Question

Where does Glycolysis occur in cellular respiration?

Answer 1

Glycolysis is the splitting of sugar, in this process each molecule of glucose(a hexose sugar) is broken down in stepwise biochemical reactions under enzymatic control into two molecules of pyruvic acid.The net gain of ATP in this process is 2 ATP molecules (four ATPs are formed but two are already used up in the reaction).

Complete answer:
The scheme of glycolysis is given by Gustav Embden, Otto Meyerhof, and J.Parnas, so it is also referred to as the EMP pathway.
Glycolysis is a common step in both aerobic and anaerobic respiration, in anaerobic organisms, it is the only process in respiration.
Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms, it does not require oxygen.
The pathway of glycolysis has two phases-
Glucose activation phase and energy extraction phase.
Steps of glycolysis-
a) Glucose is phosphorylated to glucose-6-phosphate by ATP in the presence of enzyme hexokinase.
b) Glucose-6-phosphate isomerizes to form fructose-6-phosphate by the action of the enzyme phosphohexoisomerase.
c) Fructose-6-phosphate is phosphorylated to form fructose-1,6-bisphosphate.The enzyme used in this reaction is
phosphofructokinase.During phosphorylation, ATP is converted into ADP.
d) The fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde(PGAL) by the action of Aldolase.
e) The two triose phosphates formed are interconvertible. The enzyme phosphotriose isomerase is used in this interconversion.
f) Glyceraldehyde-3-phosphate or 3-phosphoglyceraldehyde(PGAL) is converted into 1,3-bisphosphoglyceric acid or 1,3-bisphosphoglycerate(BPGA).During this conversion, $NAD^+$ is converted to $NADH^+$ and $H^+$. Enzyme Phosphoglyceraldehyde dehydrogenase catalyzes this reaction.
g) 3-phosphoglyceric acid changes to 2-phosphoglycerate by the action of phosphoglyceromutase.
h) Enolase acts on 2-phosphoglycerate and converts it into phosphoenolpyruvate(PEP).
i) Phosphoenolpyruvate is acted upon by pyruvate kinase and gets converted into pyruvic acid. This step is also substrate-level phosphorylation, therefore the ATP releasing step.

Note:
Glycolysis is the partial oxidation of glucose to form two molecules of pyruvic acid,in plants, the glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates, this sucrose is converted into glucose and fructose by the enzyme, invertase and these two monosaccharides readily enter the glycolytic pathway.