Topic 2: Cells: Diffusion

Diffusion

The net movement of particles from an area of higher concentration to a lower concentration (down a concentration gradient). It's a passive process until equilibrium is reached

Passive

Doesn't require energy

Net movement

Taking overall movement

Highly concentrated

More particles in a particular area

Distributed

A state of equilibrium where the particles are evenly spread out

Simple diffusion

When substances move through the phospholipid bilayer e.g. Oxygen and Carbon dioxide

What sort of substances move through simple diffusion?

Lipid soluble, non-polar (no charge) - can dissolve in hydrophobic core of cell membrane, very small-so can pass through spaces between phospholipids

Facilitated diffusion

Substances that are charged and larger e.g. Glucose and amino acids require proteins to move across the membrane (channel or carrierproteins)

Protein channels

Water filled hydrophilic channels that span the phospholipid bilayer - form pores in cell membrane. These channels are selective and will open only in the presence of a specific ion or molecule

Carrier proteins

Large molecule attaches to carrier protein → causes carrier protein to change shape → releases molecule on opposite side of membrane

When the number of carrier/channel proteins are the limiting factor

We say they are saturated

First factor that affects the rate of diffusion (faster)

Increased temperature - particles have more kinetic energy and so diffuse faster

Second factor that affects the rate of diffusion (faster)

Higher difference in concentration - the steeper the concentration gradient the faster the rate of diffusion

Third factor that affects the rate of diffusion (faster)

Smaller molecules/ions can diffuse faster

Fourth factor that affects the rate of diffusion (faster)

Thinner membrane - shorter distance for diffusion, particles travel shorter distances through thin exchange surfaces

Fifth factor that affects the rate of diffusion (faster)

Larger surface area - more particles can cross the membrane at once, making diffusion faster

Sixth factor that affects the rate of diffusion (faster)

More carrier/channel proteins - faster rate of facilitated diffusion