Diffusion

Facilitated diffusion

a passive process which relies on kinetic energy of the molecules that are diffusing. Carrier proteins or channel proteins speed up the rate of diffusion for large molecules like ions and polar molecules. No ATP is involved.

Passive Transport (Diffusion)

- all particles are constantly moving due to the kinetic energy that they possess. - the motion is random - particles are bouncing off one another as well as other objects.

Molecules that can NOT easily diffuse directly through the phospholipid bilayer

- charged ions - polar molecules - large - non soluble in lipids

Molecules that can easily diffuse directly through the phospholipid bilayer

- small - non polar e.g. oxygen and carbon dioxide - lipid soluble e.g steroids

Channels protein

- they form water filled hydrophilic channels across the membrane. - they allow specific water soluble ions to pass through - they are selective; they only open for specific ions, if the particular ion is not present, they will not open; this allows selective control over the entry and exit of ions. - ions bind with the protein, causing it to change shape in a way that closes it on one side of the membrane and opens it to the other side.

Carrier proteins

-A large molecule attaches to a carrier protein in the membrane -The protein changes shape -This releases the molecule on the opposite side of the membrane

Number of Channel or Carrier Proteins

Factor affecting rate of diffusion: - Once all of these are in use, the process cannot be sped up

Concentration Gradient

Factor affecting rate of diffusion: -The greater it is, the faster the rate of diffusion. As diffusion occurs, the difference between the two sides of the membrane decreases until it reaches equilibrium

Surface Area

Factor affecting rate of diffusion: -The greater this is the faster the rate of diffusion as there is more space for particles to contact the membrane

Thickness of Exchange Surface

Factor affecting rate of diffusion: -The thinner the exchange surface, the faster the rate of diffusion

Fick's law

diffusion rate of a gas across a fluid membrane is proportional to the difference in concentration, proportional to the area of the membrane, and inversely proportional to the thickness of the membrane (difference in concentration gradient x surface area) / thickness of surface.

Diffusion

example of passive transport and does not use ATP; only uses kinetic energy. Passive transport (no energy required) (does not require carrier proteins)

Definition of diffusion

the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed/until they reach equilibrium.