Passive Transport (College Board AP Biology)

• Passive transport is the net movement of molecules from a high concentration to a low concentration without the requirement for energy from respiration
• Many substances within the body are transported passively
• Examples of passive diffusion mechanisms include
  • simple diffusion
  • facilitated diffusion (covered in more detail in 2.7 Facilitated Diffusion)
  • osmosis (covered in more detail in 2.8 Tonicity & Osmoregulation)

Simple Diffusion

• Simple diffusion is a type of transportation that involves particles passing between phospholipids in the plasma membrane
• Diffusion can be defined as:
  • The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
• The molecules or ions move down a concentration gradient
• The random movement is caused by the natural kinetic energy of the molecules or ions
• As a result of diffusion, molecules or ions tend to reach an equilibrium (given sufficient time), where they are evenly spread within a given volume of space

Diagram of Simple Diffusion Across the Cell Membrane

Simple diffusion across the cell membrane

Factors that affect the rate of diffusion

• The rate at which a substance diffuses across a membrane depends on several factors:
  • 'Steepness' of the concentration gradient; the greater the difference the higher the rate of diffusion
  • Temperature; the higher the temperature, the higher the rate of diffusion, due to molecules possessing higher kinetic energy
  • Surface area; the greater the surface area, the higher the rate of diffusion
  • Properties of the molecules or ions 
    • Large molecules diffuse more slowly as they require more energy to move
    • Uncharged molecules (eg, oxygen) diffuse faster as they move directly through the phospholipid bilayer
    • Nonpolar molecules diffuse more quickly as they are soluble in the nonpolar phospholipid bilayer
    • Although polar molecules cannot easily pass through the hydrophobic part of the membrane, smaller polar molecules (eg, urea) can diffuse at low rates

Facilitated diffusion

• Certain substances cannot diffuse through the phospholipid bilayer of cell membranes
• These include:
  • Large, polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na⁺) and chloride ions (Cl^-)
• These substances can only cross the phospholipid bilayer with the help of certain proteins
• This form of diffusion is known as facilitated diffusion
• There are two types of protein that enable facilitated diffusion:
  • Channel proteins
  • Carrier proteins (these can also be used during active transport)
• They are highly specific (they each only allow one type of molecule or ion to pass through)
• Facilitated diffusion is covered in more detail in 2.7 Facilitated Diffusion

Osmosis

• All cells are surrounded by a cell membrane which is selectively permeable
• Water can move in and out of cells by osmosis
• Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution across a selectively permeable membrane
  • In doing this, water is moving down its concentration gradient
    • A dilute solution has a high concentration of water molecules and a concentrated solution has a low concentration of water molecules
• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
• Osmosis is covered in more detail in 2.8 Tonicity & Osmoregulation