Cellular Respiration

Aerobic cellular respiration

The process of extracting energy from glucose in the presence of oxygen, through glycolysis, pyruvate oxidation, Krebs cycle, and the electron transport chain/chemiosmosis, producing CO2, H2O, and ATP.

Glycolysis

A 10-step, cytoplasmic pathway that converts one glucose into two pyruvate molecules, investing 2 ATP and yielding 4 ATP and 2 NADH (net 2 ATP and 2 NADH).

Glyceraldehyde-3-phosphate (G3P)

A 3-carbon intermediate produced during glycolysis; each glucose yields two G3P molecules that are processed to pyruvate.

Glucose

A six-carbon sugar that is the main energy source for cellular respiration; taken up into cells (insulin promotes uptake) and processed to harvest energy.

Pyruvate

A 3-carbon molecule produced at the end of glycolysis; entry point for pyruvate oxidation to form acetyl-CoA; can feed into other pathways under anaerobic conditions.

Pyruvate oxidation (oxidative decarboxylation)

Process in the mitochondrial matrix that converts each pyruvate into acetyl-CoA, releasing CO2 and producing NADH; no ATP is produced in this step.

Acetyl-CoA

Two-carbon molecule formed from pyruvate oxidation; substrate that enters the Krebs cycle.

Mitochondrion

Organelle containing outer and inner membranes where aerobic respiration occurs; site of the cristae, matrix, and energy conversions.

Cristae

Folds of the inner mitochondrial membrane that increase surface area for energy-carrying reactions.

Matrix (mitochondrial matrix)

The fluid-filled space inside the inner mitochondrial membrane where many reactions of respiration occur.

NADH

Reduced form of NAD+, carries high-energy electrons to the electron transport chain; generated in glycolysis, pyruvate oxidation, and the Krebs cycle.

NAD+

Oxidized form of nicotinamide adenine dinucleotide; accepts electrons to become NADH.

FADH2

Reduced form of FAD; transfers electrons to the electron transport chain, contributing to ATP production.

FAD

Oxidized form of flavin adenine dinucleotide; accepts electrons to become FADH2.

Krebs cycle (Citric Acid Cycle)

Eight-step metabolic cycle in the mitochondrial matrix that oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP; oxaloacetate is regenerated.

Electron Transport Chain (ETC)

Series of protein complexes (cytochrome complexes) in the inner mitochondrial membrane that transfer electrons from NADH/FADH2 to O2, releasing energy to pump protons.

Cytochrome complexes

Proteins in the ETC that assist in electron transfer and contribute to proton pumping.

Ubiquinone (CoQ)

Mobile electron carrier within the inner mitochondrial membrane that transfers electrons between ETC complexes.

Cytochrome C

Mobile electron carrier protein that shuttles electrons to complex IV (cytochrome oxidase) in the ETC.

Cytochrome oxidase

Also known as Complex IV; final transfer point in the ETC that reduces oxygen to water and pumps protons.

Proton pumps

Transport proteins in the inner mitochondrial membrane that move H+ ions into the intermembrane space, creating a proton gradient.

Proton motive force (PMF)

The electrochemical gradient of protons across the inner membrane that drives ATP synthesis via ATP synthase.

ATP synthase

Enzyme that uses the proton motive force to phosphorylate ADP to ATP as protons flow back into the matrix.

Oxygen (O2)

Final electron acceptor in aerobic respiration; its reduction forms water.

Water (H2O)

End product when oxygen accepts electrons and protons at the end of the ETC.

Carbon dioxide (CO2)

Waste product produced during pyruvate oxidation and the Krebs cycle.

Anaerobic glycolysis

Glycolysis does not require oxygen; in absence of O2, pyruvate is diverted to lactic acid or ethanol fermentation.

Lactic acid fermentation (lactate)

Anaerobic process in muscle cells where pyruvate is reduced to lactate, regenerating NAD+ for glycolysis.

Alcohol fermentation (ethanol)

In yeast, pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol; used in brewing and winemaking.

Stages of Glycolysis

Stage 1: Two ATP are consumed to prepare glucose for splitting.

Stage 2: The glucose is broken down into two 3C molecules - G3P.

From this stage, all reactions occur twice.

Stage 3: G3P transforms into a 3C pyruvate molecule. The energy

from this reaction is used to produce 2 NADH and 4 ATP molecules.

Glycolysis Net: 2 Pyruvate molecules, 2 NADH, 2 ATP.

Steps in Pyruvate Oxidation

Step 1: A CO₂ molecule is removed from a pyruvate.

Stage 2: NAD+ oxidizes each pyruvate, gaining two electrons and two protons from it and converting to NADH and H+. This results in an acetic acid formation.

Stage 3: Coenzyme A (CoA) bonds with the acetic acid and forms

the acetyl-CoA complex, which is the final product

in the oxidation of pyruvate (TVO ILC, n.d.).

Pyruvate Oxidation Net: 2 CO₂, 2 Acetyl-CoA, 2 NADH, 2 H+.

Krebs Cycle

Two acetyl-CoA molecules convert into

four CO₂ molecules, gaining electrons and protons

through the metabolic pathway.

The cycle carries CoA molecules through

enzyme-facilitated oxidation reactions (8 different enzymes), dehydrogenation, decarboxylation, and phosphorylation.

8 enzymes: Citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate (also used as a reacant).

Krebs Cycle Net: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.

Steps of the Electron Transport Chain and Chemiosmosis

Step 1: NADH dehydrogenase removes high-energy electrons from NADH and transfers them to ubiquinone. The NAD+ produced during this reaction goes back into the Krebs cycle.

Step 2: Ubiquinone removes high-energy electrons from FADH₂ and transfers them cytochrome. The FAD produced during this reaction goes back into the Krebs cycle.

Step 3: Cytochrome carries electrons to the third proton pump, called cytochrome C oxidase. Two electrons passing through it are transferred to oxygen. One oxygen atom accepts two electrons, which results in the formation of one molecule of water.

Step 4: Most of the energy from the NADH and FADH₂ electrons, a by ubiquinone and cytochrome C is used to pump H+ out of the matrix into the intermembrane space, using NADH dehydrogenase, b-c1 complex, and cytochrome C oxidase pumps (TVO ILC, n.d.).

Step 5: The pumped out H+ concentrated outside of the matrix creates an electrochemical gradient across the membrane. This gradient contains potential energy, which is used to make ATP.

Step 6: H+ ions move down their concentration gradient into the matrix, using an enzyme called ATP synthase. As ATP synthase turns, it catalyzes the addition of a phosphate to ADP, capturing energy from the proton gradient as ATP (Khan Academy, n.d.).