Two Processes That Help in Obtaining Energy from Food

January 13, 2026 •

2 min read

Every living cell requires a constant supply of energy to function, with some organisms burning thousands of their own weight in ATP daily. The two fundamental processes that help in obtaining energy from food are cellular respiration and fermentation, which break down nutrients to produce the energy currency of the cell, adenosine triphosphate (ATP).

Quick Summary

Organisms extract energy from food through two primary metabolic pathways: cellular respiration and fermentation. Cellular respiration uses oxygen to fully break down glucose, yielding a large amount of ATP, while fermentation provides a smaller ATP yield in the absence of oxygen.

Key Points

Cellular Respiration: This is the primary, aerobic process where cells break down glucose using oxygen to produce a large amount of ATP.
Fermentation: This is an anaerobic process that allows cells to produce a small, quick supply of ATP from glucose when oxygen is scarce or unavailable.
ATP is Energy Currency: Both processes are designed to produce adenosine triphosphate (ATP), the universal molecule that powers most cellular activities.
Glycolysis is a Shared First Step: Both cellular respiration and fermentation begin with glycolysis, the splitting of a glucose molecule in the cytoplasm to produce a net of two ATP and two pyruvate molecules.
Oxygen Availability Dictates the Pathway: The presence of oxygen determines whether pyruvate proceeds into the high-yield Krebs cycle and electron transport chain or into the low-yield fermentation pathway.
Different Products: Cellular respiration's final products are carbon dioxide and water, while fermentation produces compounds like lactate or ethanol.
Efficiency Difference: Cellular respiration is significantly more efficient, producing about 15 times more ATP per glucose molecule than fermentation.

Cellular Respiration: The High-Efficiency Aerobic Pathway

Cellular respiration is the most efficient process for obtaining energy from food and is utilized by most organisms in the presence of oxygen. This pathway breaks down glucose into ATP, carbon dioxide, and water. The simplified equation is $C6H{12}O_6$ (glucose) + $6O_2$ (oxygen) $\rightarrow$ $6CO_2$ (carbon dioxide) + $6H_2O$ (water) + Energy (ATP).

Cellular respiration involves three main stages:

Glycolysis: Occurs in the cytoplasm without oxygen, splitting glucose into two pyruvate molecules and yielding a net of two ATP and two NADH.
The Krebs Cycle (Citric Acid Cycle): In the mitochondria with oxygen, pyruvate is converted to acetyl-CoA, entering a cycle that oxidizes food molecules to carbon dioxide, producing ATP, NADH, and FADH2.
Oxidative Phosphorylation and the Electron Transport Chain: This mitochondrial stage generates the most ATP. High-energy electrons from NADH and FADH2 move down a chain, creating a proton gradient that powers ATP synthase to produce ATP.

Fermentation: The Anaerobic Energy-Yielding Process

When oxygen is limited, fermentation provides an alternative, albeit less efficient, way to generate ATP. Starting with glycolysis to produce two ATP and pyruvate, fermentation regenerates NAD+ by converting pyruvate into different end products.

Common types include:

Lactic Acid Fermentation: Occurs in muscle cells during intense exercise, converting pyruvate to lactate and regenerating NAD+.
Alcohol Fermentation: Found in yeast and some bacteria, converting pyruvate to ethanol and carbon dioxide, also regenerating NAD+.

Comparison of Cellular Respiration and Fermentation


Feature	Cellular Respiration	Fermentation
Oxygen Requirement	Aerobic	Anaerobic
Location in Cell	Cytoplasm & Mitochondria	Cytoplasm only
ATP Yield per Glucose	High (Net 30-32 ATP)	Low (Net 2 ATP)
Glucose Breakdown	Complete oxidation	Partial breakdown
Process Speed	Slower, sustainable	Faster, less efficient
Final Electron Acceptor	Oxygen	Organic molecule

Conclusion

To summarize, organisms primarily obtain energy from food through cellular respiration when oxygen is present, and through fermentation when oxygen is scarce. Cellular respiration yields a large amount of ATP through complete glucose breakdown, while fermentation offers a rapid, lower-yield ATP production in anaerobic conditions. These pathways are crucial for meeting energy demands under varying physiological states.

Frequently Asked Questions

Before cellular respiration, the food must first be digested into its smaller monomer subunits, such as carbohydrates into glucose, proteins into amino acids, and fats into fatty acids and glycerol.

The primary energy molecule is adenosine triphosphate (ATP), which is used to store and transfer energy within cells to fuel various cellular activities.

Glycolysis, the initial phase for both, occurs in the cytoplasm. The remaining stages of cellular respiration (Krebs cycle and oxidative phosphorylation) take place in the mitochondria, while fermentation is confined to the cytoplasm.

Cellular respiration is more efficient because it fully oxidizes glucose, allowing the electron transport chain to generate a large amount of ATP. Fermentation only partially breaks down glucose and produces a small amount of ATP from glycolysis.

Fermentation's main purpose is to regenerate NAD+ for glycolysis to continue producing ATP in the absence of oxygen. This allows for a quick but temporary energy supply during high-intensity activity or in anaerobic environments.

Yes, when glucose levels are low, the body can break down triglycerides into fatty acids and glycerol. Fatty acids are then oxidized through a process called beta-oxidation to produce acetyl-CoA, which enters the Krebs cycle to generate ATP.

Vitamins and minerals serve as cofactors and coenzymes that are essential for the enzymes involved in metabolic pathways like cellular respiration and fermentation to function correctly.