Stages of Cellular Respiration

Essential Question

How do glycolysis, the Krebs cycle, and the electron transport chain produce ATP?

Cellular respiration does not release the energy of glucose all at once. It does it in three stages, and each one has its own location, inputs, and outputs. In this lesson, you’ll follow glucose through glycolysis, the Krebs cycle, and the electron transport chain, track where the carbon and the ATP go, and see what happens when one stage is blocked.

Three Stages, Three Places

• Glycolysis: cytoplasm, needs no O₂
• Krebs cycle: mitochondrial matrix
• Electron transport chain: inner membrane
• NADH and FADH₂ carry energy to the last stage

Cellular respiration has three stages. Glycolysis happens in the cytoplasm and splits glucose in half. The two halves, called pyruvate, move into the mitochondrion. There, the Krebs cycle in the matrix breaks the carbon compounds down further and releases CO₂. Finally, the electron transport chain in the inner membrane uses oxygen and makes most of the ATP. The carriers NADH and FADH₂ move energy from the first two stages to the last. This lesson focuses on inputs, outputs, locations, and approximate ATP.

Stage 1: Glycolysis

• Where: cytoplasm
• Inputs: glucose (6 C), 2 ATP, to start
• Outputs: 2 pyruvate (3 C each), 4 ATP, 2 NADH
• Net gain: about 2 ATP; no O₂ needed

Glycolysis means “splitting sugar.” It uses 2 ATP to get started, then makes 4 ATP, for a net gain of 2 ATP. It splits one 6-carbon glucose into two 3-carbon pyruvate molecules and makes 2 NADH. Glycolysis does not need oxygen, and it happens in the cytoplasm of almost every living cell. It is thought to be one of the oldest energy pathways, since it is found in nearly all organisms.

Stage 2: The Krebs Cycle

• Where: mitochondrial matrix
• Inputs: 2-carbon groups from pyruvate, NAD⁺, FAD
• Outputs: CO₂, NADH, FADH₂, about 2 ATP
• All of glucose’s carbon ends up as CO₂

Pyruvate enters the mitochondrion and loses one carbon as CO₂, leaving a 2-carbon group. The Krebs cycle then breaks down the 2-carbon groups and releases their carbon as CO₂. It also makes more NADH and FADH₂, and a small amount of ATP (about 2 per glucose). The CO₂ you breathe out comes from these steps. By the end of the cycle, every carbon atom that was in glucose has left in a CO₂ molecule.

Stage 3: The Electron Transport Chain

• Where: inner mitochondrial membrane
• Inputs: NADH, FADH₂, O₂, ADP and phosphate
• Outputs: H₂O, NAD⁺ and FAD, about 26 to 28 ATP
• Without O₂, this stage stops

The electron transport chain is in the folded inner membrane, where the large surface area of the cristae gives room for many copies of it. It uses the energy carried by NADH and FADH₂ to make most of the cell’s ATP, about 26 to 28 per glucose. Oxygen is the final acceptor at the end of the chain, and it combines with hydrogen to form water (H₂O). This is why we need to breathe oxygen. Without O₂, the chain backs up and stops, and NAD⁺ and FAD are no longer returned to the earlier stages.

Where Does the ATP Come From?

• Glycolysis: about 2 ATP
• Krebs cycle: about 2 ATP
• Electron transport chain: about 26 ATP (model)
• Total: about 30 ATP per glucose

The exact ATP yield varies from cell to cell, and textbooks give estimates from about 30 to 32 ATP per glucose. This course uses a model value of 26 for the electron transport chain, so the total is about 30 ATP: 2 + 2 + 26. Almost 90% of the ATP comes from the last stage, which is the only stage that uses oxygen directly. Without oxygen, the chain stops, and the Krebs cycle soon stops too because NAD+ and FAD are no longer returned to it. That is why cells with plenty of oxygen get so much more energy from glucose than cells that cannot use oxygen.

↑ Back to top

---

Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Percent of ATP From Each Stage

In the model, glycolysis makes 2 ATP, the Krebs cycle 2 ATP, and the electron transport chain 26 ATP per glucose. (a) Find the total. (b) Find the percent made in each stage. (c) Which stage would a cell lose if it had no oxygen, and about how much ATP would it lose?

