SAT Biology E/M Subject Test

Part II: Subject Review

Chapter 5 Cellular Respiration

Cellular respiration is a series of chemical reactions that takes place inside a cell. These chemical reactions produce energy for the cell. This chapter will explore the various molecules and mechanisms involved in cellular respiration.

LET”S TALK ABOUT CELLULAR RESPIRATION

Cellular respiration is nothing more than a series of chemical reactions that occur in a cell. The purpose of these chemical reactions is to produce energy for the cell. Cellular energy comes in the form of a molecule called ATP. ATP stands for adenosine triphosphate.

A molecule of ATP, as its name indicates, is made of adenosine bonded to three phosphate molecules. There”s a lot of energy stored in the bond that holds the third phosphate to the molecule.

When energy is required for some process in the cell (for example, active transport), the cell will hydrolyze (break) the bond between the second and third phosphate molecules on ATP. This releases the energy. What remain are a molecule of adenosine diphosphate (ADP) and one molecule of phosphate.

ATP creates the energy to fuel the cell”s processes.

ATP → ADP + 1 phosphate molecule + energy

Where the Cell Gets Its ATP: Glycolysis, Krebs Cycle, Electron Transport Chain, and Oxidative Phosphorylation

As we previously mentioned, glucose is the primary source of “cellular food,” meaning that this is the molecule that a cell can break down to make ATP. We also talked about glycogen and said it”s made of many glucose molecules bonded together; in other words, its purpose is to store energy.

Cellular Respiration

When it comes to energy,
think of it like this:

• ATP is like cash.

• Glucose is like a check.

• Glycogen is like a bank.

When a cell needs ATP, it
goes to its glycogen stores
and takes out a glucose
molecule (a check). Then
it cashes the glucose
molecule to get ATP (cash).
The “check” is “cashed”
through cellular
respiration.

This is the summary equation for cellular respiration:

It doesn”t occur, however, as a single reaction, in a single step. It occurs in a series of smaller steps designed to maximize the production of energy, as we”ll see shortly.

The table on the next page provides a summary of the events that occur in cellular respiration and where in the cell they take place. Remember that each of the chemical reactions that occurs requires its own enzyme, so there are substrates and products for each step. This is the MINIMUM amount you need to memorize about cellular respiration. Note that some substrates and some products are left out to keep this table relatively simple. In the following pages, each event will be explained in slightly greater detail.

Electron Carriers

There”s one last thing to know before the detailed discussion. Basically, cellular respiration is the breakdown of glucose to release energy. Usable energy for the body is ATP. However, the energy released from the breakdown of glucose is not all in the form of ATP. A small amount of ATP is made, but most of the energy is stored as electrons on special molecules called electron carriers. When an “empty” electron carrier accepts a pair of electrons, we say it has become reduced. When it gives those electrons up later on, we say it has become oxidized.

The two most common electron carriers in the body (and the ones that will be used during cellular respiration) are NAD⁺ and FAD.

NAD⁺ or FAD ?

NAD⁺ can accept a pair of electrons (and a hydrogen ion) to become NADH: 2e^_

NAD⁺ + H⁺ NADH

FAD can accept a pair of electrons (and two hydrogen ions) to become FADH₂:

FAD + 2 H⁺ FADH₂

These electron carriers will shuttle around the electrons until they can be used at a later time to make usable energy, ATP, for the body.

1. Glycolysis

The word “glycolysis” describes what happens during this process. Glyco- means “sugar,” and -lysis means “splitting.” So glycolysis literally means “sugar splitting,” and that”s exactly what happens. One molecule of glucose is split in half to produce two molecules of pyruvate. A pyruvate molecule is essentially half of a glucose molecule.

Two ATP molecules are needed to start the process of splitting the glucose molecule in half. So before we even accomplish this first step of cellular respiration, we are two ATP “in the hole.” However, during the chemical reactions of glycolysis, four ATP molecules are formed, and two NAD⁺ molecules accept electrons (become reduced) to become two NADH. So our net end products from glycolysis are two ATP, two NADH, and two pyruvate molecules. The ATP is, of course, immediately usable. The NADH simply carries the electrons until they can be used to make more ATP later on. Here (more or less) is the summary reaction for glycolysis. It”s not incredibly detailed, but it will give you an idea of what goes in and what comes out:

C₆H₁₂O₆ + 2 ATP + 2 NAD⁺ → 2 Pyruvate + 4 ATP + 2 NADH

Glycolysis occurs without oxygen. When a process occurs without oxygen, we describe it as anaerobic. Glycolysis is an anaerobic process. However, the remaining steps of cellular respiration all require oxygen; they are aerobic.

