How do living things use the energy in food?

January 4, 2026 •

4 min read

Over 99% of all living things, including animals, plants, and microorganisms, convert the chemical energy in food into a usable form of energy called ATP through the metabolic process of cellular respiration. Understanding how do living things use the energy in food reveals the fundamental processes that drive life itself.

Quick Summary

Organisms convert chemical energy from food into ATP, the cell's energy currency, through cellular respiration. This multi-stage process breaks down macromolecules like glucose to fuel vital functions, including growth, movement, and repair.

Key Points

ATP is the energy currency: Living things convert chemical energy from food into adenosine triphosphate (ATP), the primary molecule used to power all cellular activities.
Cellular respiration is the core process: This is a three-stage metabolic pathway (glycolysis, Krebs cycle, and oxidative phosphorylation) that breaks down organic molecules like glucose to produce ATP.
Macromolecules are the fuel: Carbohydrates, fats, and proteins from food are all used as energy sources, entering the cellular respiration pathway at different points.
Oxygen dictates efficiency: Aerobic respiration, which uses oxygen, is highly efficient and produces a large amount of ATP, while anaerobic respiration occurs without oxygen and yields much less energy.
Mitochondria are the powerhouse: In eukaryotic cells, the mitochondria are the main site for high-yield ATP production through the Krebs cycle and oxidative phosphorylation.

The Universal Energy Currency: ATP

At the heart of how living things use the energy in food is a molecule called adenosine triphosphate, or ATP. Often referred to as the "energy currency" of the cell, ATP stores and transports chemical energy within cells to fuel all cellular activities, from muscle contraction to the synthesis of new molecules. Food molecules like glucose cannot be used directly by cells for energy. Instead, they must be broken down to release their stored chemical energy, which is then captured and transferred to ATP.

Cellular Respiration: The Engine of Life

Cellular respiration is the primary metabolic pathway that makes ATP production possible. It is a series of chemical reactions that break down organic molecules, like glucose, into carbon dioxide and water, with energy captured along the way. This complex process is typically divided into three main stages: glycolysis, the Krebs cycle, and oxidative phosphorylation via the electron transport chain.

Stage 1: Glycolysis

Glycolysis is the initial stage of cellular respiration and occurs in the cytoplasm of the cell. It is an anaerobic process, meaning it does not require oxygen. During glycolysis, a single glucose molecule (a six-carbon sugar) is broken down into two molecules of pyruvate (a three-carbon compound). This process yields a small net gain of 2 ATP molecules and 2 NADH molecules, which are important electron carriers for a later stage. Glycolysis is a primitive and universal energy-generating pathway, found in nearly all living organisms.

Stage 2: The Krebs Cycle

Also known as the citric acid cycle, the Krebs cycle takes place in the matrix of the mitochondria in eukaryotic cells. Before the cycle can begin, the pyruvate from glycolysis is converted into acetyl coenzyme A (acetyl CoA). The acetyl CoA then enters the cycle and is completely oxidized to release carbon dioxide. The Krebs cycle produces a small amount of ATP, but its main function is to generate a large number of electron carriers (NADH and FADH2) that will be used in the final stage.

Stage 3: Oxidative Phosphorylation

The final and most productive stage of aerobic respiration is oxidative phosphorylation, which occurs in the inner mitochondrial membrane. The NADH and FADH2 molecules carry high-energy electrons to a series of protein complexes known as the electron transport chain. As the electrons move down the chain, they release energy, which is used to pump protons (H+) across the membrane, creating an electrochemical gradient. This gradient then powers an enzyme called ATP synthase, which phosphorylates ADP to create a large amount of ATP. Oxygen is the final electron acceptor in this process, combining with protons to form water.

The Role of Macromolecules

Food is composed of various macromolecules, and the body can extract energy from each of them, although they enter the cellular respiration pathway at different points.

Macromolecules and their Entry Points:

Carbohydrates: Broken down into simple sugars like glucose, which enter cellular respiration at glycolysis. They are the body's fastest and most preferred energy source.
Fats: Broken down into fatty acids and glycerol. Fatty acids are converted into acetyl CoA and enter the Krebs cycle. Fats are a very energy-efficient form of food, providing the slowest but most sustained energy release.
Proteins: Digested into amino acids. Depending on the type of amino acid, they can be modified and enter cellular respiration at various points in the glycolysis or Krebs cycle pathways.

Aerobic vs. Anaerobic Respiration

Not all organisms or cells have access to oxygen at all times, leading to two distinct types of cellular respiration.


Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Requirement	Requires oxygen to be the final electron acceptor.	Does not require oxygen.
Location	Begins in the cytoplasm (glycolysis), but most ATP is made in the mitochondria.	Occurs entirely in the cytoplasm.
Glucose Breakdown	Complete breakdown of glucose into carbon dioxide and water.	Partial breakdown of glucose.
Energy Yield	Very high (approx. 30-32 ATP per glucose).	Very low (net 2 ATP per glucose).
End Products	Carbon dioxide, water, and ATP.	Lactic acid (in animals) or alcohol and CO2 (in yeasts).
Examples	Most plants and animals, including humans.	Muscle cells during intense exercise, bacteria, and yeast.

Conclusion: The Foundation of Biological Activity

The process by which living things use the energy in food is a marvel of biological engineering. Through the intricate pathways of cellular respiration, the chemical energy locked within carbohydrates, fats, and proteins is systematically harvested and converted into the universally usable form of ATP. This energy currency powers every function, from the most basic cellular maintenance to complex motor movements. Whether through the efficient, oxygen-dependent process of aerobic respiration or the less efficient, oxygen-independent anaerobic respiration, this conversion of food into functional energy is the fundamental process that sustains all life on Earth. For a more detailed look into this process, the NCBI provides comprehensive resources on cellular energy conversion.

Frequently Asked Questions

ATP, or adenosine triphosphate, is the main energy-carrying molecule in cells. It is crucial because it provides the readily usable energy that fuels nearly all cellular activities, much like money in an economy.

Food must first be digested and broken down into smaller molecules, such as glucose, fatty acids, and amino acids. These simpler molecules can then be transported to cells to be processed for energy.

The main stages are glycolysis (in the cytoplasm), the Krebs cycle (in the mitochondrial matrix), and oxidative phosphorylation (in the inner mitochondrial membrane).

No. While aerobic respiration uses oxygen and is very efficient, anaerobic respiration occurs in the absence of oxygen and produces less energy. Some organisms rely entirely on anaerobic processes.

The vast majority of ATP is produced during the final stage, oxidative phosphorylation, via the electron transport chain within the mitochondria.

Yes, organisms can obtain energy from various macromolecules. Fats are broken down into fatty acids, and proteins into amino acids, which enter the cellular respiration pathway at different stages.

The main difference is the presence of oxygen. Aerobic respiration uses oxygen and yields a high amount of ATP, while anaerobic respiration does not use oxygen and yields a much lower amount of ATP.