Diffusion

define diffusion.

The passive and random net movement of molecules and ions from an area of high concentration to an area of low concentration down a concentration gradient. 

when will diffusion stop?

when equilibrium is reached.

what is equilibrium?

Equilibrium is when the molecules have spread out evenly; when the same number of molecules are on both sides of the membrane; there does not have to be a membrane present e.g. perfume.  

does diffusion require a membrane?

no

does diffusion require energy?

no.

what 3 factors effect diffusion

• concentration gradient. 

• temperature. 

• diffusion distance

how does concentration gradient effect diffusion?

the greater the concentration difference between two areas, the faster the diffusion will occur.  

how does temperature effect diffusion

• the greater the temperature the faster diffusion will occur.  

^ this is because the molecules have more kinetic energy (from the thermal energy) and so will move around more (although their movement is still random) 

how does diffusion distance effect diffusion

the greater the distance, the longer diffusion will take (as molecules have further to travel)  

what are the 2 diffusion calculations, and are they effected by surface area

Distance travelled / time (not affected by surface area) 

Volume filled / time (this IS affected by surface area)  

3 characteristics of diffusion across membranes 

• It is passive transport across membranes 

• It does not require energy 

• It is movement of molecules down their concentration gradient 

3 types of diffusion 

• Simple diffusion (straight through the phospholipid bilayer)  

• Facilitated diffusion using channel proteins 

• Facilitated diffusion using carrier proteins  

 

what are the 5 things facilitated diffusion is depends on?

• temp

• conc gradient 

• membrane surface area

• membrane thickness 

• number of channel or carrier proteins present 

how does temp affect facilitated diffusion

too high temperature (above 40 degrees Celsius) WILL denature channel and carrier proteins  

how does membrane surface area affect the rate of facilitated diffusion?

the greater the surface area, the more molecules that will diffuse across it in a certain length of time – the greater the SA, the faster the rate of diffusion 

how does membrane thickness effect facilitated diffusion?

the greater the thickness the slower the rate of diffusion 

what type of molecules is simple diffusion for?

• diffusion of small molecules 

• diffusion of lipid based molecules 

how does simple diffusion work?

• molecules can simply pass through the membrane because they are too small. 

• lipid based molecules can pass through the membrane because the membrane is a phospholipid bilayer (and lipids are non-polar)

what type of molecules use facilitated diffusion?

• polar molecules and ions (e.g. calcium ions)

• large molecules (e.g. glucose)

How does facilitated diffusion work using channel proteins?

• channel proteins only allow one type of ion through. 

• they are often ‘gated’ (open or closed - like a gate)

what is facilitated diffusion?

When a molecule needs either a channel or a carrier protein to move across a membrane.

How does facilitated diffusion work using carrier proteins?

• allow only one type of molecule through 

• when molecule fits into protein, the carrier protein changes shape

• molecule moves through to the other side of membrane 

• as carrier protein has changed shape, the molecule can’t diffuse back to the other side 

• once the molecule has moved through, the carrier protein changes shape so that another molecule can move through the membrane.