Chapter 5: Ground Rules of Metabolism

How Living Organisms Use and Convert Energy

Fireflies and Bioluminescence: How do they produce light?

  • B. Researchers transferred genes for bioluminescence into strains of Salmonella so that the course of infection could be tracked by visualization.
  • I. Energy and the Underlying Organization of Life

  • A. Defining Energy

    1. Potential energy is the capacity to do work; in molecules it is called chemical energy.

  • 2. Kinetic energy is the energy of motion.

    3. Energy transfers release heat.

  • B. What Can Cells Do With Energy?

  • 1. Energy can be obtained from the sun, inorganic, and organic substances.

    2. Cells use energy for work: chemical, mechanical, and electrochemical.

  • C. How Much Energy Is Available?

  • 1. First law of thermodynamics states that the total amount of energy in the universe is constant; it cannot be created or destroyed; it can only change form.

    2. Energy cannot be produced by a cell; it can only be borrowed from someplace else.

  • D. The One-Way Flow of Energy

  • 1. Energy can be of high quality, that is, highly concentrated and usable; or it can be of low quality, such as heat that is released into the universe.

    2. Second law of thermodynamics states that the spontaneous direction of energy flow is from high- to low-quality forms.

  • a. Each conversion results in production of energy (usually heat) that is unavailable for work.

    b. As systems lose energy, they become more disorganized; the measure of this disorder is called entropy.

  • 3. The world of life (plant and animal) maintains a high degree of organization only because it is being resupplied with energy from the sun.

  • II. Doing Cellular Work

  • A. When cells convert one form of energy to another, there is a change in the amount of potential energy.
  • 1. Endergonic (energy in) reactions result in products with more energy than the reactants had.

    2. Exergonic (energy out) reactions result in products with less energy than the reactants had.

  • B. ATP, The Cell’s Energy Currency

  • 1. Before cells can use the energy of sunlight or that stored in carbohydrates, they must transfer the energy to molecules of ATP.
  • a. ATP is composed of adenine, ribose, and three phosphate groups.

    b. ATP transfers energy to many different chemical reactions; almost all metabolic pathways directly or indirectly run on energy supplied by ATP.

  • 2. Energy input links phosphate to ADP to produce ATP.

  • a. ATP can donate a phosphate group (phosphorylation) to another molecule, which then becomes primed and energized for specific reactions.

    b. ADP can be recycled to ATP very rapidly.

  • C. Electron Transfers

  • 1. Oxidation-reduction reactions are simply electron transfers between molecules.
  • a. The donor molecule loses an electron and is oxidized.

    b. The receptor molecule gains an electron and is reduced.

  • 2. Certain electron transfers proceed in an orderly, stepwise fashion to control the release of energy.

  • D. Metabolic Pathways

  • 1. Metabolic pathways form series of reactions that regulate the concentration of substances within cells by enzyme-mediated linear and circular sequences.
  • a. In biosynthetic pathways, small molecules are assembled into large molecules; for example, simple sugars are assembled into complex carbohydrates.

    b. In degradative pathways, large molecules such as carbohydrates, lipids, and proteins are broken down to form products of lower energy. Released energy can be used for cellular work.

  • 2. Terms used in describing metabolic pathways include:

  • a. Substrates (= reactants )are substances that enter into a reaction.

    b. Intermediates are substances that form between the start and conclusion of metabolic pathway.

    c. End products are the substances present at the conclusion of a reaction or pathway.

    d. Energy carriers donate energy to substances by transferring functional groups to them; ATP is the main type.

    e. Enzymes are proteins that catalyze (speed up) specific reactions.

    f. Cofactors are organic molecules or metal ions that assist enzymes or transport electrons/atoms.

    g. Transport proteins adjust the concentration gradients at cell membranes in way that influence the direction of metabolic reactions.

  • III. Enzyme Structure and Function

  • A. Enzymes mediate reversible reactions that tend to run toward chemical equilibrium.

    B. Four Features of Enzymes

  • 1. Enzymes are proteins that serve as catalysts; they speed up reactions.

