Cellular Respiration

What is the main purpose of cellular respiration?

To convert the chemical energy of glucose into ATP that powers cellular work.

Write the balanced equation for cellular respiration.

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

Is cellular respiration endergonic or exergonic?

Exergonic (ΔG = -686 kcal/mol), energy is released.

What are the main stages of cellular respiration?

1. Glycolysis 2. Pyruvate Oxidation 3. Krebs Cycle 4. Electron Transport Chain & Chemiosmosis

Where does cellular respiration occur?

Glycolysis → Cytoplasm; Link Reaction & Krebs Cycle → Mitochondrial Matrix; ETC → Inner Mitochondrial Membrane

What types of phosphorylation generate ATP?

Substrate-level phosphorylation (glycolysis & Krebs) and oxidative phosphorylation (ETC/chemiosmosis)

Define oxidation and reduction.

Oxidation: Loss of electrons or hydrogen; gain of oxygen. Reduction: Gain of electrons or hydrogen; loss of oxygen.

What are oxidizing and reducing agents?

Oxidizing agent gains electrons (is reduced); reducing agent loses electrons (is oxidized).

In cellular respiration, what is oxidized and what is reduced?

Glucose is oxidized; oxygen is reduced to water.

What is the role of NAD+ in cellular respiration?

Acts as an electron carrier; accepts electrons and hydrogen to form NADH.

What enzyme catalyzes redox reactions involving NAD+?

Dehydrogenases.

Why is energy released in small steps in respiration?

Controlled, stepwise reactions prevent energy loss as heat and allow efficient ATP production.

Trace the path of electrons during cellular respiration.

Glucose → NADH/FADH2 → Electron Transport Chain → O2 → H2O.

What are the two major phases of glycolysis?

Energy Investment Phase and Energy Payoff Phase.

How many carbons does glucose have, and what does it become after glycolysis?

Glucose (6C) → 2 Pyruvate (3C each).

How many ATP are used and produced in glycolysis?

2 ATP used; 4 ATP produced → Net gain of 2 ATP.

How many NADH are produced in glycolysis?

2 NADH.

What enzyme converts glucose to glucose-6-phosphate?

Hexokinase.

What enzyme is the rate-limiting step of glycolysis?

Phosphofructokinase (PFK).

What happens to DHAP in glycolysis?

It is converted into G3P (glyceraldehyde-3-phosphate).

What are the final products of glycolysis?

2 Pyruvate, 2 NADH, 2 Net ATP, and 2 H2O.

What type of phosphorylation forms ATP in glycolysis?

Substrate-level phosphorylation.

Has any CO2 been released yet?

No — all carbons remain in pyruvate.

Where does the link reaction occur?

Mitochondrial matrix.

What happens to pyruvate during this step?

Each pyruvate (3C) loses one CO2, forming acetate (2C), which joins with CoA to form Acetyl-CoA.

What are the products per glucose molecule from the link reaction?

2 Acetyl-CoA, 2 NADH, and 2 CO2.

What enzyme complex catalyzes the link reaction?

Pyruvate dehydrogenase complex.

What is the purpose of Acetyl-CoA?

To carry acetyl groups into the Krebs cycle for oxidation.

What does Acetyl-CoA combine with to start the Krebs cycle?

Oxaloacetate (4C) → forms Citrate (6C).

Name the main intermediates of the Krebs cycle (in order).

Oxaloacetate → Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate.

What is released as a waste product in the Krebs cycle?

CO2 (two per turn).

How many turns of the Krebs cycle occur per glucose molecule?

Two turns (one per Acetyl-CoA).

Per turn, what does the Krebs cycle produce?

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

What is the total yield per glucose from the Krebs cycle?

6 NADH, 2 FADH2, 2 ATP, and 4 CO2.

What type of phosphorylation produces ATP in the Krebs cycle?

Substrate-level phosphorylation.

Where do the NADH and FADH2 go next?

To the electron transport chain.

Where does the ETC occur?

Inner mitochondrial membrane (cristae).

What are the four protein complexes in the ETC?

I - NADH dehydrogenase; II - Succinate dehydrogenase; III - Cytochrome bc1 complex; IV - Cytochrome c oxidase.

What are the mobile electron carriers?

Ubiquinone (Coenzyme Q) and Cytochrome c.

What is the final electron acceptor in the ETC?

Oxygen (O2), forming H2O.

What is pumped across the membrane during the ETC?

H+ ions into the intermembrane space.

What enzyme synthesizes ATP using the proton gradient?

ATP synthase.

What is chemiosmosis?

The diffusion of H+ ions through ATP synthase to drive ATP formation.

Why is it called oxidative phosphorylation?

Because ATP synthesis is powered by oxidation of electron carriers (NADH/FADH2).

How many ATP are made per NADH and per FADH2?

NADH → ~2.5 ATP; FADH2 → ~1.5 ATP.

What is the total ATP yield per glucose molecule?

~30-32 ATP.

Where do regenerated NAD+ and FAD return?

NAD+ → glycolysis and Krebs cycle; FAD → Krebs cycle.

What is fermentation?

The anaerobic breakdown of glucose to make ATP without using oxygen.

What molecule must be regenerated for glycolysis to continue?

NAD+.

How many ATP does fermentation yield per glucose?

2 ATP (from glycolysis only).

What are the two types of fermentation?

Alcohol fermentation and lactic acid fermentation.

What happens in alcohol fermentation?

Pyruvate → ethanol + CO2 (performed by yeasts and some bacteria).

What happens in lactic acid fermentation?

Pyruvate → lactate (no CO2 released; occurs in muscle cells and some bacteria).

What is the main purpose of fermentation?

To regenerate NAD+ so glycolysis can continue producing ATP.

Compare fermentation and cellular respiration.

Both start with glycolysis; fermentation is anaerobic and produces 2 ATP, respiration is aerobic and produces ~30-32 ATP.

What are obligate aerobes, obligate anaerobes, and facultative anaerobes?

Obligate aerobes require O2; obligate anaerobes cannot survive in O2; facultative anaerobes can switch between aerobic respiration and fermentation.

Why is glycolysis considered an ancient pathway?

It occurs in all living organisms, in the cytoplasm, and does not require oxygen or organelles.

What does this suggest about early Earth?

Glycolysis evolved before oxygen was abundant (~3.5 billion years ago).

Total yield from 1 molecule of glucose in aerobic respiration.

Glycolysis: 2 ATP + 2 NADH; Link Reaction: 2 NADH; Krebs: 2 ATP + 6 NADH + 2 FADH2; ETC: ~32-34 ATP → Total ≈ 36-38 ATP.

What happens to CO2 generated in respiration?

Diffuses out of the mitochondria, enters bloodstream, and is exhaled.

Why does O2 pull electrons through the ETC?

It has high electronegativity — strong attraction for electrons.

What type of reaction is respiration overall?

A catabolic, exergonic redox reaction that releases energy by oxidizing glucose.