2. The Pyruvate Dehydrogenase Complex (PDC)

If oxygen is available, the pyruvate formed during glycolysis can continue on, through a series of reactions designed to produce more reduced electron carriers and more ATP. The pyruvate dehydrogenase complex (PDC) is a group of enzymes that prepares pyruvate to enter the next (third) step of cellular respiration: the Krebs cycle. Pyruvate contains three carbon atoms. However, the Krebs cycle can only accept a molecule that contains two carbon atoms. So the PDC”s job is to remove one of the carbons from pyruvate and to attach the remaining two-carbon structure to a coenzyme called coenzyme A. In the process, another molecule of NADH is produced, as shown in the following diagram.

The carbon that is removed leaves in the form of carbon dioxide (CO₂). And don”t forget, because there are two molecules of pyruvate at the end of glycolysis, two molecules of acetyl Co-A are produced, two molecules of NADH are produced, and two molecules of carbon dioxide are produced.

Remember, we said earlier that this would occur only if oxygen were available. That”s correct, but where”s the oxygen? Oxygen is not used directly in this step of cellular respiration, but, as we will see later, if oxygen is not available, this step can”t proceed. So this is an aerobic process; it requires oxygen.

Where Does It All Happen?

As you read about the steps of cellular respiration, you may begin to wonder where within the cell this all occurs. Glycolysis takes place in the cytoplasm. An important organelle in the process of cellular respiration is the mitochondria. Recall that mitochondria have an outer membrane as well as an intricately folded inner membrane. Glycolysis takes place outside the mitochondria. The enzymes that drive the Krebs cycle are found in the matrix of the mitochondria, and the electron transport chain takes place along the inner membrane of the mitochondria.

Quick Quiz #1

Fill in the blanks and check the appropriate boxes:

1. The process of glycolysis produces ATP and NADH by converting one molecule of _________________________ to two molecules of ______________________________.

2. The pyruvate dehydrogenase complex is found in the _____________of the mitochondria.

3. The process of glycolysis [ does does not ] require oxygen.

4. _________________________ is made of many glucose molecules bonded together, and its function is to store energy.

5. When a molecule (such as an electron carrier) accepts a pair of electrons, we say it has become [ reduced oxidized ].

6. During the PDC, a molecule of pyruvate is converted to _________, a molecule of _________________________ is produced, and _________________________ is lost.

7. The PDC [ is is not ] an aerobic process.

Correct answers can be found in Chapter 15.

3. The Krebs Cycle

The Krebs cycle is called such because the first molecule is regenerated each time the cycle is completed. That”s why, if you look back at the summary table under “Krebs cycle,” you see oxaloacetic acid listed as both a substrate and a product.

Here”s an overview of what happens in the Krebs cycle. Acetyl Co-A is combined with oxaloacetic acid to form citric acid. (In fact, the Krebs cycle is sometimes referred to as the citric acid cycle for this reason.) Citric acid is broken down, one carbon at a time (released as carbon dioxide), then rearranged to form the original oxaloacetic acid molecule. In the process, three molecules of NADH, one molecule of FADH₂, and one molecule of ATP are made. Also, carbon dioxide is released.

Here”s a picture of the Krebs cycle. You don”t need to memorize it, but if you see a picture like this on the SAT Biology E/M Subject Test, you”ll know it”s the Krebs cycle.

Two last points to remember: First, for every glucose molecule, we get two pyruvate molecules and therefore two acetyl Co-A molecules. So the Krebs cycle runs twice for each glucose molecule. Second, the Krebs cycle requires oxygen. Just like in the PDC, oxygen is not used directly, but if it isn”t available, the Krebs cycle can”t run. The Krebs cycle is an aerobic process.

So far we”ve made a little bit of usable energy (ATP). We made two ATP during glycolysis and another two during the Krebs cycle. But most of our energy is stored in the form of reduced electron carriers. It would be great to take these reduced electron carriers and use them to make more ATP … and that”s exactly what the cell does during electron transport and oxidative phosphorylation.

