chapter 08

Bio1151 Chapter 8 9

consists of all the chemical reactions of an organism, organized into metabolic .
- A metabolic pathway is a sequence of chemical reactions that begins with a starting molecule and ends with a product. These reactions are usually catalyzed by enzymes. The pathways can be either catabolic (exergonic) or anabolic (endergonic),
- Catabolic reactions are exergonic: they release energy (in the form of ATP) from the break down of complex molecules into simpler compounds.
  Anabolic reactions are endergonic: they consume energy to build large molecules from simpler ones.
Energy is associated with the relative of objects, and abides by laws of physics called .
- Forms of energy.
  An object not in motion (such as a diver on a platform) may possess potential energy if it is capable of motion later.
  The potential energy is converted to kinetic energy when the object is in motion (as when the diver jumps off the platform).
  After the dive, the diver has less potential energy than before, and must expand more energy to get back on the platform.
  Heat is released when energy is converted from one form to another.
- First Law
  - First law of thermodynamics.
    Energy cannot be created nor destroyed (conservation of energy), but can be transferred or transformed from one form to another.
    For example, the chemical (potential) energy in food is converted to the kinetic energy of the cheetah's movement through metabolism.
- Second Law
- Second law of thermodynamics.
  Every energy transformation increases the disorder (entropy) of a closed system.
  For example, entropy is added to the cheetah's surroundings in the form of heat and the small molecules that are the by-products of metabolism.
- Order is evident in the root tissue from a buttercup plant.
  As open systems, organisms can increase their order by increasing disorder (entropy) of their surroundings through metabolism.
Review: Energy Transformations
ATP drives chemical reactions for cellular work by , transferring a phosphate to other molecules, and is replenished by cellular (aerobic) .
- Adenosine TriPhosphate (ATP) is the cell's energy shuttle. An Adenine ribose nucleoside is attached to 3 phosphate groups that provide energy. Review:
- ATP is hydrolyzed to inorganic phosphate ( P[i] ) and Adenosine DiPhosphate (ADP) when a terminal phosphate bond is broken by the addition of a molecule of water.
  This process releases energy that can be used for cellular work.
- ATP drives cellular work.
  Phosphate group transfer provides energy for most cellular work.
- ATP drives active transport by phosphorylating membrane proteins.
- ATP drives mechanical work by phosphorylating motor proteins, such as those that move vesicles along cytoskeleton "tracks".
  After work is done, the phosphate is released as inorganic phosphate (Pi).
- Hydrolysis of ATP drives endergonic reactions that consume energy (anabolism). Energy released by exergonic reactions (catabolism) is used to phosphorylate ADP, regenerating ATP in a process called cellular (aerobic) respiration.
An is a protein that speeds up a reaction without being consumed by the reaction.
- An enzyme as a catalyst.
  The enzyme sucrase catalyzes the hydrolysis of sucrose.
  The starting molecules ( sucrose and H[2]O ) in a chemical reaction are called reactants or substrates.
  The molecules produced by the reaction ( glucose and fructose ) are called products.
  The enzyme does not take part in the reaction, but serves as a catalyst, speeding up the reaction.
An enzyme facilitates an reaction by lowering its _ANI_8,3 energy.
- Exergonic reaction.
  In an exergonic reaction, the products have lower free energy than the reactants (DG < 0), and the reaction occurs spontaneously.
  However, the reaction usually involves first breaking some bonds and requires activation energy (E[A]).
  The E[A] provides a barrier that determines the rate of the reaction.
- Free energy (G) is a measure of a system's instability: its tendency to change to a more stable state. A process that decreases the free energy (DG < 0) of the system can proceed spontaneously. Examples include a diver jumping off a platform, diffusion, and exergonic reactions.
- Activation energy.
  An enzyme speeds up an exergonic reaction by reducing its activation energy (E[A]).
  The free-energy change (DG) of the reaction is not affected.
- Active site.
  An enzyme catalyzes the conversion of reactant (substrate) molecules to product molecules.
  The substrates bind to the active site of the enzyme by weak forces such as hydrogen bonds.
  The enzyme lowers the E[A] by orienting substrates properly within the active site.
An enzyme is often activated when its substrate binds to the protein's site.
- Induced fit between an enzyme and its substrate.
- The active site of this enzyme (hexokinase) forms a groove on its surface.
- When the substrate (glucose) binds the active site, it induces a change in the protein's shape (conformation).
  This change allows more weak bonds to form, causing the active site to embrace the substrate and hold it in place.
An can be regulated by altering its shape in response to the binding of a molecule.
- Enzyme activation and inhibition. A substrate binding at the protein's active site activates the enzyme. In competitive inhibition, the inhibitor interferes with the active site of the enzyme. In noncompetitive inhibition, the inhibitor binds elsewhere on the protein, changing the shape (conformation) of the active site.
- Many enzymes are allosteric and can change their shape (conformation).
  They have 2 kinds of binding sites, the active site for the substrate and the regulatory site for a signal molecule.
  When a signal molecule binds to its regulatory site, the shape of the enzyme changes.
  This change may activate or inhibit enzyme activity.
Enzymes must be in the proper 3-dimensional (conformation) to be active. Factors that affect enzyme function include and , and most enzymes have an optimal temperature and pH range.
- Enzyme activity.
  The activity of an enzyme is influenced by both pH and temperature.
  Most human enzymes, such as the intestinal trypsin work best at about 35�C - 40�C and neutral pH.
  Gastric enzymes such as pepsin are adapted to the acidic environment of the stomach.
Review: Chemical Reactions and ATP