Cellular Respiration: Processes, Key Molecules, and Energy Production

What is the overall purpose of cellular respiration?

To break down food molecules (like glucose) and harvest energy in the form of ATP for cellular processes.

What is ATP's role in cells?

It acts as an energy currency, transferring energy from catabolic to anabolic reactions.

What is carbohydrate catabolism?

The breakdown of carbohydrate molecules (mainly glucose) to release energy stored in chemical bonds.

What is catabolism in general?

The metabolic breakdown of large molecules into smaller ones, releasing energy.

Why do we need to eat?

To obtain organic molecules that can be broken down to produce ATP.

Cells have two main options for glucose catabolism — what are they?

Aerobic respiration (requires O₂) and anaerobic fermentation (no O₂).

Which glucose catabolic pathway yields more energy?

Aerobic respiration.

List three things aerobic respiration and fermentation have in common.

1️⃣ Both start with glycolysis. 2️⃣ Both involve oxidation/reduction reactions. 3️⃣ Both produce ATP (though at different yields).

List three differences between aerobic respiration and fermentation.

1️⃣ Aerobic uses oxygen; fermentation doesn't. 2️⃣ Aerobic yields ~36 ATP; fermentation yields 2 ATP. 3️⃣ Aerobic fully oxidizes glucose; fermentation partially oxidizes it.

What is oxidation? → The loss of electrons (or hydrogens) from a molecule.

What is oxidation?

The loss of electrons (or hydrogens) from a molecule.

In biology, oxidation often involves what?

The loss of hydrogen atoms, which represent stored energy.

What is reduction?

The gain of electrons (or hydrogens).

Why do oxidation and reduction always occur together?

Because when one molecule loses electrons, another must gain them — forming a redox pair.

Example redox reaction:

AH + B → A + BH → Here, A is oxidized, B is reduced.

What molecule is oxidized in cellular respiration?

Glucose.

What molecule is reduced in cellular respiration?

Oxygen (O₂) → forms H₂O.

What is a redox reaction?

A reaction involving both oxidation and reduction processes.

Which molecule is the most oxidized? Glucose, lactic acid, ethanol, or CO₂?

CO₂ (fully oxidized carbon).

Which molecules are products of fermentation?

Lactic acid or ethanol + CO₂.

Which molecules are products of aerobic respiration?

CO₂ and H₂O.

Why do cells make NADH and FADH₂?

They temporarily store energy released from glucose oxidation.

Are NADH and FADH₂ oxidized or reduced forms?

They are the reduced forms (store high-energy electrons).

What happens to NADH and FADH₂ at the end of cellular respiration?

They donate electrons to the electron transport chain, producing ATP.

When NADH donates electrons, what is regenerated?

NAD⁺, which can be reused in glycolysis and other reactions.

The Four Stages of Aerobic Respiration

1️⃣ Glycolysis (in cytoplasm) 2️⃣ Pyruvate oxidation (in mitochondrial matrix) 3️⃣ Citric acid cycle (Krebs cycle) (in mitochondrial matrix) 4️⃣ Electron transport chain + ATP synthesis (inner mitochondrial membrane)

Where does glycolysis occur?

In the cytoplasm of both eukaryotes and prokaryotes.

Does glycolysis require oxygen?

No, it is anaerobic.

Inputs of glycolysis:

1 Glucose, 2 NAD⁺, 2 ADP + 2 Pi.

Outputs of glycolysis:

2 Pyruvate, 2 NADH, 2 ATP (net gain).

How many total ATP are produced in glycolysis?

4 ATP (2 net gain).

Why are there so many steps in glycolysis?

So energy can be released gradually and captured in usable chemical forms (ATP, NADH) instead of lost as heat.

What is the purpose of glycolysis?

To partially oxidize glucose and generate ATP and NADH.

If O₂ is available after glycolysis, what happens next?

Pyruvate oxidation → Citric Acid Cycle → ETC.

If O₂ is absent after glycolysis, what happens next?

Fermentation.

Where does pyruvate oxidation occur?

In the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes).

What happens during pyruvate oxidation?

Each pyruvate (3C) is converted to acetyl-CoA (2C) and CO₂ is released.

Inputs of pyruvate oxidation:

2 Pyruvate, 2 CoA, 2 NAD⁺.

Outputs of pyruvate oxidation:

2 Acetyl-CoA, 2 CO₂, 2 NADH.

How many carbons from glucose remain in acetyl-CoA?

