Overview of Cellular Respiration - OpenStax CNX · · 2014-07-13This is a general overview of cellular respiration. ... Aerobic cellular respiration (Figure 3) ... Note that no

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Overview of Cellular Respiration∗

Robert Bear

David Rintoul

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0†

Abstract

This is a general overview of cellular respiration.

Introduction

Surely the mitochondrion that �rst entered another cell was not thinking about the future bene�ts

of cooperation and integration; it was merely trying to make its own living in a tough Darwinian

world.

Stephen Jay Gould, in Wonderful Life: the Burgess Shale and the Nature of History, (1990)All living organisms require energy, and for all organisms this energy comes from the chemical energy

found in compounds that they acquire from their environment. The mitochondrion, a descendent of anaerobically-respiring bacteria, is the site of energy generation in eukaryotes. As we learned previously, theprocess of photosynthesis uses solar energy (sunlight) and converts this energy into chemical energy in theform of carbohydrates. In order for the chemical energy in the carbohydrates to be made available to docellular work, the energy must be converted into a useable form known as ATP. Adenosine Triphosphate isthe energy currency of the cell, and everything you do from walking down the street to reading this bookrequires energy in the form of ATP. Organisms need a constant supply of ATP, and the potential energystored in food is the source of energy to meet this need. By connecting all this together, you should realizethat your daily activities are fueled by the energy from the sun and that even on the cellular level nutrientscycle and energy �ows (Figure 1).

∗Version 1.7: Jul 9, 2014 9:47 am -0500†http://creativecommons.org/licenses/by/4.0/

http://cnx.org/content/m49546/1.7/


Figure 1: This image illustrates the relationship between photosynthesis and cellular respiration. (Imageby Eva Horne and Robert Bear)



All organisms need ATP, but not all organisms use the same pathways to generate ATP from the foodthat is consumed. Aerobic cellular respiration, the main subject of this chapter, uses oxygen (O2) andglucose to generate ATP. Organisms (plants, animals, fungi and microbes) that live in an oxygen (O2) richenvironment use this process to generate ATP. The overall equation for aerobic cellular respiration is thereverse of photosynthesis, is an exergonic reaction, and supplies the ATP for cellular functions (Figure 2).

Figure 2: This image illustrates the overall equation for aerobic cellular respiration and how the amountsof free energy di�ers between the reactants and the products. (Image by Robert Bear)

As the aerobic cellular respiration equation shows (Figure 2), an organism needs to acquire the O2 fromits surroundings and to get rid of the CO2 that is produced. The acquisition of O2 and the release of CO2 isaccomplished in a variety of ways. In single celled organisms, the movement of O2 and CO2 (gas exchange)is done by simple di�usion. However, in complex organisms there are specialized organs that allow for gasexchange; for example, gills in aquatic organisms and lungs in terrestrial animals.

A common misconception is that plants do not undergo cellular respiration because they make theirown energy by photosynthesis. Plants do perform cellular respiration using the carbohydrates produced viaphotosynthesis; this occurs in tissues that are not photosynthetically active (e.g., roots), as well as in leavesand stems. Approximately half of the glucose produced by photosynthesis is consumed by the plant, mostlyto generate ATP during aerobic cellular respiration. Other uses of glucose in the plant include synthesis ofcell walls, starch, and other plant carbohydrates. So, plants harvest light energy via photosynthesis, makingcarbohydrates, and then they use the energy stored in those carbohydrates to perform various cellular



functions. This is the reason why they are called autotrophs, or self feeders.Another pathway that organisms can use to extract energy from carbohydrates is anaerobic cellular

respiration; this process occurs in the absence of oxygen. In this chapter, we will explore one type ofanaerobic cellular respiration called fermentation. You may already be familiar with a one type of fermenta-tion, lactic acid fermentation, especially if you have recently over-exerted your muscles. Anaerobic cellularrespiration is used by many organisms to produce ATP when oxygen is not available and thus pathwayswhich require oxygen cannot be used. The amount of ATP produced by fermentation is much less then thatproduced by aerobic cellular respiration, so there is a cost and bene�t associated with organisms utilizingfermentation.

