Cellular Respiration

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Cellular respiration is the process by which cells break down glucose and other organic molecules to produce ATP (adenosine triphosphate), the energy currency of cells. This process occurs in all living organisms and is essential for life.

Cellular respiration is a series of metabolic reactions that convert biochemical energy from nutrients (primarily glucose) into ATP, releasing waste products (carbon dioxide and water) in the process.

The Overall Equation

The complete equation for aerobic cellular respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (approximately 36-38 ATP)

In words: Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)

The Three Stages of Cellular Respiration

Stage	Location	Input	Output	ATP Produced
Glycolysis	Cytoplasm	1 Glucose (6C)	2 Pyruvate (3C each), 2 NADH	2 ATP (net)
Krebs Cycle	Mitochondrial Matrix	2 Acetyl-CoA (2C each)	6 NADH, 2 FADH₂, 4 CO₂	2 ATP
Electron Transport Chain	Inner Mitochondrial Membrane	NADH, FADH₂, O₂	H₂O, ATP	32-34 ATP

Stage 1: Glycolysis

Glycolysis ("sugar splitting") occurs in the cytoplasm and does not require oxygen (anaerobic). Key points:

One 6-carbon glucose molecule is split into two 3-carbon pyruvate molecules
Uses 2 ATP but produces 4 ATP (net gain of 2 ATP)
Produces 2 NADH molecules (electron carriers)
Can occur with or without oxygen present

Stage 2: The Krebs Cycle (Citric Acid Cycle)

The Krebs Cycle occurs in the mitochondrial matrix and requires oxygen indirectly:

Pyruvate is first converted to Acetyl-CoA (releases 1 CO₂ per pyruvate)
Acetyl-CoA enters the cycle and is completely oxidized
Each turn produces: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂
The cycle turns twice per glucose (once for each pyruvate)

Stage 3: Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is located on the inner mitochondrial membrane and produces the most ATP:

NADH and FADH₂ donate electrons to protein complexes
Electrons move through the chain, releasing energy
Energy pumps H⁺ ions across the membrane (chemiosmosis)
H⁺ ions flow back through ATP synthase, producing ATP
Oxygen is the final electron acceptor, forming water

Aerobic vs. Anaerobic Respiration

Aerobic respiration requires oxygen and produces 36-38 ATP per glucose. Anaerobic respiration (fermentation) occurs without oxygen and produces only 2 ATP per glucose. In humans, lactic acid fermentation occurs during intense exercise when oxygen supply is limited.

SAT/ACT Connection

Science passages may ask you to interpret data about ATP production, compare aerobic and anaerobic processes, or analyze graphs showing respiration rates under different conditions. Understanding the inputs and outputs of each stage is essential.

Examples

Example 1: Calculating Total ATP

Problem: Calculate the maximum ATP produced from one glucose molecule undergoing complete aerobic respiration.

Step 1: Glycolysis produces 2 ATP directly.

Step 2: Krebs Cycle produces 2 ATP (1 per turn x 2 turns).

Step 3: Electron Transport Chain: 10 NADH x 3 ATP = 30 ATP; 2 FADH₂ x 2 ATP = 4 ATP.

Step 4: Total = 2 + 2 + 30 + 4 = 38 ATP maximum (actual yield is often 36 due to transport costs).

Answer: Approximately 36-38 ATP per glucose molecule.

Example 2: Identifying the Stage

Problem: A scientist observes a cell producing carbon dioxide but no oxygen is being consumed. Which stage of respiration is occurring, and where?

Step 1: Identify that CO₂ is being produced, which happens in the Krebs Cycle and during pyruvate conversion.

Step 2: Note that O₂ is not being consumed - this means the ETC is not running.

Step 3: However, the Krebs Cycle indirectly needs O₂ to regenerate NAD⁺. Without ETC, NADH accumulates.

Step 4: This scenario is unusual - likely observing early stages before O₂ depletion stops the Krebs Cycle.

Answer: The observation suggests pyruvate oxidation and early Krebs Cycle activity, occurring in the mitochondrial matrix.

Example 3: Comparing Energy Efficiency

Problem: How much more efficient is aerobic respiration compared to fermentation?

Step 1: Aerobic respiration produces approximately 36-38 ATP per glucose.

Step 2: Fermentation (anaerobic) produces only 2 ATP per glucose.

Step 3: Calculate the ratio: 36 / 2 = 18.

Answer: Aerobic respiration is approximately 18 times more efficient than fermentation in terms of ATP production.

Example 4: Role of Oxygen

Problem: Explain why cells die quickly without oxygen, even though glycolysis doesn't require oxygen.

Step 1: Glycolysis produces 2 ATP without oxygen, which seems like it could sustain cells.

Step 2: However, glycolysis requires NAD⁺ to accept electrons.

Step 3: Without oxygen, the ETC cannot regenerate NAD⁺ from NADH.

Step 4: Fermentation can regenerate some NAD⁺, but only 2 ATP per glucose is insufficient for most cells.

Answer: Cells die without oxygen because the ETC cannot regenerate NAD⁺, and the 2 ATP from glycolysis/fermentation cannot meet cellular energy demands.

Example 5: Interpreting a Respiration Experiment

Problem: A student measures CO₂ production in yeast under two conditions: with oxygen and without oxygen. With oxygen, 6 mL of CO₂ is produced per gram of glucose. Without oxygen, 22 mL is produced. Explain this result.

