[生物] Cell Respiration

Respiration is the process by which organisms burn food to produceenergy. The starting material of cellular respiration is the sugar glucose,which has energy stored in its chemical bonds. You can think of glucoseas a kind of cellular piece of coal: chock-full of energy, but uselesswhen you want to power a stereo. Just as burning coal produces heat andenergy in the form of electricity, the chemical processes ofrespiration convert the energy in glucose into usable form.
    Adenosine triphosphate (ATP) is theusable form of energy produced by respiration. ATP is like electricity:it contains the same energy as coal, but it’s easier to transport andis just what’s needed when the cell needs some power to carry out atask.

    ATP

    ATP is a nucleic acid similar to RNA. Ithas a ribose sugar attached to the nitrogenous base adenine. However,instead of the single phosphate group typical of RNA nucleotides, ATPhas three phosphate groups. Each of the ATP phosphate groups carries anegative charge. In order to hold the three negative charges in suchproximity, the bonds holding the phosphate groups have to be quitepowerful. If one or two of the bonds are broken and the additionalphosphates are freed, the energy stored in the bonds is released andcan be used to fuel other chemical reactions. When the cell needsenergy, it removes phosphates from ATP by hydrolysis, creating energyand either adenosine diphosphate (ADP), which has two phosphates, oradenosine monophosphate (AMP), which has one phosphate.

    Respiration is the process of making ATPrather than breaking it down. To make ATP, the cell burns glucose andadds new phosphate groups to AMP or ADP, creating new power molecules.

    There are actually two general types ofrespiration, aerobic and anaerobic. Aerobic respiration occurs in thepresence of oxygen, while anaerobic respiration does not use oxygen.Both types of cell respiration begin with the process of glycolysis,after which the two diverge. We’ll first discuss aerobic respirationand then move to anaerobic.

    Aerobic Cell Respiration

    Aerobic respiration is more efficient andmore complicated than anaerobic respiration. Aerobic respiration usesoxygen and glucose to produce carbon dioxide, water, and ATP. Moreprecisely, this process involves six oxygen molecules for every sugarmolecule:

6O2 + C6H12O66CO2 + 6H2O + ATP energy

    This general equation for aerobicrespiration (which you should know for the test) is actually theproduct of three separate stages: glycolysis, the Krebs cycle, and theelectron transport chain. Typically, the SAT II Biology only asksquestions about the starting and ending products of each stage and thelocation where each takes place. Understanding the internal details ofstages will help you remember these key facts and prepare you in casethe testers throw in a more difficult question, but the details of allthe complex reactions will probably not be tested by the SAT II.

    Glycolysis

    Glycolysis is the first stage of aerobic(and anaerobic) respiration. It takes place in the cytoplasm of thecell. In glycolysis (“glucose breaking”), ATP is used to split glucosemolecules into a three-carbon compound called pyruvate. This splitting produces energy that is stored in ATP and a molecule called NADH. The chemical formula for glycolysis is:

C6H12O6 + 2ATP + 2NAD+2pyruvate + 4ATP + 2NADH

    As the formula indicates, the cell must invest 2 ATP molecules in order to get glycolysis going. But by the time glycolysis is complete, the cell has produced 4 new ATP, creating a net gain of 2 ATP. The 2NADH molecules travel to the mitochondria, where, in the next twostages of aerobic respiration, the energy stored in them is convertedto ATP.

    The most important things to remember about glycolysis are:

• Glycolysis is part of both aerobic and anaerobic respiration.
• Glycolysis splits glucose, a six-carbon compound, into two pyruvate molecules, each of which has three carbons.
• In glycolysis, a 2 ATP investment results in a 4 ATP payoff.
• Unlike the rest of aerobic respiration, which takes place inthe mitochondria, glycolysis takes place in the cytoplasm of the cell.
• Unlike the rest of aerobic respiration, glycolysis does not require oxygen.

    The Krebs Cycle

    After glycolysis, the pyruvate sugars aretransported to the mitochondria. During this transport, thethree-carbon pyruvate is converted into the two-carbon molecule calledacetate. The extra carbon from the pyruvate is released as carbondioxide, producing another NADH molecule that heads off to the electrontransport chain to help create more ATP. The acetate attaches to acoenzyme called coenzyme A to form the compound acetyl-CoA. Theacetyl-CoA then enters the Krebs cycle. The Krebs cycle is called acycle because one of the molecules it starts with, the four-carbonoxaloacetate, is regenerated by the end of the cycle to start the cycleover again.

    The Krebs cycle begins when acetyl-CoA andoxaloacetate interact to form the six-carbon compound citric acid. (TheKrebs cycle is also sometimes called the citric acid cycle.)This citric acid molecule then undergoes a series of eight chemicalreactions that strip carbons to produce a new oxaloacetate molecule.The extra carbon atoms are expelled as CO2(the Krebs cycle is the source of the carbon dioxide you exhale). Inthe process of breaking up citric acid, energy is produced. It isstored in ATP, NADH, and FADH2. The NADH and FADH2 proceed on to the electron transport chain.

    The entire Krebs cycle is shown in the figurebelow. For the SAT II Biology, you don’t have to know the intricaciesof this figure, but you should be able to recognize that it shows theKrebs cycle.

