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Cellular Respiration/Fermentation Problem Leading to Lack of Energy

Cellular Respiration/Fermentation Problem Leading to Lack of Energy


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A young animal has never had much energy. He is brought to a veterinarian for help and is sent to the animal hospital for some tests. There they discover his mitochondria can use only fatty acids and amino acids for respiration, and his cells produce more lactate than normal. Of the following, which is the best explanation of his condition?

A) His mitochondria lack the transport protein that moves pyruvate across the outer mitochondrial membrane.

B) His cells cannot move NADH from glycolysis into the mitochondria.

C) His cells contain something that inhibits oxygen use in his mitochondria.

D) His cells lack the enzyme in glycolysis that forms pyruvate.

E) His cells have a defective electron transport chain, so glucose goes to lactate instead of to acetyl CoA.

Answer: A

A) So, from this assumption, pyruvate would build up in the cytosol of cell. However, I do not see why lactate would be formed. Lactate forms when there is a lack of NAD+. In this case, there is no problem with NAD+. All NADH will be recycled into NAD+ by the Electron Transport Chain in the mitochondria.

B) I do not know what will happen in this case. Too many contradicting factors. Can anyone explain?

C) This is clearly not correct. If there were no O2,then there would only be lactic acid fermentation; the protein and fatty acids would not be used.

D) If lactate is formed, pyruvate must have been formed.

E) The electron transport chain needs to work. See C). Same explanation.

So, I am debating between A and B. Can anyone explain why the textbook says A?

EDIT : If anyone could give me a step-by-step approach to solving this problem, I would really appreciate it. This is supposed to be a challenge problem not representative of test questions, but I still want to fully get it.


Short answer
(A) is a possible answer and is indeed cause for fatigue, as pyruvate is needed for the Krebs cycle to run. The krebs cycle is an essential step in the generation of ATP in aerobic organisms. (B) is incorrect because NADH is never transported into the mitochondria in any organism (it is a nonsense answer).

Background
NADH is not transported into mitochondria. Instead, its electrons are transported into mitochondria, and I quote from Berg et al. (2002):

NADH cannot simply pass into mitochondria for oxidation by the respiratory chain, because the inner mitochondrial membrane is impermeable to NADH and NAD+. The solution is that electrons from NADH, rather than NADH itself, are carried across the mitochondrial membrane. One of several means of introducing electrons from NADH into the electron transport chain is the glycerol 3-phosphate shuttle (Figure 18.37).

Electrons from NADH can enter the mitochondrial electron transport chain by being used to reduce dihydroxyacetone phosphate to glycerol 3-phosphate. Glycerol 3-phosphate is reoxidized by electron transfer to an FAD prosthetic group in a membrane-bound glycerol 3-phosphate dehydrogenase. Subsequent electron transfer to Q to form QH2 allows these electrons to enter the electron-transport chain.

Hence, (B) is incorrect.

EDIT: Now why would lactate be formed in (A)? Pyruvate builds up in the cell, as it cannot be transported into the mitochondria. Since lactate dehydrogenase has pyruvate as a substrate, the equilibrium of this reaction will be pushed to the right and lactate will be formed:

Source: wikipedia page on lactate dehydrogenase

The lack of glucose oxidation takes away the main power supply of aerobic organisms, causing fatigue. Lactate buildup may also induce fatigue.

Reference
Berg et al. Biochemistry 2002, Freeman, New York


AP Biology 3.4 - Cellular Energy

This section of the AP Biology curriculum is meant to be an introduction to what energy is, the rules that apply to energy, and how cells must operate in order to maintain these rules, survive, grow, and reproduce. In this section, we’ll look at a broad overview of how cells gather and distribute energy. We’ll also see how the first and second laws of thermodynamics apply to how cells utilize energy. This includes how cells gather energy via photosynthesis in chloroplast and how cells utilize energy via aerobic respiration in the mitochondria. Though later sections of the AP Biology curriculum dive into the complexities of those processes, in this section we will be looking at how cells create highly ordered systems of enzymes and couple exothermic reactions with endothermic reactions to be as efficient as possible to conserve energy.


Cellular Respiration/Fermentation Problem Leading to Lack of Energy - Biology

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

6.5. Anaerobic Cellular Respiration

Although aerobic cellular respiration is the fundamental process by which most organisms generate ATP, some organisms do not have the necessary enzymes to carry out the Krebs cycle and ETS. Most of these are Bacteria or Archaea, but there are certain eukaryotic organisms, such as yeasts, that can live in the absence of oxygen and do not use their Krebs cycle and ETS. Even within multicellular organisms, there are differences in the metabolic activities of cells. For example some of your cells are able to survive for periods of time without oxygen. However, all cells still need a constant supply of ATP. An organism that does not require O2 as its final electron acceptor is called anaerobic (an = without aerob = air) and performs anaerobic cellular respiration. Although some anaerobic organisms do not use oxygen, they are capable of using other inorganic or organic molecules as their final electron acceptors. The acceptor molecule might be sulfur, nitrogen, or other inorganic atoms or ions. It might also be an organic molecule, such as pyruvic acid (CH3COCOOH). Anaerobic respiration is an incomplete oxidation and results in the production of smaller electron-containing molecules and energy in the form of ATP and heat (figure 6.10).

FIGURE 6.10. Anaerobic Cellular Respiration in Perspective

This flowchart shows the relationships among the various types of cellular respiration and the descriptive terminology used. Notice that all begin with a molecular source of energy and end with the generation of ATP.

Many organisms that perform anaerobic cellular respiration use the glycolytic pathway to obtain energy. Fermentation is the word used to describe anaerobic pathways that oxidize glucose to generate ATP by using an organic molecule as the ultimate hydrogen electron acceptor. Electrons removed from sugar in the earlier stages of glycolysis are added to the pyruvic acid formed at the end of glycolysis. Depending on the kind of organism and the specific enzymes it possesses, the pyruvic acid can be converted into lactic acid, ethyl alcohol, acetone, or other organic molecules (figure 6.11).

The upper portion of this figure is a simplified version of glycolysis. Many organisms can carry out the process of glycolysis and derive energy from it. The ultimate end product is determined by the kinds of enzymes the specific organism can produce. The synthesis of these various molecules is the organism’s way of oxidizing NADH to regenerate NAD + and reducing pyruvic acid to a new end product.

