Enzymes, Activation Energy & Cell Structure: Quick Review

Enzymes and Activation Energy

  • Activation energy (Ea) is the energy input required to start a chemical reaction.
  • Enzymes decrease Ea, enabling faster reactions; reductions can reach up to \sim 10^{6}×.
  • Substrate binds to the enzyme's active site; induced fit enhances catalysis.
  • Enzymes are reusable and not consumed in reactions.
  • Specificity: each enzyme catalyzes one type of reaction; many enzyme names end with -ase.

How Enzymes Work: Catabolic vs. Anabolic Reactions

  • Catabolic (decomposition): bonds broken; e.g., sucrose breakdown by sucrase.
  • Anabolic (synthesis): bonds formed; e.g., amino acids form a dipeptide via a peptide bond; enzyme remains unchanged.
  • Hydrolysis example: sucrose + H2O → glucose + fructose; enzyme catalyzes.

Enzyme Characteristics and Examples

  • Active site shape determines substrate compatibility; only specific substrates fit.
  • Enzymes are reusable; not altered by the reaction.
  • Examples: sucrase digests sucrose; lactase digests lactose; maltase digests maltose.

Enzyme Conditions: Temperature and pH

  • Each enzyme has an optimal temperature and pH.
  • Human body temperature is about 37°C; pH optimum varies by enzyme; stomach is acidic and small intestine is slightly basic.

Digestive Tract pH Adaptation

  • The body adjusts pH along the tract so digestive enzymes work best.

Cellular Environment: Cytoplasm and Cytosol

  • Cytoplasm includes cytosol (cellular fluid) and organelles.

Plasma Membrane: Structure and Components

  • Phospholipid bilayer: hydrophilic heads and hydrophobic tails; forms a fluid bilayer.
  • Cholesterol: intercalated in the membrane; stiffens and reduces temperature sensitivity; animals have it, plants do not.
  • Membrane proteins: integral/transmembrane span the membrane; peripheral proteins attach to one side.
  • Functional classes: channel proteins, carrier proteins, cell recognition proteins, receptor proteins, junction proteins.

Specific Membrane Proteins and Functions

  • Channel proteins: form pores; may be gated.
  • Carrier proteins: bind and transport substances via shape change.
  • Cell recognition proteins: identify self vs non-self for immune surveillance.
  • Receptor proteins: receive signals.
  • Junction proteins: anchor cells together in tissues.

Cytoskeleton and Cellular Extensions

  • Cytoskeleton: three main fiber types – microfilaments (thinnest), intermediate filaments, microtubules (thickest).
  • Microfilaments: support plasma membrane; form microvilli; microvilli increase absorption surface; terminal web supports microvilli.
  • Microtubules: move organelles and chromosomes; originate from centrosome (microtubule organizing center).
  • Cilia vs. Flagella: cilia beat; flagella in humans mainly on sperm for locomotion.