will complex with ADP, ATP and the substrates to aid the enzymatic reactions.