Organisms that produce ethyl alcohol have genes for the production of enzymes that guide electrons onto pyruvic acid. This reaction results in the conversion of pyruvic acid to ethyl alcohol (ethanol) and carbon dioxide. Other organisms have different genes, produce different enzymes, carry out different reactions, and, therefore, lead to the formation of different end products of fermentation. The formation of molecules such as alcohol and lactic acid is necessary to regenerate the NAD + needed for continued use in glycolysis. It must be done here, because it is not being regenerated by an ETS, as happens in aerobic respiration. Although many products can be formed from pyruvic acid, we will look at only two fermentation pathways in more detail.

Alcoholic Fermentation

Alcoholic fermentation is the anaerobic respiration pathway that yeast cells follow when oxygen is lacking in their environment. In this pathway, the pyruvic acid (CH3COCOOH) is converted to ethanol (a 2-carbon alcohol, CH3CH2OH) and carbon dioxide. Yeast cells then are able to generate only 4 ATPs from glycolysis. The cost for glycolysis is still 2 ATPs thus, for each glucose a yeast cell oxidizes, it profits by 2 ATPs.

Although during alcoholic fermentation yeasts get ATP and discard the waste products ethanol and carbon dioxide, these waste products are useful to humans. In making bread, the carbon dioxide is the important end product it becomes trapped in the bread dough and makes it rise—the bread is leavened. Dough that has not undergone this process is called unleavened. The alcohol produced by the yeast evaporates during the baking process. In the brewing industry, ethanol is the desirable product produced by yeast cells. Champagne, other sparkling wines, and beer are products that contain both carbon dioxide and alcohol. The alcohol accumulates, and the carbon dioxide in the bottle makes them sparkling (bubbly) beverages. In the manufacture of many wines, the carbon dioxide is allowed to escape, so these wines are not sparkling they are called “still” wines.

Summary of Alcohol Fermentation

a. Glucose is metabolized to pyruvic acid.

2. During alcoholic fermentation

a. pyruvic acid is reduced to form ethanol.

b. carbon dioxide is released.

Lactic Acid Fermentation

In lactic acid fermentation, the pyruvic acid (CH3COCOOH) that results from glycolysis is converted to lactic acid (CH3CHOHCOOH) by the transfer of electrons that had been removed from the original glucose. In this case, the net profit is again only 2 ATPs per glucose. The buildup of the waste product, lactic acid, eventually interferes with normal metabolic functions and the bacteria die. The lactic acid waste product from these types of anaerobic bacteria are used to make yogurt, cultured sour cream, cheeses, and other fermented dairy products. The lactic acid makes the milk protein coagulate and become pudding-like or solid. It also gives the products their tart flavor, texture, and aroma (Outlooks 6.2).

In the human body, different cells have different metabolic capabilities. Nerve cells must have a constant supply of oxygen to conduct aerobic cellular respiration. Red blood cells lack mitochondria and must rely on the anaerobic process of lactic acid fermentation to provide themselves with energy. Muscle cells can do either. As long as oxygen is available to skeletal muscle cells, they function aerobically. However, when oxygen is unavailable—because of long periods of exercise or heart or lung problems that prevent oxygen from getting to the skeletal muscle cells—the cells make a valiant effort to meet energy demands by functioning anaerobically.

When skeletal muscle cells function anaerobically, they accumulate lactic acid. This lactic acid must ultimately be metabolized, which requires oxygen. Therefore, the accumulation of lactic acid represents an oxygen debt, which must be repaid in the future. It is the lactic acid buildup that makes muscles tired when we exercise. When the lactic acid concentration becomes great enough, lactic acid fatigue results. As a person cools down after a period of exercise, breathing and heart rate stay high until the oxygen debt is repaid and the level of oxygen in the muscle cells returns to normal. During this period, the lactic acid that has accumulated is converted back into pyruvic acid. The pyruvic acid can then continue through the Krebs cycle and the ETS as oxygen becomes available. In addition to what is happening in the muscles, much of the lactic acid is transported by the bloodstream to the liver, where about 20% is metabolized through the Krebs cycle and 80% is resynthesized into glucose.

Summary of Lactic Acid Fermentation

a. Glucose is metabolized to pyruvic acid.

2. During lactic acid fermentation

a. pyruvic acid is reduced to form lactic acid.

b. no carbon dioxide is released.

3. Muscle cells have the enzymes to do this, but brain cells do not.

a. Muscle cells can survive brief periods of oxygen deprivation, but brain cells cannot.

b. Lactic acid “burns” in muscles.

The fermentation of carbohydrates to organic acid products, such as lactic acid, is commonly called souring. Cultured sour cream, cheese, and yogurt are produced by the action of fermenting bacteria. Lactic-acid bacteria of the genus Lactobacillus are used in the fermentation process. While growing in the milk, the bacteria convert lactose to lactic acid, which causes the proteins in the milk to coagulate and come out of solution to form a solid curd. The higher acid level also inhibits the growth of spoilage microorganisms. Spoilage, or putrefaction, is the anaerobic respiration of proteins with the release of nitrogen and sulfur-containing organic compounds as products. Protein fermentation by the bacterium Clostridium produces foul-smelling chemicals such as putrescine, cadaverine, hydrogen sulfide, and methyl mercaptan. Clostridium perfringens and C. sporogenes are the two anaerobic bacteria associated with the disease gas gangrene. A gangrenous wound is a foul-smelling infection resulting from the fermentation activities of those two bacteria.

9. Why are there different end products from different forms of fermentation?


Sulli's Biology

*Organisms get the energy they need from food. A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius. The Calorie (capitol C) that is used on food labels is a kilocalorie, or 1000 calories.

Cells can use all sorts of molecules for food, including fats, proteins, and carbohydrates.

1 gram of carb = 4 Calories
1 gram of protein = 4 Calories
1 gram of fat (lipid) = 9 Calories

Photosynthesis and Cellular Respiration are opposite processes.

For 9.1 Powerpoint click here

Lesson 9.2 The Process of Cellular Respiration



Flour is so flammable that it has caused several explosions, including the one seen here at London's City Flour Mills in 1872. More recently, a sugar factory exploded along the Savannah river.
Glycolysis
**The benefits of glycolysis are that it happens very fast and does not require oxygen!**

To initiate glycolysis, an investment of 2 ATP (activation energy) must be made.

During Glycolysis, 1 molecule of glucose, a 6-carbon compound, is transformed into 2 molecules of Pyruvic Acid, a 3-carbon compound. Energy is released in the form of high energy electrons when the bonds are broken and is captured by the "energy police" NAD+ and carried away to the ETC.

