Living organisms depend on a steady supply of energy available to do work. It takes energy to produce new cellular materials, to maintain the organization of membranes and organelles, and to fuel movement and active transport. The common energy for use in living systems is chemical energy released when chemical bonds are broken.
Metabolism encompasses all of the chemical reactions within the cell. Catabolic reactions are breakdown reactions that reduce large molecules to smaller components, releasing energy. Digestion and cell respiration are primarily catabolic processes. Anabolic reactions are synthetic reactions that produce larger molecules from small ones, storing energy in chemical bonds. Photosynthesis and the manufacture of proteins are examples of anabolic processes. Metabolic pathways, both anabolic and catabolic, are chains of reactions involving multiple steps. Among the most important of metabolic pathways are those involved in glycolysis and cellular respiration -- the chain of reactions that allows basic nutrients, such as the simple sugar glucose, to be metabolized to release energy in a form that can be easily used by the cell.
Potential energy is stored in the chemical bonds of molecules. Carbon-carbon and carbon-hydrogen bonds are relatively rich in energy. This energy is released when the chemical bond is broken. Some compounds contain high energy bonds. Notable among theses is adenosine triphosphate [ATP] with its high-energy phosphate bond. The high-energy bond is the one between the last two phosphate groups.
Adenosine-Phosphate-Phosphate~Phsophate
This bond has high energy because both of the phosphate groups contain several oxygen atoms, and the electronegative oxygen atoms tend to pull electrons toward themselves. The resulting tension produces a high energy condition. When this bond is broken as the phosphate group is removed, energy is released.
Free energy is the energy that can be made available to do work. Essentially, this is the amount of energy that can be released by breaking chemical bonds in a molecule. When the products of a reaction have less free energy [symbolized by G] than the reactants, energy is released by the reaction. These reactions that release energy are exergonic. oxidation reactions and dephosphorylation reactions are exergonic. When the products of a reaction have more free energy than the reactants, energy must be added. These reactions are endergonic. Reduction reactions and phosphorylation reactions are endergonic.
Under standard conditions [sea-level atmospheric pressure, 25 deg.C, pH 7, and 1 molar concentrations of all reactants and products], exergonic reactions tend to move forward, to produce more product, and to release energy. They are referred to as spontaneous reactions. Endergonic reactions do not progress under standard conditions. They are non-spontaneous.
The change in standard free energy in an exergonic reaction is graphed below. Initially, the energy of the reactants, labeled Greactants, is high and the final Gproducts, the energy of the products of the reaction is low.
In an endergonic reaction, the graph would look like the one below; the initial Greactants is low and the final Gproducts is high.
Endergonic reactions may be described either by the change in G value [[[Delta]]G] or by the equilibrium constant. [[Delta]]G is the difference between Gproducts and Greactants. Exergonic reactions will have a -[[Delta]]G value; exergonic reactions will have a +[[Delta]]G.
[[Delta]]G = Gproducts - Greactants
The equilibrium constant [Keq] is the ratio of the concentration of products to the concentration or reactants at equilibrium. [The equilibrium point of a reaction is the point at which it is equally likely to progress forward or to go in reverse. At equilibrium, there is no net reaction.]
Keq = [products]
[reactants]
Exergonic reactions, which tend to move forward, will have Keq values greater than 1 [since the reaction will reach equilibrium only when relatively large amounts of product have been formed, and when little reactant remains. Endergonic reactions have Keq values less than one. Endergonic reactions tend to reach equilibrium when the concentration of reactants is still high and the concentration of products is low.
Endergonic reactions are important to many cell processes. Endergonic reactions may be 'pushed' forward by coupling them with exergonic reactions. The energy released from the exergonic reaction can be used to drive the endergonic reaction.
Endergonic reactions may also be 'pulled' forward by removing the products of the reaction as fast as they form, so that the reaction is prevented from reaching its equilibrium point.
Chemical reactions, both spontaneous and non-spontaneous, require an initial input of energy, the energy of activation, to begin the reaction. This energy is needed to bring reactant molecules into position and condition for reaction.
In non-living systems, this energy can be supplied by heat. Extreme elevated temperatures are not compatible with living systems; an alternative mechanism for the initiation of chemical reactions in living systems is needed.
Enzymes serve as organic catalysts that lower the energy of activation, thus facilitating reactions. The enzyme is able to interact with the reactants of a reaction to 'set them up' by distorting the molecules slightly, weakening their bonds and making it more likely that the molecules undergo a chemical change. Enzymes can also act to 'collect' molecules in a local area and to position them in such a way that bonds are easily formed between them.
The convenient 'energy package' that recaptures energy for the cell's use is the molecule ATP [adenosine triphosphate]. The bonds between the phosphate groups in this molecule are high energy bonds that release a standard amount [7.3 kcal/mole] ] of energy when the bond is broken. A basic goal of the respiratory pathways is to release the chemical energy of glucose or other nutrients and recapture that energy in the high energy bonds of ATP molecules.
When glucose is fully oxidized, it is broken down to carbon dioxide and water. The chemical equation for this reaction is:
C6H12O6 + 6 O2 ----> 6 CO2 + 6 H2O
In the process, energy is released. It is possible to measure the energy released by a reaction in an instrument called a calorimeter. A calorimeter measures the amount of heat released when the sugar [or any other substance] is oxidized. When glucose is fully oxidized, 686 kilocalories of energy are released from each mole of glucose. When oxidation occurs in a single step -- as it does during combustion, when a substance is burned -- the energy is lost as heat and light, and is no longer available to do biological work. The slow step-wise controlled release of energy during glycolysis and respiration allows a significant proportion of the energy of the sugar to be converted to energy stored in ATP molecules.
The process can be broken into four groups of reactions: glycolysis, the transition reactions, the Krebs cycle, and the electron transport system.
Glycolysis is a universal metabolic pathway. All organisms -- plant, animal, and bacterial cells -- use the same glycolytic pathway. The diagrams on the next page show the 10 steps of glycolysis. Note that all of the steps in the second column [after the formation of phosphoglyceraldehyde [PGAL] ] are doubled, since there are two molecules of PGAL formed from one original glucose molecule.
If oxygen is available [and if an organism is aerobic, and able to use oxygen in its respiration], the pyruvate formed during glycolysis continues on through the transition reactions, the Krebs cycle, and the electron transport system. Although oxygen is not involved in the reactions until the end of the electron transport system, the entire series [beginning from pyruvate] is considered to be aerobic or oxygen-dependent. Aerobic respiration extracts far more usable energy from the glucose molecule than glycolysis alone.
If oxygen is not available, the pyruvate formed during glycolysis is sent along a different metabolic pathway. Anaerobic respiration requires no oxygen and produces no additional usable energy, but it does allow glycolysis to continue. Some organisms, such as yeast cells, use alcoholic fermentation as the anaerobic pathway; others, such as human muscle cells, use lactic acid fermentation.