Step 1: With oxygen (aerobic): Complete oxidation produces 6 CO₂ per glucose.

Step 2: Without oxygen: Yeast performs alcoholic fermentation.

Step 3: Fermentation produces 2 ATP per glucose vs. ~36 for aerobic respiration.

Step 4: To meet energy needs, yeast must process much more glucose anaerobically.

Step 5: Each fermented glucose produces 2 CO₂ (not 6), but many more glucose molecules are processed.

Answer: The higher CO₂ production without oxygen indicates that yeast must process more glucose through fermentation to meet energy needs, since fermentation is less efficient than aerobic respiration.

Practice

Test your understanding of cellular respiration with these questions.

1. Where does glycolysis occur in the cell?

A) Mitochondrial matrix B) Inner mitochondrial membrane C) Cytoplasm D) Nucleus

2. What is the final electron acceptor in the electron transport chain?

A) Carbon dioxide B) NADH C) Oxygen D) Glucose

3. Which stage of cellular respiration produces the most ATP?

A) Glycolysis B) Krebs Cycle C) Pyruvate oxidation D) Electron Transport Chain

4. What is the net ATP gain from glycolysis?

A) 0 ATP B) 2 ATP C) 4 ATP D) 36 ATP

5. During which process is carbon dioxide released?

A) Glycolysis only B) ETC only C) Krebs Cycle and pyruvate oxidation D) All three stages

6. What happens to NADH produced during glycolysis and the Krebs Cycle?

A) It is excreted as waste B) It donates electrons to the ETC C) It is converted to glucose D) It stores energy permanently

7. A cell is placed in an environment with no oxygen. Which process can still occur?

A) Krebs Cycle B) Electron Transport Chain C) Oxidative phosphorylation D) Glycolysis

8. What is the role of ATP synthase?

A) Break down glucose B) Transport electrons C) Use H⁺ gradient to produce ATP D) Accept electrons from NADH

9. How many turns of the Krebs Cycle are needed to fully process one glucose molecule?

A) 1 turn B) 2 turns C) 3 turns D) 6 turns

10. What product of cellular respiration is used by plants during photosynthesis?

A) ATP B) Glucose C) Oxygen D) Carbon dioxide

Click to reveal answers

C) Cytoplasm - Glycolysis occurs in the cytoplasm and does not require a mitochondrion.
C) Oxygen - Oxygen accepts electrons at the end of the ETC and combines with H⁺ to form water.
D) Electron Transport Chain - The ETC produces 32-34 ATP, far more than any other stage.
B) 2 ATP - Glycolysis uses 2 ATP and produces 4 ATP, for a net gain of 2 ATP.
C) Krebs Cycle and pyruvate oxidation - CO₂ is released when pyruvate becomes Acetyl-CoA and during the Krebs Cycle.
B) It donates electrons to the ETC - NADH carries high-energy electrons to the ETC where energy is extracted to make ATP.
D) Glycolysis - Glycolysis is anaerobic and can occur without oxygen.
C) Use H⁺ gradient to produce ATP - ATP synthase allows H⁺ ions to flow down their gradient, using the energy to phosphorylate ADP to ATP.
B) 2 turns - Each glucose produces 2 pyruvate, which become 2 Acetyl-CoA, requiring 2 turns of the Krebs Cycle.
D) Carbon dioxide - Plants use CO₂ from respiration (and the atmosphere) as a carbon source for photosynthesis.

Check Your Understanding

1. Explain why the mitochondrion is called the "powerhouse of the cell."

Reveal Answer

The mitochondrion is called the powerhouse because it is where most ATP production occurs. While glycolysis (in the cytoplasm) produces only 2 ATP, the Krebs Cycle and Electron Transport Chain within the mitochondria produce the remaining 34-36 ATP. The mitochondria's structure (double membrane creating compartments) is essential for establishing the proton gradient that drives ATP synthesis.

2. Compare and contrast photosynthesis and cellular respiration in terms of their reactants and products.

Reveal Answer

Photosynthesis and cellular respiration are essentially reverse processes. Photosynthesis uses CO₂ and H₂O (plus light energy) to produce glucose and O₂. Cellular respiration uses glucose and O₂ to produce CO₂, H₂O, and ATP. Together, they form a cycle: the products of one process are the reactants of the other. Photosynthesis stores energy in glucose bonds; respiration releases that energy.

3. Why do you breathe more heavily during intense exercise?

Reveal Answer

During intense exercise, muscle cells require more ATP for contraction. To meet this demand, cellular respiration must speed up, which requires more oxygen (for the ETC) and produces more carbon dioxide. You breathe heavily to: (1) bring in more O₂ to serve as the final electron acceptor, and (2) expel the increased CO₂ produced. If oxygen supply cannot meet demand, muscles switch to anaerobic fermentation, producing lactic acid and causing fatigue.

4. How would a poison that blocks ATP synthase affect cellular respiration?

Reveal Answer

If ATP synthase is blocked, H⁺ ions cannot flow back through it to produce ATP. This would: (1) halt ATP production from the ETC (losing ~34 ATP), (2) cause H⁺ to accumulate, eventually stopping the ETC because the gradient becomes too strong to pump against, (3) stop the regeneration of NAD⁺ and FAD, which would eventually halt glycolysis and the Krebs Cycle as well. The cell would die from ATP depletion.

🚀 Next Steps

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