The products of Glycolysis, 2 Pyruvic Acids, can now move into the mitochondria and go through the 2nd process of respiration, the Krebs Cycle.

In the Krebs Cycle, which occurs in the matrix of the mitochondria, Pyruvic Acid is broken down to CO2 in a series of energy-extracting reactions.

Pyruvic Acid
to
Acetic Acid + C(exhaled as CO2)
to
Acetic Acid is attached to CoenzymeA
becoming
Acetyl-CoA
which bonds with a 4-Carbon cyclic molecule forming

Citric Acid

Citric Acid is broken to

a 5-Carbon molecule + CO2
then
a 4-Carbon molecule + CO2

**Each time bonds are broken, "Energy police" NAD+ and FAD swoop in and collect the high energy electrons lost and carry them away to the ETC!

In addition, 1 ATP comes from each turn of the Krebs Cycle. Each glucose molecule spins 2 Krebs cycles from it's 2 molecules of Pyruvic Acid. So, Krebs spits out 2 ATPS from each glucose!


As a result of Kreb's Cycle, each Pyruvic Acid (3 Carbons) is broken down releasing 3 CO2s into the air while the energy from these bonds was collected and sent to the ATP Factory, the Electron Transport Chain (ETC).

THE KREBS CYCLE

Similar to the ETC in photosynthesis, the ETC in respiration uses high energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.

The "energy police" (high energy electron carriers) NAD+ and FAD carry electrons loaded with energy to the ETC.

NAD+ + 2e- + H+ -------> NADH
and
FAD + 2e- + 2H+ --------> FADH2

The ETC occurs in the inner membrane of the mitochondria where transport proteins are embedded. NADH and FADH2 drop off their high energy electrons in the membrane where they move from protein to protein passing on their energy to them. The job of the transport proteins is to pump H+ ions across the membrane against their concentration gradient from low to high concentration (active transport). This causes an extreme concentration gradient with the high being in the Intermembrane Space (between the two membranes) and the low being in the matrix. As are the laws of nature, H+ will gladly diffuse (passive transport) from high to low concentration and does this through the good sport enzyme protein, ATP Synthase. When H+ ions fly through ATP Synthase trying to escape the crowd, this causes ATP Synthase to spin, and this energy is used to add P (phosphates) to ADP making ATPs.



What if Oxygen is not around?

What happens when you hold your breath and dive under water, or use up oxygen more quickly than you can replace it?

Recall that glycolysis is an anaerobic process which can produce ATP quickly. But, when a cell generates large amts of AP from glycolysis, it runs into a problem.

In just a few seconds, all of the cell's available NAD+ molecules used to collect high energy electrons from Glycolysis will get used up if the ETC does not occur (an aerobic process). This will leave the cell with no available NAD+ an therefore, will shut down Glycolysis and production of ATPs.

In the absence of O2, fermentation, which takes place in the cytoplasm, releases energy from food molecules producing ATP!

During fermentation, cells convert NADH to NAD+ by passing high energy electrons back to Pyruvic Acid. This action concerts NADH back into the electron carrier NAD+, allowing glycolysis to produce a steady supply of ATP.

Fermentation can take two pathways:

Alcoholic Fermentation
and
Lactic Acid Fermentation

In Alcoholic Fermentation, which is carried out by yeasts and a few other microorganisms, the Pyruvic Acid accepts the high energy electrons from glycolysis and is broken down to ethyl alcohol and carbon dioxide while relieving NADH of it's electrons so that it becomes NAD+ again and recycles back through glycolysis, keeping it going.



Aerobic Respiration and Fermentation (With Diagram)

The reactions of glycolysis have no specific requirement for oxygen. Oxidation reactions do occur, such as the removal of two hydrogen’s from glyceraldehyde- 3-phosphate, and NAD + is reduced to NADH, but oxygen per se is not consumed.

Under anaerobic con­ditions (i.e., in the absence of oxygen), pyruvate may be reduced to a variety of different compounds.

Alco­holic fermentation (reactions 10-19 and 10-20, Fig. 10-4) is a common pathway in microorganisms and is of industrial importance.

In these two steps, 1 mole of CO2 is removed from each mole of pyruvate (i.e., 2 miles of CO2 per mole of monosaccharide) and NADH is reoxidized to NAD + , thereby producing ethanol.

The stoichiometric relationships and the cyclic in­volvement of NAD are especially important in this pathway. During the oxidation of each mole of glyceraldehyde-3-phosphate (reaction 10-10, Fig. 10- 2), 1 mole of NAD + is reduced to form NADH and during the conversion of acetaldehyde to ethanol (re­action 10-20, Fig. 10-4), a mole of NADH is oxidized to form NAD + .

Therefore, the levels of NAD + and NADH are unaffected by the conversion of glyceraldehyde-3-phosphate to ethanol. The quanti­ties of these coenzymes in cells are very small. There­fore, if the NAD + reduced in reaction 10-10 was not reoxidized, this central pathway would soon be blocked at the glyceraldehyde-3-phosphate step by the lack of NAD + and the pathway would necessarily have to cease.

Like NAD + and NADH, ATP and ADP are cycled between the ATP-requiring reactions in the early steps of glycolysis and the ATP-producing reactions in the later steps. Cells contain small pools of ATP, ADP, and AMP when needed, these compounds are drawn from the pools and later returned to the pools.

During the early stages of glycolysis, 2 moles of ATP are con­sumed per mole of glucose, whereas during the later stages 4 moles of ATP are produced. The early stages proceed by drawing ATP from the pool, while the lat­ter reactions return ATP to the pool. However, there is a new generation of ATP from glycolysis that is then available for other energy-requiring reactions within the cell.

Another common fate of pyruvate that occurs in the absence of oxygen is its conversion to lactate. This is a normal process in active muscle cells that are not re­ceiving adequate amounts of oxygen and in many plant and bacterial cells that live under anaerobic con­ditions.

In a single reaction, pyruvate is converted to lactate (reaction 10-21, Fig. 10-4). During the reac­tion, 1 mole of NADH is converted to NAD + . This stoichiometrically balances NADH production during the earlier glyceraldehyde oxidation step.


Anaerobic Pathways | Back to Top

Under anaerobic conditions, the absence of oxygen, pyruvic acid can be routed by the organism into one of three pathways: lactic acid fermentation, alcohol fermentation, or cellular (anaerobic) respiration. Humans cannot ferment alcohol in their own bodies, we lack the genetic information to do so. These biochemical pathways, with their myriad reactions catalyzed by reaction-specific enzymes all under genetic control, are extremely complex. We will only skim the surface at this time and in this course.

