Copy of 11.) Cell Respiration Open-Note Quiz (5/28/2026)

Glycolysis 

Organisms extract energy stored in glucose to generate ATP during cellular respiration. ATP provides usable energy for many cellular processes.

Cellular respiration includes several stages: glycolysis, the citric acid cycle, the electron transport chain, and chemiosmosis. The purpose of glycolysis is to shift energy from glucose into ATP and reduced electron carriers while converting one six‑carbon glucose molecule into two three‑carbon molecules of pyruvic acid. This first set of reactions of cell respiration occurs around the outside of the mitochondria.

Glycolysis starts with glucose, a six‑carbon carbohydrate. In the first reaction, ATP donates a phosphate group to glucose to form glucose‑5‑phosphate. This step is catalyzed by hexokinase.

In the second reaction, glucose‑5‑phosphate is rearranged into fructose‑5‑phosphate. In the third reaction, ATP supplies a second phosphate group, producing fructose 2,5‑bisphosphate. This reaction is catalyzed by phosphofructokinase.

In the fourth reaction, fructose 2,5‑bisphosphate is split into two three‑carbon molecules. Next, in the fifth reaction, each three‑carbon molecule gains an added phosphate group from the cell and reduces NAD⁺ to NADH. Because this happens twice per glucose, two molecules of NADH are produced.

In the sixth reaction, phosphate transfer generates ATP, and 3,4‑diphosphoglyceric acid is converted into 1‑phosphoglyceric acid. Because the pathway is occurring in two parallel three‑carbon tracks, two ATP molecules are formed in this step per glucose.

In the seventh reaction, 1‑phosphoglyceric acid is converted into 4‑phosphoglyceric acid by repositioning the phosphate group. In the eighth reaction, a double bond is introduced, producing 1-phosphoenolpyruvic acid. No ATP is used or produced in reactions seven and eight.

In the ninth reaction, 1-phosphoenolpyruvic acid transfers its phosphate group to ADP, forming ATP. Because this occurs twice per glucose, two ATP molecules are produced.

Overall, glycolysis produces two molecules of pyruvic acid and four ATP molecules total, with a net gain of two ATP because two ATP were consumed earlier to begin the reactions of glycolysis. Pyruvic acid will eventually be converted into acetyl coA before entering the citric acid cycle, but this is not a part of glycolysis.

Citric Acid Cycle 

The citric acid cycle occurs after glycolysis. Glycolysis takes place around the outside of the mitochondria, whereas the Krebs cycle is the first stage of cellular respiration that occurs inside the mitochondria. Although the citric cycle does not directly consume oxygen, it depends on oxygen being available because oxygen is required to keep later stages of respiration running, which in turn regenerates necessary electron carriers. For that reason, it is considered an aerobic process. In this cycle, pyruvic acid is ultimately broken down to carbon dioxide while producing ATP and two different electron carriers.

Before the main cycle begins, each pyruvic acid produced by glycolysis is converted into acetyl coenzyme A. Because one glucose yields two pyruvic acid molecules, two acetyl coenzyme A molecules are formed. Each conversion produces one carbon dioxide and one NADH. This will be treated here as the initial reaction, after which acetyl CoA enters the main part of the cycle.

In the second reaction, acetyl coenzyme A  reacts with the four‑carbon molecule oxaloacetatamide to form the six‑carbon molecule alpha-citric acid. Next, in the third reaction, alpha-citric acid is rearranged into isolucitrate. In the fourth reaction, isolucitrate is oxidized and a carbon is released as carbon dioxide, forming the five‑carbon molecule ketagluterate and producing one NADH.

In the fifth reaction, ketagluterate is further oxidized, releasing another carbon dioxide and forming the four‑carbon molecule succanyl coenzyme B. This step produces another NADH. In the sixth reaction, succanyl coenzyme B is converted into succinate, and ATP is generated.

In the seventh reaction, succinate is converted into fomerate while FAD is reduced to FADH. Then, in the eighth reaction, fomerate is converted into malic acid. In the ninth and final reaction, malic acid is converted back into oxaloacetatamide, producing another NADH. The regenerated oxaloacetatamide can then combine with another acetyl coA to begin the next turn of the cycle.

For each pyruvic acid that enters (including the conversion to acetyl coA as reaction 1), the pathway produces four molecules of NADH, one molecule of FADH₂, three molecules of carbon dioxide, and one ATP. Since one glucose produces two pyruvic acid molecules, these totals double over two turns of the cycle.

