Cellular respiration is the set of chemical reactions cells use to release usable energy from nutrients, especially glucose. That energy is captured mainly in the molecule ATP (adenosine triphosphate), which cells use to power activities such as movement, growth, active transport, repair, and the synthesis of new molecules.
In most human and other animal cells, cellular respiration takes place partly in the cytoplasm and partly in the mitochondria. The first stage, glycolysis, occurs in the cytoplasm. The later stages occur inside mitochondria. In cells that lack mitochondria, such as mature human red blood cells, energy must be obtained through processes that do not require mitochondria.
Although cellular respiration is often described as simply using oxygen to “burn” glucose, the actual process is a carefully controlled series of enzyme-driven reactions. Oxygen is important for aerobic cellular respiration, but not every step of cellular respiration uses oxygen directly.
What cellular respiration does
Cells need a continuous supply of energy. They obtain much of that energy from organic molecules such as carbohydrates, fats, and proteins. These molecules contain chemical energy that can be transferred into forms the cell can use.
For glucose, the overall aerobic process can be summarized as:
glucose + oxygen → carbon dioxide + water + usable energy (ATP and heat)
This equation describes the overall transformation, not a single reaction. In a living cell, glucose is broken down through many smaller steps. Energy released along the way is captured in stages rather than being released all at once.
ATP acts as a convenient energy carrier. When a cell needs energy for a particular task, it can use ATP and convert it into ADP and inorganic phosphate. Cellular respiration continually helps regenerate ATP from these lower-energy forms.
Where cellular respiration happens
The location depends on which stage of the process is being considered.
| Stage | Main location in a eukaryotic cell | Main role |
|---|---|---|
| Glycolysis | Cytoplasm | Splits glucose into pyruvate and produces some ATP and NADH |
| Pyruvate oxidation | Mitochondrial matrix | Converts pyruvate into acetyl-CoA and produces NADH |
| Citric acid cycle | Mitochondrial matrix | Further breaks down carbon compounds and produces NADH and FADH₂ |
| Electron transport and oxidative phosphorylation | Inner mitochondrial membrane | Uses electrons to produce most of the ATP in aerobic respiration |
The term cytoplasm refers to the contents of a cell outside the nucleus, including the fluid portion called the cytosol. Glycolysis takes place in the cytosol.
The mitochondria are membrane-bound organelles found in most eukaryotic cells. Their internal organization allows the later stages of aerobic respiration to occur efficiently.
Glycolysis: the first stage
Cellular respiration begins with glycolysis, a pathway that breaks one six-carbon glucose molecule into two three-carbon molecules called pyruvate.
Glycolysis occurs in the cytosol and does not require oxygen. It involves a sequence of enzyme-controlled reactions that first require a small investment of ATP and then produce more ATP. The pathway also transfers some of the glucose’s energy to NADH, a molecule that carries high-energy electrons to later reactions.
For each molecule of glucose, glycolysis produces a net gain of two ATP and two NADH, along with two molecules of pyruvate.
Because glycolysis does not depend directly on mitochondria or oxygen, it can occur in cells under conditions where oxygen is unavailable. What happens to pyruvate afterward depends on the type of cell and the availability of oxygen and other metabolic conditions.
What happens in the mitochondria
When oxygen is available and aerobic respiration can proceed, pyruvate enters the mitochondrion in eukaryotic cells.
Pyruvate oxidation
Inside the mitochondrial matrix, each pyruvate molecule is converted into acetyl-CoA. During this process, carbon dioxide is released and NADH is produced.
Acetyl-CoA then enters the next major stage, the citric acid cycle.
The citric acid cycle
The citric acid cycle, also called the Krebs cycle or tricarboxylic acid (TCA) cycle, takes place in the mitochondrial matrix.
Rather than directly producing large amounts of ATP, this cycle transfers much of the remaining chemical energy from the original glucose into electron carriers, primarily NADH and FADH₂. It also produces a small amount of ATP or an equivalent energy-carrying molecule and releases carbon dioxide.
Importantly, the carbon dioxide exhaled by a person ultimately comes from the breakdown of carbon-containing fuel molecules through pathways that include pyruvate oxidation and the citric acid cycle.
The electron transport chain produces most ATP
The largest share of ATP from aerobic respiration is generated during oxidative phosphorylation, which occurs at the inner mitochondrial membrane.