Alcohol fermentation is the formation of alcohol from sugar. Yeast, when under anaerobic conditions, convert glucose to pyruvic acid via the glycolysis pathways, then go one step farther, converting pyruvic acid into ethanol, a C-2 compound.

Fermentation of ethanol. Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Many organisms will also ferment pyruvic acid into, other chemicals, such as lactic acid. Humans ferment lactic acid in muscles where oxygen becomes depleted, resulting in localized anaerobic conditions. This lactic acid causes the muscle stiffness couch-potatoes feel after beginning exercise programs. The stiffness goes away after a few days since the cessation of strenuous activity allows aerobic conditions to return to the muscle, and the lactic acid can be converted into ATP via the normal aerobic respiration pathways.

Fermentation of lactate (lactic acid). Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.


Anaerobic Cellular Respiration

The production of energy requires oxygen. The electron transport chain, where the majority of ATP is formed, requires a large input of oxygen. However, many organisms have developed strategies to carry out metabolism without oxygen, or can switch from aerobic to anaerobic cell respiration when oxygen is scarce.

During cellular respiration, some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD + from NADH are collectively referred to as fermentation. In contrast, some living systems use an inorganic molecule as a final electron acceptor. Both methods are called anaerobic cellular respiration, where organisms convert energy for their use in the absence of oxygen.

Certain prokaryotes, including some species of bacteria and archaea, use anaerobic respiration. For example, the group of archaea called methanogens reduces carbon dioxide to methane to oxidize NADH. These microorganisms are found in soil and in the digestive tracts of ruminants, such as cows and sheep. Similarly, sulfate-reducing bacteria and archaea, most of which are anaerobic, reduce sulfate to hydrogen sulfide to regenerate NAD + from NADH.

Figure (PageIndex<1>): Anaerobic bacteria: The green color seen in these coastal waters is from an eruption of hydrogen sulfide-producing bacteria. These anaerobic, sulfate-reducing bacteria release hydrogen sulfide gas as they decompose algae in the water.

Eukaryotes can also undergo anaerobic respiration. Some examples include alcohol fermentation in yeast and lactic acid fermentation in mammals.


CELLULAR METABOLISM AND FERMENTATION

Glycolysis, the Universal Process | Back to Top

Nine reactions, each catalyzed by a specific enzyme, makeup the process we call glycolysis . ALL organisms have glycolysis occurring in their cytoplasm .

At steps 1 and 3 ATP is converted into ADP, inputting energy into the reaction as well as attaching a phosphate to the glucose. At steps 6 and 9 ADP is converted into the higher energy ATP. At step 5 NAD + is converted into NADH + H + .

The process works on glucose, a 6-C, until step 4 splits the 6-C into two 3-C compounds. Glyceraldehyde phosphate (GAP, also known as phosphoglyceraldehyde, PGAL) is the more readily used of the two. Dihydroxyacetone phosphate can be converted into GAP by the enzyme Isomerase. The end of the glycolysis process yields two pyruvic acid (3-C) molecules, and a net gain of 2 ATP and two NADH per glucose.

Graphic summary of the glycolysis process. Image from W.H. Freeman and Sinauer Associates, used by permission.

Anaerobic Pathways | Back to Top

Under anaerobic conditions, the absence of oxygen, pyruvic acid can be routed by the organism into one of three pathways: lactic acid fermentation, alcohol fermentation, or cellular (anaerobic) respiration. Humans cannot ferment alcohol in their own bodies, we lack the genetic information to do so. These biochemical pathways, with their myriad reactions catalyzed by reaction-specific enzymes all under genetic control, are extremely complex. We will only skim the surface at this time and in this course.

Alcohol fermentation is the formation of alcohol from sugar. Yeast, when under anaerobic conditions, convert glucose to pyruvic acid via the glycolysis pathways, then go one step farther, converting pyruvic acid into ethanol, a C-2 compound.

Fermentation of ethanol. Image from W.H. Freeman and Sinauer Associates, used by permission.

Many organisms will also ferment pyruvic acid into, other chemicals, such as lactic acid. Humans ferment lactic acid in muscles where oxygen becomes depleted, resulting in localized anaerobic conditions. This lactic acid causes the muscle stiffness couch-potatoes feel after beginning exercise programs. The stiffness goes away after a few days since the cessation of strenuous activity allows aerobic conditions to return to the muscle, and the lactic acid can be converted into ATP via the normal aerobic respiration pathways.

Fermentation of lactate (lactic acid). Image from W.H. Freeman and Sinauer Associates, used by permission.

Aerobic Respiration | Back to Top

When oxygen is present (aerobic conditions), most organisms will undergo two more steps, Kreb's Cycle , and Electron Transport , to produce their ATP. In eukaryotes, these processes occur in the mitochondria, while in prokaryotes they occur in the cytoplasm.

Overview of the cellular respiration processes. Image from W.H. Freeman and Sinauer Associates, used by permission.

Acetyl Co-A: The Transition Reaction

Pyruvic acid is first altered in the transition reaction by removal of a carbon and two oxygens (which form carbon dioxide). When the carbon dioxide is removed, energy is given off, and NAD+ is converted into the higher energy form NADH. Coenzyme A attaches to the remaining 2-C (acetyl) unit, forming acetyl Co-A . This process is a prelude to the Kreb's Cycle.

Kreb's Cycle (aka Citric Acid Cycle)

The Acetyl Co-A (2-C) is attached to a 4-C chemical (oxaloacetic acid). The Co-A is released and returns to await another pyruvic acid. The 2-C and 4-C make another chemical known as Citric acid, a 6-C. Kreb's Cycle is also known as the Citric Acid Cycle. The process after Citric Acid is essentially removing carbon dioxide, getting out energy in the form of ATP, GTP, NADH and FADH 2 , and lastly regenerating the cycle. Between Isocitric Acid and a -Ketoglutaric Acid, carbon dioxide is given off and NAD+ is converted into NADH. Between a -Ketoglutaric Acid and Succinic Acid the release of carbon dioxide and reduction of NAD + into NADH happens again, resulting in a 4-C chemical, succinic acid. GTP (Guanine Triphosphate, which transfers its energy to ATP) is also formed here (GTP is formed by attaching a phosphate to GDP).

