Cellular respiration is a process that all living things use to convert glucose into energy. Autotrophs (like plants) produce glucose during photosynthesis. Heterotrophs (like humans) ingest other living things to obtain glucose. While the process can seem complex, this page takes you through the key elements of each part of cellular respiration.
Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis is an anaerobic process, while the other two pathways are aerobic. In order to move from glycolysis to the citric acid cycle, pyruvate molecules (the output of glycolysis) must be oxidized in a process called pyruvate oxidation.
Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half. The first half is known as the “energy requiring” steps. This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. In the second half, the “energy releasing: steps, 4 molecules of ATP and 2 NADH are released. Glycolysis has a net gain of 2 ATP molecules and 2 NADH.
Some cells (e.g., mature mammalian red blood cells) cannot undergo aerobic respiration, so glycolysis is their only source of ATP. However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration.
In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only happen if oxygen is available. In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A (CoA) to form acetyl CoA. This process also releases CO2.
Citric Acid Cycle
The citric acid cycle (also known as the Krebs cycle) is the second pathway in cellular respiration, and it also takes place in the mitochondria. The rate of the cycle is controlled by ATP concentration. When there is more ATP available, the rate slows down; when there is less ATP the rate increases. This pathway is a closed loop: the final step produces the compound needed for the first step.
The citric acid cycle is considered an aerobic pathway because the NADH and FADH2 it produces act as temporary electron storage compounds, transferring their electrons to the next pathway (electron transport chain), which uses atmospheric oxygen. Each turn of the citric acid cycle provides a net gain of CO2, 1 GTP or ATP, and 3 NADH and 1 FADH2.
Electron Transport Chain
Most ATP from glucose is generated in the electron transport chain. It is the only part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it occurs in the plasma membrane.
The electron transport chain is made up of 4 proteins along the membrane and a proton pump. A cofactor shuttles electrons between proteins I–III. If NAD is depleted, skip I: FADH2 starts on II. In chemiosmosis, a proton pump takes hydrogens from inside mitochondria to the outside; this spins the “motor” and the phosphate groups attach to that. The movement changes from ADP to ATP, creating 90% of ATP obtained from aerobic glucose catabolism.
Now that you’ve reviewed cellular respiration, this practice activity will help you see how well you know cellular respiration:
Click here for a text-only version of the activity.
Did you have an idea for improving this content? We’d love your input.
Improve this pageLearn More
If you're seeing this message, it means we're having trouble loading external resources on our website.
If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.
Which nutrients function as coenzymes and are needed in only small amounts?
Which of the following is a water-soluble vitamin?
Neurons and red blood cells rely exclusively on __________ to meet their energy needs.
__________ is a substance in food used by the body to promote normal growth, maintenance, and repair.
The term essential nutrient refers to the chemicals that can be interconverted in the liver so that the body can maintain life and good health.
Which of the following is represented by the "membrane" indicated in the figure?
inner mitochondrial membrane
Which of the following molecules is most directly involved in the transfer of "energy from food" to the "proton pumps" depicted in this figure?
Which of the following is a common feature of the processes indicated by both letters D and E?
movement of hydrogen atoms associated with NADH
Which of the following is following is a true statement regarding the reactions indicated by the letters F and G?
Both reactions F and G involve the transfer of a phosphate group from an organic substrate to ADP.
__________ refers to reactions in which large molecules are broken down into smaller molecules.
__________ refers to reactions in which large molecules are broken down into smaller molecules.
Redox reactions __________.
are characterized by one substance gaining an electron while another substance loses an electron
Glycolysis occurs in the __________ of cells and is an __________ process.
Which nutrients are the most important anabolic nutrients?
Cellular respiration is an anabolic process.
The "proton pumps" indicated in the figure are physically associated with ______.
the electron transport chain
Which of the following is a common feature of the both sets of reactions represented by the letters A and B in the figure?
Both include steps involving the oxidation of organic molecules.
Where do the processes indicated by the letter C occur?
along the cristae of the mitochondria
Oxygen is consumed during which of the lettered processes?
Proton pumps within the electron transport chain transport H+ ions ______.
from matrix to intermembrane space
Which of the following statements regarding the electron transport chain is FALSE?
