Glycolysis

Glycolysis (glycos = sugar, lysis = splitting) is the first stage of breaking down glucose. It was worked out by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. It takes place in the cytoplasm of all living organisms. In organisms that live without oxygen (anaerobes), it is the only process of respiration.

In plants, the glucose comes from sucrose (table sugar), which the enzyme invertase splits into glucose and fructose, or from stored carbohydrates like starch. The enzyme hexokinase adds a phosphate to the sugar, and it is then changed into fructose-6-phosphate.

In ten enzyme-controlled steps, one glucose (6 carbons) becomes two pyruvic acid (3 carbons each). Along the way:

• ATP is used twice: when glucose becomes glucose-6-phosphate, and when fructose-6-phosphate becomes fructose 1,6-bisphosphate. It is like spending a little money to start a business.
• Fructose 1,6-bisphosphate splits into two 3-carbon sugars: dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
• When PGAL becomes BPGA (1,3-bisphosphoglycerate), NAD⁺ picks up hydrogen and becomes NADH + H⁺.
• ATP is made twice per 3-carbon unit: when BPGA becomes PGA, and when PEP becomes pyruvic acid.

Counting for one glucose: 4 ATP made, 2 ATP used, so a net gain of 2 ATP, plus 2 NADH + H⁺. Glycolysis does not need oxygen.