The remaining energy carrier-generating steps involve the shifting of atomic arrangements within the 4-C molecules. Between Succinic Acid and Fumaric Acid, the molecular shifting releases not enough energy to make ATP or NADH outright, but instead this energy is captured by a new energy carrier, Flavin adenine dinucleotide (FAD). FAD is reduced by the addition of two H's to become FADH 2 . FADH 2 is not as rich an energy carrier as NADH, yielding less ATP than the latter.

The last step, between Malic Acid and Oxaloacetic Acid reforms OA to complete the cycle. Energy is given off and trapped by the reduction of NAD + to NADH. The carbon dioxide released by cells is generated by the Kreb's Cycle, as are the energy carriers (NADH and FADH 2 ) which play a role in the next step.

Summary of the Krebs' (or citric acid) cycle. Image from W.H. Freeman and Sinauer Associates, used by permission.

Electron Transport Phosphorylation

Whereas Kreb's Cycle occurs in the matrix of the mitochondrion, the Electron Transport System (ETS) chemicals are embedded in the membranes known as the cristae . Kreb's cycle completely oxidized the carbons in the pyruvic acids, producing a small amount of ATP, and reducing NAD and FAD into higher energy forms. In the ETS those higher energy forms are cashed in, producing ATP. Cytochromes are molecules that pass the "hot potatoes" (electrons) along the ETS chain. Energy released by the "downhill" passage of electrons is captured as ATP by ADP molecules. The ADP is reduced by the gain of electrons. ATP formed in this way is made by the process of oxidative phosphorylation. The mechanism for the oxidative phosphorylation process is the gradient of H + ions discovered across the inner mitochondrial membrane. This mechanism is known as chemiosmotic coupling. This involves both chemical and transport processes. Drops in the potential energy of electrons moving down the ETS chain occur at three points. These points turn out to be where ADP + P are converted into ATP. Potential energy is captured by ADP and stored in the pyrophosphate bond. NADH enters the ETS chain at the beginning, yielding 3 ATP per NADH. FADH 2 enters at Co-Q, producing only 2 ATP per FADH 2 .


Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen and is therefore anaerobic (processes that use oxygen are called aerobic). Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways.

  1. Through secondary active transport in which the transport takes place against the glucose concentration gradient.
  2. Through a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist in the facilitated diffusion of glucose.

Glycolysis begins with the six carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate (Figure 1).

Figure 1. Reactants and products of glycolysis.

Glycolysis consists of ten steps divided into two distinct halves. The first half of the glycolysis is also known as the energy-requiring steps. This pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. The second half of glycolysis (also known as the energy-releasing steps) extracts energy from the molecules and stores it in the form of ATP and NADH, the reduced form of NAD.

First Half of Glycolysis (Energy-Requiring Steps)

Figure 2. The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.

Step 1. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.

Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, fructose-6-phosphate. An isomerase is an enzyme that catalyzes the conversion of a molecule into one of its isomers. This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules.

Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when the concentration of ADP is high it is less active when ADP levels are low and the concentration of ATP is high. Thus, if there is “sufficient” ATP in the system, the pathway slows down. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.

Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. Thus, the pathway will continue with two molecules of a single isomer. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule.

Second Half of Glycolysis (Energy-Releasing Steps)

So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules. Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules.

Figure 3. The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.

Step 6. The sixth step in glycolysis (Figure 3) oxidizes the sugar (glyceraldehyde-3-phosphate), extracting high-energy electrons, which are picked up by the electron carrier NAD + , producing NADH. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.

Here again is a potential limiting factor for this pathway. The continuation of the reaction depends upon the availability of the oxidized form of the electron carrier, NAD + . Thus, NADH must be continuously oxidized back into NAD + in order to keep this step going. If NAD + is not available, the second half of glycolysis slows down or stops. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP. In an environment without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH to NAD + .

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. (This is an example of substrate-level phosphorylation.) A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.

Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing this step is a mutase (a type of isomerase).

Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).

Step 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate). Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions.

Outcomes of Glycolysis

Glycolysis starts with glucose and ends with two pyruvate molecules, a total of four ATP molecules and two molecules of NADH. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and two NADH molecules for its use.

If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. Mature mammalian red blood cells are not capable of aerobic respiration—the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP. If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die.

The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half. Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.

In Summary: Glycolysis

Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD + . Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.

Figure 4 shows the entire process of glycolysis in one image:


Cellular Respiration/Fermentation Problem Leading to Lack of Energy - Biology

IN THIS CHAPTER

Summary: This chapter covers the basics behind the energy-creation process known as respiration. This chapter also teaches you the difference between aerobic and anaerobic respiration and takes you through the steps that convert a glucose molecule into ATP.

Aerobic respiration: glycolysis &rarr Krebs cycle &rarr oxidative phosphorylation &rarr 36 ATP.

Anaerobic respiration: glycolysis &rarr regenerate NAD + &rarr much less ATP.

Oxidative phosphorylation results in the production of large amounts of ATP from NADH and FADH2 .

Chemiosmosis is the coupling of the movement of electrons down the electron transport chain with the formation of ATP using the driving force provided by the proton gradient.

Introduction

In this chapter, we explore how cells obtain energy. It is important that you do not get lost or buried in the details. You should finish this chapter with an understanding of the basic process. The AP Biology exam will not ask you to identify by name the enzyme that catalyzes the third step of glycolysis, nor will it require you to name the fourth molecule in the Krebs cycle. But it will ask you questions that require an understanding of the respiration process.

BIG IDEA 2.A.1

All living things require constant input of energy .

There are two major categories of respiration: aerobic and anaerobic. Aerobic respiration occurs in the presence of oxygen, while anaerobic respiration occurs in situations where oxygen is not available. Aerobic respiration involves three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation. Anaerobic respiration, sometimes referred to as fermentation, also begins with glycolysis, and concludes with the formation of NAD + .

Aerobic Respiration

BIG IDEA 2.A.2

Heterotrophs capture free energy present in the food they eat through cellular respiration .

Glycolysis occurs in the cytoplasm of cells and is the beginning pathway for both aerobic and anaerobic respiration. During glycolysis, a glucose molecule is broken down through a series of reactions into two molecules of pyruvate. It is important to remember that oxygen plays no role in glycolysis. This reaction can occur in oxygen-rich and oxygen-poor environments. However, when in an environment lacking oxygen, glycolysis slows because the cells run out (become depleted) of NAD + . For reasons we will discuss later, a lack of oxygen prevents oxidative phosphorylation from occurring, causing a buildup of NADH in the cells. This buildup causes a shortage of NAD + . This is bad for glycolysis because it requires NAD + to function. Fermentation is the solution to this problem&mdashit takes the excess NADH that builds up and converts it back to NAD + so that glycolysis can continue. More to come on fermentation later . . . be patient.