Stage	ATP per glucose (model)
Glycolysis	2
Krebs cycle	2
Electron transport chain	26

Show the solution

1. (a) Total: 2 + 2 + 26 = 30 ATP
2. (b) Percents: glycolysis: 2 ÷ 30 × 100 = 6.7%. Krebs cycle: 2 ÷ 30 × 100 = 6.7%. Electron transport chain: 26 ÷ 30 × 100 = 86.7%.
3. Check: 6.7% + 6.7% + 86.7% = 100.1%, which is 100% after rounding.
4. (c) Without O₂: the electron transport chain stops, so the cell loses about 26 of its 30 ATP (about 87%).
5. Answer: 30 ATP in all. 6.7%, 6.7%, and 86.7%. Without oxygen, about 26 ATP are lost.

Example 2: Follow the Carbon

Follow the 6 carbon atoms of one glucose molecule through cellular respiration. (a) How many carbons are in each pyruvate? (b) One carbon is lost from each pyruvate before the Krebs cycle. How many CO₂ is that? (c) The Krebs cycle releases the remaining carbons. How many more CO₂ are made, and how many in all?

Show the solution

1. (a) Glycolysis: 1 glucose (6 C) → 2 pyruvate. 6 ÷ 2 = 3 carbons in each pyruvate.
2. (b) Pyruvate to 2-carbon group: each pyruvate loses 1 C as CO₂, so 2 × 1 = 2 CO₂. Each pyruvate becomes a 2-carbon group (2 × 2 = 4 C left).
3. (c) Krebs cycle: the 4 remaining carbons are released as 4 CO₂. Total: 2 + 4 = 6 CO₂.
4. Check: 6 carbons in glucose, 6 carbons in 6 CO₂. Carbon atoms are conserved.
5. Answer: 3 carbons in each pyruvate; 2 CO₂ before the Krebs cycle, 4 in it, 6 in all. All the carbon leaves as CO₂.

Example 3: Blocking the Electron Transport Chain

Cyanide blocks the electron transport chain. Cells were given cyanide at 7 minutes, and ATP production was measured (sample data). (a) By what percent did ATP production fall? (b) Which stage kept working, and what does the leftover ATP production tell you?

Show the solution

1. (a) Read the graph: ATP production went from 100% to 7%, a drop of 100 − 7 = 93 percentage points, so it fell by 93%.
2. (b) Compare with the model: glycolysis makes 2 of the 30 ATP, which is 2 ÷ 30 × 100 = 6.7%, close to the 7% left.
3. Reasoning: glycolysis does not need the electron transport chain or oxygen, so it kept working. The cells lost the Krebs cycle and the chain, because without the chain, NADH is not recycled to NAD⁺.
4. Answer: ATP production fell by about 93%. Glycolysis kept working, and that is about 7% of the normal ATP.

↑ Back to top

---

Practice

Name the inputs, outputs, and locations of each stage, calculate the percent of ATP from each stage, and explain what happens when a stage is blocked. Try it on your own first, then check your work with the answer key.

• Stages of Cellular Respiration Practice Worksheet (PDF)
• Stages of Cellular Respiration Practice Answer Key (PDF)

↑ Back to top

---

Lesson Slides

• 8.5 Stages of Cellular Respiration Slides (PDF)

↑ Back to top

---

Big Ideas

• Glycolysis: cytoplasm, 2 ATP, no O₂ needed
• Krebs cycle: matrix, releases CO₂, makes NADH and FADH₂, about 2 ATP
• Electron transport chain: inner membrane, uses O₂, makes H₂O and about 26 ATP
• About 30 ATP per glucose (model)
• No O₂: the chain stops and most ATP is lost

Back to the essential question: glycolysis, the Krebs cycle, and the electron transport chain each have their own location, inputs, and outputs. Glycolysis makes about 2 ATP without oxygen. The Krebs cycle releases the carbon of glucose as CO₂. The electron transport chain uses oxygen to make most of the ATP, about 26 of about 30. Next, you’ll find out what cells do when there is no oxygen at all.

↑ Back to top

---

↑ Back to top