Passive Transport (College Board AP® Biology)

Passive Transport

• 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