    2. Enzymes can be reused.

    3. Enzyme actions are reversible.

    4. Enzymes are selective and act upon specific substrates.

  • C. Enzyme-Substrate Interactions

  • 1. Activation energy is the amount of energy needed to bring colliding molecules to the transition state.

    2. Enzymes increase the rate of a reaction by lowering the activation energy through extensive bonding of substrate at the active site.

  • a. The active site is a crevice where the substrate binds to the enzyme during a reaction according to the induced-fit model.

    b. In order to proceed reactants must reach a "transition" state.

  • IV. Factors Influencing Enzyme Activity

  • A. Enzymes and the Environment
  • 1. Because enzymes operate best within defined temperature ranges, high temperatures decrease reaction rate by disrupting the bonds that maintain three-dimensional (globular) shape (denaturation occurs).

    2. Most enzymes function best at a pH near 7.
    Higher or lower pH values disrupt enzyme shape and keep them from functioning.

  • B. How Is Enzyme Action Controlled?

  • 1. Some methods of control regulate the number of enzyme molecules available by speeding up or slowing down their synthesis.

    2. Feedback inhibition operates when a substance triggers a cellular change that shuts down production of that substance.

    3. Allosteric enzymes have (in addition to active sites) regulatory sites where control substances can bind to alter enzyme activity; if this control substance is the end product in the enzyme’s metabolic pathway, feedback inhibition occurs.

    4. Hormones are the signaling molecules in enzyme control.

  • V. Reactants, Products, and Cell Membranes

  • A. Every cell membrane shows selective permeability;
    i.e., some substances, but not others, can cross them at certain times.

    B. Gases, nonpolar molecules, and water can move through the lipid bilayer by Diffusion.

    C. Glucose and other large polar molecules must cross the membrane through transport proteins, in some cases via diffusion and in other cases by Active Transport.

  • VI. Working With and Against Concentration Gradients

  • A. A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions.
  • 1. Molecules constantly collide and tend to move according to existing concentration gradients.

    2. The net movement of like molecules down a concentration gradient (high to low) is simple diffusion.

    3. Gradients in temperature, electric charge, and pressure, can influence movements.

  • B. Passive Transport

  • 1. In passive transport, solutes pass through the cell membrane with assistance from transport proteins in accordance with the concentration gradient.

    2. Transport proteins change shape to move substances into and out of the cell.

  • C. Active Transport

  • 1. In active transport, solutes can move against concentration gradients with assistance from transport proteins that can change their shape with energy supplied by ATP.

    2. The sodium-potassium pump is a major cotransport system in that it can set up concentration gradients that can in turn drive other transport activities.

  • VII. Movement of Water Across Membranes

  • A. Osmosis
  • 1. Bulk flow is the mass movement of one or more substances in response to pressure, gravity, of some other external force, like the flow of blood in the circulatory system.

    2. Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.

  • B. Effects of Tonicity

  • 1. Osmotic movements are affected by the relative concentrations of solutes (called tonicity) in the fluids inside and outside the cell.

    2. Three conditions can occur:

  • a. A hypotonic fluid has a lower concentration of solutes than does the fluid in the cell; cells immersed in it may swell.

    b. A hypertonic fluid has a greater concentration of solutes than does the fluid in the cell; cells in it may shrivel.

    c. An isotonic fluid has the same concentration of solutes as the fluid in the cell; immersion in it causes no net movement of water.

  • C. Effects of Fluid Pressure

  • 1. Any volume of fluid exerts hydrostatic pressure against a cell membrane.

    2. Osmotic pressure is the amount of force that prevents any further increase in the volume of solution inside a cell.

  • VIII. Exocytosis and Endocytosis

  • A. Exocytosis
  • 1. Vesicles, small sacs made of membranes, can transport and store substances within the cytoplasm.

    2. Exocytosis moves substances from cytoplasm to plasma membrane during secretion.

  • B. Endocytosis

  • 1. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm.

    2. Phagocytic cells (amoebas and white blood cells) digest the contents of the endocytic vesicles by means of enzymes within lysosomes which fuse with the vesicles.