Question

What happens in the investment phase of glycolysis?

Answer 1

Glycolysis is a biological process in which glucose is broken down to produce energy. It takes place in the cytosol of the cell, in the presence or absence of oxygen. At the end of the glycolysis reaction two molecules of pyruvate, ATP, NADH and water are produced. Glycolysis occurs in two phases- the investment phase and pay-out phase. Only in the pay-out phase, ATP molecules are produced.

Complete answer:
The glucose molecule enters the glycolysis pathway through three ways. Dietary glucose obtained from food directly enters glycolysis via blood stream. Glucose can be released from hepatic stores of glycogen and enter glycolysis. The monosaccharide intermediates of various biochemical reactions also enter the pathway.
Glycolysis is a two-part process. In the first part, energy is consumed to produce high energy intermediates. This part is known as the energy investment phase. In the second part or the pay-out phase, the high energy intermediates are metabolized to produce ATP and NADH molecules.
In the investment phase, two ATP molecules are utilized to produce the intermediates glyceraldehyde $3$-phosphate (GA$3$P) and dihydroxyacetone phosphate (DHAP) from glucose. The DHAP will be further converted into GA$3$P and two molecules of GA$3$P will go into the pay-out phase.

Note:
In the energy pay-out phase, one molecule of NADH and two molecules of ATP are produced per molecule of GA$3$P entering the pathway. One molecule of glucose generates two molecules of GA$3$P and the total pay-out from the pay-out phase will be two NADH and four ATP. Since two ATP molecules are used in the investment phase, the net gain from the first molecule of glucose is $2$NADH and $2$ATP.