8.1 What happens to Energy in Chemical Reactions?

• Kinetic energy is energy of motion.

• Potential energy is energy that is stored in position or configuration.

• First law of thermodynamics: the first law is that energy cannot be created or destroyed, it can only be transferred and transformed.

• The total energy in a molecule is referred to as its enthalpy (represented by H).

• When a reaction releases heat (products have less potential energy than the reactants), it is exothermic and the change in energy is negative.

• If heat is taken up during the reaction, generating products that have higher potential energy than the reactants, the reaction is endothermic and change in H is positive.

• Another factor that changes during a chemical reaction is the amount of disorder or entropy

• The second law of thermodynamics, in fact, states that total entropy always increases in a system that includes the surroundings as well as the products of the reaction

• To determine whether a chemical reaction is spontaneous, it’s necessary to assess the amount of energy in the reaction available to do work-what chemists call Gibbs free energy

• Chemical reactions are spontaneous when change in Gibbs free energy is less than zero which is an exergonic reaction.

• Reactions are non spontaneous when change in Gibbs free energy is greater than zero which is an endergonic reaction.

• The number of collisions occurring between the substances in a mixture depends on their temperature and concentration:

  • When the concentration of reactants is high, more collisions will occur and reactions should proceed more quickly.

  • When their temperature is high, reactants will move faster and should collide more frequently.

8.2 Nonspontaneous Reactions May Be Driven Using Chemical Energy

• Chemical reactions that involve the loss or gain of one or more electrons are called reduction-oxidation reactions, or redox reactions

• Oxidation Is Loss of electrons

• Reduction Is Gain of electrons.

• Redox reactions represent the energetic coupling of two half reactions, one exergonic and one endergonic.

• In such a reaction, the molecule giving up an electron is called the electron donor, while the recipient molecule is called the electron acceptor.

• Adenosine triphosphate (ATP) is a ribonucleotide used for RNA synthesis, but it is more commonly known for its ability to make things happen in cells.

• A kilocalorie (kcal) of energy raises 1 kilogram (kg) of water 1 °C.

• The addition of a phosphate group to a molecule is called phosphorylation.

8.3 How Enzymes Work

• In all reactions, even spontaneous ones, a certain minimum amount of kinetic energy-called the activation energy-is required to sufficiently strain the chemical bonds in molecules so they can react to form products.

• The transition state is the intermediate point between breaking old bonds and forming new ones.

• The more unstable the transition state, the higher the activation energy and the less likely a reaction is to proceed quickly.

• When reactants undergo a chemical reaction by binding to an enzyme, they are referred to as substrates.

• Enzymes are catalysts-they bring substrates together in a precise orientation that makes reactions more likely to occur.

• Active-site binding helps substrates to collide in a precise orientation so that particular bonds can break and new bonds can form to generate products.

• The steps of a enzyme catalyze reaction include:

  • Initiation

  • Transition state facilitation

  • Termination

• There are three different types of enzyme “helpers”:

  • Cofactors are inorganic ions, such as the metal ions Zn2+ (zinc), Mg2+ (magnesium), and Fe2+ (iron), that reversibly interact with enzymes.

  • Coenzymes are organic molecules that reversibly interact with enzymes.

  • Prosthetic groups are atoms or non-amino acid molecules that are permanently attached to proteins.

8.4 What Factors Affect Enzyme Function?

• The rate of an enzyme-catalyzed reaction depends not only on substrate concentration and the enzyme’s intrinsic affinity for the substrate, but also on temperature and pH.

• The regulatory molecule is similar in size and shape to the enzyme’s natural substrate and inhibits catalysis by binding to the enzyme’s active site which is a tactic called competitive inhibition.

• The regulatory molecule binds at a location other than the active site and changes the shape of the enzyme.

• This type of interaction is called allosteric regulation because the binding event changes the shape of the enzyme in a way that makes the active site available or unavailable.

8.5 Enzymes Can Work Together in Metabolic Pathways

• Each of the molecules are built by a series of reactions, each catalyzed by a different enzyme and this multistep process is referred to as metabolic pathways.

• A convenient way to regulate metabolic pathways is to use the final product of the reaction sequence to inactivate one of the pathway’s own enzymes and this type of regulation is called feedback inhibition.

• The metabolic activity of other microbes is now being scrutinized and engineered to clean up a variety of human-made pollutants-giving rise to a new technology called bioremediation

• Those that break down molecules for sources of energy and carbon building blocks are called catabolic pathways.

• Those that use energy and carbon building blocks to synthesize molecules are called anabolic pathways.