4.2 Diffusion

Question 1

What type of transport is simple diffusion

Answer 1

. Passive transport, so the energy for movement of particles comes from the natural kinetic energy of the particles rather than an external support eg ATP

. The motion is random so there is no set way for how the particles move around
. Particles bounce off each other and off sides of vessel in which they are containes
. Particles are constantly in motion due to kinetic energy they posess

Question 2

Definition of diffusion

Answer 2

The net movement of molecules or ions from a region of high concentration to a region of low concentration until they are evenly distributed.

Eg there are equal numbers on both sides of the vessel, however particles will constantly moving to both sides

Question 3

What particles can and can’t diffuse easily across the plasma membrane

Answer 3

. Plasma membranes aren’t readily permeable to molecules so only small, non-polar molecules like oxygen can diffuse across them easily

. Charged ions and polar molecules don’t diffuse easily because of the hydrophobic nature of the fatty acid tails of the phospholipids in the membrane

Question 4

What is facilitated diffusion

Answer 4

Since many particles are unable to do simple diffusion through the cell surface membrane, the movement is facilitated by transmembrane channels and carriers that span the membrane.

. It is a passive process so only requires the inbuilt motion (kinetic energy) of the diffusing molecules and there is no external input of ATP.

. It occurs down a concentration gradient however it happens at specific points in the plasma membrane where there are special protein molecules (two types)
Called protein channels and carrier proteins

Question 5

What are protein channels

Answer 5

. These proteins form water filled hydrophilic channels across the membrane
. They allow water soluble ions to pass through
. The channels are selective so only open for a specific ion and if the particular ion isn’t present, the channel stays closed.

Ions can bind with protein causing it to change shape in a way that closes one side of the membrane and opens it to the other side

. As a result there is control over the entry and exit of ions

Question 6

What do carrier proteins do

Answer 6

. Alternate form of facilitated diffusion
. Carrier proteins span the plasma membrane

When a molecule eg glucose that is specific to the protein is present, it binds with the protein
. This causes it to change shape in a way that the molecule is released to the inside of the membrane, so they can move inside the cell.

. No external energy is needed for it, and the molecules move from a region of high to low concentration using only the kinetic energy of the molecules themselves