Aerobic Respiration for AP Biology (page 2)
Practice problems for these concepts can be found at: Respiration Review Questions for AP Biology
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—it 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 the process of respiration, to understand the basic process, and become comfortable with respiration as a whole. Major concepts are the key. I will explain the specific steps of glycolysis because they will help you understand the big picture—but 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—seems stupid … but be patient … the genius is 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 molecu es 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—the 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 herebecause 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.
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 H2O 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 a-ketoglutarate, carbon dioxide, and a molecule of NADH. The aketoglutarate 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 mole le: 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.
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.
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—those 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 four 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 / 2 O2 is the final 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.
The electron transport chain (ETC) is the chain of 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—they are unimportant for this exam. When thinking of the ETC, I am 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 theenergy level with each pass is akin to the water sloshed out as the bucket is hurriedly passed along, and the 1 / 2 O2 represents the fire onto which the water is dumped at the end of the chain. As the 1 / 2 O2 accepts the 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 my baseball card collecting days helps me better understand this coupling concept. I needed money to buy baseball cards. I would babysit or do yardwork for my neighbors and use that money to buy cards. I 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 7.4a, 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 7.4b). This creates a proton gradient that rives 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 7.4c). 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.
Chemiosmosis is not oxidative phosphorylation per se; rather, it is a major part of oxidative phosphorylation. An important fact I 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):
Practice problems for these concepts can be found at:
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