Chapter 6: An Introduction To Metabolism

Energy is defined as the capacity to perform work. It refers to the ability of a system or object to exert force and cause a change or movement. Work is done when a force is applied to an object and it moves in the direction of the force. Energy can exist in various forms such as kinetic, potential, thermal, electrical, and chemical energy. It is the fundamental concept in physics that describes the ability to do work or cause a change in the state or motion of an object.

The first law of thermodynamics states that energy is neither created nor destroyed. This principle, also known as the law of conservation of energy, suggests that the total energy of a closed system remains constant. Energy can be transferred or transformed from one form to another, but the total amount of energy within the system remains constant. This law forms the foundation of energy conservation and is applicable to various fields, including physics, chemistry, and engineering.

Plants store energy in the form of chemical energy. This is because plants undergo photosynthesis, a process in which they convert sunlight into chemical energy by combining carbon dioxide and water to produce glucose. Glucose is then stored in the plant cells as starch or other carbohydrates. When the plant needs energy, it can break down these stored molecules and release the chemical energy in the form of ATP (adenosine triphosphate) through cellular respiration. Therefore, chemical energy is the most common form of energy storage in plants.

Organisms are described as thermodynamically open systems because they acquire energy from their surroundings. This means that they are not isolated from their environment and rely on external sources for energy to carry out their metabolic processes. This energy acquisition allows them to maintain their internal order and function in accordance with the second law of thermodynamics, which states that entropy in a closed system tends to increase over time.

When DG is positive, it indicates that the reaction or process requires an input of energy in order to proceed. This means that the reaction or process is not spontaneous and is termed endergonic. In an endergonic reaction, the products have higher energy than the reactants, and energy is absorbed from the surroundings.

Enzymes lower the energy of activation of a reaction by providing an alternative pathway for the reaction to occur. They do this by binding to the substrate and stabilizing the transition state, making it easier for the reaction to proceed. This lowers the amount of energy required for the reaction to start, allowing it to happen more quickly and efficiently.

The second law of thermodynamics does not state that energy is neither created nor destroyed. This statement is actually the first law of thermodynamics, which is the law of energy conservation. The second law of thermodynamics deals with entropy, heat content, and spontaneity. It states that in any spontaneous process, the total entropy of the system and its surroundings always increases.

The explanation for this answer is that succinylcholine is structurally almost identical to acetylcholine, and when it is combined with the enzyme that normally hydrolyzes acetylcholine, the enzyme is unable to hydrolyze acetylcholine. This suggests that succinylcholine competes with acetylcholine for the active site of the enzyme, acting as a competitive inhibitor.

Allosteric proteins are sensitive to environmental conditions, meaning that changes in factors such as temperature, pH, or the presence of certain molecules can affect their activity. They are also acted on by inhibitors, which can bind to the protein and reduce its activity. Allosteric proteins exist in active and inactive conformations, meaning that they can switch between different shapes that correspond to different levels of activity. Lastly, allosteric proteins typically have more than one subunit, meaning that they are composed of multiple individual protein units that work together. Therefore, all of the given statements are true about allosteric proteins.

All of the listed environments or actions, including heating the enzyme, cooling the enzyme, salt concentration, and pH, can affect enzyme activity. Enzymes are sensitive to changes in temperature, as heating can denature the enzyme and affect its shape and function. Cooling the enzyme can slow down its activity. Salt concentration can also affect enzyme activity by altering the electrostatic interactions between the enzyme and its substrate. pH can affect enzyme activity by changing the ionization state of amino acid residues in the active site, which can in turn affect the enzyme's ability to bind to its substrate and catalyze reactions. Therefore, all of the listed environments or actions can have an impact on enzyme activity.

Cracking a nut by using a nutcracker does not represent a transformation of one type of energy to another. This is because the energy used to crack the nut comes from the person using the nutcracker, and there is no conversion or transformation of energy involved in this process. The energy input and output remain the same, making it a mechanical process rather than an energy transformation.

All of the statements concerning enzymes are true and correct. Most enzymes are proteins, an enzyme is not consumed by the catalytic process, an enzyme is very specific in terms of which substrates it can bind to, and an enzyme lowers the activation energy of a chemical reaction. Therefore, the correct answer is "All of the above."

An exergonic (spontaneous) reaction is a chemical reaction that releases energy when proceeding in the forward direction. This means that the reaction will naturally occur and release energy without the need for any external input. It is not dependent on the presence of enzymes or the specific environment of living cells. Therefore, the correct answer is that an exergonic reaction releases energy when proceeding in the forward direction.

Adenosine is a component of both RNA and DNA, but it is more easily modified in RNA to form ATP. ATP is synthesized through the addition of phosphate groups to adenosine, and RNA already contains a ribose sugar that is compatible with the formation of these phosphate bonds. In contrast, DNA nucleotides have deoxyribose sugars, which do not readily form these bonds. Therefore, adenosine in RNA is the most easily modified monomer to form ATP.