1 Summary of Aerobic Cellular Respiration

Aerobic cellular respiration (Figure 3) is series of linked chemical reactions that can be best understood ifit is separated into four stages. These are glycolysis, pyruvate oxidation, the Krebs Cycle, and oxida-

tive phosphorylation. Similar to photosynthesis, cellular respiration uses a series of oxidation-reductionreactions. During these reactions, electrons are stripped from the chemical bonds of the original glucosemolecule and eventually added to oxygen, via a series of intermediate steps. This series of reactions releasessmall amounts of energy at each step; this energy is used to drive the formation of ATP. This section is abrief introduction to the stages of aerobic respiration with more detail to follow in the chapter.



Figure 3: This image illustrates aerobic cellular respiration. (Image by Eva Horne and Robert Bear)

The �rst stage of cellular respiration is called Glycolysis and occurs in the cytoplasm of the cell. Duringglycolysis, 1 glucose molecule (with 6 carbon atoms) is broken down into 2 pyruvate molecules (with threecarbon atoms each). This is accompanied by the production of a few ATP molecules and the storage of somehigh-energy electrons on the electron carrier NADH. Note that no O2 is needed for this set of reactions,which means that glycolysis can proceed in the absence of oxygen.

The second stage is a short series of reactions called the oxidation of pyruvate during which pyruvate (3carbon atoms) is converted to Acetyl-CoA (two carbon atoms), accompanied by the production of CO2 (onecarbon atom). This process occurs on the mitochondrial inner membrane, and as a result the Acetyl-CoA isformed inside the mitochondria. Pyruvate is made in the cytoplasm, and this step moves the next compoundin the pathway into the mitochodria. This is critical, since all subsequent steps in the pathway occur withinthe mitochondria. The other important event of this stage is the addition of high-energy electrons to NAD+,generating another molecule of the electron carrier NADH.

Acetyl-CoA enters into the Krebs cycle, a series of mitochondrial reactions that completes the break-down of the original glucose, thereby releasing CO2. In this third stage of the process, energy is harvestedin the form of high-energy electrons being used to generate NADH as well as another high-energy electroncarrier, FADH2. The reactions of the Krebs cycle also produce a small amount of ATP.

So far, a minimal amount of ATP has been produced, but a lot of energy has been stored in the electroncarriers NADH and FADH2. In the �nal stage of aerobic cellular respiration, Oxidative Phosphorylation,



a series of enzymes known as the electron transport chain uses those high-energy electrons to produce alarge amount of ATP. The high energy electrons harvested in the �rst three stages, and ferried by electroncarriers (NADH and FADH2) to the electron transport chain, are used to produce large amounts of ATPvir the mitochondrial membrane protein known as the ATP synthase. During this �nal stage is also whenatmospheric oxygen is used as the �nal electron and hydrogen ion acceptor, in a reaction which produceswater. The need for O2 in this �nal step means that these reactions are part of aerobic cellular respiration.

2 Location and Structures of Aerobic Cellular Respiration

All eukaryotic cells (protists, fungi, plants and animals) have mitochondria, and mitochondria are often calledthe power plants of the cell because these organelles produce a large amount of ATP. As you may rememberfrom a previous module, the mitochondrion is an organelle that is hypothesized to have originated as anendosymbiotic aerobic bacteria. Some of the evidence for this hypothesis comes from the relationship of thefunctional parts of the mitochondria (Figure 4) to the structure of a typical aerobic bacteria. There is anouter membrane which de�nes the organelle and represents the membrane which enveloped the bacteriawhen it was taken into the cell via endocytosis. The inner membrane represents the plasma membraneof the bacteria; the inner and outer membranes together form the intermembrane space. The innermembrane is highly folded; these folds are called cristae. The extensive folding increases the surface areafor the numerous electron transport chain enzymes and the ATP synthases that are used to make ATP. Inbacteria all of these enzymes are packed into the plasma membrane, as one would expect if the endosymbiotichypothesis is correct. The production of ATP is driven by a concentration gradient between the outer andinner compartment; in aerobic bacteria this concentration gradient is between the inside and the outside ofthe cell. The innermost compartment, derived from the cytoplasm of the ancestral bacteria, is called thematrix, and this compartment (just like the cytoplasm of today's bacteria) contains ribosomes and DNA;It is also the location of the Krebs Cycle reactions.



Figure 4: This image illustrates the structures within the mitochondria. (Image by Eva Horne andRobert Bear)


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Overview of Cellular Respiration - OpenStax CNX · · 2014-07-13This is a general overview of cellular respiration. ... Aerobic cellular respiration (Figure 3) ... Note that no