Concept of Movement of Substances Across a Plasma Membrane

3.2 Concept of Movement of Substances Across a Plasma Membrane
  The characteristics of substances that are able to move across a plasma membrane:  
  • There are three common factors that determine whether a molecule can pass through a plasma membrane, which are molecule size, polar molecule, and 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

Polar molecules; water

Non-polar molecules; oxygen, carbon dioxide, and ion: K+, Na+, Ca2+, Mg2+

Glucose and amino acids

Passive transport:

  • This process does not require energy
  • Examples of passive transport are simple diffusion,osmosis and facilitated diffusion.

Simple diffusion:

  • A process where the substances pass through the plasma membrane follows the concentration gradient.
  • The substances move from a high concentration region to a low concentration region.
  • The moving molecules are said to move down the concentration gradient until a dynamic equilibrium is achieved.
  • This may occur with or without the presence of a plasma membrane.
  • Lipid soluble molecules (fatty acids and glycerol), oxygen, and carbon dioxide diffuse through the phospholipid bilayer through simple diffusion.


  • Osmosis is a passive transport process that is similar to diffusion but it involves only water molecules.
  • Osmosis refers to the net movement of water molecules from a higher water potential to a low water potential.
  • Osmosis occurs through the phospholipid bilayer.

Facilitated diffusion:

  • Lipid-insoluble molecules such as ions, large molecules such as amino acids, and glucose are unable to pass through the phospholipid bilayer.
  • These substances move across the membrane with the aid of transport proteins (carrier or pore proteins).
  • Facilitated diffusion does not require energy because the transport proteins transport molecules down a concentration gradient.
  • The process continues until a dynamic equilibrium is achieved when the concentration of molecules is the same at both sides of membranes.
Active transport:
  • The movement of a molecule or ion substances across a plasma membrane occurs against a concentration gradient.
  • It requires energy from ATP (adenosine triphosphate) molecules generated during cellular respiration.
  • It requires specific carrier proteins with specific active sites to bind with certain molecules or ions.
  • Carrier proteins also possess receptors to bind with ATP molecules.
  • Carrier proteins change shape when a phosphate group attaches to it.
  • As a result, molecules or ions move across a membrane.
  The simillarities between passive and active transport:  
  • Occurs through a selectively permeable membrane
  • Moving substance across a membrane
  The differences between passive and active transport:  
Passive Transport Active Transport
  • Energy does not required
  • Requires energy
  • Occurs following the concentration of the gradient.
  • Occurs againts the concentration of the gradient.
  • Occurs untill a dynamic equillibrium is achieved.
  • There are accumulation and disposal of molecule or ions.