Passive Transport: Diffusion and Facilitated Diffusion

Definition : movement is particles from a higher concentration to a lower concentration, down a concentration gradient.

—> a passive process : no metabolic energy (ATP) is needed.
—> happens because the particles in a gas or liquid have their own kinetic energy that is random causing an unequal distribution of particles too eventually become equal.

—> will continue until there is a concentration equilibrium between the two areas.
—> particles are still moving but movement of equal in all directions.

—> diffusion is one of the processes that particles can use to move into cells.
—> involves particles passing through the phospholipid layer.
—> this can only happen if the membrane is permeable to the particle.

—> to pass through the phospholipid layer, particles have to come into contact with the membrane.

—> means that very small uncharged and lipid soluble molecules such as O2 can diffuse freely down a concentration gradient across a membrane through the phospholipid bilayer.
—> this is because any particle that has a charge can’t interact with the hydrophobic fatty acid tails of the membrane so can’t easily pass through.
—> large molecules can’t fit between the phospholipids.

Rate at which diffusion across a membrane can occur is affected by :

Temperature : higher the temperature, the higher the rate of diffusion because particles have more kinetic energy and thus move at higher speeds.

Concentration Difference : the greater the difference in concentration between the 2 areas; the faster the rate of diffusion because the overall movement will be larger.

Surface Are : the larger the exchange surface, the higher the rate of diffusion.

Thickness of Membrane : the thinner the exchange surface, the higher the rate of diffusion.

Definition : The phospholipid bilayer is a barrier to large and charged particles (ions), but they need to get into the cell somehow. They do this through intrinsic proteins. This is known as facilitated diffusion and makes the membrane selectively permeable.

—> as it’s a form of diffusion, particles move from areas of higher concentration to areas of lower concentrations until an equilibrium is reached.
—> at this point, movement is equal in both directions.

—> a passive process : no metabolic energy (ATP) is needed.

There are 2 types of intrinsic proteins :

Channel proteins : channels that can open and close for specific ions (Na+ , K+ , Ca2+).

Carrier proteins : proteins that transport larger molecules (e.g. carbohydrates).

The rate of facilitated diffusion depends on :

—> Temperature
—> Concentration difference
—> Surface Area
—> Thickness of Membrane
—> Number of Channel and Carrier Proteins