To reiterate, the AP Biology exam will not require you to memorize the various steps of respiration. Your time is better spent studying the broad explanation of respiration, to understand the basic process, and become comfortable with respiration as a whole. Major concepts are the key. We will explain the specific steps of glycolysis because they will help you understand the big picture&mdashbut do not memorize them all. Save the space for other facts you have to know from other chapters of this book.

Examine Figure 7.1 , which illustrates the general layout of glycolysis. The beginning steps of glycolysis require energy input. The first step adds a phosphate to a molecule of glucose with the assistance of an ATP molecule to produce glucose-6-phosphate (G6P). The newly formed G6P rearranges to form a molecule named fructose-6-phosphate (F6P). Another molecule of ATP is required for the next step, which adds another phosphate group to produce fructose 1,6-biphosphate. Already, glycolysis has used two of the ATP molecules that it is trying to produce&mdashseems stupid . . . but be patient . . . the genius has yet to show its face. F6P splits into two 3-carbon-long fragments known as PGAL (glyceraldehyde phosphate). With the formation of PGAL, the energy-producing portion of glycolysis begins. Each PGAL molecule takes on an inorganic phosphate from the cytoplasm to produce 1,3-diphosphoglycerate. During this reaction, each PGAL gives up two electrons and a hydrogen to molecules of NAD + to form the all-important NADH molecules. The next step is a big one, as it leads to the production of the first ATP molecule in the process of respiration&mdashthe 1,3-diphosphoglycerate molecules donate one of their two phosphates to molecules of ADP to produce ATP and 3-phosphoglycerate (3PG). You’ll notice that there are two ATP molecules formed here because before this step, the single molecule of glucose divided into two 3-carbon fragments. After 3PG rearranges to form 2-phosphoglycerate, phosphoenolpyruvate (PEP) is formed, which donates a phosphate group to molecules of ADP to form another pair of ATP molecules and pyruvate. This is the final step of glycolysis. In total, two molecules each of ATP, NADH, and pyruvate are formed during this process. Glycolysis produces the same result under anaerobic conditions as it does under aerobic conditions: two ATP molecules. If oxygen is present, more ATP is later made by oxidative phosphorylation.

Figure 7.1 Glycolysis.

If you are going to memorize one fact about glycolysis, remember that one glucose molecule produces two pyruvate, two NADH, and two ATP molecules.

One glucose &rarr 2 pyruvate, 2 ATP, 2 NADH

The Krebs Cycle

The pyruvate formed during glycolysis next enters the Krebs cycle, which is also known as the citric acid cycle . The Krebs cycle occurs in the matrix of the mitochondria. The pyruvate enters the mitochondria of the cell and is converted into acetyl coenzyme A (CoA) in a step that produces an NADH. This compound is now ready to enter the eight-step Krebs cycle, in which pyruvate is broken down completely to H2 O and CO2 . You do not need to memorize the eight steps.

As shown in Figure 7.2 , a representation of the Krebs cycle, the 3-carbon pyruvate does not enter the Krebs cycle per se. Rather, it is converted, with the assistance of CoA and NAD + , into 2-carbon acetyl CoA and NADH. The acetyl CoA dives into the Krebs cycle and reacts with oxaloacetate to form a 6-carbon molecule called citrate . The citrate is converted to a molecule named isocitrate, which then donates electrons and a hydrogen to NAD + to form 5-carbon &alpha-ketoglutarate, carbon dioxide, and a molecule of NADH. The &alpha-ketoglutarate undergoes a reaction very similar to the one leading to its formation and produces 4-carbon succinyl CoA and another molecule each of NADH and CO2 . The succinyl CoA is converted into succinate in a reaction that produces a molecule of ATP. The succinate then transfers electrons and a hydrogen atom to FAD to form FADH2 and fumarate. The next-to-last step in the Krebs cycle takes fumarate and rearranges it to another 4-carbon molecule: malate. Finally, in the last step of the cycle, the malate donates electrons and a hydrogen atom to a molecule of NAD + to form the final NADH molecule of the Krebs cycle, at the same time regenerating the molecule of oxaloacetate that helped kick off the cycle. One turn of the Krebs cycle takes a single pyruvate and produces one ATP, four NADH, and one FADH2 .

Figure 7.2 The Krebs cycle.

If you are going to memorize one thing about the Krebs cycle, remember that for each glucose dropped into glycolysis, the Krebs cycle occurs twice. Each pyruvate dropped into the Krebs cycle produces

4 NADH, 1 FADH2 , 1 ATP, and 2 CO2

Therefore, the pyruvate obtained from the original glucose molecule produces:

8 NADH, 2 FADH2 , and 2 ATP

Up to this point, having gone through glycolysis and the Krebs cycle, one molecule of glucose has produced the following energy-related compounds: 10 NADH, 2 FADH2 , and 4 ATP. Not bad for an honest day’s work . . . but the body wants more and needs to convert the NADH and FADH2 into ATP. This is where the electron transport chain, chemiosmosis, and oxidative phosphorylation come into play.

Oxidative Phosphorylation

After the Krebs cycle comes the largest energy-producing step of them all: oxidative phosphorylation. During this aerobic process, the NADH and FADH2 produced during the first two stages of respiration are used to create ATP. Each NADH leads to the production of up to three ATP, and each FADH2 will lead to the production of up to two ATP molecules. This is an inexact measurement&mdashthose numbers represent the maximum output possible from those two energy components if all goes smoothly. For each molecule of glucose, up to 30 ATP can be produced from the NADH molecules and up to 4 ATP from the FADH2 . Add to this the 4 total ATP formed during glycolysis and the Krebs cycle for a grand total of 38 ATP from each glucose . Two of these ATP are used during aerobic respiration to help move the NADH produced during glycolysis into the mitochondria. All totaled, during aerobic respiration, each molecule of glucose can produce up to 36 ATP.

Do not panic when you see the illustration for the electron transport chain (Figure 7.3 ). Once again, the big picture is the most important thing to remember. Do not waste your time memorizing the various cytochrome molecules involved in the steps of the chain. Remember that the 1&frasl2 O2 is the final electron acceptor in the chain, and that without the O2 (anaerobic conditions), the production of ATP from NADH and FADH2 will be compromised. Remember that each NADH that goes through the chain can produce three molecules of ATP, and each FADH2 can produce two.