4. Electron Transport and Oxidative Phosphorylation

Electron transport and oxidative phosphorylation have two primary goals:

1. to return the electron carriers to their “empty” state (oxidize them), and

2. to use the energy from those electrons to make ATP

If we don”t oxidize the electron carriers back to their “empty” states, we can”t keep running glycolysis, PDC, or the Krebs cycle. We have to have empty electron carriers so that they can accept electrons during those three processes.

The electron transport chain is a process in which NADH and FADH₂ hand down electrons to a chain of carrier molecules. The electrons are passed along the chain until they”re given to oxygen, which forms water.

Electron transport is an aerobic process. Oxygen is known as the final electron acceptor, because it is the last molecule in the electron transport chain to accept electrons.

So we finally see how oxygen is used. Clearly, electron transport is an aerobic process.

If oxygen is unavailable, it”s not there to accept electrons from the transport chain. The transport chain backs up because each member of the chain is “stuck” with its electrons. Ultimately, it backs up all the way to NADH and FADH₂, and they”re unable to get rid of their electrons (and thus are not oxidized to “empty”). And if they”re unable to be oxidized, there will be no “empty” carriers available for glycolysis, PDC, and the Krebs cycle. Glycolysis, luckily, has another method of acquiring “empty” electron carriers (we”ll see it in a minute), but the PDC and the Krebs cycle are stuck. Without empty electron carriers available, these processes shut down. That”s why, even though PDC and the Krebs cycle don”t use oxygen directly, they still rely on it to run.

So the first goal has been accomplished. But what about the second goal? How can we use the energy of these electrons to make ATP?

Remember where the steps of cellular respiration are taking place:

Electron transport takes place along the inner membrane of the mitochondria. Remember that mitochondria are double-membraned organelles. The carrier molecules of the electron transport chain use the energy of the electrons they”re transporting to pump H⁺ ions out of the mitochondrial matrix and into the space between the membranes.

This creates a gradient of H⁺ ions—lots of H⁺ in the space between the membranes, and less H⁺ in the matrix. The H⁺ ions would really like to get back into the matrix, but they can”t cross the membrane very easily because they are charged (remember that charged substances can cross membranes only by facilitated diffusion). However, if we provide the H⁺ ions with a method for getting across, they will. And we can use that to our advantage.

The method of getting across the inner membrane is provided by a special protein called an ATP synthase. ATP synthase allows the H⁺ ions back into the matrix, and on the way in, that energy is used to phosphorylate an ADP to an ATP. Here”s the picture:

Now both goals have been accomplished. NADH and FADH₂ have been oxidized back to NAD⁺ and FAD and are free to accept electrons from glycolysis, PDC, and the Krebs cycle. And we”ve used the energy stored on those electron carriers to make some usable energy for the body in the form of ATP.

Altogether, the four steps of cellular respiration produce about 36 ATP molecules for each molecule of glucose that”s broken down.

Quick Quiz #2

Fill in the blanks and check the appropriate boxes:

1. The Krebs cycle [ does does not ] require oxygen.

2. The principal substance that enters the Krebs cycle is ____________.

3. Oxygen is also known as the ______________________________.

4. One of the goals of electron transport is to [ reduce oxidize ] the electron carriers back to “empty.”

5. Electron transport occurs along the __________________________of the mitochondria.

6. The products of the Krebs cycle are three molecules of ___________, one molecule of _______________________, and one molecule of _______________________.

7. ATP synthase relies on the facilitated diffusion of ________________ions down their gradient to produce ATP.

8. In the last step of the electron transport chain, oxygen accepts electrons to form _________________________.

Correct answers can be found in Chapter 15.

What Happens If Oxygen Is Not Available?

If oxygen is not available, the electron carriers cannot be oxidized back to “empty.” Because the PDC and the Krebs cycle absolutely rely on these electron carriers to run, if the electron carriers are not available, PDC and the Krebs cycle shut down. Theoretically, glycolysis should shut down as well, but glycolysis has its own method of creating empty electron carriers that does not rely on oxygen. That”s why glycolysis is an anaerobic process.

Regenerating empty electron carriers in the absence of oxygen is called fermentation. Remember that in order to “empty” (oxidize) an electron carrier, the electrons must be removed and given to something else (something else must be reduced).

