Cellular Respiration & Fermentation: Study Notes

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Chapter 7: Cellular Respiration & Fermentation

Overview: Life Is Work

All living organisms require energy from external sources to sustain life processes. Animals obtain energy by consuming other organisms, while plants and photosynthetic organisms (such as algae) capture energy from sunlight.

Energy is essential for cellular activities, growth, and maintenance.
Animals depend on plants or other animals for energy.
Photosynthetic organisms convert solar energy into chemical energy.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuel

Cells extract energy from organic molecules through catabolic pathways, primarily by oxidation. These pathways break down complex molecules, releasing energy that is captured in the form of ATP.

Catabolic pathways degrade organic molecules, releasing energy.
ATP (Adenosine Triphosphate) is the main energy currency of the cell.
Energy stored in chemical bonds is released and used for cellular work.
Some energy is lost as heat during these processes.

Cellular Respiration

Includes both aerobic and anaerobic processes.
Aerobic respiration consumes oxygen and organic fuel, producing carbon dioxide, water, and ATP.
General equation for aerobic respiration:

Anaerobic respiration uses molecules other than oxygen (e.g., sulfate, nitrate) as electron acceptors, generating ATP and various byproducts.
Fermentation is a partial degradation of organic fuel that occurs without oxygen, producing less ATP and various waste products.

Redox Reactions: Oxidation and Reduction

Energy transfer in cells often involves redox reactions, where electrons are transferred between molecules. These reactions are fundamental to cellular respiration.

Oxidation: Loss of electrons from a molecule, resulting in an increase in charge (becomes more positive).
Reduction: Gain of electrons by a molecule, resulting in a decrease in charge (becomes more negative).
Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain (of electrons).
Redox reactions release energy, which is used to synthesize ATP.

Redox Reaction Table

Process	Electron Change	Result
Oxidation	Loses electrons (e-)	More positive charge, less energy
Reduction	Gains electrons (e-)	More negative charge, more energy

Oxidation of Organic Fuel Molecules During Cellular Respiration

During cellular respiration, organic molecules such as glucose are oxidized, and oxygen is reduced. Glucose is a highly reduced molecule, containing many hydrogen atoms that act as sources of electrons.

Glucose () is the primary fuel for most cells.
Oxidation of glucose releases electrons and hydrogen ions:

These electrons are transferred to electron carriers and ultimately to oxygen, forming water.

Pathways of Cellular Respiration and Fermentation

Cellular respiration and fermentation are two major metabolic pathways for extracting energy from glucose.

Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate.
Pyruvate can enter the mitochondria for aerobic respiration or be used in fermentation if oxygen is absent.
Aerobic respiration: Pyruvate is converted to Acetyl CoA, enters the citric acid cycle, and is fully oxidized to CO2.
Fermentation: Pyruvate is converted to ethanol or lactate in the absence of oxygen.

Pathway	Location	Oxygen Required?	Main Products
Glycolysis	Cytosol	No	Pyruvate, ATP, NADH
Aerobic Respiration	Mitochondria	Yes	CO2, H2O, ATP
Fermentation	Cytosol	No	Ethanol or Lactate, ATP

Example: Alcoholic Fermentation

Occurs in yeast and some bacteria.
Pyruvate is converted to ethanol and CO2.
Used in brewing and baking industries.

Example: Lactic Acid Fermentation

Occurs in muscle cells and some bacteria.
Pyruvate is converted to lactate.
Responsible for muscle fatigue during intense exercise.

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