Cellular Respiration: Brief Look at the Citric Acid and Oxidative Phosphorylation

Just recently, I blabbed about cells and cellular respiration. Then toward the end of my last post I got really excited about cellular respiration as fundamental to all living creatures, including you and me. But all I had really talked about was the fundamental units of biological organization, the smallest living things, thermodynamics and biological chemistry, and how we get a little ATP and a bunch of energy stored as pyruvate from each glucose molecule that gets broken by glycolysis. Nothing very important, really...

Perhaps you remembered vaguely when you first learned this material. Perhaps you wondered why anyone would care. Perhaps you were amazed to be reminded of the chemical basis of all life on earth, the far-reaching implications of the chemistry which forms the foundation of biological science. If you're in the latter group, right on! And read on! Because glycolysis is only one process linked to cellular respiration. It is not considered a part of real respiration because oxygen is not involved. And it yields a relatively small amount of ATP dividends per unit starting molecule and per unit effort (10 steps, two molecules of ATP burnt off). What really gets my engine burning, and hopefully yours too, are the citric acid cycle (CAC) and oxidative phosphorylation (OP).

Before I talk about these, allow me to just say that these aren't just singular discoveries written down in books so that students get a view of the big picture from their biology education--at least, that's not all these are. Instead, the CAC and OP pathways represent active areas of scientific research. Recent studies illustrate this point: a group of scientists from Germany and other places have been looking into fluorescent imaging of CAC intermediates (here), while American scientists have examined the origin and self-organization of biochemical cycles, not to mention the evolution of the enzymes involved.

So, on to CAC and OP and what these should mean to you and me.

The CAC (Fig. 1 below) is an eight-step biochemical pathway turning pre-packaged Acetyl-CoA (a modified form of pyruvate from glycolysis; two per glucose molecule, just like pyruvate) into two molecules of CO2, three NADH, one ATP, and one FADH2 (an electron carrier similar to NADH). On the other hand, OP takes electrons from NADH and FADH2 made available from glycolysis and the citric acid cycle and uses these to generate up to 32 or 34 molecules of ATP per starting molecule of glucose!!! As an aside for now, Acetyl-CoA used in the CAC may also come from proteins and lipids--not just glucose--but this is beyond the scope of our discussion. All in all, factoring in CAC and OP in view of the classical glucose metabolism perspective, each molecule of glucose broken down in your cells has the potential to generate up to 36 or 38 molecules of ATP!!!


Figure 1. The citric acid cycle (Orgel 2000, see link at end of text).



So, we see from this overview how oxidative phosphorylation and chemiosmosis (linked processes) are together responsible for the majority of ATP produced by the respiration of our cells (eventually linked to the respiration we do with our circulatory system and respiratory systems, or blood and lungs). These ATP generated by OP and chemiosmosis are largely the result of what is known as the electron transport chain (ETC), making this one of the most important sub-cellular structures in living things.

The ETC is a collection of multiprotein complexes found all over the inner membrane of our mitochondria. NADH and FADH2 deliver electrons to specific pieces of this chain. In turn, these electrons move down the chain using well-known avenues. At the end of this chain, the last protein in the ETC sequence (these proteins are mostly particular kinds, called cytochromes. My DNA Barcoding work studies cytochrome oxidase 1 to explore nearshore fishes of the Carribbean/Western Central Atlantic for cryptic speciation events), "Cyt a3," transfers its electrons to oxygen (O2), which grabs two hydrogens to become water (H2O). Because electrons from FADH enter the ETC at a different protein complex than those donated by NADH, they provide less energy for ATP synthesis than those from NADH.

So, electrons release energy as they fall down the ETC. "But how is this energy harnessed to make ATP?" you might ask. Simple. Coupling electron transport to ATP synthesis (exergonic reactions coupled to endergonic ones, as mentioned elsewhere). This is accomplished by the process of chemiosmosis.