Let”s look at what we have at the end of glycolysis:

• 2 ATP

• 2 NADH

• 2 pyruvate

The ATP is, of course, usable energy. The NADH cannot be used to make more ATP, because the electron transport chain is shut down in the absence of oxygen. The pyruvate cannot be converted to acetyl Co-A, because, in the absence of oxygen, the PDC is shut down also. So, because these two substances are not being used, why not take the electrons off the NADH and donate them to pyruvate? That would regenerate an empty electron carrier that can be reused to run glycolysis again, and that”s exactly what happens.

The NADH gives its electrons up to pyruvate, thereby becoming NAD⁺. Pyruvate, having accepted these electrons, becomes reduced. But to what?

What pyruvate gets reduced to depends upon what organism is running fermentation. In yeast, pyruvate is reduced (in two steps) to ethanol, and carbon dioxide is released as a by-product. In human muscle cells, pyruvate is reduced to lactic acid. Here”s the picture:

There are only two problems with fermentation. First, the end products, ethanol or lactic acid, are toxic. Yeast die when the ethanol concentration reaches about 12 percent. Muscle cells stop contracting if lactic acid levels get too high (and pH subsequently drops).

Second (and this is the big one to remember for the exam):

The only ATP you get from fermentation are the two net ATP from glycolysis. So instead of a big 36 ATP per glucose from aerobic cellular respiration, only two ATP per glucose are produced.

Two ATP per glucose is enough for yeast, a single-celled organism, to survive. And it”s enough for muscle cells to keep functioning for a short while. But it is certainly not enough for large, multicellular organisms like humans, other animals, and plants to survive. This is why humans, other animals, and plants absolutely MUST have oxygen to survive.

Quick Quiz #3

Fill in the blanks and check the appropriate boxes:

1. Fermentation produces _______________________ in yeast and ______________________ in muscle cells.

2. Anaerobic organisms [ do do not ] conduct glycolysis.

3. Anaerobic respiration (fermentation) produces [ less more ] ATP than aerobic respiration.

4. In fermentation, NADH is _________________________ to NAD⁺, whereas pyruvate is _________________________.

5. Anaerobic organisms [ do do not ] conduct the Krebs cycle.

Correct answers can be found in Chapter 15.

Key Words

adenosine triphosphate (ATP)

hydrolyze

adenosine diphosphate (ADP)

electron carriers

reduced

oxidized

glycolysis

pyruvate

anaerobic

aerobic

pyruvate dehydrogenase complex (PDC)

coenzyme A

Krebs cycle (citric acid cycle)

electron transport

oxidative phosphorylation

final electron acceptor

ATP synthase

fermentation

alcoholic fermentation

lactic acid fermentation

Summary

• When glycolysis, PDC, the Krebs cycle, electron transport, and oxidative phosphorylation have all finished, the cell is left with 36 molecules of ATP per glucose.

• During glycolysis, glucose is converted to two molecules of pyruvic acid. A net total of two ATP and two NADH are formed along the way.

• During PDC, each molecule of pyruvate is converted to acetyl Co-A. This produces one molecule of NADH per pyruvate and releases carbon dioxide.

• During the Krebs cycle, the acetyl Co-A from the PDC is combined with oxaloacetic acid to form citric acid, then carbons are removed and rearranged to form oxaloacetic acid again. In the process, three NADH are produced, one FADH₂ is produced, and one ATP is produced (per acetyl Co-A). Carbon dioxide is released.

• During electron transport, NADH and FADH₂ give up electrons to the electron transport chain (are oxidized), and the electrons are handed down through a series of carrier molecules and finally to oxygen to form water. Oxygen is known as the final hydrogen acceptor. As the electrons are passed along the carrier molecules, H⁺ ions are pumped out of the matrix and into the space between the inner and outer mitochondrial membranes (an H⁺ gradient is formed).

• During oxidative phosphorylation, the H⁺ gradient created during electron transport is used to drive the production of ATP from ADP and P_i. A protein known as an ATP synthase allows the H⁺ ions back into the matrix.

• Without oxygen, fermentation occurs. Fermentation produces toxic by-products and two net ATP. This process is more useful for unicellular organisms than multicellular organisms.