chapter 4

• Energy is the ability to do work—to move matter
• Energy has different forms:

  • Kinetic energy = energy of motion/movement
  • Potential energy = stored energy that is available to do work
• Energy is converted from one form to another

  • Energy changes form within biological systems
  • Energy is never created or destroyed, according to the first law of thermodynamics
• Energy transformations are inefficient

  • Heat energy is lost at each step
  • Heat energy is disordered and cannot be used or converted back to a useful form of energy
• Entropy is a measure of disorder

  • The randomness of the universe can be described as “disorder” or entropy
  • The entropy of the universe is increasing due to heat energy constantly being lost
• Metabolism includes all chemical reactions in cells

  • Chemical reactions rearrange atoms
  • Building complex molecules out of simple parts forms new chemical bonds
  • Breaking complex molecules into simple parts breaks apart chemical bonds
• Chemical reactions can require or release energy

  • Endergonic reactions require energy input to form bonds and build molecules
  • Exergonic reactions release energy stored in the bond when bonds are broken
• Some chemical reactions transfer electrons

  • Most energy transformations in organisms occur in oxidation-reduction reactions
• Oxidation reactions release energy

  • Oxidation is the loss of electrons from an atom or molecule, releasing energy
• Reduction reactions require energy

  • Reduction is the gain of electrons by an atom or molecule, requiring energy
• Oxidations and reductions occur simultaneously (“redox”)

  • An electron transport chain is a series of membrane proteins participating in linked oxidation-reduction reactions
• Redox reactions release a small amount of energy at each step

  • Photosynthesis and cellular respiration use electron transport chains to store and use released energy
• Enzymes speed biochemical reactions

  • Enzymes are proteins that act as catalysts, speeding up chemical reactions without being consumed
• Each enzyme fits the shape of a substrate

  • Substrate molecules bind to the enzyme’s active site, where the chemical reaction occurs
• Enzymes alter substrates to form products

  • Once the chemical reaction occurs, product molecules are released, and the enzyme retains its original form
• Enzymes lower the activation energy

  • Activation energy is the energy required to start a reaction
  • Enzymes lower the activation energy when they bind to the substrate
• Some enzymes require cofactors

  • Cofactors help catalyze reactions and increase enzyme activity
  • Metal ions and vitamins are common cofactors
• Cells control their biochemical reactions

  • Enzyme inhibition prevents unneeded reactions from taking place
• Inhibitors lower enzyme activity

  • Substrate molecules typically bind to the active site of enzymes, but inhibitors can prevent this binding
• Noncompetitive enzyme inhibitors change the shape of the active site

  • Cells can produce molecules that bind to an enzyme outside of its active site to alter enzyme shape and prevent substrate binding
• Noncompetitive enzyme inhibitors change the shape of the active site
• Competitive enzyme inhibitors block access to the active site
• Inhibitors shut down unneeded reactions
• In a process called negative feedback, the product of a reaction slows the production of more product
• In the opposite process, called positive feedback, the product of a reaction stimulates its own production
• Temperature affects enzyme activity
• Enzymes also have optimal salt concentrations and pH, at which they function most quickly
• It maintains this difference by regulating transport of dissolved substances (solutes) across its membrane
• Regulating what is inside of a cell is an example of homeostasis
• Maintaining a gradient requires energy
• Passive transport includes simple diffusion, osmosis, and facilitated diffusion
• Active transport occurs against a concentration gradient and can happen in vesicles through endocytosis or exocytosis
• Simple diffusion does not require energy
• Only small, nonpolar molecules can cross membranes by simple diffusion
• Osmosis does not require energy
• Water moves across cell membranes by osmosis
• In an isotonic solution, water moves equally into and out of cells
• In a hypotonic solution, water rushes into cells from outside
• In a hypertonic solution, water rushes out of cells
• Plants usually keep the solute concentration inside their cells higher than the outside, so that water enters the cells