Cellular Respiration and Fermentation: Study Notes

Cellular Respiration and Fermentation

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules. This energy is stored in the form of adenosine triphosphate (ATP), which is the main energy currency of the cell. Cells cannot use food directly for energy; instead, they convert the energy in food into ATP through a series of biochemical pathways.

• ATP: The primary molecule that stores and provides energy for cellular processes.

• Organic molecules such as carbohydrates, fats, and proteins serve as fuel for cellular respiration.

Types of Respiration

There are several pathways by which cells can generate ATP, depending on the availability of oxygen.

• Aerobic respiration: Consumes organic molecules and oxygen (O2), yielding ATP.

• Anaerobic respiration: Similar to aerobic respiration but uses compounds other than oxygen as the final electron acceptor.

• Fermentation: Partial degradation of sugars that occurs without oxygen.

Redox Reactions in Cellular Respiration

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred between molecules, releasing energy.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• OIL RIG: "Oxidation Is Losing, Reduction Is Gaining" (mnemonic).

• Reducing agent: Electron donor (becomes oxidized).

• Oxidizing agent: Electron acceptor (becomes reduced).

Example: In the reaction of methane and oxygen to produce carbon dioxide and water, methane is oxidized and oxygen is reduced.

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{Energy}$

Stages of Cellular Respiration

The breakdown of glucose to extract energy occurs in three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating ATP, NADH, and FADH2.

3. Oxidative Phosphorylation: Driven by the electron transport chain, generates most of the ATP.

Glycolysis

Glycolysis is the first stage of cellular respiration and occurs in the cytosol. It does not require oxygen and consists of two phases:

• Energy investment phase: ATP is used to phosphorylate glucose.

• Energy payoff phase: ATP and NADH are produced.

Net reaction:

$\text{Glucose} \rightarrow 2\text{Pyruvate} + 2\text{H}_2\text{O}$ $4\text{ATP formed} - 2\text{ATP used} = 2\text{ATP net gain}$ $2\text{NAD}^+ \rightarrow 2\text{NADH} + 2\text{H}^+$

Key facts for exams:

• Location: Cytosol

• Phases: Energy investment and energy payoff

• End products: 2 pyruvate, 2 ATP (net), 2 NADH

Citric Acid Cycle (Krebs Cycle)

After glycolysis, pyruvate is transported into the mitochondrion and converted to acetyl coenzyme A (acetyl CoA), which enters the citric acid cycle. This cycle completes the oxidation of organic molecules.

• Each glucose molecule results in two turns of the cycle.

• Generates ATP, NADH, and FADH2 per turn.

• Produces CO2 as a waste product.

Oxidative Phosphorylation and Electron Transport Chain

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced. It involves the electron transport chain (ETC) and chemiosmosis.

• Electron transport chain (ETC): A series of proteins in the mitochondrial inner membrane that transfer electrons from NADH and FADH2 to oxygen.

• Chemiosmosis: The use of a proton (H+) gradient to drive ATP synthesis via ATP synthase.

• Proton-motive force: The H+ gradient across the membrane.

Equation for ATP yield:

$\text{Glucose} \rightarrow \text{NADH} \rightarrow \text{ETC} \rightarrow \text{Proton-motive force} \rightarrow \text{ATP}$

About 34% of the energy in glucose is transferred to ATP, yielding approximately 32 ATP molecules per glucose.

Fermentation

Fermentation allows cells to produce ATP without oxygen. When oxygen is absent, glycolysis is coupled with fermentation to regenerate NAD+ so glycolysis can continue.

• Alcohol fermentation: Converts pyruvate to ethanol and CO2.

• Lactic acid fermentation: Converts pyruvate to lactic acid.

Process	Final Electron Acceptor	ATP Yield (per glucose)
Fermentation	Organic molecule	2
Cellular Respiration	Oxygen	~32

Key differences: Fermentation produces much less ATP than cellular respiration.

Energy Sources and Exercise

Cells can use carbohydrates, fats, and proteins as energy sources. Fats are more energy-dense than carbohydrates because they have more reduced carbon atoms.

• ATP/phosphocreatine (PCr): Provides rapid ATP for short bursts of activity (anaerobic).

• Glycolysis/Fermentation: Supplies ATP anaerobically for moderate activity.

• Citric Acid Cycle/Electron Transport: Supplies ATP aerobically for sustained activity.

Maximum Heart Rate (MHR) formula:

$\text{MHR} = 220 - \text{age}$

Review Questions and Applications

• Why do we breathe oxygen? Oxygen is the final electron acceptor in the electron transport chain, allowing for efficient ATP production.

• What happens to cellular respiration in the absence of oxygen? The electron transport chain ceases, and cells rely on fermentation for ATP production.

• What makes fatty acids more energy dense than carbohydrates? Fatty acids have more reduced carbon atoms, allowing for more electrons to be transferred and more ATP to be generated.

• If you lose 10 pounds of fat, where does it go? Fat is broken down into CO2 and H2O, which are exhaled and excreted.

Key Vocabulary

• Oxidative phosphorylation: ATP formation using energy from redox reactions in the electron transport chain.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a substrate.

• Chemiosmosis: Use of a proton gradient to drive ATP synthesis.

• Proton-motive force: The force generated by the transmembrane proton gradient.

Additional info: Some diagrams and tables were inferred from context and standard biology knowledge to ensure completeness and clarity.