Chemiosmosis is the phenomenon whereby motion of an ion across its gradient through a membrane releases energy which is used to do cellular work. ATP synthase, also found in the inner wall of the mitochondrion, is the enzyme responsible for ATP formation from ADP (the "used up" form of ATP in our cells) and inorganic phosphate (P-). In the case of ATP synthase, the ion used to generate the energy of ATP regeneration is hydrogen, which accumulates in the mitochondrial intermembrane space as a result of the action of the ETC. This combined action of the ETC and chemiosmosis make up what we know as oxidative phosphorylation, what I have called OP. One key difference between this process and the ATP-yielding pathways we've discussed, glycolysis and the citric acid cycle, is that OP generates ATP from phosphorylation using these two processes instead of substrate-level phosphorylation, the method of glycolysis and CAC. Are you with me??

Hope so.


There you have it... in a nutshell, the difference between glycolysis and cellular respiration, the difference between glycolysis/CAC and oxidative phosphorylation (ETC + chemiosmosis/ATP synthesis), and all fit into an overview of how we get chemical energy from our food. Holla!

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See Campbell and Reece (2007) for more information, or try a search of Google Scholar to learn more about cellular respiration, glycolysis, the CAC or OP. Or, read this Orgel (2000) paper. Peace out ~ JB

Cellular Respiration: Cells, Energy and Glycolysis

Living cells perform work. Essentially, work is the amount of energy transferred by force, thus energy is required for work to take place. My fingers are performing work by pressing down on and transferring their kinetic energy to the keys on this keypad. This is a coordinated effort of nervous impulses in my brain receiving and dictating signals to and from the muscles and other tissues of my eyes, hands, and fingers. An input of energy is required for each type of tissue involved to accomplish the task of each keystroke. My brain cells are not performing the same type of mechanical work as my hands and fingers; however, they are costing my body an enormous amount of energy through maintenance and metabolic costs from not only my mental activity, but also the basic needs of my neural cells. Recall how two major themes of biological science are (a) that energy flows through ecosystems from the sun to producers to consumers and (b) that living things place energetic demands on their environments in order to do the work characteristic of life. At this point, we will delve into these in greater detail, all from the standpoint of cellular respiration.

Cellular respiration (CR) might be defined as the sum of all the processes involved in the catabolism (breakdown) of food molecules (protein, carbohydrates, and lipids) into carbon dioxide, water, and energy bound in the form of ATP and heat released from chemical reactions involved. This relationship between different chemical and physical variables of cellular respiration is similar to the oxidation reactions generating motion in heat combustion engines. A major goal of any education in cellular respiration should be founded upon an understanding of the way cells arrive at ATP and the way this molecule is utilized by cells to supply the energy necessary for life. Before this can be grasped, however, it is necessary we embellish more on the nature of energy and metabolism in natural systems.

The first law of thermodynamics: Energy is neither created nor destroyed, it may only be transformed or transferred from one form to another. Second law of thermodynamics: all energy transformations and transferences increase the overall entropy of the universe. Notice how this second law of thermodynamics need not apply to individual transactions between organisms and their environment, whether the events we consider are single physical interactions of adults or we are concerned with development or evolution of populations. What I mean by this is that, simply, we learn from studying nature how many ordered phenomena coexist with random, chaotic, or stochastic processes and events. The order we find in the universe, in the beauty and symmetry of life or its evolution for example, does not contradict the second law of thermodynamics because these are single occurrences while organisms and their populations increase the entropy of the universe in other ways, including heat loss. Thus, while individual structures or events may decrease the entropy of the universe (by introducing orderliness; e.g., cleaning your room), the universe is more than its individual systems (organisms, ecosystems, planets, etc.), extending to includes each system’s surroundings (e.g., where heat is lost), so entropy may drop locally but is always increasing in the universe as a whole.

The Gibbs free energy (ΔG) of a system describes the energy available for work in a system, such as a cell, when pressure and temperature are constant. This quantity is equivalent to the total energy in a biological system minus the entropy change over a given time period at a given temperature. Spontaneous processes use available energy and in doing so decrease the energy in a system, and therefore are represented by negative Gibbs free energy values; conversely, a positive value of ΔG suggests a process is not spontaneous. Systems in the biosphere often run down energy gradients towards energetically more stable states, the most stable of these being that of equilibrium, where free energy change is near zero or positive and more work cannot be accomplished. Our cells represent dynamic systems where many different types of chemical and mechanical forces are at work to maintain and alter living things. Here, in our cells, reactions where energy is lost (ΔG <> 0), or endergonic reactions, may occur, often in simul. Because of the many chemical processes carried out in our cells, our bodies never reach a state of energetic equilibrium; rather, materials constantly flow through our physiological systems.

