Chapter 9 - Cellular Respiration and Fermentation

Reducing agent

Electron donor of a redox reaction; it becomes oxidized

Oxidizing Agent

Electron acceptor in a redox reaction; it becomes reduced

Reduction

Gain of electrons by a molecule, atom, or ion

Oxidation

Loss of electrons by a molecule, atom, or ion

Substrate-Level Phospholyration

Enzyme-catalyzed formation of ATP by direct transfer of a phosphate group to ADP from a high-energy intermediate. Occurs in glycolysis and the citric acid cycle.

Chemiosis

Movement of ions (H⁺) across a semipermeable membrane, down their electrochemical gradient, driving ATP synthesis via ATP synthase

Producers

Organisms that use sunlight to convert carbon dioxide (CO₂) and water into carbohydrates and oxygen (O₂) through photosynthesis

Consumers

Organisms that must ingest organic compounds to produce ADP and release CO₂ through cellular respiration

Anaerobic Respiration

Makes energy without using oxygen—A less efficient catabolic pathway where inorganic molecules other than oxygen serve as final electron acceptors in the electron transport chain (ETC), allowing ATP production without oxygen.

Aerobic Respiration

Uses oxygen to make energy—the most efficient catabolic pathway used by eukaryotic and prokaryotic cells. Oxygen acts as the final electron acceptor in the ETC, where exergonic reactions release energy to drive ATP synthesis.

Potential energy

Stored covalent bonds where atoms share electrons

Electronegativity Effect

Atoms like oxygen pull electrons closer, causing those electrons to lose energy

Energy Comparision

Electrons in bonds with oxygen have lower potential energy than those bonds with carbon or hydrogen

Closer electrons =

Lower energy

Oxygen bonds =

Low energy

Carbon/hydrogen =

Higher energy

Redox Reaction

A chemical reaction where electrons are transferred between molecules

Reducing Agent 

Gives away electrons (gets oxidized). This reduces the other molecule.

Oxidizing agent

Takes electrons (gets reduced). This oxidizes the other molecule.

Electron Position in Covalent Bonds

Electrons shared in a bond can be closer to one atom than the other

Potential Energy of Electrons

• Closer to electrons = lower energy 

• Farther from nucleus = higher energy

Redox Reaction in Cellular Respiration

Glucose (C₆H₁₂O₆) is oxidized to carbon dioxide (CO₂), and oxygen (O₂) is reduced to water (H₂O)

NADH

A high-energy electron carrier. It holds electrons from earlier reactions and delivers them to the electron transport chain (ETC), where they help make ATP.

Role of NADH in the ETC

NADH transfers electrons to oxygen through the ETC. This helps form water and powers ATP production.

Enzyme that Reduces NAD⁺

Dehydrogenases - enzymes that add electrons (and usually protons) to NAD⁺, turning it into NADH

What happens during the conversion of NAD⁺ to NADH, and how does NADH help make ATP?

• NAD⁺ + electrons → NADH (via dehydrogenase)

• NADH → donates electrons → ATP made + water formed (through the electron transport chain)

What is the Electron Transport Chain (ETC)?

A series of proteins and molecules in the inner mitochondrial membrane that:

• Take electrons from NADH and FADH₂

• Pass them to oxygen (the final electron acceptor)

• Release energy with each step

• Use that energy to pump protons across the membrane, creating a gradient that powers ATP production

What’s the basic flow of energy in the Electron Transport Chain?

Electrons → Oxygen → Energy → Proton Pump → ATP

What is the main goal of glycolysis?

Break glucose into 2 pyruvate molecules and make a small amount of ATP and NADH

What energy investment is made in glycolysis?

Input = 2 ATP

What are the net products of glycolysis?

2 Pyruvate, 2 ATP, 2 NADH

What is the main reaction of the intermediate “step”?

Pyruvate → Acetyl-CoA (via pyruvate dehydrogenase)

What energy investment is put into the intermediate “step”?

2 Pyruvate + 2 NAD⁺ + 2 CoA

What outputs are from the intermediate “step”?

2 Acetyl-CoA + 2 NADH + 2 CO₂

What is the main goal of the Citric Acid Cycle?

Oxidize Acetyl-CoA to CO₂ and produce NADH, FADH₂, and ATP.

What is the starting step of the Citric Acid Cycle?

4-carbon molecule + 2-carbon Acetyl-CoA → 6-carbon citrate

What is the finishing step of the Citric Acid Cycle?

1 ATP, 2 CO₂, 3 NADH, 1 FADH₂

ETC Concept

Electrons from NADH/FADH₂ pass through protein complexes, releasing energy to pump protons across the membrane.

Proton pump complexes

Complex I, III, and IV

Electron entry points

• Complex I accepts electrons from NADH

• Complex II accepts electrons from FADH₂

Electron shuttles

• Ubiquinone: Carries 2 electrons from Complex I/II → III

• Cytochrome C: Carries 1 electron from Complex III → IV

Final electron acceptor

Oxygen (O₂)

Proton Motive Force

Electrochemical gradient of H⁺ across the inner mitochondrial membrane that powers ATP synthesis.

ATP Synthase Role

Uses proton motive force to convert ADP + Pi → ATP

Which molecule produces more ATP when donating electrons to the ETC — NADH or FADH₂?

NADH

Is cellular respiration 100% efficient?

No. Some energy from glucose is always lost as heat, so not all of it is converted into ATP.

Is cellular respiration always a step-by-step A → B → C process?

No. There are multiple entry points, depending on which molecules are available in the cell.

What is the main control point of cellular respiration?

Glycolysis. It’s the key regulatory step that controls whether glucose enters the

When do cells undergo fermentation?

When oxygen isn’t available for cellular respiration

Why recycle NADH into NAD⁺ during fermentation?

To keep glycolysis going and make more ATP

What types of cells use alcohol fermentation?

Bacteria and Yeast (fungus)

What are the products of alcohol fermenation?

Pyruvate → Ethanol + CO₂ + NAD⁺

What type of cells use lactic acid fermentation?

Muscle cells, some bacteria, and fungi when oxygen is low

What is the product of lactic acid fermentation?

Pyruvate → Lactate + NAD⁺

Do humans use alcohol fermentation or lactic acid fermentation?

Lactic Acid fermentation