2.2

• Characteristics of substances that are able to move across a plasma membrane

  • Molecule size
  • Polar molecule
  • Ionic charge
• Characteristics of substances across the plasma membrane

  • Lipid-soluble substances
  • Lipid insoluble substances
  • Small molecules and ions
  • Large molecules
• Non-polar molecules


  • Vitamin A, D, E, K
  • Steroid compounds
  • Fatty acids and glycerol
• Non-polar molecules and ions

  • Oxygen
  • Carbon dioxide
  • K+
  • Na+
  • Ca2+
  • Mg2+
• Polar molecules


  • Glucose
  • Amino acids
• Passive transport


  Process that does not require energy
• Examples of passive transport


  • Simple diffusion
  • Osmosis
  • Facilitated diffusion
• Simple diffusion


  Substances pass through the plasma membrane following the concentration gradient, from high to low concentration
• Osmosis


  Passive transport process involving only water molecules, from higher to lower water potential
• Facilitated diffusion


  Lipid-insoluble molecules and large molecules move across the membrane with the aid of transport proteins, down a concentration gradient
• Active transport


  Movement of molecules or ions across a plasma membrane against a concentration gradient, requires energy from ATP
• Active transport


  • Requires specific carrier proteins with active sites to bind with certain molecules or ions
  • Carrier proteins possess receptors to bind with ATP molecules
  • Carrier proteins change shape when a phosphate group attaches to it
• Similarities between passive and active transport

  • Occurs through a selectively permeable membrane
  • Moving substance across a membrane
• Differences between passive and active transport

  • Passive transport does not require energy, occurs following the concentration gradient, and achieves dynamic equilibrium
  • Active transport requires energy, occurs against the concentration gradient, and involves accumulation and disposal of molecules or ions