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Study GuideBiology–Cellular Respiration1. Introduction to Cellular RespirationLiving things need energy to grow, repair themselves, and carry out daily activities. Someorganisms—especiallyplants—can capture energy directly from sunlight throughphotosynthesis.During this process, plants makeglucose, a type of carbohydrate that stores energy in its chemicalbonds.However, many organisms cannot perform photosynthesis.Animals, fungi, many protozoa, andmost bacteriadepend on plants (or on organisms that eat plants) for their energy. They obtain thisenergy by breaking down carbohydrates like glucose.1.1 How Cells Get Energy from FoodCells release the energy stored in carbohydrates through a process calledcellular respiration.During cellular respiration, glucose enters the cell’s cytoplasm and is gradually broken down througha series of chemical reactions.The energy released is not usually used right away. Instead, it is stored in a special molecule calledadenosine triphosphate (ATP). ATP is made by combiningadenosine diphosphate (ADP)with aphosphate group. You can think of ATP like arechargeable battery—it stores energy and releases itwhenever the cell needs power for activities such as movement, growth, or making new molecules.1.2 The Role of Oxygen and Carbon DioxideCellular respiration producescarbon dioxide (CO₂)as a waste product. This carbon dioxide isreleased into the environment, where plants can reuse it during photosynthesis to make moreglucose.Oxygen (O₂)is also essential for cellular respiration. It acts as the finalelectron acceptor, allowingthe process to continue efficiently. This oxygen is the same gas that plants release duringphotosynthesis.Because of this exchange,photosynthesis and cellular respiration are closely connected.Photosynthesis stores energy from sunlight in glucose, and cellular respiration releases that energyso cells can use it.

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Study Guide1.3 The Major Stages of Cellular RespirationCellular respiration does not happen all at once. It occurs infour main stages, each with a specificrole:1. GlycolysisGlycolysis takes place in thecytoplasm. During this stage, one glucose molecule is broken down intotwo smaller molecules calledpyruvic acid.2. Krebs CycleThe pyruvic acid molecules enter themitochondrion, where they are converted intoacetyl CoAandthen enter theKrebs cycle. In this stage, the molecules are broken down further, and energy istransferred to high-energy carriers such asNADH.3. Electron Transport SystemThe electrons carried by NADH move through a series of proteins calledcoenzymes andcytochromes. As electrons pass along this chain, they release energy.4. ChemiosmosisThe energy from the electrons is used to pumpprotonsacross a membrane. This creates a gradientthat drives the production of large amounts ofATP.1.4 The Overall Chemical ReactionThe entire process of cellular respiration can be summarized by this chemical equation:C₆H₁₂O₆+ 6 O₂→ 6 H₂O + 6 CO₂+ energyThis equation shows thatglucose reacts with oxygen to produce water, carbon dioxide, and usableenergy in the form of ATP.

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Study Guide1.5 Where Cellular Respiration Happens•Glycolysisoccurs in thecytoplasm.•Pyruvic acid conversion, the Krebs cycle, electron transport, andchemiosmosisalloccur in themitochondrion.Together, these steps allow cells to efficiently release and store energy so organisms can survive andfunction.2. GlycolysisGlycolysisis the first major step of cellular respiration. In this process,one glucose moleculeisbroken down intotwo molecules of pyruvic acid(also calledpyruvate). This pathway happens inthecytoplasmof all types of cells, includinganimal cells, plant cells, and microorganisms.Glycolysis is not a single reaction. Instead, it is amulti-step metabolic pathwaythat usesat leastsix different enzymesto guide each step safely and efficiently.2.1 Energy Investment: Starting the ProcessAt the beginning of glycolysis, the cell mustinvest energy.In thefirst and third steps,ATP molecules provide energyto activate glucose and its intermediatecompounds.•Two ATP molecules are usedduring these early steps.•This energy input helps prepare the glucose molecule for later breakdown.Think of this as “spending energy to make energy.”

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Study Guide2.2 Splitting the Glucose MoleculeAs glycolysis continues, the originalsix-carbon glucose moleculechanges into several intermediatecompounds. Eventually, itsplits into two three-carbon molecules.These three-carbon molecules go through additional reactions, and by the end of glycolysis, each onebecomespyruvic acid. Since glucose splits into two parts,two pyruvic acid moleculesareproduced in total.2.3 Energy Payoff: Making ATPDuring the later stages of glycolysis, energy is released from chemical reactions. This energy is usedto makeATP.•Four ATP molecules are produced

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Study Guide•Two ATP molecules were used earlierThis results in anet gain of two ATP moleculesfor each glucose molecule broken down.2.4 Formation of NADHGlycolysis also produces another important energy carrier. Some of the released energy is used toconvertNADintoNADH(along with a hydrogen ion).•Two NADH moleculesare formed during glycolysis•NADH stores energy that will be released later in theelectron transport system2.5 Glycolysis Does Not Need OxygenOne important feature of glycolysis is that itdoes not require oxygen. Because of this, glycolysis isdescribed as ananaerobic process.For some organisms—such as certainbacteriaandfermentation yeasts—glycolysis is theonly waythey can produce energy.2.6 Efficiency of GlycolysisGlycolysis is consideredsomewhat inefficient. Although it produces ATP,most of the energy fromglucose remainsstored in the two pyruvic acid molecules. This remaining energy can bereleased later during the next stages of cellular respiration.Interestingly, glycolysis issomewhat similar to a reversal of photosynthesis, where energy-richmolecules are built rather than broken down.3. Quiz Glycolysis1. QuestionFill in the blank:How many ATP molecules are produced through glycolysis?

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Study GuideAnswer Choices•Six molecules of ATP are created in the second half of glycolysis after two are required in thefirsthalf, for a net gain of four molecules of ATP.•Two ATP molecules are used in the initial steps and four are produced in the latter stages, fora net gain of two ATP molecules.•Two molecules of ATP are created in glycolysis, and none are used in the process.Correct AnswerTwo ATP molecules are used in the initial steps and four are produced in the latter stages, for anet gain of two ATP molecules.Why This Is CorrectDuring glycolysis, the cell firstinvests 2 ATP moleculesin the early reactions. Later,4 ATPmolecules are producedthrough substrate-level phosphorylation.Because 4 ATP are made and 2 ATP are used, thenet gain is 2 ATP moleculesper glucosemolecule.2.QuestionFill in the blank:Which of the following statements about glycolysis isFALSE?Answer Choices•It occurs in the cytoplasm.•It requires atmospheric oxygen.•It results in two molecules of pyruvic acid.Correct AnswerIt requires atmospheric oxygen.

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