Aerobic Respiration Ap Biology Essay

The AP Biology course has so many different concepts and ideas within it that it may seems like there are way too many to understand.  We get that, and want to make sure that you know the most important concepts as well as possible.  We’ve picked out a fewof the most important AP Bio concepts for you which fall under the umbrella of two critical processes: photosynthesis and cellular respiration.


The process of photosynthesis is an essential AP Biology concept to understand.  Photosynthesis is the means by which most of the plant life on Earth gets its energy.  All animals receive nutrients either from plants or from animals that get their energy from plants.  This means that plants are the basis for all of the ecosystems on Earth,making photosynthesis a very important process.  So, in order to understand how photosynthesis happens, biologists have to break it down into two parts, the light and dark reactions.

1. Light Reaction (or the Light-Dependent Reactions)

This is the first step in the process of photosynthesis.  The light reaction is the process by which the leaf of a plant will absorb energy from the sun (in the form of photons) and will transfer it into electron carriers and energy (in the form of Adenosine Triphosphate, or ATP).

First, light will enter the leaf and will be absorbed by pigments within photosystem II, which is the first half of the light reaction.  After the light is absorbed by the pigments, the photon is bounced around inside the photosystem until it reaches the pigment that is correct for that wave of light.  Once it reaches that pigment, it will excite an electron and will move the electron up to the primary electron acceptor.

After reaching the primary electron acceptor, the electron will be moved to photosystem I.  It will be transported across what is known as the electron transport chain.  All you need to know about the ETC is that the electron will move down it and as it moves down it will transfer energy, which is used by the plant to combine ADP and a phosphate group into ATP.

Once the electron reaches photosystem I (just remember that photosystems go in reverse, II is first and I is second), it is excited by another photon and the electron again is bounced to a primary electron acceptor.  Only this time when the electron falls, it is used to combine two hydrogen ions and one NADP+ ion to create NADPH.  This will be used later on in the dark reactions to further the process.  Once the NADPH has been made, the ATP and the NADPH move into the dark reactions, the second part of the cycle.

2. Dark Reaction (or the Light Independent Reaction/Calvin Cycle)

The next and final process of photosynthesis is the Calvin cycle.  This is the process that takes the NADPH, ATP and CO2 and and converts them into G3P (which can be turned into glucose, the basic unit of energy).  This process is a cycle, so part of the product, G3P, will be used to start and end the cycle.

The cycle starts with three five-carbon chains that have a phosphorus attached to each of them using some of the ATP, creating three RuBPs.  After this, carbon dioxide is fixed (carbon will be added) to the RuBP and will create three unstable six-carbon compounds.  Then the compound will fall apart and will make six three-carbon chains.  Next, an electron will be donated to the three carbon chain and it will create the end product, 6 G3Ps.  One of these G3Ps will be removed from the cycle and the other five will be used to repeat the cycle and create the five-carbon chain.  Finally, after enough G3P is made, it will be turned into glucose and thus photosynthesis is complete.

The Different Parts of Cellular Respiration

Cellular respiration is what cells will do in order create energy from glucose.  The currency of energy that is used by cells is called ATP (which pops up often on the AP Biology exam), which is the product of this reaction.  Cell respiration can be broken down into five different steps: glycolysis, formation of acetyl COA, Krebs cycle, oxidative phosphorylation and chemiosmosis.

1. Glycolysis and Formation of Acetyl COA

The first step in the essential AP Biology concept of cellular respiration is glycolysis, which is a simple reaction that moves in a linear chain.  All you need to know for the AP Biology exam is that one glucose will be turned into two pyruvate, two ATP and two NADH (similar to NADPH, it is an electron carrier).  The only other thing you will need to know about glycolysis is that it takes place in the cytoplasm of the cell.

The next step is another simple process that takes place in the cytoplasm of the cell.  This step takes one pyruvate and will turn it into one acetyl COA using coenzyme A.  This process will also create two NADH during the process.  Now, onto the Krebs cycle.

2. The Krebs Cycle

This is again another cycle, which is common among AP Biology concepts, but this takes place in the matrix of the mitochondria.  It will take the acetyl COA and will use it to create electron carriers and energy for the next part of the reaction.  This process will create one ATP, three NADHs and one FADH2.  In this process acetyl COA will be combined with oxaloacetate, which will then turn into citric acid.  After this, this compound will go around the cycle, releasing two carbons along the cycle and creating two carbon dioxides in the process.  This process will then repeat itself and allow the electron carriers to move onto the next cycle.

3. Oxidative Phosphorylation and Chemiosmosis

These two steps will occur at the same time.  All of those electron acceptors that were made in the previous steps will be used to release the electrons that will go down the electron transport chain.  As these electrons move down the chain, protons will be pumped out of the matrix of the mitochondria.  This will cause a higher concentration of protons to exist, increasing the proton concentration  Because of this, the protons will want to re-enter the matrix, and they will do so using a channel known as ATP Synthase.  This will cause this protein to create more ATP as the protons travel across it.  Finally after the ATP is made, the electrons will be accepted by the final electron acceptor, oxygen.  This will combine with the protons (also called hydrogen ions) to make water.

How to Get Better?

If you have read this and you understand these AP Biology concepts, then you are that much closer to succeeding on your AP Biology exam!  If you want to test your knowledge of these topics, practice questions are a great way to do this.

Photo by Light-dependent reactions of photosynthesis at the thylakoid membrane” Licensed under Public Domain via Wikipedia.

By the way, you should check out for your AP Biology review. We have hundreds of AP Biology practice questions written just for you!

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.


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.


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! If you want to study more with us, check out our article, Chloroplasts and Mitochondria: AP Biology Crash Course Review!

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