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Advanced Placement (AP)

AP Biology is known for being one of the tougher AP exams , and, for most students, the free-response section is the hardest part of the test. In 2021 , the average score for every free-response question was less than a 50%! However, knowing what to expect can make it easier to get a great score on AP Biology FRQ. And in this guide, we explain everything you need to know to ace this section. Read on to learn the format of AP Biology FRQ, what graders are looking for, what the questions will look like, and what you can do to be well-prepared on exam day.

What's the Format of the AP Biology Free Response Section?

The AP Biology exam has two sections: multiple choice and free response. The free-section comes second and contains six questions:

• Two long-response questions , both with a focus on analyzing experimental results. The second long question will require you to create a graph.
• Four short-answer questions on the following topics in this order:
• Scientific Investigation
• Conceptual Analysis
• Analysis of Model or Visual Representation
• Analysis of Data

Additionally:

• The free-response section is 90 minutes long
• It's worth 50% of your total score
• You're able to use the AP Biology formula sheet for the entire section

Long questions are worth 8-10 points each, whereas short-answer questions are each worth 4 points. It's recommended that you spend about 25 minutes on each long question and about 10 minutes on each short question (although you'll decide yourself how long you spend on each question).

The AP Biology test expects you to know how to:

• Understand how graphical and mathematical models can be used to explain biological principles and concepts
• Make predictions and justify events based on biological principles
• Implement your knowledge of proper experimental design
• Interpret data

AP Biology Sample Free Response Questions 

Now we'll go through two AP Biology free response example questions: one long question and one short question. These questions both were used for the 2021 AP Biology exam . You can see answers and scoring for each of the 2021 AP Biology FRQs here .

Long Question

First let's look at one of the long questions. This is Question 2, so remember you'll need to create a graph for at least one part of it. The entire question is worth 8 points.

Part A (1 point)

Part B (4 points)

First, you need to create a graph based on the data in Table 1. The graph is worth 3 points: 1 for axis labels, 1 for the correct plotting in the bar graph, and 1 for the error bars. Here's an example of a graph that would get full points:

Part C (1 point)

Part D (2 points)

• The data do not support the claim because females III-2 and III-6 have the disorder and, if inheritance was X-linked recessive, they'd only have the disorder if their father II-1 had the disorder, which he does not.
• The data supports mitochondrial inheritance because all of the offspring of individual II-2 , not just the sons, have the disorder.

Giving one of those answers is worth one point.

Short Question

Next is a short question. It's question 3 in the free-response section which means it will focus on scientific investigation. It's worth a total of four points.

Part B (1 point)

• The researchers must run the experiment without adding resveratrol.
• The researchers must treat the cells with DMSO alone.

There are two potential answers; you only need to include one:

• No ATP production
• Reduced ATP production

Part D (1 point)

For Part D, you must state that more electrons can be transferred so that more oxygen is required as the final electron acceptor.

Where to Find AP Biology FRQs

Taking practice tests and answering practice questions is one of the best ways to prepare for any AP exam, including AP Biology FRQs. Fortunately, the College Board, who creates and administers AP courses and exams, has made dozens of old AP Bio FRQs available for free online. Because there are so many official FRQs available, we recommend only using them instead of looking online for unofficial questions (those not created by the College Board), which can be hit or miss in terms of quality. However, if you're using an AP Biology prep book, they often have solid FRQs. For advice on which prep book to get, check out our guide on the best AP Biology prep books.

Here are links to the FRQs:

Additionally, the AP Biology Course and Exam Description includes two up-to-date FRQs, beginning on page 206.

Note that, until 2020, the AP Bio exam had six short-answer questions instead of the current four. This means that questions from 2019 and earlier will have a different format and slightly different content. They can still be useful to study, but be aware of the differences between them and the current free-response section.

4 Tips for AP Biology FRQs

When you're studying for AP Bio FRQs and actually taking the exam, there are a lot of things to remember to ensure you do your absolute best. Keep these four tips in mind throughout the year and on exam day.

#1: Know Your 13 Required Labs

There are 13 labs you're required to complete during the AP Biology course. Questions that relate at least in part to these labs make up 25% of the AP Biology exam. It’s important to understand how these labs are conducted and how the principles behind them relate to the main ideas of the course. This will help in answering both free-response and multiple-choice questions that deal with lab scenarios on the test. There's a nice overview of each of the 13 labs on this site that can refresh your memory, and we link to in-depth explanations of each of the labs in our AP Biology study guide .