Biologists generally recognize three kinds of work cells accomplish using the energy harvested from biochemical reactions making up cellular respiration (oxidizing food particles into useful energy), (1) mechanical work involving movement of various cellular structures, (2) transport work which pumps substances across membranes against concentration gradients, and (3) chemical work, which involves supplying energy from outside of chemical products to force processes to completion which are not naturally spontaneous. Much of this work is accomplished through energy coupling, or using exergonic processes to generate energy fueling endergonic processes. I have already commented on the role of mitochondria in ATP production, the ability of this molecule to trap energy, and subsequently have alluded to its importance in cellular respiration and metabolism. But what is the structure and importance of ATP really like? Why, more specifically, is its role in the reactions of cellular respiration important, and what are those reactions themselves like?

Each ATP (adenosine triphosphate) molecule is composed of a nitrogenous base, adenine, linked to a ribose (pentose, or five-carbon ring structured) sugar, which is in turn connected to three inorganic phosphate molecules. The region of ATP containing these inorganic phosphate molecules is negatively charged and unstable, making ATP an effective reducing agent (it oxidizes other molecules). When bonds between ATP phosphates are broken by hydrolysis in our cells by enzymes, a P group is transferred to other molecules, which are then said to have been “phosphorylated” by ATP, which is oxidized to ADP, adenosine diphosphate. This process of phosphorylation represents a coupling of exergonic (ATP hydrolysis) and endergonic processes used by the cell to accomplish different tasks, i.e., cellular work—either involved in mechanical motion, transport, or chemical changes. ATP phosphorylation is a ubiquitous solution to problems involved in cellular work, which is taking place in our cells constantly and generates a high demand for ATP in our cells. Luckily, our bodies are equipped with mechanisms for quickly and efficiently regenerating hydrolyzed ATP from by-products of catabolic processes (more on this later).

Metabolic (energetic) and catabolic (break down) pathways of cellular respiration, as a whole, represent a complex and tangled web of biochemical interactions. Given this complexity, biologists have seen fit to break this complexity into two basic processes comprising cellular respiration: (1) the citric acid cycle (a.k.a., the Krebs cycle) and (2) oxidative phosphorylation. Broadly speaking, this order represents a potentially but not necessarily in-line time series of events classically taught from the perspective of breaking down glucose molecules into energy in the form of ATP. Now, we take a closer look at each of these steps in cellular respiration.

Glycolysis, considered by many the first stage of cellular respiration although making this designation is committing a semantic error (cellular respiration only means processes using oxygen in CR), refers to biochemical processing our cells carry out in the cytosol, or liquid matrix comprising the extra-organellar (NS) space within cells, which serves the purpose of breaking individual glucose molecules (6-carbon ringed polysaccharide monomers) into two pyruvate molecules (3-carbon chemical compound). This evolves a small amount of ATP. Glycolysis is comprised of a ten-step process. In the early stages, the energy investment phase, cells burn two molecules of ATP; in the latter half of the whole series of reactions, two molecules of pyruvate (3-carbon molecule formed from breaking the 6-carbon glucose ring in two using Aldolase), four ATP molecules (net of two per glucose molecule) and two molecules of NADH are generated per starting glucose monomer. Pyruvate, after being shuttled into the mitochondrion, may be further oxidized through the citric acid cycle (CAC) and or by oxidative phosphorylation (OP) machinery, but this only can happen if oxygen is present in the cell. Alternatively, NADH, the reduced form of NAD+, holds electrons which can be used by CAC and OP processes to yield waaaayyy more energy than glycolysis per unit glucose starting material (as you'll see next post).

What I really want to harp on here is that this stuff is important! Really, really, really important, actually. Everyone should be interested in learning about this because biology is everyone's problem by virtue of our biological nature. Learning about biology is learning about your history, your form, your function, your relatives, your disease, your world. No biological entities could move or do any work without a means of converting energy in food to useable form in our cells, which are uniquely equipped with machinery to carry out highly specialized functions maintaining life as we know it. Next, I'll review another troubling set of ideas forming the remainder of cellular respiration: the citric acid cycle and oxidative phosphorylation.