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.
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