Diffusion: simple and facilitated diffusion

Diffusion

• Diffusion is the movement of solute particles within a gas or a liquid from a region of high concentration to an area of lower concentration

• The particles keep moving until they are evenly dispersed

• The difference in the concentration of the particles between the 2 areas is called a concentration gradient

• The particles move down the concentration gradient, therefore don’t require any energy to move other than the kinetic energy they all ready have

• Diffusion is a passive process

Fick’s Law

Rate of diffusion = (surface area x concentration difference) / Length of diffusion pathway

Plasma membranes

• Permeability is affected by the polarity of molecules

• Hydrophobic molecules (phospholipid tails) have an equal distribution of electrons in their molecules and therefore have no polarity

• Non-polar substances that are lipid soluble can easily diffuse through the bilayer (O₂ and CO₂)

• Hydrophilic molecules (proteins) have an unequal distribution of electrons which makes one part of them slightly negative and one part slightly positive

Facilitated diffusion

• Polar substances (certain ions) and large molecules (glucose) can’t pass through the lipid bilayer

• Instead they travel through polar molecules, proteins

  • Channel Proteins: ions

  • Carrier Proteins: glucose

• Diffusion of substances through these proteins is called facilitated diffusion

Channel Proteins

• Hydrophilic channels that act as pores that allow substances to move from a high concentration to a low concentration

• They allow water soluble ions such as Na⁺, K⁺, Ca⁺ to pass through

• The protein doesn’t bind to the ions but simply allows them to pass through

• They’re very selective

Gated Channels

• Some channel pores are grated, which means that they can open and close

• This is important to the function of the cell

• E.g. nerve cells contain Na gated channels. When they’re open, Na⁺ enter the cell and cause the propagation of a nerve impulse. When the gates are closed the impulse stops. Constant propagation of impulses would result in permanently contacted muscles

Carrier Proteins

• They’re specific (i.e. they only take in certain substances that are too large to pass through the bilayer)

• As soon as diffusing molecules arrives at one side of the carrier protein a change in its shape takes place and the diffusing molecule ends up on the other side of the membrane where it’s released

• The molecules are travelling down a concentration gradient and so the process is passive

• Carrier proteins can also be used in active transport

Compare and Contrast Carrier Proteins and Channel Proteins