NADH and FADH₂ deliver high-energy electrons to the electron transport chain. As electrons move through a series of protein complexes in the inner mitochondrial membrane, their energy is used to pump hydrogen ions across the membrane.
This creates an electrochemical gradient. Hydrogen ions then flow back across the membrane through an enzyme called ATP synthase. The movement of hydrogen ions provides the energy ATP synthase uses to produce ATP.
At the end of the electron transport chain, oxygen accepts electrons and combines with hydrogen ions to form water. This is why oxygen is essential for aerobic respiration: without a suitable final electron acceptor, the electron transport chain cannot continue operating normally.
Why mitochondria are central to cellular respiration
Mitochondria are particularly well suited for aerobic energy production because their membranes create separate compartments with different chemical conditions.
The inner mitochondrial membrane contains the protein complexes of the electron transport chain and ATP synthase. The mitochondrial matrix contains the enzymes responsible for pyruvate oxidation and the citric acid cycle.
This compartmentalization allows cells to establish the hydrogen-ion gradient needed for efficient ATP production.
Mitochondria also have their own small genomes and several features that distinguish them from most other cellular organelles. However, it would be misleading to describe mitochondria as the place where all cellular respiration happens. Glycolysis, for example, takes place outside them in the cytosol.
Does cellular respiration always require oxygen?
No. Cellular respiration is a broader concept than aerobic respiration.
Aerobic respiration uses oxygen as the final electron acceptor in the electron transport chain. Some organisms can instead carry out forms of anaerobic respiration, using substances other than oxygen as final electron acceptors.
Cells can also use fermentation, which is different from respiration. Fermentation allows glycolysis to continue when oxidative phosphorylation cannot proceed by regenerating NAD⁺ from NADH. In human muscle cells, for example, pyruvate can be converted to lactate under conditions in which the demand for energy temporarily exceeds the capacity of aerobic metabolism.
Fermentation does not produce the large additional yield of ATP associated with oxidative phosphorylation. The ATP produced directly during glycolysis remains available, allowing cells to generate some ATP without relying on the mitochondrial electron transport chain.
How cellular respiration connects to the food we eat
Glucose is an important fuel, but cells are not limited to glucose.
Fats can be broken down into fatty acids and glycerol. Fatty acids can be converted into acetyl-CoA through a pathway called beta-oxidation, allowing their carbon atoms to enter pathways associated with the citric acid cycle and oxidative phosphorylation.
Proteins can also contribute energy. Their amino acids must first have their amino groups removed or otherwise processed before their remaining carbon structures can enter various metabolic pathways.
This flexibility allows cells to adjust their fuel use according to nutritional conditions and energy demands.
Where cellular respiration happens in different kinds of cells
The locations described above apply primarily to eukaryotic cells, including those of humans, animals, plants, fungi, and many other organisms.
Prokaryotic cells, such as bacteria and archaea, do not have mitochondria. Their glycolysis occurs in the cytoplasm, while processes analogous to the citric acid cycle and electron transport take place in the cytoplasm and across the cell membrane, respectively, depending on the organism and its metabolic pathways.
Plants carry out cellular respiration too. Photosynthesis captures and stores energy in organic molecules, but plant cells still need cellular respiration to make usable ATP from those molecules. Photosynthesis and cellular respiration are therefore related but distinct processes.
The important distinction is that photosynthesis stores energy in organic molecules, while cellular respiration releases energy from those molecules for cellular use. Plants use both processes.
Cellular respiration is more than “making energy”
A common shorthand says that cellular respiration “makes energy,” but technically, energy is not created by the cell. Instead, cellular respiration transfers and captures chemical energy from nutrient molecules into ATP and other usable forms.
The process also supplies important intermediate molecules that cells can use to build other compounds. Metabolism is therefore not simply a matter of extracting energy from food; it is an interconnected network in which molecules can be broken down for energy or diverted into pathways that support growth, maintenance, and synthesis.
For aerobic respiration, the central picture is straightforward: glycolysis begins glucose breakdown in the cytosol, mitochondrial pathways continue extracting energy from its products, and the electron transport chain uses that energy to drive most ATP production at the inner mitochondrial membrane. Oxygen ultimately accepts electrons, and carbon dioxide and water are among the major end products of the overall process.