Figure 7.3 Electron transport chain (ETC).

The electron transport chain (ETC) is the chain of enzyme molecules, located in the mitochondria, that passes electrons along during the process of chemiosmosis to regenerate NAD + to form ATP. Each time an electron passes to another member of the chain, the energy level of the system drops. Do not worry about the individual members of this chain&mdashthey are unimportant for this exam. When thinking of the ETC, we are reminded of the passing of a bucket of water from person to person until it arrives at and is tossed onto a fire. In the ETC, the various molecules in the chain are the people passing the buckets the drop in the energy level with each pass is akin to the water sloshed out as the bucket is hurriedly passed along, and the 1&frasl2 O2 represents the fire onto which the water is dumped at the end of the chain. As the 1&frasl2 O2 (each oxygen atom, or half of an O2 molecule) accepts a pair of electrons, it actually picks up a pair of hydrogen ions to produce water.

Chemiosmosis is a very important term to understand. It is defined as the coupling of the movement of electrons down the electron transport chain with the formation of ATP using the driving force provided by a proton gradient. So, what does that mean in English? Well, let’s start by first defining what a coupled reaction is. It is a reaction that uses the product of one reaction as part of another reaction. Thinking back to our baseball card collecting days helps us better understand this coupling concept. We needed money to buy baseball cards. We would babysit or do yardwork for our neighbors and use that money to buy cards. We coupled the money-making reaction of hard labor to the money-spending reaction of buying baseball cards.

Let’s look more closely at the reactions that are coupled in chemiosmosis. If you look at Figure 7.4 a, a crude representation of a mitochondrion, you will find the ETC embedded within the inner mitochondrial membrane. As some of the molecules in the chain accept and then pass on electrons, they pump hydrogen ions into the space between the inner and outer membranes of the mitochondria (Figure 7.4 b). This creates a proton gradient that drives the production of ATP. The difference in hydrogen concentration on the two sides of the membrane causes the protons to flow back into the matrix of the mitochondria through ATP synthase channels (Figure 7.4 c). ATP synthase is an enzyme that uses the flow of hydrogens to drive the phosphorylation of an ADP molecule to produce ATP. This reaction completes the process of oxidative phosphorylation and chemiosmosis. The proton gradient created by the movement of electrons from molecule to molecule has been used to form the ATP that this process is designed to produce. In other words, the formation of ATP has been coupled to the movement of electrons and protons.

Figure 7.4 Chemiosmosis.

Chemiosmosis is not oxidative phosphorylation per se rather, it is a major part of oxidative phosphorylation. An important fact we want you to take out of this chapter is that chemiosmosis is not unique to the mitochondria. It is the same process that occurs in the chloroplasts during the ATP-creating steps of photosynthesis (see Chapter 8 ). The difference is that light is driving the electrons along the ETC in plants. Remember that chemiosmosis occurs in both mitochondria and chloroplasts.

Remember the following facts about oxidative phosphorylation (Ox-phos):

1. Each NADH &rarr 3 ATP.

2. Each FADH2 &rarr 2 ATP.

3. 1&frasl2 O2 is the final electron acceptor of the electron transport chain, and the chain will not function in the absence of oxygen.

4. Ox-phos serves the important function of regenerating NAD + so that glycolysis and the Krebs cycle can continue.

5. Chemiosmosis occurs in photosynthesis as well as respiration.

Anaerobic Respiration

Anaerobic respiration, or fermentation, occurs when oxygen is unavailable or cannot be used by the organism. As in aerobic respiration, glycolysis occurs and pyruvate is produced. The pyruvate enters the Krebs cycle, producing NADH, FADH2 , and some ATP. The problem arises in the ETC&mdashbecause there is no oxygen available, the electrons do not pass down the chain to the final electron acceptor, causing a buildup of NADH in the system. This buildup of NADH means that the NAD + normally regenerated during oxidative phosphorylation is not produced, and this creates an NAD + shortage. This is a problem, because in order for glycolysis to proceed to the pyruvate stage, it needs NAD + to help perform the necessary reactions. Fermentation is the process that begins with glycolysis and ends when NAD + is regenerated. A glucose molecule that enters the fermentation pathway produces two net ATP per molecule of glucose, representing a tremendous decline in the efficiency of ATP production.

Under aerobic conditions, NAD + is recycled from NADH by the movement of electrons down the electron transport chain. Under anaerobic conditions, NAD + is recycled from NADH by the movement of electrons to pyruvate, namely, fermentation. The two main types of fermentation are alcohol fermentation and lactic acid fermentation. Refer to Figures 7.5 and 7.6 for the representations of the different forms of fermentation. Alcohol fermentation (Figure 7.5 ) occurs in fungi, yeast, and some bacteria. The first step involves the conversion of pyruvate into two 2-carbon acetaldehyde molecules. Then, in the all-important step of alcohol fermentation, the acetaldehyde molecules are converted to ethanol, regenerating two NAD + molecules in the process.

Figure 7.5 Alcohol fermentation.

Lactic acid fermentation (Figure 7.6 ) occurs in human and animal muscle cells when oxygen is not available. This is a simpler process than alcoholic fermentation&mdashthe pyruvate is directly reduced to lactate (also known as lactic acid) by NADH to regenerate the NAD + needed for the resumption of glycolysis. Have you ever had a cramp during exercise? The pain you felt was the result of lactic acid fermentation. Your muscle was deprived of the necessary amount of oxygen to continue glycolysis, and it switched over to fermentation. The pain from the cramp came from the acidity in the muscle.

Figure 7.6 Lactic acid fermentation.

Review Questions

1 . Most of the ATP creation during respiration occurs as a result of what driving force?

A. Electrons moving down a concentration gradient

B. Electrons moving down the electron transport chain

C. Protons moving down a concentration gradient

D. Sodium ions moving down a concentration gradient

E. Movement of pyruvate from the cytoplasm into the mitochondria

2 . Which of the following processes occurs in both respiration and photosynthesis?

3 . What is the cause of the cramps you feel in your muscles during strenuous exercise?

A. Lactic acid fermentation

D. Too much oxygen delivery to the muscles

E. Oxidative phosphorylation

4 . Which of the following statements is in correct?

A. Glycolysis can occur with or without oxygen.

B. Glycolysis occurs in the mitochondria.

C. Glycolysis is the first step of both anaerobic and aerobic respiration.

D. Glycolysis of one molecule of glucose leads to the production of 2 ATP, 2 NADH, and 2 pyruvate.

For questions 5–8, use the following answer choices:

C. Oxidative phosphorylation

D. Lactic acid fermentation

5 . This reaction occurs in the matrix of the mitochondria and includes FADH2 among its products.

