09: Cellular Respiration

What is the overall goal of cellular respiration?

To convert the chemical energy in food molecules (mainly glucose) into usable ATP energy that powers cellular work.

What type of metabolic process is cellular respiration?

It’s a catabolic, exergonic, redox process that breaks down complex molecules into simpler ones, releasing energy.

Write the overall balanced equation for aerobic cellular respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat)

How is respiration related to photosynthesis?

It’s essentially the reverse — respiration oxidizes glucose to CO₂, while photosynthesis reduces CO₂ to glucose.

Why is respiration considered a redox process?

Because it involves electron transfers — glucose is oxidized (loses e⁻ and H), and oxygen is reduced (gains e⁻ to form water).

What are the four main stages of aerobic respiration?

Glycolysis, Pyruvate Oxidation, Citric Acid Cycle (Krebs Cycle), and Oxidative Phosphorylation (ETC + Chemiosmosis).

Where does glycolysis occur?

In the cytoplasm of the cell — it does not require oxygen.

What is the carbon flow in glycolysis?

1 glucose (6C) → 2 pyruvate (3C each)

What are the two phases of glycolysis?

The Energy Investment Phase and the Energy Payoff Phase.

What happens during the energy investment phase?

2 ATP are used to phosphorylate glucose, forming 2 G3P (3C) molecules — this primes the molecule for energy extraction.

What happens during the energy payoff phase?

Each G3P is oxidized to pyruvate, generating 4 ATP and 2 NADH (net gain = 2 ATP and 2 NADH).

What type of reaction is glycolysis overall — endergonic or exergonic?

Exergonic overall — it releases more energy than it consumes.

What are the final products of glycolysis per glucose molecule?

2 pyruvate, 2 NADH, and 2 net ATP.

Why can glycolysis occur without oxygen?

It doesn’t require the mitochondria or O₂; NAD⁺ is regenerated by fermentation when O₂ is absent.

Where does pyruvate oxidation occur?

mitochondrial matrix.

What happens to pyruvate’s carbons during this step?

Each 3C pyruvate loses one carbon as CO₂, producing acetyl-CoA (2C).

What enzyme catalyzes pyruvate oxidation?

Pyruvate dehydrogenase.

What are the products of pyruvate oxidation per glucose molecule?

2 acetyl-CoA, 2 CO₂, and 2 NADH.

Is ATP produced directly during pyruvate oxidation?

No — energy is captured in NADH, not ATP.

How many of glucose’s original 6 carbons have been released as CO₂ by this point?

2 out of 6 carbons (one from each pyruvate).

Where does the Citric Acid Cycle occur?

In the mitochondrial matrix.

What is the starting molecule for each turn of the cycle?

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

What happens to carbon atoms in the cycle?

Two carbons are released as CO₂ per turn, completing the oxidation of glucose.

What are the major products of one turn of the Citric Acid Cycle?

3 NADH, 1 FADH₂, 1 ATP (via GTP), and 2 CO₂.

How many turns occur per glucose molecule?

Two turns (one per acetyl-CoA).

What is regenerated at the end of the cycle to allow it to continue?

Oxaloacetate (4C).

By the end of the Citric Acid Cycle, how many of glucose’s carbons have been released as CO₂?

All six.

Where is most of the energy from glucose now stored?

NADH and FADH₂, the electron carriers.

What two processes make up oxidative phosphorylation?

The Electron Transport Chain (ETC) and Chemiosmosis.

Where does oxidative phosphorylation occur?

Along the inner mitochondrial membrane.

What is the main role of the ETC?

To transfer electrons from NADH and FADH₂ to O₂, using their energy to pump H⁺ into the intermembrane space.

Why is oxygen essential for the ETC?

It acts as the final electron acceptor, forming H₂O. Without O₂, the chain halts and ATP synthesis stops.

What is the purpose of the proton (H⁺) gradient?

It stores potential energy used to power ATP synthase.

What is chemiosmosis?

The movement of protons down their gradient through ATP synthase, coupling exergonic diffusion to endergonic ATP production.

How does ATP synthase work?

It spins like a molecular turbine as H⁺ flows through, physically joining ADP and Pi to make ATP.

Why does the ETC use a series of steps instead of releasing energy all at once?

To prevent energy loss as heat and to efficiently capture energy in the proton gradient.

How many ATP are produced per glucose molecule in aerobic respiration (theoretical max)?

About 30–32 ATP total (2 from glycolysis, 2 from citric acid cycle, ~26–28 from oxidative phosphorylation).

How many CO₂ molecules are released per glucose molecule?

6 CO₂ total — 2 from pyruvate oxidation, 4 from the Citric Acid Cycle.

At what points is NADH produced during respiration?

Glycolysis, Pyruvate Oxidation, and Citric Acid Cycle.

At what points is FADH₂ produced?

Only during the Citric Acid Cycle.

Why is NADH worth more ATP than FADH₂?

NADH donates electrons earlier in the ETC, driving more proton pumping and ATP synthesis.

How are carbons, electrons, and energy connected in respiration?

Carbon atoms are oxidized → electrons transferred to carriers → energy from electrons used to pump protons → proton gradient drives ATP synthesis.

Why is respiration stepwise instead of one big reaction?

Controlled oxidation allows energy capture in small, usable amounts instead of wasted heat.

How does the mitochondrion’s structure support its function?

The folded inner membrane increases surface area for ETC and ATP synthase, maximizing ATP production.

How are photosynthesis and respiration connected in ecosystems?

Photosynthesis stores energy in glucose; respiration releases that energy for biological work — forming an energy cycle.