Cellular respiration: flashcards, quiz and summary

Flashcards

1. 1What is the overall equation for aerobic cellular respiration, and what does it release?

   C6H12O6 + 6 O2 gives 6 CO2 + 6 H2O + energy, where the energy is captured as ATP and some is released as heat.

2. 2Why does glycolysis not need oxygen, yet still count as part of aerobic respiration?

   Glycolysis splits glucose in the cytoplasm without oxygen, so it can happen in aerobic and anaerobic conditions. It is still the first step of aerobic respiration, because its pyruvate continues into the mitochondrion when oxygen is present.

3. 3How does the electron transport chain make most of the ATP in cellular respiration?

   NADH and FADH2 give electrons to the chain. As electrons move down it, their energy pumps H+ ions into the space between the two membranes. The H+ ions then flow back through ATP synthase, and this flow drives the production of ATP (chemiosmosis).

4. 4Why does the electron transport chain stop when there is no oxygen?

   Oxygen is the final electron acceptor. It picks up electrons and H+ to form water. Without oxygen, electrons have nowhere to go, so the chain backs up and stops.

5. 5What happens to each pyruvate during pyruvate oxidation?

   Each pyruvate enters the mitochondrion and is converted into acetyl CoA. One CO2 is released per pyruvate, and NADH is made.

6. 6What does the Krebs cycle produce per glucose molecule?

   Two turns of the cycle produce 2 ATP, 6 NADH, 2 FADH2 and 4 CO2 per glucose.

7. 7What is the job of fermentation when oxygen is absent?

   Fermentation turns NADH back into NAD+. Glycolysis needs NAD+ to keep running, so fermentation allows the small ATP yield of glycolysis to continue even without oxygen.

8. 8Why do newer textbooks give a total of 30 to 32 ATP per glucose instead of 36 to 38?

   The lower number accounts for real losses in the process. The older 36 to 38 figure comes from earlier textbooks, so you may see both totals.

Practice quiz

Pick an answer to check it.

1. 1Which stage of cellular respiration takes place in the cytoplasm?

   1. Pyruvate oxidation
   2. Electron transport chain
   3. Krebs cycle
   4. Glycolysis
   Show the answer

   D. Glycolysis Glycolysis is the only stage that takes place in the cytoplasm. The other three stages take place in the mitochondrion.

2. 2What is the net ATP gain from glycolysis per glucose molecule?

   1. 4 ATP
   2. 36 ATP
   3. 2 ATP
   4. 30 ATP
   Show the answer

   C. 2 ATP Glycolysis uses 2 ATP to start and makes 4, so the net gain is 2 ATP. The 4 is a gross number, not the net gain.

3. 3What is the role of oxygen in cellular respiration?

   1. It is split to release energy directly as ATP during glycolysis
   2. It is the final electron acceptor that combines with electrons and H+ to form water
   3. It is released as CO2 at the end of the electron transport chain
   4. It is the molecule that glucose is broken down into during the Krebs cycle
   Show the answer

   B. It is the final electron acceptor that combines with electrons and H+ to form water Oxygen accepts electrons and H+ at the end of the electron transport chain to form water. Without it, the chain stops. CO2 comes from pyruvate oxidation and the Krebs cycle, not from oxygen.

4. 4Which process allows yeast to make ethanol when oxygen is absent?

   1. Lactic acid fermentation
   2. Krebs cycle
   3. Chemiosmosis
   4. Alcoholic fermentation
   Show the answer

   D. Alcoholic fermentation Alcoholic fermentation turns pyruvate into ethanol and CO2 in yeast. Lactic acid fermentation produces lactate, which is a different outcome.

5. 5Where does chemiosmosis take place, and what drives it?

   1. In the mitochondrial matrix, driven by the release of CO2
   2. In the inner mitochondrial membrane, driven by H+ flowing back through ATP synthase
   3. In the cytoplasm, driven by glucose splitting into pyruvate
   4. In the outer mitochondrial membrane, driven by NADH entering the cell
   Show the answer

   B. In the inner mitochondrial membrane, driven by H+ flowing back through ATP synthase Chemiosmosis happens at the inner mitochondrial membrane. The H+ gradient built by the electron transport chain drives H+ back through ATP synthase, which makes ATP.

6. 6What are the products of pyruvate oxidation?

   1. Acetyl CoA, CO2 and NADH
   2. Ethanol and CO2
   3. Citrate, ATP and water
   4. Lactate and NAD+
   Show the answer

   A. Acetyl CoA, CO2 and NADH Pyruvate oxidation turns each pyruvate into acetyl CoA, releasing one CO2 and making NADH. Citrate is formed later in the Krebs cycle, and lactate and ethanol come from fermentation.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing the net gain of 2 ATP from glycolysis with the gross 4 ATP made, since the 2 ATP used to start the process is subtracted.
• Saying glycolysis needs oxygen because it is the first step of aerobic respiration. Glycolysis does not need oxygen, though it is still part of aerobic respiration.
• Placing the Krebs cycle or pyruvate oxidation in the cytoplasm, or the electron transport chain in the matrix. Pyruvate oxidation and the Krebs cycle are in the matrix; the electron transport chain is in the inner membrane.
• Saying oxygen is used in the Krebs cycle. Oxygen's role is as the final electron acceptor in the electron transport chain.
• Mixing up breathing and cellular respiration. Breathing supplies oxygen and removes CO2, but respiration happens inside the cells.
• Quoting the ATP total as 36 to 38 without noting that newer textbooks give 30 to 32 as the more accurate figure, or the reverse. Use the number your teacher uses.

Summary

• Cellular respiration breaks glucose down to release its stored energy and capture it as ATP.
• Glycolysis happens in the cytoplasm, splits one glucose into two pyruvate, and gives a net gain of 2 ATP and 2 NADH.
• Pyruvate oxidation and the Krebs cycle happen in the mitochondrial matrix, release CO2, and produce NADH and FADH2.
• The electron transport chain in the inner mitochondrial membrane uses electrons from NADH and FADH2 to pump H+ ions and drive ATP synthase.
• Oxygen is the final electron acceptor, and without it the electron transport chain stops.
• The electron transport chain makes most of the ATP, roughly 26 to 28 per glucose, for about 30 to 32 ATP total with oxygen.
• Without oxygen, only glycolysis runs, and fermentation regenerates NAD+ so glycolysis can continue.

Key terms

Glycolysis

The first stage of cellular respiration, in the cytoplasm, where one glucose (6 carbons) is split into two pyruvate (3 carbons each), with a net gain of 2 ATP and 2 NADH.

Pyruvate oxidation

The step in the mitochondrial matrix where each pyruvate is turned into acetyl CoA, releasing one CO2 and making NADH.

Krebs cycle (citric acid cycle)

The cycle in the mitochondrial matrix where acetyl CoA joins a 4-carbon molecule to form citrate, which is broken back down, producing ATP, NADH, FADH2 and CO2.

Electron transport chain (ETC)

Protein complexes in the inner mitochondrial membrane that receive high-energy electrons from NADH and FADH2 and pass them along the chain, using their energy to pump H+ ions and build a gradient.

Chemiosmosis

The flow of H+ ions back through ATP synthase, which makes ATP. It is also called oxidative phosphorylation.

Final electron acceptor

Oxygen, which picks up electrons and H+ at the end of the electron transport chain to form water. Without it, the chain stops.

Fermentation

A process without oxygen that turns NADH back into NAD+ so glycolysis can keep running. The two main kinds are lactic acid fermentation and alcoholic fermentation.

ATP

The energy molecule cells spend on nearly every task. Cellular respiration captures energy from glucose in this form.

How to study it

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Questions