    It is also important to remember that eachglucose molecule that enters glycolysis is split into two pyruvatemolecules, which are then converted into the acetyl-CoA that movesthrough the Krebs cycle. This means that for every glucose moleculethat enters glycolysis, the Krebs cycle runs twice. Therefore, for oneglucose molecule running through aerobic cell respiration, the equationfor the Krebs cycle is:

2acetyl-CoA + 2oxaloacetate4CO2 + 6NADH + 2FADH2 + 2ATP + 2oxaloacetate

    For the SAT II Biology, the most important things to remember about the Krebs cycle are:

• The Krebs cycle results in 2 ATP molecules for each glucose molecule run through glycolysis.
• The Krebs cycle sends energy-laden NADH and FADH2molecules on to the next step in respiration, the electron transportchain. It does not export carbon molecules for further processing.
• The Krebs cycle takes place in the mitochondrial matrix, the innermost compartment of the mitochondria.
• Though the Krebs cycle does not directly require oxygen, itcan only take place when oxygen is present because it relies onby-products from the electron transport chain, which requires oxygen.The Krebs cycle is therefore an aerobic process.

    The Electron Transport Chain

    A great deal of energy is stored in the NADH and FADH2molecules formed in glycolysis and the Krebs cycle. This energy isconverted to ATP in the final phase of respiration, the electrontransport chain:

10NADH + 2FADH234ATP

    The electron transport chain consists of a set of three protein pumps embedded in the inner membrane of the mitochondria. FADH2 and NADH are used to power these pumps. Using the energy in NADH and FADH2, these pumps move positive hydrogen ions (H+) from the mitochondrial matrix to the intermembrane space. This creates a concentration gradient over the membrane.

    In a process called oxidative phosphorylation, H+ions flow back into the matrix through a membrane protein called an ATPsynthase. This channel is the opposite of the standard membrane pumpsthat burns ATP to transport molecules against their concentrationgradient: ATP synthase uses the natural movement of ions along theirconcentration gradient to make ATP. All told, the flow of ions throughthis channel produces 34 ATP molecules. The waste products from thepowering of the electron transport chain protein pumps combine withoxygen to produce water molecules. By accepting these waste products,oxygen frees NAD+ andFAD to play their roles in the Krebs cycle and the electron transportchain. Without oxygen, these vital energy carrier molecules would notperform their roles and the processes of aerobic respiration could notoccur.

    For the SAT II Biology, the most importantthings to remember about the electron transport chain and oxidativephosphorylation are:

• Four ATP molecules are produced by glycolysis and the Krebs cycle combined. The electron transport chain produces 34 ATP.
• The electron transport chain occurs across the inner membrane of the mitochondria.
• The electron transport chain requires oxygen.

    Anaerobic Respiration

    Aerobic respiration requires oxygen. However,some organisms live in places where oxygen is not always present.Similarly, under extreme exertion, muscle cells may run out of oxygen.Anaerobic respiration is a form of respiration that can functionwithout oxygen.

    In the absence of oxygen, organisms continue tocarry out glycolysis, since glycolysis does not use oxygen in itschemical process. But glycolysis does require NAD+. In aerobic respiration, the electron transport chain turns NADH back to NAD+ with the aid of oxygen, thereby averting any NAD+ shortage and allowing glycolysis to take place. In anaerobic respiration, cells must find another way to turn NADH back to NAD+.

    This “other way” is called fermentation. Fermentation’s goal is not to produce additional energy, but merely to replenish NAD+supplies so that glycolysis can continue churning out its slow butsteady stream of ATP. Because pyruvates are not needed in anaerobicrespiration, fermentation uses them to help regenerate NAD+.While employing the pyruvates in this way does allow glycolysis tocontinue, it also results in the loss of the considerable energycontained in the pyruvate sugars.

    There are two principle forms of fermentation, lactic acid fermentation and alcoholic fermentation. For the SAT II Biology, remember that no matter what kind of fermentation occurs, anaerobic respiration only produces 2 net ATP in glycolysis.

    Lactic Acid Fermentation

    In lactic acid fermentation, pyruvate is converted to a three-carbon compound called lactic acid:

pyruvate + NADHlactic acid + NAD+

    In this reaction, the hydrogen from the NADH molecule is transferred to the pyruvate molecule.
Lactic acid fermentation is common in fungi andbacteria. Lactic acid fermentation also takes place in human musclecells when strenuous exercise causes temporary oxygen shortages. Sincelactic acid is a toxic substance, its buildup in the muscles producesfatigue and soreness.

    Alcoholic Fermentation

    Another route to NAD+ produces alcohol (ethanol) as a by-product:

pyruvate + NADHethyl alcohol + NAD+ + CO2

    Alcoholic fermentation is the source ofethyl alcohol present in wines and liquors. It also accounts for thebubbles in bread. When yeast in bread dough runs out of oxygen, it goesthrough alcoholic fermentation, producing carbon dioxide. These carbondioxide bubbles create spaces in the dough and cause it to rise.

    Like lactic acid, the ethanol produced byalcoholic fermentation is toxic. When ethanol levels rise to about 12percent, the yeast dies.