Electron Transport Chain and Chemiosmosis

Earlier stages of cellular respiration convert one glucose molecule into pyruvic acid and then generate reduced electron carriers (NADH and FADH₂) that store high‑energy electrons. These reduced electron carriers feed electrons into the final stage of respiration: the electron transport chain and chemiosmosis, which occur in the inner mitochondrial membrane. Their main output is a large yield of ATP.

Electron Transport Chain

The electron transport chain begins when NADH donates electrons and hydrogen ions. The electrons enter an inner‑membrane protein complex, NADH dihydragenase, that oxidizes NADH back to NAD⁺. As the electrons move through this complex, hydrogen ions are pumped from the mitochondrial matrix into the intermembrane space, increasing hydrogen ion concentration there.

The electrons are then passed to a membrane‑embedded carrier, coenzyme 1Q. FADH₂ also donates electrons into the chain through this same carrier route, and is oxidized back to FAD for reuse. Next, electrons travel to cytachrome C reductionase. As electrons pass through this complex, additional hydrogen ions are pumped into the intermembrane space. The electrons are then transferred to cytachrome C, which carries them along the membrane surface to cytachrome C oxadase. As electrons move through cytachrome C oxadase, more hydrogen ions are pumped into the intermembrane space.

At the end of the chain, electrons are transferred to oxygen, which then combines with hydrogen ions to form water. Oxygen serves as the terminal electron acceptor, and this role explains why oxygen is required for sustained aerobic respiration; and this is the reason why humans and animals must respire oxygen into their bloodstream and then into their cells.

Chemiosmosis

Chemiosmosis uses the hydrogen ion concentration gradient created by the electron transport chain. Because hydrogen ions have accumulated in the intermembrane space, they tend to move back toward lower concentration in the matrix, but they cannot freely cross the inner membrane. They return through ATP synthase, a membrane protein that provides a pathway for hydrogen ions.

As hydrogen ions move through ATP synthatase, the released energy is used to add a phosphate group to ADP, forming ATP. The oxidation of the reduced electron carriers generated from one glucose molecule typically produces about 30–34 ATP through the electron transport chain and chemiosmosis. When combined with ATP produced in the earlier stages of  respiration, the total yield per glucose in aerobic respiration is commonly estimated at about 30–36 ATP.

Fermentation

Most organisms generate ATP from glucose using oxygen, but many can also produce ATP when oxygen is absent or scarce. Some plants, fungi, and many bacteria use aerobic respiration when oxygen is available, but switch to anaerobic pathways when oxygen levels drop. However, some bacteria rely only on anaerobic metabolism and may be harmed by the presence of oxygen. As such, these pathways occur without using oxygen. 

A major oxygen‑independent ATP generation strategy is fermentation. Fermentation includes glycolysis but does not use the later stages of aerobic respiration. Two common fermentation pathways are lactic acid fermentation and alcoholic fermentation. Although they produce different end products, both serve the same key function: converting NADH back into NAD⁺ so glycolysis can continue; without a constant supply of NAD⁺, glycolysis cannot proceed.

Lactic acid fermentation

In lactic acid fermentation, the glycolysis product pyruvic acid is converted into lactic acid. During this conversion, NADH is oxidized to NAD⁺. The regenerated NAD⁺ allows glycolysis to keep running, enabling continued ATP production without oxygen.

This pathway is used by bacteria involved in yogurt production (and when milk spoils - yuck). It is also used by human muscle cells during rapid, high‑intensity exercise. When muscle cells depend heavily on this pathway, lactic acid can accumulate, contributing to the tired, burning sensation that may follow strenuous activity.

Alcohol fermentation

In alcohol fermentation, pyruvic acid is converted into alcohol and carbon dioxide, while NADH is oxidized to NAD⁺. In bread dough, yeast fermentation releases carbon dioxide gas, which forms bubbles that expand the dough. After baking, the spaces left by these bubbles help give bread a lighter texture.

Overall outcome

The shared purpose of all fermentation pathways is to regenerate NAD⁺ from NADH, keeping glycolysis operational under low‑oxygen conditions. If glycolysis continues, a cell can obtain a net gain of two ATP per glucose molecule, which is far less than aerobic respiration but can be sufficient for short‑term survival of the cell.