Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Glycolysis consists of an energy-requiring phase followed by an energy-releasing phase.

Glycolysis (from glycose, an older term for glucose + -lysis degradation) is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy molecules ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).

Introduction

All microorganisms (or the majority of them) can break down glucose to produce energy. Or at least, it is a very common way to produce energy. Lactobacillus acidophilus is the bacterium that turns milk into yogurt. Overall, the metabolism of glucose in one of your cells would be pretty different from its metabolism in Lactobacillus, yet, the first steps would be the same in both cases. Glycolysis is the process to break down the glucose into two molecules and finally produce pyruvate and both you and the bacterium would follow identical steps.

What is glycolysis?

Glycolysis is constituted by a series of reactions that extract energy from glucose by splitting it into two three-carbon molecules called pyruvates. Glycolysis is an ancient metabolic pathway, meaning that it evolved long ago; this is also confirmed by the fact that bacteria and humans use the same enzymes and this metabolic pathway is also found in the great majority of organisms alive today.

Glycolysis is only the first stage of this process in those organisms that perform cellular respiration. However, glycolysis does not necessarily require oxygen, and many anaerobic organisms (which can survive in environments without oxygen) can also use this pathway.

Highlights of glycolysis

The cytosol of a cell is the compartment where glycolysis takes place. Glycolysis can be broken down into two main phases: the energy-requiring phase and the energy-releasing phase.

Energy-requiring phase:

This phase requires the rearrangement of the starting molecule of glucose; two phosphate groups are attached to it. The enzyme hexokinase or glucokinase (liver) is responsible for the first phosphorylation which will create glucose-6-phosphate; this step is important since the modified glucose cannot leave the cell now since it cannot cross the membrane.
Glucose-6-phosphate is later converted into fructose-6-phosphate by phosphoglucose isomerase. Later, fructose-6-phosphate is converted into fructose-1,6-bisphosphate by phosphofructokinase. Each steps convert one ATP to ADP.

The phosphate groups make the modified sugar called fructose-1,6-bisphosphate which can be later split in half and form two phosphate-bearing three-carbon sugars. The phosphates used in these steps come from ATP and two ATP molecules are used up. Phosphofructokinase is one of the most important enzyme for regulation of glycolysis since it catalyzes the formation of the unstable, two-phosphate sugar molecule, fructose-1,6-bisphosphate. In addition, the role of the phosphofructokinase is to speed up or slow down glycolysis according to the energy needs of the cell.
Fructose bisphosphate aldolase is the enzyme that catalyzes the conversion of fructose-1,6-bisphosphate which is split into two three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP).
The DHAP is converted into glyceralde-3-phosphate by an enzyme called triose phosphate isomerase. This reaction can go in either direction, but because glyceraldehyde-3-phosphate is continually being used up in the rest of the pathway, the equilibrium favors conversion of DHAP to glyceraldehyde-3-phosphate.

Energy producing phase:

In the second half of glycolysis, the three-carbon sugars formed in the first half of the process go through a series of additional transformations, ultimately turning into pyruvate.
Indeed, with a series of steps producing one NADH and two ATP, each molecule of glyceraldehyde-3-phosphate is converted into a pyruvate molecule. This happens twice for each molecule of glucose, making four ATP and two NADH moleculae overall.

Glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate. This is a redox reaction in which NAD+ is converted to NADH (with the release of an H+ ion) by using two electrons and two proton from this molecule. An inorganic phosphate is also a reactant for this reaction, which is catalyzed by glyceraldehyde-3-phosphate dehydrogenase forming 1,3-bisphosphoglycerate.
1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase. This step converts an ADP to an ATP through the donation of one of its phosphate groups.
3-phosphoglycerate is converted to the isomer 2-phosphoglycerate by phosphoglycerate mutase.
2-phosphoglycerate is converted to phosphoenolpyruvate (PEP) by enolase by losing a molecule of water.
Phosphoenolpyruvate (PEP), an unstable molecule is finally converted to pyruvate by pyruvate kinase. An ADP is converted to an ATP in this reaction.

At the end of glycolysis, we’re left with two ATP, two NADH molecules and two pyruvate molecules. If oxygen is available, the pyruvate can be broken down (oxidized) all the way to carbon dioxide in cellular respiration (the citric acid cycle and oxidative phosphorylation), making many more molecules of ATP. Cells need to continuously generate the electron transport molecule called NAD+ to continue performing glycolysis. Without oxygen, this molecule is not generated in sufficient amounts. Thus, cells must undergo a different chemical reaction in order to generate NAD+. This set of reactions is called fermentation and it is common in yeats for examples, but it can also be carried out by humans where it generates lactic acid.

When people exercise, sometimes the oxygen supply can be insufficient. When this happens, muscle cells can temporarily switch to anaerobic respiration to generate energy. Lactic acid accumulate in the muscles as a result of this process. Oversaturation of lactic acid leads to muscle cramps and fatigue.

So, in absence or low level of of oxygen, fermentation pathways are a primary metabolic choice for lots of bacteria, including Lactobacillus acidophilus. Even some cells in your body, such as red blood cells, rely on fermentation to make their ATP.

What about the supposed relationship between high carbohydrate intake and fat accumulation in human body?
The news are quite contrasting. Several friends training regularly in the gym are convinced that carbohydrate are the real enemy of the muscles and can easily make you fat. On the other hand, it seems that excess dietary fat leads to greater fat accumulation than does excess dietary carbohydrate (Hort 1995). Glucose is rapidly broken down in every cell of the body or, if in excess, stored in glycogen, a multibranched polysaccharide of glucose that serves as a form of energy storage in humans, animals, fungi, and bacteria. However, the amount of glycogen in the humna body is rather limited.

References
https://www.ncbi.nlm.nih.gov/pubmed/7598063
https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/a/glycolysis
https://quizlet.com/135033449/glycolisi-flash-cards/
http://hyperphysics.phy-astr.gsu.edu/hbasees/Biology/glycolysis.html

Video!