6 . This reaction is performed to recycle NAD + needed for efficient respiration.

7 . This process uses the proton gradient created by the movement of electrons to form ATP.

8 . This process includes the reactions that use NADH and FADH2 to produce ATP.

9 . Which of the following molecules can give rise to the most ATP?

10 . Which of the following is a proper representation of the products of a single glucose molecule after it has completed the Krebs cycle?

Answers and Explanations

1 . C &mdashThis is the concept of chemiosmosis: the coupling of the movement of electrons down the electron transport chain and the formation of ATP via the creation of a proton gradient. The protons are pushed out of the matrix during the passage of electrons down the chain. They soon build up on the other side of the membrane, and are driven back inside because of the difference in concentration. ATP synthase uses the movement of protons to produce ATP.

2 . B &mdashThis is an important concept to understand. The AP examiners love this topic!

3 . A &mdashLactic acid fermentation occurs in human muscle cells when oxygen is not available. Answer choice B would be incorrect because alcohol fermentation occurs in yeast, fungi, and some bacteria. During exercise, if your muscle becomes starved for oxygen, glycolysis will switch over to fermentation. The pain from the cramp is due to the acidity in the muscle caused by the increased concentration of lactate.

4 . B &mdashGlycolysis occurs in the cytoplasm. All the other statements are correct.

9 . D &mdashA glucose molecule can net 36 ATP, an NADH molecule can net 3, an FADH2 molecule can net 2, and a pyruvate molecule can net 15.

10 . D &mdashDuring glycolysis, a glucose molecule produces 2 ATP, 2 NADH, and 2 pyruvate. The 2 pyruvate then go on to produce 8 NADH, 2 FADH2 , and 2 ATP during the Krebs cycle to give the total listed in answer choice D.

Rapid Review

Try to rapidly review the material presented below.

There are two main categories of respiration: aerobic and anaerobic.

Aerobic respiration: glycolysis &rarr Krebs cycle &rarr oxidative phosphorylation &rarr 36 ATP per glucose molecule

Anaerobic respiration (fermentation ): glycolysis &rarr regenerate NAD + &rarr 2 ATP per glucose molecule

Glycolysis: conversion of 1 glucose molecule into 2 pyruvate, 2 ATP, and 2 NADH occurs in the cytoplasm, and in both aerobic and anaerobic respiration must have NAD + to proceed.

Total energy production to this point &rarr 2 ATP + 2NADH

Krebs cycle: conversion of 1 pyruvate molecule into 4 NADH, 1 FADH2 , 1 ATP, H2 O, and CO2 occurs twice for each glucose to yield 8 NADH, 2 FADH2 , and 2 ATP occurs in mitochondria.

Total energy production per glucose molecule to this point &rarr 4 ATP + 10 NADH + 2 FADH2

Oxidative phosphorylation: production of large amounts of ATP from NADH and FADH2 .

• Occurs in the mitochondria requires presence of oxygen to proceed.

• NADH and FADH2 pass their electrons down the electron transport chain to produce ATP.

• Each NADH can produce up to 3 ATP each FADH2 up to 2 ATP.

• 1&frasl2 O2 is the final acceptor in the electron transport chain.

• Movement of electrons down the chain leads to movement of H + out of matrix.

• Ox-phos regenerates NAD + so that glycolysis and the Krebs cycle can continue!

Chemiosmosis: coupling of the movement of electrons down the ETC with the formation of ATP using the driving force provided by the proton gradient occurs in both cell respiration and photosynthesis to produce ATP.

ATP synthase: enzyme responsible for using protons to actually produce ATP from ADP.

Total energy production per glucose molecule to this point &rarr 38 ATP (use 2 in process) &rarr 36 ATP total

Fermentation (general ): process that regenerates NAD + so glycolysis can begin again.

• Occurs in the absence of oxygen.

• Begins with glycolysis: 2 ATP, 2 pyruvate, and 2 NADH are produced from 1 glucose molecule.

• Because there is no oxygen to accept the electron energy on the chain, there is a shortage of NAD + , which prevents glycolysis from continuing.

Fermentation (alcohol ): occurs in fungi, yeast, and bacteria causes conversion of pyruvate to ethanol.

Fermentation (lactic acid ): occurs in human and animal muscle cells causes conversion of pyruvate &rarr lactate causes cramping sensation when oxygen runs low in muscle cells.

Respiration

1 . What is the ratio of ATP produced via the chemi­osmotic principle for a molecule of NADH relative to a molecule of FADH2 as they transfer high-energy electrons to the electron transport chain?

2 . The absence of which of the following substrates would directly block the progression of oxidative phosphorylation?

3 . Which process occurs in the cytoplasm of cells and is the beginning pathway for both aerobic and anaerobic respiration?

(A) Oxidative phosphorylation

4 . Each pyruvate dumped into the Krebs cycle yields

(A) 4 NADH, 1 FADH2 , and 1 ATP.

(B) 8 NADH, 2 FADH2 , and 2 ATP.

(C) 2 NADH, 1 FADH2 , and 2 ATP.

(D) 4 NADH, 2 FADH2 , and 2 ATP.

Answers and Explanations

1 . A &mdashThree molecules of ATP are produced for every molecule of NADH that transfers high-energy electrons to the electron transport chain (ETC). Two molecules of ATP are produced for every molecule of FADH2 that transfers high-energy electrons to the ETC. So the ratio would be 3:2.

2 . C &mdashFADH2 is the only choice listed that is an input to the OX-PHOS process.

3 . D &mdashOxidative phosphorylation is the aerobic process in which NADH and FADH2 pass their electrons down the electron transport chain (ETC) to produce ATP. Chemiosmosis represents the coupling of the movement of electrons down the ETC with the formation of ATP, using the driving force provided by a proton gradient. Carbon fixation is the attachment of the carbon from CO2 to a molecule that is able to enter the Calvin cycle. This process occurs in photosynthesis rather than respiration. Glycolysis occurs in the cytoplasm of cells and is, in fact, the first step of both aerobic and anaerobic respiration.

4 . A &mdashChoice B is sneaky because the Krebs cycle occurs twice for each glucose dropped into glycolysis. Each pyruvate produces what is listed in choice A (which is what the question is asking). But if the question were to ask about the yield from the pyruvate produced from each glucose molecule, it would be choice B.

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