Enzymes are biological catalysts that can greatly speed up reactions by lowering the activation energy required for the reaction to occur. However, enzymes cannot change the equilibrium point of reactions or the net energy output of the reaction. Therefore, the correct answer is "lower the activation energy of reactions ... change the equilibrium point ... change the net energy output".

If the reaction AB + CD -> AC + BD occurs spontaneously, it means that the reaction is thermodynamically favorable. This implies that the change in Gibbs free energy (ΔG) of the reaction must be negative. Additionally, since the reaction is exergonic, it releases energy. The environment must have adequate thermal energy to meet the activation energy requirement for the reaction to proceed. Lastly, for the reaction to occur, the bonds in the reactants (AB and CD) must have absorbed enough energy to become unstable and break. Therefore, all of the statements mentioned in the answer are true.

In an exergonic reaction, the potential energy of the products is lower than the potential energy of the reactants. This means that energy is released during the reaction, making it spontaneous and favoring the formation of products. In contrast, an endergonic reaction requires an input of energy because the potential energy of the products is higher than the potential energy of the reactants. Therefore, an exergonic reaction is designated as such when the potential energy of the products is lower than the potential energy of the reactants.

ATP is an energy currency in cells, and its main role is to provide energy for cellular processes. It does so by transferring its high-energy phosphate group to other molecules, forming a phosphorylated intermediate. This transfer of phosphate group allows ATP to couple with endergonic reactions, which require energy input, and provide the necessary energy for these reactions to occur. Therefore, the statement "Its free energy is coupled to an endergonic process via the formation of a phosphorylated intermediate" best characterizes the role of ATP in cellular metabolism.

The reaction "maltose + fructose -> sucrose" would be endergonic because it requires an input of energy to form the product, sucrose. In this reaction, two molecules are combining to form a larger, more complex molecule, which requires energy input. The other reactions listed either involve the breaking of bonds or the conversion of ATP to ADP, which release energy and are therefore exergonic.

The correct answer is DG. DG, or Gibbs free energy change, tells us the way a process will go spontaneously without any additional information. It is a measure of the balance between the enthalpy (DH) and entropy (DS) changes in a system. A negative DG indicates that the process will proceed spontaneously in the forward direction, while a positive DG indicates that the process will not occur spontaneously. The values of DH, DS, and TDS alone do not provide enough information to determine the spontaneity of a process.

The second law of thermodynamics states that in any energy transfer or transformation, the total entropy of a closed system will always increase over time. In the case of the aerobic respiration of glucose, energy is being transferred and transformed, and one of the by-products of this process is heat. Heat is a form of energy that is usually considered to have high entropy, so the generation of heat during aerobic respiration is an example of the second law of thermodynamics.

The statement "The action of competitive inhibitors may be reversible or irreversible" is true. Competitive inhibitors are molecules that compete with the substrate for binding to the active site of an enzyme. The binding of a competitive inhibitor to the active site prevents the substrate from binding, thus inhibiting the enzyme's activity. The action of competitive inhibitors can be reversible, meaning that the inhibitor can dissociate from the enzyme and the enzyme can regain its activity. However, in some cases, the binding of a competitive inhibitor can be irreversible, leading to permanent inhibition of the enzyme.

According to the second law of thermodynamics, the ordering of one system depends on the disordering of another. This means that in any energy conversion or process, there will always be an increase in the overall entropy or disorder of the universe. Energy conversions do not increase order in the universe, the total amount of energy in the universe is constant, and the entropy of the universe is constantly increasing, not decreasing. While all reactions do produce some heat, this is not the main focus of the second law of thermodynamics.

From the equation DG = DH - TDS, it can be concluded that all of the above statements are true. A decrease in the system's total energy will increase the probability of spontaneous change because it indicates a more favorable state. Increasing the entropy of a system will also increase the probability of spontaneous change as it leads to more disorder and a greater number of possible arrangements. Increasing the temperature of a system will increase the probability of spontaneous change because it provides more energy for reactions to occur. Lastly, the capacity of a system to perform work is indeed related to the total energy of the system, as work is the transfer of energy.

The correct answer is "all of the above". This is because all of the listed examples - the beating of cilia, the movement of glucose into an adipose cell, and the synthesis of new protein - require the use of ATP hydrolysis to provide the necessary energy for cellular work. Additionally, pumping blood through the circulatory system also requires ATP hydrolysis for mechanical work. Therefore, all of these examples are valid illustrations of cellular work accomplished with the free energy of ATP hydrolysis.

The hydrolysis of ATP drives cellular work by transferring a phosphate group to some other molecule. This transfer of the phosphate group provides energy to the recipient molecule, allowing it to undergo a specific reaction or perform a specific function. This process of transferring a phosphate group is a key mechanism for energy transfer and storage in cells.

