Lesson 8- Cellular Respiration (Citric Acid Cycle, Electron Transport System)

Question 1

What is the purpose of cellular respiration?

Answer 1

Cellular respiration breaks down glucose to generate ATP, the cell’s primary energy source.

Question 2

What are the four stages of cellular respiration?

Answer 2

1. Glycolysis – Glucose → 2 Pyruvate + 2 ATP, 2 NADH (Occurs in cytoplasm).
2. Preparatory Step – Pyruvate → Acetyl-CoA + CO₂, NADH (Occurs in mitochondria).
3. Citric Acid Cycle – Acetyl-CoA → ATP, NADH, FADH2, CO₂ (Occurs in mitochondrial matrix).
4. Electron Transport System (ETS) & Oxidative Phosphorylation – Uses NADH & FADH2 to make most ATP (Occurs in inner mitochondrial membrane).

Question 3

Where does the citric acid cycle occur?

Answer 3

In the mitochondrial matrix.

Question 4

Why does the citric acid cycle occur twice for each glucose molecule?

Answer 4

Because one glucose molecule produces two Acetyl-CoA molecules, and each enters the cycle separately.

Question 5

What is the starting molecule of the citric acid cycle?

Answer 5

Acetyl-CoA (2C), which combines with oxaloacetate (4C) to form citrate (6C)

Question 6

What happens in the first step of the citric acid cycle?

Answer 6

Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

Question 7

What happens when citrate is converted into α-ketoglutarate?

Answer 7

One CO₂ is released, and NADH is produced.

Question 8

What happens when α-ketoglutarate is converted into succinate?

Answer 8

One ATP, one NADH, and another CO₂ are produced

Question 9

What happens in the final steps of the citric acid cycle?

Answer 9

Succinate → Fumarate (producing FADH2), then Fumarate → Oxaloacetate (producing NADH).

Question 10

What are the total products of the citric acid cycle per Acetyl-CoA?

Answer 10

• 3 NADH
• 1 FADH2
• 1 ATP
• 2 CO₂

Question 11

What are the total products of the citric acid cycle per glucose molecule?

Answer 11

• 6 NADH
• 2 FADH2
• 2 ATP
• 4 CO₂

Question 12

What happens to the NADH and FADH2 produced in the citric acid cycle?

Answer 12

They carry high-energy electrons to the electron transport system (ETS) to generate more ATP.

Question 13

Where does the electron transport system (ETS) occur?

Answer 13

In the inner mitochondrial membrane.

Question 14

What is the main purpose of the ETS?

Answer 14

To use NADH and FADH2 to create a proton gradient that drives ATP production through oxidative phosphorylation.

Question 15

What molecule acts as the final electron acceptor in the ETS?

Answer 15

Oxygen (O₂), which combines with H+ and electrons to form water (H₂O)

Question 16

What happens first in the ETS?

Answer 16

NADH and FADH2 donate electrons to the electron transport chain (ETC).

Question 17

What happens to the electrons as they pass through the ETC?

Answer 17

They lose energy, which is used to pump H+ ions into the intermembrane space, creating a proton gradient.

Question 18

How is ATP produced in the ETS?

Answer 18

H+ ions diffuse back into the mitochondrial matrix through ATP synthase, which uses this energy to convert ADP + Pi → ATP.

Question 19

What is the final step of the ETS?

Answer 19

lectrons and H+ combine with oxygen to form H₂O

Question 20

How many ATP molecules are produced in the ETS?

Answer 20

34 ATP molecules.

Question 21

How many water molecules are produced in the ETS?

Answer 21

6 H₂O molecules.

Question 22

What happens to NADH and FADH2 after the ETS?

Answer 22

They are oxidized back into NAD+ and FAD, which are recycled for reuse in earlier steps of cellular respiration.

Question 23

What is the total ATP yield from one glucose molecule?

Answer 23

Stage ATP Yield
Glycolysis 2 ATP
Citric Acid Cycle 2 ATP
Electron Transport System 34 ATP
Total ATP 38 ATP (Net: 36 ATP due to transport costs)

Question 24

What are the three main macromolecules used for energy?

Answer 24

Glycogen, fats, and proteins

Question 25

Which macromolecule provides the most energy?

Answer 25

Fats (~78% of body’s energy reserves) because they produce twice as much ATP as glucose.

Question 26

How are fats used for energy?

Answer 26

• Triglycerides are broken down into glycerol and fatty acids. • Glycerol is converted into glucose or pyruvate for glycolysis. • Fatty acids are converted into Acetyl-CoA, entering the citric acid cycle.

Question 27

How are proteins used for energy?

Answer 27

• Amino acids are deaminated (NH₂ removed) and converted into intermediates of the citric acid cycle. • The removed amine group is converted into urea and excreted in urine. • Protein breakdown increases during starvation, leading to muscle loss.

Question 28

What happens if oxygen is not available?

Answer 28

Glycolysis is the only ATP source, leading to anaerobic metabolism.

Question 29

What happens to pyruvate in anaerobic metabolism?

Answer 29

It is converted into lactic acid, since it cannot enter the citric acid cycle.

Question 30

How much ATP is produced in anaerobic respiration?

Answer 30

Only 2 ATP per glucose (compared to 36 ATP in aerobic respiration).

Question 31

What are the consequences of anaerobic metabolism?

Answer 31

Lactic acid buildup, leading to muscle fatigue.

Question 32

Final summary of cellular respiration

Answer 32

1. Glycolysis → Glucose → Pyruvate + 2 ATP + 2 NADH. 2. Preparatory Step → Pyruvate → Acetyl-CoA + CO₂. 3. Citric Acid Cycle → Acetyl-CoA → ATP, NADH, FADH2, CO₂. 4. Electron Transport System → NADH & FADH2 → 34 ATP + H₂O.