The AP® Biology Exam tests the principles of cellular respiration every year. Cellular respiration is an important topic to study, and it tends to be one of the more challenging topics for students. In this AP® Biology Crash Course Review, we will review the parts of cellular respiration that you may see on your AP® Bio exam. We will first review what cellular respiration is and a few other things that you must know before we delve into the details of the process. We will then divide cellular respiration into four major steps and review each step to the extent that you could be tested on. We will then use the information that we have reviewed to answer a question about cellular respiration that was seen on a past AP® Biology exam.

What is Cellular Respiration?

Cellular respiration is the process that cells use to release energy from chemical bonds in food. The cell can then use this energy for the essential processes of life that require energy. It is possible for cellular respiration to be aerobic and anaerobic. Aerobic respiration is more favorable and produces more energy. All cells must go through cellular respiration. In eukaryotic cells cellular respiration will occur in the cytoplasm and mitochondria and in prokaryotic cells it will occur in the cytoplasm.

ATP

Before we begin, there are a few items worth discussing. First of all, in this process, ATP will be used as energy. You probably know this by now that ATP is the cellular “currency” for energy, but it is important to understand why. ATP is made up of three phosphate groups that each have negative charges. The negative charges that each of the phosphates possesses are causing repulsion to occur in the molecule, and are effectively pulling the molecule apart. We use ATP to store energy because when we remove one of the phosphate groups from ATP a large amount of energy will be released. The released energy from this reaction is then coupled to an unfavorable reaction.

Step One: Glycolysis

Glycolysis is the breaking down of sugar or glucose. Glycolysis occurs in the cytoplasm and regardless of aerobic or anaerobic respiration, must occur first. To begin, the cell must use two molecules of ATP as activation energy for the rearrangement of glucose to fructose diphosphate. Fructose will then be split into two three-carbon molecules of PGAL. The cell will then harvest energy from the rearrangement of PGAL to pyruvate. From each PGAL molecule, two ATP molecules and one NADH molecule is made. You must be able to recall the names of these precursor molecules for the AP® Biology exam.

It is important to study how much energy is created at this step total and net total. The total amount of energy created during glycolysis from a glucose molecule is four ATPs and two NADHs. The total net energy is two ATPs and two NADHs due to the use of two ATPs at the beginning of the reaction.

Step Two: Oxidation of Pyruvate

Before entering the Krebs Cycle(also known as the citric acid cycle), pyruvate must be made into acetyl coA. This process occurs in the matrix of the mitochondria. The pyruvate molecule is oxidized, losing two electrons and a hydrogen molecule. The oxidation results in the creation of a NADH molecule and the loss of CO₂. After pyruvate is made into acetyl coA, the molecule enters the Krebs Cycle.

Step Three: Krebs Cycle

The Krebs Cycle will take place in the matrix of the mitochondria. The acetyl coA molecule made in step two of cellular respiration will then enter into the Krebs Cycle. Acetyl-CoA will first be bonded to a four carbon molecule called oxaloacetate. Oxaloacetate is made in the final step of the Krebs Cycle. When the two join together, they form citric acid, a six carbon molecule. There are several intermediates that occur in the Krebs Cycle but for the purpose of the AP® Biology exam, you do not need to know all of their names or shapes, it is just important to know what is produced.

During the Krebs Cycle electrons and hydrogen ions are removed from the citric acid molecule. The high energy electrons that are moved are added to electron carriers to form NADH and FADH₂. The electron carriers are important because they must carry the electrons to step four of the process, the electron transport chain. The Krebs Cycle produces 6 NADH, 2 FADH₂, 2 ATP, and 4CO₂ molecules. Most of the energy produced in this step is contained in the electron carriers.

Step Four: Electron Transport Chain

The electron transport chain is the step in cellular respiration that creates the most energy. ATP is generated by the step wise release of energy using the folds of the cristae in the mitochondria. The first step of the electron chain is when one of the electron carriers that were created in the Krebs Cycle will release an electron. The electron will be taken by a different carrier that will move through three different membrane proton pump proteins. As the electron passes through the proton pump, its energy is harnessed to pump a proton to the other side of the membrane.

The pumping of the proton creates a chemiosmoticgradient. A chemiosmotic gradient essentially means that the concentration of protons on one side of the membrane is significantly different than the concentration of the protons on the other side of the membrane. When the protons are held at this gradient, they begin to repel each other and entrance to the other side of the membrane would release energy.

After the electron has moved through the entire electron transport chain, will undergo reactions to form H₂O and exit the third protein. On the end of the three proteins is a very important membrane protein called ATP synthase. ATP synthase will then allow a proton to move from the high to low concentration, and it will use the energy that it releases to create ATP.

The electron transport chain produces 34 ATPs using this process of oxidative phosphorylation. Overall, aerobic cellular respiration will produce: 6CO2, 6H2O and 38 ATP molecules. As you can see the electron transport chain is responsible for the majority of ATP production.

Fermentation

Remember that we said cellular respiration could occur with or without oxygen? Without oxygen, the cell will not be able to use the electron transport chain because oxygen is the final electron acceptor (when H₂O is made using the electron). Fermentation is the anaerobic respiration process. There are two common ways that fermentation occurs.

Alcoholic Fermentation

In alcoholic fermentation, after glycolysis pyruvate is converted to carbon dioxide and ethanol. In this process, the NADH is recycled to NAD+ which allows for glycolysis to keep occurring. This fermentation occurs in yeast and certain bacteria which are used to create bread and wine.

Lactic Acid Fermentation

In lactic acid fermentation, pyruvate is converted to lactic acid. Again, the NADH is recycled to NAD+ which allows for glycolysis to keep occurring. This process occurs in animal and bacterial cells. After strenuous exercise, this fermentation will occur in muscle cells causing fatigue and lactic acid build up. Lactic Acid Fermentation is responsible for the “burn”.

AP® Biology Exam Question

Here is an example of a question about cellular respiration from the AP® Biology Exam. Let’s see how you can use your knowledge to get full credit!

All of the following provide evidence of an increased rate of cellular respiration EXCEPT

(A) increase in the concentration of CO2

(B) decrease in the concentration of O2

(C) a low pH in the inner membrane space

(D) increased activity of ATPsynthase

(E) an increase in the concentration of lactic acid

If you chose E, you are correct. As we have just learned lactic acid is a product of lactic acid fermentation, not cellular respiration.

Thank you for reading this article, Cellular Respiration: AP® Biology Crash Course Review! We really appreciate your feedback, let us know how we did!

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