You should also know general lab skills. Many free-response questions ask you to identify the components of a proposed experiment (dependent and independent variables) or to design a lab to test a certain hypothesis. You might have forgotten about the labs you did toward the beginning of the year, so take extra care to go over them. Make sure that you understand just how they were conducted and what the results mean.

#2: Eliminate Irrelevant Information

Free-response AP Biology questions (especially the long questions) include lots of scientific terminology and visual aids, and this kind of format might be intimidating if you’re not used to it. It’s important to practice sorting through this jumble of information so that you can quickly get to the root of the question rather than obsessing over small details you don’t understand.

Try underlining important words and phrases in the question to help you stay focused on the main points and avoid misleading distractions.

You should also practice responding to free-response questions in a straightforward way without any unnecessary fluff. Remember, this isn’t an English test; the graders are just looking for clear facts and analysis. Make it easy for them to give you points!

#3: Draw During Studying

If you're feeling shaky on your knowledge of a process or system in AP Biology, one helpful strategy is to draw it. This will both reinforce what you know and highlight what you still need to work on learning. Once you're able to draw an accurate diagram of a system or process without looking at your notes, you can feel confident that you know exactly how it works.

For example, you could challenge yourself to draw a diagram of a cell membrane, label its different components, and explain their significance. You could also draw a process like mitosis that happens in clear visual stages, or a more complex process like cellular respiration where you might focus on one aspect at a time (glycolysis, Krebs cycle, electron transport chain). You can also apply this tip during the exam, if you need help visualizing part of an AP Bio FRQ.

#4: Pay Attention to the Clock

Time is always tight on AP exams. For the AP Biology free response section, you get 90 minutes to answer six questions. It can be easy to get caught up on one question and suddenly realize you're nearly out of time but haven't had a chance to look at some of the questions, let alone answer them. Don't let this happen to you! We recommend spending 25 minutes on each of the two long questions and 10 minutes on each of the four short questions. You don't need to keep perfectly to that plan, but don't get too far off it, either. 

At the very least, make note of where you are halfway through the free-response section (that's 45 minutes in). If you're roughly halfway finished with the section (taking into account that long questions take about twice as much time to complete as short questions), you're doing well. If you're significantly behind that, you know you need to pick up the pace.

Also, don't feel you need to answer the FRQ in the order they're listed. We recommend skimming through each of the questions at the start of the section, then tackling the questions that seem easiest first so you can spend more time on trickier questions.

Summary: Acing the AP Biology Free Response Section

The AP Biology free-response section can be tough, but if you prepare well for it, you can go into exam day confident and knowing what to expect. The section consists of two long questions and four short questions, lasts 90 minutes, and is worth half of your total score. You'll need to create a graph for the second AP biology FRQ. Old exam questions are a great study resource and, when you're preparing for the free-response section, keep these four tips in mind:

• Know your labs
• Eliminate irrelevant information
• Make drawings while studying
• Stay aware of time

What's Next?

How should you study for the AP Biology exam?  Our expert article goes over all 5 steps to take during your AP Biology review.

What is the rest of the AP Biology exam like? Our article on the AP Biology exam goes over every question type you can expect to see as well as tips for answering them.

Looking for an easier AP class than Biology? Learn which AP classes tend to be the least challenging for students .

Looking for help studying for your AP exam?

Our one-on-one online AP tutoring services can help you prepare for your AP exams. Get matched with a top tutor who got a high score on the exam you're studying for!

Christine graduated from Michigan State University with degrees in Environmental Biology and Geography and received her Master's from Duke University. In high school she scored in the 99th percentile on the SAT and was named a National Merit Finalist. She has taught English and biology in several countries.

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3.6 Cellular Respiration

5 min read • december 28, 2022

Caroline Koffke

Haseung Jun

Cellular Respiration is a chemical process with the following equation: C6H12O6 + O2 → H2O + CO2. All organisms, including those capable of photosynthesis, go through the process of cellular respiration . The overall reaction breaks down a carbohydrate, most frequently modeled by glucose, and converts the energy stored in that molecule into the most basic cellular energy, ATP. Respiration is almost the complete opposite of photosynthesis. So if you understood photosynthesis, understanding respiration should be relatively easy.

