Glycolysis

Let’s say that you just ate a big slice of pizza with onions, mushrooms, bell peppers, and jalapenos. To pull energy out of the glucose in that pizza or really any food, requires glycolysis.

Glycolysis is a series of enzymatic reactions in which glucose, a 6 carbon sugar molecule, is broken down into two 3 carbon pyruvate molecules.

And as glucose gets processed, energy is produced in the form of adenosine triphosphate, or ATP.

Now, glycolysis happens in the cytoplasm of cells, and no special organelles or even oxygen are needed to turn glucose into ATP.

Therefore, all cells can use glucose to make energy; and it’s possible to do glycolysis even when oxygen levels are low.

Glycolysis can be divided into two phases: an energy-consuming phase, and an energy-producing phase.

It’s like a business investment - the cell needs to spend some energy before it can start making energy, and like any good investment the cell gets more energy back than it puts in.

The energy-consuming phase requires ATP, and the energy-producing phase generates ATP, as well as other molecules like reduced nicotinamide adenine dinucleotide, or NADH, which can be used to make ATP.

We can keep track of all of this using an energy counter.

Going back to that delicious pizza, first, glucose from those ingredients has to first get from the small intestine into the bloodstream.

In response to high blood glucose, the pancreatic beta-cells secrete insulin.

Now, to get inside the cells, glucose utilizes glucose transporters, or GLUT, which are on the cell membrane.

In fact, some GLUTs like GLUT2 in the liver and pancreatic beta-cells are particularly responsive to glucose in the presence of insulin.

Once glucose gets inside the cell, it’s prevented from diffusing across the cell membrane back into the circulation by enzymes called kinases which phosphorylate the glucose.

Adding a phosphate group changes the shape of the glucose molecule, which means it can’t easily diffuse out of the cell, a bit like a criminal that’s handcuffed to the table in an interrogation room.

The phosphate comes from the breakdown of ATP into ADP and phosphate - so this initial phosphorylation step drops us to -1 on that energy counter.

Specifically, there are two enzymes called hexokinase and glucokinase, and they both add a phosphate group to the 6th carbon in the glucose molecule, turning it into glucose-6-phosphate.

Both enzymes pretty much do the same thing, but hexokinase is found in all cells, whereas glucokinase, like GLUT2, is induced by the presence of insulin, and is found in the liver cells and the beta-cells of the pancreas.