Lecture Outline - Chapter 6

6.1. Energy (p. 98)

1. Energy is capacity to do work and occurs as light, electricity, heat, etc.
2. Kinetic energy is energy of motion; includes contracting muscles and falling water.
3. Potential energy is stored energy; examples include water behind a dam, a rock at the top of a hill, or the molecule ATP.
4. Two Laws of Thermodynamics
   • a. Thermodynamics is study of energy relationships and exchanges.
   • b. First law of thermodynamics states energy can neither by created nor destroyed, but can be converted from one form to another; for example, food we eat is converted into chemical energy of ATP and then this potential energy is converted to mechanical energy of muscle contraction. (Fig. 6.1)
   • c. Second law of thermodynamics states one usable form of energy cannot be completely converted into another usable form; for example, muscles convert chemical-bond energy within ATP to mechanical energy of contraction with some energy lost to surroundings as heat.
5. Entropy is a measure of randomness or disorder; it takes a constant input of usable energy from the environment to maintain the organization of living organisms.
6. Energy is measured in joules or kilocalories; the latter is amount of energy needed to raise the temperature of one kilogram of water one degree Celsius.

1. Metabolism is the sum of all chemical activities occurring inside a cell.
2. A reaction will occur spontaneously if the products of the reaction have less free energy than the reactants.
3. Free energy is the amount of energy available to do work after a chemical reaction has occurred.
4. Exergonic and Endergonic Reactions
   • a. Exergonic reactions result in release of excess energy; such reactions occur spontaneously.
   • b. Endergonic reactions require input of energy; the products contain more free energy than the reactants.
5. Reversible Reactions
   • a. When free energy is about equal on both sides of an equation, the reaction is reversible and at equilibrium.
   • b. If cells use a product as soon as it is formed, this pulls a reversible reaction in that direction.
6. Coupling Reactions
   • a. Energy released by exergonic reactions can be used to drive endergonic reactions.
   • b. Breakdown of ATP is exergonic and often coupled to cell reactions that require energy input.
   • c. Even coupled reactions represent an overall loss of energy; as when ATP breaks down.
7. ATP: Energy for Cells
   • a. ATP is common energy currency of cells.
   • b. Our body produces a lot of ATP but it is the same ATP being constantly recycled from ADP and (P).
   • c. ATP is a nucleotide made of adenine base and ribose sugar (together called adenosine) plus three phosphate groups.
   • d. Under cellular conditions, the amount of energy released when ATP becomes ADP plus (P) is 7.3 kilocalories per mole.
   • e. ATP is a carrier of cell energy because:
     • i. Many different cell reactions can utilize its common energy currency.
     • ii. Energy released when ATP becomes ADP plus (P) is just enough for biological purposes and little energy is wasted.
     • iii. ATP breakdown can couple with endergonic reactions to minimize energy loss.
   • f. ATP Functions
     • i. Chemical work requires energy to synthesize cell molecules.
     • ii. Transport work is done when substances are pumped across cell membrane.
     • iii. Mechanical work is done when muscles contract, cilia and flagella beat, chromosomes move, etc.

1. Metabolic pathways:
   • a. Begin with a reactant and end with an end product.
   • b. Involve many small steps.
   • c. Proceed in an orderly, step-by-step manner.
   • d. Allow one pathway to lead to other pathways if there are several molecules in common.
   • e. Capture and use energy more easily since it is released in small increments.
2. 2. Reactants in enzymatic reactions are called substrates and are used to form a product, which in turn becomes the substrate for the next reaction.
3. 3. Enzymes
   • a. Every reaction in a cell requires a specific enzyme.
   • b. Enzyme names are often formed by adding "-ase" to name of its substrate or by action they perform; for example, dehydrogenase. (p. 102)
   • c. Enzymes speed reactions are usually protein molecules that act as organic catalysts to speed up chemical reactions.
   • d. Enzymes lower energy of activation, which is amount of heat needed for a reaction to occur. (Fig. 6.4) Less heat is therefore needed to bring about a reaction in a cell.
   • e. Enzymes lower energy of activation by forming an enzyme-substrate complex.
     • i. Substrate attaches to an enzyme at active site.
     • ii. Enzymes are not permanently altered by the reaction; only a small amount of enzyme is needed since it can be used over again.
     • iii. Shape of an enzyme allows an enzyme-substrate complex to form, which explains specificity of an enzyme, similar to a key that fits just one lock.
     • iv. In the induced fit model, enzyme undergoes a slight change in shape in order to achieve best fit of the substrate.
   • f. Presence or absence of enzymes determines what reactions take place.
4. Factors That Affect Enzymatic Speed
   • a. Enzymatic reactions can be rapid if substrate fills active sites most of the time; for example, the enzyme catalase assists breakdown of hydrogen peroxide to water and oxygen at 600,000 times per second.
   • b. As temperature increases up to a certain point, enzymatic activity increases. Beyond that point, the reaction levels off and then declines as enzyme is denatured.
   • c. Each enzyme has optimal pH, which helps maintain normal shape; change in pH alters bonding of R groups resulting in a change in shape (denaturation).
   • d. Activity increases as substrate concentration increases.
5. Enzyme Inhibition
   • a. An inhibitor is any molecule that can bind to an enzyme and inhibit the reaction.
   • b. Inhibition is a common method cells use to regulate enzyme activity.
   • c. In competitive inhibition, the inhibitor is similar in shape to substrate and competes with substrate for the active site.
   • d. In noncompetitive inhibition, a molecule will bind to a site other than the active site (the allosteric site) so that active site is altered.
   • e. Feedback inhibition is normal way reactions are regulated in cells; a product produced by an enzymatic reaction in turn inhibits its activity; when the product is used up, inhibition is reduced and more product is produced. (Fig. 6.6)
   • f. An end product can also bind an allosteric site on the first enzyme in a pathway and shut down the pathway completely. (Fig. 6.7)
6. Cofactors and Coenzymes Help Enzymes
   • a. Many enzymes require a nonprotein cofactor to assist in carrying out their function.
   • b. Magnesium (Mg⁺⁺), potassium (K⁺), and calcium(Ca⁺⁺) are ions that are cofactors.
   • c. Other enzymes require coenzymes, organic molecules that bind to enzymes and serve as carriers for chemical groups or electrons.
   • d. Coenzymes include vitamins such as niacin, folic acid, and thiamin.

   6.4. Metabolic Pathways and Living Things

   1. Energy flows through organisms.
   2. The metabolic pathways of photosynthesis and cellular respiration convert one form of energy to another.
   3. Energy is temporarily stored in ATP and cellular molecules.
   4. Eventually, all solar energy received from the sun is lost as heat, a nonusable form of energy.
   
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