6.1 Glycolysis and Fermentation

6.1 Glycolysis and Fermentation

Cells release usable energy from glucose in stages. The first stage is glycolysis, which converts one glucose molecule into two molecules of pyruvate. The enzymes sit in the cytosol of prokaryotes and of eukaryotes, and the pathway does not require oxygen. A bacterium with no mitochondria can still run glycolysis, and so can a eukaryotic cell that is temporarily short of oxygen.

Where glycolysis runs

Cytosol, with oxygen optional

Prokaryotes keep glycolytic enzymes in the cytoplasm. Eukaryotes keep the same pathway in the cytosol, outside both mitochondrial membranes. Oxygen is not a reactant in these steps. The pathway runs when oxygen delivery lags, as in hard-working muscle, and it also runs as the opening stage of aerobic respiration. What stops it is a shortage of NAD+, not a move into the mitochondrial matrix.

From one glucose to two pyruvate

Glucose has six carbons. Pyruvate has three. After the sugar is prepared, it is split into two three-carbon sugars, and each of those becomes pyruvate. One glucose therefore yields two pyruvate. The energy result is a ledger.

Adenosine triphosphate (ATP) is spent before it is earned. Hexokinase, or glucokinase in liver cells, transfers a phosphate from ATP to glucose and forms glucose 6-phosphate. Phosphofructokinase then transfers a phosphate from a second ATP to fructose 6-phosphate and forms fructose 1,6-bisphosphate. That second transfer commits the sugar to the rest of glycolysis.

Fructose 1,6-bisphosphate is cleaved into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Only glyceraldehyde 3-phosphate goes forward directly. The other three-carbon sugar is converted into a second glyceraldehyde 3-phosphate, so every later reaction happens twice per glucose.

Payoff: NADH and ATP

Each glyceraldehyde 3-phosphate is oxidized. Electrons pass to nicotinamide adenine dinucleotide, and NAD+ becomes NADH. Inorganic phosphate is attached in that same oxidation. The next enzyme donates the phosphate to ADP, forming ATP without a membrane and without oxygen. This is substrate-level phosphorylation. A second transfer, from phosphoenolpyruvate to ADP, makes another ATP as pyruvate is released. Two glyceraldehyde 3-phosphate molecules therefore produce 2 NADH and 4 ATP. Subtract the 2 ATP invested at the start. The net from one glucose is 2 ATP and 2 NADH, together with two pyruvate. Those 2 NADH are reduced carriers, not extra ATP already in hand.

Phosphofructokinase sets the pace

Cells do not push every glucose molecule through the pathway merely because the enzymes are present. Phosphofructokinase is a major control step because its reaction strongly favors products and is essentially irreversible in the cell. When ATP is abundant, it binds a regulatory site and slows the commitment of fructose 6-phosphate. When ATP is scarce, more sugar enters the payoff phase. The major control step is phosphofructokinase.

Fermentation restores NAD+

Glycolysis keeps reducing NAD+ to NADH. The cytosol holds a limited pool of the carrier. If NADH is not oxidized back to NAD+, oxidation of glyceraldehyde 3-phosphate stops, and the ATP-forming steps after it stop too.

When oxygen is available and a respiratory membrane is working, the electron-transport chain can oxidize NADH. When oxygen is absent, that route is closed. Fermentation solves the carrier problem in the cytosol. Electrons move from NADH onto an organic molecule made from pyruvate, NAD+ is regenerated, and glycolysis can continue. The fermentation reactions do not themselves produce ATP. Under fermentation the net remains 2 ATP per glucose, and those 2 ATP come from glycolysis. The NADH is used to regenerate the carrier, so it is not cashed in for a further ATP harvest.

Lactic acid fermentation

In lactic acid fermentation, pyruvate accepts the electrons and becomes lactate. Animal skeletal muscle uses this route during intense exercise, when NADH is produced faster than oxygen delivery can clear it. Some bacteria use the same chemistry to regenerate NAD+. No carbon dioxide is released. The three carbons of pyruvate remain the three carbons of lactate. From one glucose the carbon product is two lactate, and the net ATP is 2.

Alcohol fermentation

Alcohol fermentation is the yeast pathway, and some bacteria use it too. It has two steps. Pyruvate is converted to acetaldehyde, and carbon dioxide leaves. Acetaldehyde then accepts electrons from NADH and becomes ethanol, which restores NAD+. From one glucose, two pyruvate become two ethanol and two CO2. The net ATP is still 2. Animal muscle uses lactic acid fermentation rather than this yeast route.

A worked count from one glucose

Start with one glucose in a yeast cell that cannot use oxygen. Glycolysis invests 2 ATP and produces 4 ATP, for a net of 2 ATP, and it reduces 2 NAD+ to 2 NADH. The carbon product at the end of glycolysis is two pyruvate. Each pyruvate loses one carbon as CO2 and becomes acetaldehyde, then ethanol. The carbon products of the whole route are two ethanol and two CO2. The two NADH pass electrons to the two acetaldehyde molecules and regenerate two NAD+. Those redox steps make no ATP.

The named result is two ethanol, two carbon dioxide, and a net of 2 ATP. Repeat the count for a muscle fiber using lactic acid fermentation. The carbon product is two lactate, carbon dioxide is not produced, and the net ATP is still 2. If an item asks only about glycolysis, stop at two pyruvate, a net of 2 ATP, and 2 NADH. Convert pyruvate to lactate or ethanol only when the item specifies fermentation or the absence of oxygen.

Fermentation is the anaerobic pathway paired with glycolysis here. It has no electron-transport chain and makes no ATP of its own. Some prokaryotes instead end a membrane chain on an acceptor such as nitrate, which is a different anaerobic pathway.