The electron carriers within complex IV have a greater affinity for electrons than the electron carrier within complex III. Oxidation of electron carriers within the electron transport chain results in the transport of protons across the inner mitochondrial membrane. xxx Each enzyme complex in the electron transport chain catalyzes the conversion of oxygen to water.
Transfer of electrons to the electron transport chain from NADH results in more ATP synthesis than transfer of electrons from FADH2.
Which of the following describes the event depicted on the right side of the figure?
Which of the following pathways act as a source for the electrons carried by the NADH and FADH2 molecules shown in this figure?
Which of the following is NOT an end product of the Krebs cycle?
xxx citric acid FADH2 NADH
Which of the following is a characteristic of the electron transport chain (ETC)?
The ETC occurs in the mitochondria.
Ammonia, which is a byproduct of protein metabolism, is converted to __________ primarily in the __________.
__________ is the key hormone regulator of the absorptive state.
__________ are considered "bad" cholesterol; high blood levels are believed to increase the risk of cardiovascular disease.
Which nutrient molecule is the pivotal fuel molecule in the oxidative pathways?
The primary goal during the postabsorptive state is to __________.
maintain blood glucose levels within an adequate range
Which of the following is NOT a pathway in the oxidation of glucose?
xxxx gluconeogenesis Krebs cycle electron transport chain and oxidative phosphorylation
During ketosis caused by inadequate ingestion of carbohydrates, ______.
the glycogen content of the liver is reduced
If cyanide entered cells, the result would be ______.
a net gain of only 4 ATP molecules produced from each glucose molecule
Which term describes the breakdown of stored fats into glycerol and fatty acids?
The hyperglycemia that occurs during diabetes mellitus is accompanied by ______.
Which hormone directs essentially all events of the absorptive state?
The body's rate of kilocalorie consumption needed to fuel all ongoing activities is called the __________.
A growing child is likely to exhibit negative nitrogen balance.
Which of the following is NOT an essential role of the liver?
xxx urea disposal protein metabolism biotransformation functions
One of the reasons why statins are effective in reducing the risk of cardiovascular disease is ______.
they decrease the incidence of cholesterol deposition in arterial walls
Most ATP in cellular respiration is generated in glycolysis.
The body is able to form glucose from non-carbohydrate precursors.
Which hormone is called the "metabolic" hormone?
Which brain region is the main integrating center for thermoregulation?
Leptin is a short-term regulator of food intake.
Metabolism includes both anabolism and catabolism. In hyperthyroidism, the metabolic rate is increased because ______.
the rate of exergonic reactions is increased
Although both conditions share the common characteristic of elevated body temperature, hyperthermia is technically different from fever because ______.
fever is a condition in which there is a change in the body's temperature set-point
Frostbite is LEAST likely to occur in the skin of a body region (e.g., abdominal region, finger region, etc.) that ______.
is closer to the heart than skin of a body region that is farther from the heart
Heavy sweating can induce heat cramps due to ______.
abnormal electrolyte levels in muscle cells abnormal electrolyte levels in neurons abnormal electrolyte levels in interstitial fluid
xxx All of the listed responses are correct.
Peptides called NPY and AgRP are powerful appetite enhancers.
Ghrelin, produced by the stomach, is a powerful appetite stimulant.
An effective way to treat galactosemia would be to stop ingesting galactose and ______.
The primary reason elderly people should decrease their caloric intake is that ________.
muscle mass and metabolism decline with age
What is the primary function of cellular respiration - its end-purpose?
Which of the following would decrease body temperature?
dilation of cutaneous blood vessels
Diets high in cholesterol and saturated fats tend to produce high HDL concentrations.
Select the correct sequence of steps as energy is extracted from glucose during cellular respiration.
glycolysis → acetyl CoA → citric acid cycle → electron transport chain
What is the correct general equation for cellular respiration?
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP energy
Which of the following processes takes place in the cytosol of a eukaryotic cell?
In what organelle would you find acetyl CoA formation, the citric acid cycle, and the electron transport chain?
Which statement describes glycolysis?
This process splits glucose in half and produces 2 ATPs for each glucose.
Which statement describes the citric acid cycle?
This process produces some ATP and carbon dioxide in the mitochondrion.
Which statement describes the electron transport chain?
This process uses energy captured from electrons flowing to oxygen to produce most of the ATPs in cellular respiration.