Cellular Respiration is broken down into three major steps which are dependent on one another: glycolysis , the Krebs cycle , and the electron transport chain . While glycolysis takes place in the cytoplasm of the cell, the Krebs cycle and the electron transport chain take place inside of the mitochondria .

Image courtesy of WikiMedia Commons .

Glycolysis is the most evolutionarily conserved process in cellular respiration . The process takes place in all living organisms in almost the exact same way. Fundamentally, glycolysis involves breaking down glucose, which possesses 6 carbons, into two 3-carbon molecules of pyruvate .

In the process, a small amount of energy is released due to the breaking of bonds. This is captured as 2 molecules of ATP. Similarly, the breaking of bonds releases a few electrons that are picked up by electron carriers, NADH . These electrons will be dropped off to the electron transport chain later.

Before pyruvate can continue on into the mitochondria to enter the Krebs cycle , pyruvate oxidation takes place. Oxidation is the loss of electrons. In this process, pyruvate becomes a 2-carbon molecule called acetyl CoA . A molecule of carbon dioxide is released from each pyruvate molecule that is oxidized.

Krebs Cycle

The Krebs Cycle takes place in the mitochondria . In this cycle, similarly to the Calvin Cycle , a number of enzymes process a number of reactions that… you DON’T need to know about! (unless you go to medical school one day… good luck!)

The moral of the story is that a number of highly specific enzymes break down acetyl CoA in reactions that create a number of electrons and a little bit of energy. The process results in the creation of a lot of electron carriers (around 8) such as NADH and FADH2 . These electron carriers will allow a lot of ATP production in the electron transport chain . 2 ATP are also produced in the Krebs Cycle .

Electron Transport Chain

The electron transport chain is where the majority of ATP is produced. The chain works in the same way as the electron transport chain in photosynthesis. A concentration gradient is formed, and ATP synthase is responsible for creating ATP.

When hydrogen ions are dropped off by electron carriers to the electron transport chain , the hydrogen ion is pumped across the plasma membrane to form a high concentration gradient of hydrogen ions . These will be used by ATP synthase .

The electron travels through the electron transport chain on a number of electronegative proteins . It eventually ends up binding with oxygen, the final electron acceptor. When oxygen accepts the electron, it forms a bond with hydrogen ions and water is created.

The concentration gradient of hydrogen travels through ATP synthase , in the same way as it does in photosynthesis, the kinetic energy is used to phosphorylate ADP into ATP. This process is called chemiosmosis , as ions are moving down their concentration gradient. This process produces somewhere between 30 and 40 ATP molecules. Don’t worry, you don’t need to know specific numbers! Just know that a TON more ATP is produced through this process than through either glycolysis or the Krebs cycle .

Another important aspect of the electron transport chain is the recycling of electron carriers . This takes place when they drop off their electron and can then be refilled in glycolysis or the Krebs cycle . If these carriers were not emptied, the cycle would not be able to continue.

Fermentation

In organisms without access to oxygen, anaerobic respiration takes place. This happens in a number of bacteria, and in other organisms when oxygen is being used up faster than it can be inhaled (think crazy workout). 

Without oxygen, the Krebs cycle and electron transport chain cannot take place, because there is no final electron acceptor. Instead, electron carriers must be recycled elsewhere. This happens through the process of fermentation . 

Organisms find other molecules to drop off their electrons. Some examples include creating lactic acid , ethanol , and carbon dioxide . This is how beer and wine are fermented by various bacteria and yeast. In humans, our body produces lactic acid when oxygen is in short supply, such as in a tough workout. This can create sore muscles the next day.

Image courtesy of Pixabay .

The main takeaway about fermentation is that cells MUST recycle their electron carriers in order to continue to reuse them to produce ATP. They will find another molecule to drop their electrons off on. Secondly, during anaerobic respiration , glycolysis , alone, is producing ATP . This means that ATP production is MUCH lower than in aerobic respiration.

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AP® Biology

Cellular respiration: ap® biology crash course review.

• The Albert Team
• Last Updated On: March 1, 2022

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 CO 2 . 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 FADH 2 . 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 FADH 2 , 2 ATP, and 4CO 2 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 H 2 O 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 H 2 O 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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