Two carbons per pyruvate (four total per glucose).

What molecule now carries energy into the Citric Acid Cycle?

Acetyl-CoA.

Where does the citric acid cycle occur?

In the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes).

Inputs of the citric acid cycle (per glucose):

2 Acetyl-CoA, 6 NAD⁺, 2 FAD, 2 ADP + 2 Pi.

Outputs (per glucose):

4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.

Does the citric acid cycle require oxygen directly?

No, but it depends on oxygen indirectly (ETC must regenerate NAD⁺ and FAD).

What makes it a cycle?

The starting molecule oxaloacetate is regenerated at the end of each turn.

How many turns of the citric acid cycle per glucose?

Two turns (one per acetyl-CoA).

How is energy stored during the citric acid cycle?

In the reduced coenzymes NADH and FADH₂.

Where does the electron transport chain occur?

Inner mitochondrial membrane.

What are the inputs of the ETC?

NADH, FADH₂, O₂, ADP + Pi.

What are the outputs of the ETC?

NAD⁺, FAD, H₂O, ATP.

Where did the energy in NADH and FADH₂ originally come from?

From oxidation of glucose during glycolysis and the citric acid cycle.

What is the purpose of the ETC?

To use energy from electrons to pump protons (H⁺) across the membrane, creating a gradient.

What is this proton gradient called?

The proton-motive force.

What process uses the proton gradient to make ATP?

Chemiosmosis.

What enzyme synthesizes ATP?

ATP synthase.

How many ATP molecules are generated (approximately) by oxidative phosphorylation?

About 26-28 ATP per glucose.

What role does oxygen play in the ETC?

It is the final electron acceptor, combining with electrons and protons to form H₂O.

What happens if oxygen is not available?

The ETC stops, NADH and FADH₂ can't unload electrons, and ATP production halts

When does fermentation occur?

When oxygen is unavailable after glycolysis.

What is the purpose of fermentation?

To regenerate NAD⁺ so glycolysis can continue.

Does fermentation produce additional ATP?

No — it only regenerates NAD⁺; ATP comes solely from glycolysis.

What are the two main types of fermentation?

Lactic acid fermentation and ethanol (alcohol) fermentation.

Lactic acid fermentation occurs in...

Animals (muscle cells during oxygen debt) and some bacteria.

Ethanol fermentation occurs in...

Yeast and some plant cells.

End products of lactic acid fermentation:

Lactic acid + NAD⁺.

End products of ethanol fermentation:

Ethanol + CO₂ + NAD⁺.

How does fermentation differ from aerobic respiration?

It produces no additional ATP, uses no oxygen, and does not oxidize glucose completely.

Why does the cell need fermentation at all?

To regenerate NAD⁺, allowing glycolysis (and thus ATP production) to continue temporarily.

Can cells harvest energy from other molecules besides carbohydrates?

Yes — fats, proteins, and other biomolecules can enter the pathway at various steps (e.g., acetyl-CoA, pyruvate).

What molecule links catabolism of carbohydrates, fats, and proteins?

Acetyl-CoA.

How does fat enter the respiration pathway?

Fatty acids are broken into acetyl-CoA units via beta-oxidation.

How do proteins enter the respiration pathway?

Amino acids are deaminated and their carbon skeletons enter as intermediates (e.g., pyruvate, acetyl-CoA).

Why is cellular respiration stepwise instead of one reaction?

Stepwise oxidation releases energy gradually, which can be captured efficiently in ATP rather than lost as heat.

In biology, oxidation often involves what?

The loss of hydrogen atoms, which represent stored energy.

What is reduction?

The gain of electrons (or hydrogens).

Why do oxidation and reduction always occur together?

Because when one molecule loses electrons, another must gain them — forming a redox pair.

Example redox reaction:

AH + B → A + BH → Here, A is oxidized, B is reduced.

What molecule is oxidized in cellular respiration?

Glucose.

What molecule is reduced in cellular respiration?

Oxygen (O₂) → forms H₂O.

What is a redox reaction?

A reaction involving both oxidation and reduction processes.

Which molecule is the most oxidized? Glucose, lactic acid, ethanol, or CO₂?

CO₂ (fully oxidized carbon).

Which molecules are products of fermentation?

Lactic acid or ethanol + CO₂.

Which molecules are products of aerobic respiration?

CO₂ and H₂O.