A starch molecule has the most free energy per molecule compared to the other options listed. Starch is a complex carbohydrate made up of multiple glucose molecules. Glucose is a high-energy molecule that can be broken down through cellular respiration to release energy in the form of ATP. As a result, starch, being composed of multiple glucose molecules, has a higher potential for energy release compared to a single sugar molecule, an amino acid molecule, a fatty acid molecule, or a cholesterol molecule.

The correct answer is that the enzyme changes its shape slightly as it binds to the substrate. This is known as the induced fit of an enzyme. When the substrate binds to the enzyme's active site, the enzyme undergoes a conformational change, causing its shape to slightly alter. This induced fit allows for a better alignment of the enzyme and substrate, enhancing the catalytic activity of the enzyme. This conformational change can also help in stabilizing the transition state of the reaction, leading to a more efficient conversion of the substrate to product.

If the entropy of a system is increasing, it means that the system is becoming more disordered and chaotic. In order to maintain organization and counteract this increase in entropy, energy input will be required. This is because energy is needed to create and sustain order within a system. Without energy input, the system would continue to become more disordered and lose its organization. Therefore, the correct answer is that energy input will be needed to maintain organization.

The active site of an enzyme resembles a groove or crevice into which the substrate fits. This means that the active site has a specific shape that is complementary to the shape of the substrate. Just like a lock and key, the substrate can fit into the active site in a specific orientation, allowing the enzyme to catalyze the reaction. The rigidity of the active site is important for maintaining the specificity and efficiency of the enzyme-substrate interaction. The other options are incorrect because they do not accurately describe the active site of an enzyme.

High or low pH can disrupt the hydrogen bonding and change the shape of the active site. This change in shape can prevent the substrate from properly binding to the active site, leading to a decrease in enzyme activity. Therefore, most substrates do not function well at extreme pH levels.

Most enzymes are chains of amino acids. Enzymes are proteins that act as catalysts in biological reactions. They are made up of long chains of amino acids that are folded into specific three-dimensional structures. This structure allows enzymes to bind to specific substrates and facilitate chemical reactions. Enzymes are highly specific in their function and their activity is dependent on their amino acid composition and structure. Therefore, the statement that most enzymes are chains of amino acids is true.

Allosteric regulation refers to the process of stabilizing the quaternary structure of an enzyme in its active form by the binding of a molecule. In this process, the binding of a molecule to a specific site on the enzyme causes a conformational change in the enzyme's structure, leading to either activation or inhibition of the enzyme's activity. This regulation can occur through positive allosteric regulation, where the binding of a molecule enhances the enzyme's activity, or negative allosteric regulation, where the binding of a molecule inhibits the enzyme's activity. This mechanism allows for fine-tuning of enzyme activity in response to the concentration of specific molecules in the cell.

The correct answer is that the negatively charged phosphate groups vigorously repel one another. This is because the phosphate groups in ATP carry negative charges, and like charges repel each other. The repulsion between the phosphate groups makes the bonds between them unstable.

When one substrate molecule binds to the active site of one subunit in an enzyme molecule with multiple subunits, it causes all the subunits to assume their active conformation. This phenomenon is known as cooperativity. In cooperativity, the binding of one substrate molecule induces a conformational change in the enzyme, making it more receptive to other substrate molecules. This allows for the efficient and coordinated binding of multiple substrate molecules, enhancing the enzyme's catalytic activity.

The most reasonable explanation for the low velocity at 0°C is that there is too little activation energy available. Activation energy is the energy required to start a chemical reaction, and at lower temperatures, there is less thermal energy available to provide this activation energy. Therefore, the reaction rate is slower at 0°C compared to higher temperatures where there is more thermal energy available.

A competitive inhibitor competes with the product at the active site of an enzyme. This means that the inhibitor molecule and the product molecule both bind to the same active site on the enzyme, and they compete for the binding site. This competition can prevent the product from binding to the enzyme and carrying out its normal function.

When one molecule is broken down into six component molecules, the entropy (DS) of the system increases. This is because the number of possible microstates or arrangements of the molecules has increased, leading to a greater disorder or randomness. Since DS represents the change in entropy, it is definitely true that DS is positive in this case.

Glucose-6-phosphate is formed from glucose through a process called phosphorylation. This involves the transfer of a phosphate group from ATP to glucose, resulting in the formation of glucose-6-phosphate and ADP. Therefore, the correct answer is ATP —> ADP, as ATP is being converted to ADP during the reaction.

If, during a process, the system becomes more ordered, it means that the entropy of the system decreases. Entropy is a measure of the randomness or disorder in a system. When the system becomes more ordered, the randomness decreases, resulting in a negative change in entropy (ΔS). This negative change in entropy affects the Gibbs free energy (ΔG) of the system, making it negative. Therefore, the correct answer is that TDS (change in entropy) is negative.