STPM Biology Biological Molecules (Part 21)

Movement of Substances through Membrane 1

Substances can move cell membrane in the following ways:

• diffusion
• facilitated diffusion
• osmisis
• active transport
• phagocytosis
• pinocytosis

Diffusion

• Diffusion = the passive movement of particles (molecules, ions, atoms and other) from a region of high concentration to a region of lower concentration.
• For example, the diffusion of oxygen from alveolus into pulmonary capillary.

  

  Diffusion

Facilitated diffusion

• Facilitated diffusion = the diffusion of certain particles (molecules or ions) across the plasma membranes with the help of carrier proteins or channel proteins. It is a passive process, does not require energy (ATP).
• For example,
• glucose molecules are taken up by the epithelial cells of the intestinal villi through facilitated diffusion. 
• E.coli bacteria take lactose molecules by facilitated diffusion. When lactose is present, pemease enzymes are synthesized. These enzymes will form the channel protein on the plasma membrane to facilitate the uptake of glucose molecules.

  

  Facilitated diffusion

 

 

 

 

 

 

Osmosis

• Definition
• the movement of water from hypotonic colution into hypertonic solution, through a semi-permeable membrane.
• the movement of water from solution of higher water potential into solution of lower water potential, through a semi-permeable membrane.

  

  Osmosis

• If both solutions have the same solute potential (Ψ_sol) value, both solution are isotonic to each other.
• A solution with higher water potential is hypertonic to a solution with lower water potential.
• There is net water flow from region of higher water potential to region of lower water potential.