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| style="height:20px; width:600px; text-align:center;" |This animation shows a high [[concentration]] of red [[particle]]s initially at the top. When [[diffusion]] begins the red [[particle]]s spread to the area of lower [[concentration]] at the bottom. [[Diffusion]] continues until all [[particle]]s are equally spread.
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==Key Stage 4==
===Meaning===
'''[[Diffusion]] is when [[substance]]s spread from a high [[concentration]] to a low [[concentration]] along the [[Concentration Gradient|concentration gradient]].
===About Diffusion across a Membrane===
: [[Diffusion]] in [[Cell (Biology)|cells]] is a passive process which means it does not require [[energy]] to take place.
: When [[Cell (Biology)|cells]] are surrounded by a high [[concentration]] of a [[nutrient]] then the [[nutrient]] will [[Diffusion|diffuse]] through the [[Cell Membrane|cell membrane]] and into the [[Cell (Biology)|cells]].
: When a [[Cell (Biology)|cell]] has filled with a large number of waste [[chemical]]s they will [[Diffusion|diffuse]] through the [[Cell Membrane|cell membrane]] out of the [[Cell (Biology)|cell]].
===Factors Affecting Diffusion across a Membrane===
*[[Surface Area]] - The larger the [[Surface Area|surface area]] the more quickly [[diffusion]] can take place across the [[membrane]].
*Thickness - The thinner the [[membrane]] the more quickly [[diffusion]] can take place across the [[membrane]].
*[[Concentration Gradient]] - The greater the difference between the [[concentration]] on each side of the [[membrane]] the more quickly [[diffusion]] can take place across the [[membrane]].
===Adaptations for Diffusion===
: The [[lung]]s and [[Small Intestine|small intestine]] are specially [[Adaptation|adapted]] exchange surfaces that maximise the rate of [[diffusion]].
====Lungs====
The [[lung]]s must be able to [[Absorb (Biology)|absorb]] [[Oxygen]] into the [[blood]] and release [[Carbon Dioxide]] from the [[blood]] back into the [[lung]]s.
: The [[lung]]s are [[Adaptation|adapted]] with thousands of [[alveoli]] which increase the surface area of the [[lung]]s.
: The walls of the [[alveoli]] separating the [[air]] from the [[blood]] are very thin.
: The [[blood]] supply to the [[lung]]s is very good to constantly supply [[blood]] with little [[Oxygen]] and a lot of [[Carbon Dioxide]] keeping a high [[Concentration Gradient|concentration gradient]].
====Small Intestine====
: The [[Small Intestine]] is long and folded to increase the [[Surface Area|surface area]].
: The [[Small Intestine]] has structures called [[Villi]] which are extra folds in the surface to increase the [[Surface Area|surface area]].
: The [[Cell (Biology)|cells]] on the [[villi]] have [[microvilli]] which are folds on the [[Cell Membrane|cell membrane]] to increase the [[Surface Area|surface area]].
: : The walls of the [[villi]] separating the food from the [[blood]] are very thin.
: The [[blood]] supply to the [[villi]]s is very good to constantly supply [[blood]] with little [[nutrients]] keeping a high [[Concentration Gradient|concentration gradient]] between the food and the [[blood]].