Currently Internet Explorer is not working properly with our website. Please use the Chrome, Firefox or Safari browsers for a better user experience.

Metabolic Respiration Overview


Krebs Cycle

Crabs = Krebs Cycle
Fire = Pyruvate
Ox-Fossil = Oxidative Phosphorylation

Anaerobic Respiration

Moon = No=oxygen
Milk = Lactic Acid

Metabolic Respiration

Metabolic respiration, also called cellular respiration, is the process that allows the body to break down glucose and turn it into energy. When we hear the word respiration, we think of the act of breathing in and out. Air goes into our lungs and the oxygen is absorbed. Exhaling releases carbon dioxide waste. That may sound like a simple process, and it is something that people do without thinking. However, when we talk about metabolic or cellular respiration, we are concerned with the process that happen inside the cells that allow the body to make usable energy. There are two types of respiration, anaerobic, needing no oxygen, and aerobic, which does use oxygen. There are three processes involved with cellular respiration

metabolic-respirationThe First Process – Glycolysis

Glycolysis occurs in the cytosol. This is a process that takes glucose and converts it into energy. The process of Glycolysis involves 9 steps, during which that 6 carbon Glucose molecule is broken down. When the process is complete, the result is two 3-carbon molecules. The process also gives the body a net gain of 2 molecules of Adenosine triphosphate (ATP) and two molecules of the Nicotinamide Adenine Dinucleotide Hydrate (NADH), as well as pyruvate.

ATP is a nucleic acid used by the body to create energy through a process called hydrolysis. Metabolic respiration is the process in the body that is able to create the ATP that we need. NADH works as an enzyme to help facilitate the reactions needed for the body to turn glucose into energy.

Whether oxygen is present or not, the first step in respiration is Glycolysis. It is a process that can be broken down into 2 stages and 9 steps. The process can be written out as:

C6H12O6 + 2ATP + 2NAD+2pyruvate + 4ATP + 2NADH

Glycolysis is the only one of the three processes of metabolic respiration that does not require oxygen.

The Second Process – The Krebs Cycle

This process is called a cycle because what goes into the Krebs cycle will eventually come back out the same at the end of the cycle. This allows the process to start all over again, thus creating a cycle of events.

During this step, we see the action move from the cytoplasm of the cell to the mitochondria. In the mitochondria, the pyruvate, which is currently a 3-carbon molecule, will become a 2-carbon molecule. When this happens, the extra carbon molecule is released as carbon dioxide. This will then lead to the formation of another NADH molecule, which will then be used to help create more ATP molecules.

The Third Process – Oxidative Phosphorylation

The Krebs Cycle sees the formation of a 6-carbon molecule called citric acid, and as such, is also sometimes called the citric acid cycle. The citric acid is broken up during the cycle to create energy. During this cycle, the cells begin with 2 molecules of pyruvate, which means the cycle will run twice. During the Krebs Cycle, oxygen is needed for electron transport.

This process takes place in the mitochondria and involves the three protein pumps that can be found embedded in the inner membrane of the cell.

Pumps require something to power them. The power comes from the NADH and the Flavin adenine dinucleotide (FADH2) that have been created in the previous processes of glycolysis and the Krebs Cycle. The pumps are responsible for moving the Hydrogen positive ions from the mitochondrial matrix to the intermembrane space.

This results in the formation of more molecules of ATP. The 10 NADH and 2 FADH2 will produce 34 ATP molecules. The protein pumps are also responsible for combining the waste products from the oxidative phosphorylation process with oxygen to form water. Without the presence of oxygen, these molecules would be unable to form ATP.

Respiration without Oxygen

Some organisms do not have the oxygen needed for aerobic respiration, and have to rely on anaerobic respiration. Instead of going through the processes above, the cells go through glycolysis and fermentation to create energy.

Fermentation does not create a surplus of energy, as the Krebs cycle and oxidative phosphorylation do. Instead, it merely resupplies the body with what is needed to keep glycolysis going.


Life requires energy, and that energy comes from sugars. Metabolic, or cellular respiration is set of processes by which organisms break down sugars, mostly glucose, and release that energy. In all cases, the first step of metabolic respiration is glycolysis. During glycolysis, glucose is broken down into two 3-carbon molecules, called pyruvate. In the presence of sufficient oxygen, glycolysis is followed by the Krebs cycle and oxidative phosphoraylation, which makes a lot of energy available in the form of ATP. When there isn't sufficient oxygen for those two steps, fermentation occurs. Fermentation doesn't release nearly as much energy as the Krebs cycle and oxidative phosphorylation, but it does keep the process going.

Metabolic Respiration

 1. Zoom: whole scene

Hot Spot: Geico-Lizard = Glycolysis

Learning: In this CoursePic, we'll take a very broad look at metabolic respiration and its three steps: the glycolysis, the Krebs cycle, and anaerobic respiration. Each of these three steps takes the form of a race. The first stage is glycolysis, represented by the geico-lizards the first team of racers is riding. During this phase, glucose, represented by the candy the riders are eating, is broken down by enzymes, creating pyruvic acid.

Story: The Great Metabolic Race has begun! The first team, the Geico-Lizards, includes a team of three who ride on the back of their own lizard mounts. These riders love candy and treat themselves to some of their favorite sugary snacks as they race.

2. Zoom: The second race in the desert

Hot Spot: Crabs = Krebs Cycle; fire = pyruvate; ox-fossil = oxidative phosphorylation

Learning: Next, the pyruvate generated from glycolysis fuels pyruvate oxidation. This occurs in the Krebs cycle. This part of respiration requires oxygen and consumes the pyruvate. At the end of the Krebs cycle, oxidative phosphorylation occurs. This is where 90% of all energy from metabolic respiration is generated. This phase is represented by a team of racers riding crabs. They’re on fire, which represents the pyruvate, and finish up on a large ox-fossil, representing oxidative phosphorylation.

Story: While the Geico-Lizard team is doing well, they’re not in first place. They’re beaten to the finish line by the Crabs. This team is on fire literally! Fortunately, they put out the flames by running through a pool of water before reaching the finish line. At the end, they jump on a large Ox-fossil to take a break.

3. Zoom: Runners on the moon

Hot Spot: Moon = No oxygen; milk = lactic acid

Learning: Anaerobic respiration is a little different. Glycolysis occurs, but instead of using oxygen, the process uses other types of electron acceptors such as those created through alcohol or lactic acid fermentation. This type of respiration is represented by racers running on the moon, where there is no oxygen. Lactic acid fermentation is represented by the bottles-of-milk.

Story: The final team has run so far they’re on the moon! This team is also on fire, but luckily they have bottles-of-milk they can use to douse the flames.

This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.