Diffusion: Difference between revisions
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Diffusion is a mechanism used to transport materials across [[Cell membranes|cell membranes]], using no energy ([[ATP|ATP]]). | |||
There are three types of diffusion: | There are three types of diffusion<ref>Biochemistry (6th Edition) Berg et. al. W. H. Freeman and Company New York</ref>: | ||
*simple diffusion | *[[Simple diffusion|simple diffusion]] | ||
*facilitated | *[[Facilitated diffusion|facilitated diffusion]] | ||
*co-transport | *co-transport (eg. Coupling of Sodium + Glucose) | ||
=== Simple Diffusion === | === Simple Diffusion === | ||
The movement of [[Ions|ions]] and [[Molecules|molecules]] from an area of high concentration to a low concentration across a [[Cell membrane|cell membrane]] (down their [[Concentration|concentration gradient]]). This agrees with the [[Second Law of Thermodynamics|Second Law of Thermodynamics]] which says that molecules will spontaneously move towards an area of lower concentration, from an area of higher concentration. Molecules that can travel across cell membranes are called [[Lipophilic|lipophilic]] molecules as they dissolve in the [[Lipid bilayer|lipid bilayer]]. | |||
[[Molecules|Molecules]] which are transported by simple diffusion include: | [[Molecules|Molecules]] which are transported by simple diffusion include: | ||
*[[Oxygen|Oxygen]] | *[[Oxygen|Oxygen]] | ||
*[[ | *[[Carbon dioxide|Carbon dioxide]] | ||
*[[Steroid Hormone|Steroid Hormones | *[[Steroid Hormone|Steroid Hormones ]]- a good physiological example of simple diffusion. Steroid hormones (similar to [[Cholesterol|cholesterol]]) will move freely across [[Cell membrane|membranes]] - provided it is down their [[Concentration|concentration]] gradient. | ||
=== Facilitated | === Facilitated Diffusion === | ||
Facilitated [[Diffusion|diffusion]] is the diffusion of ions, [[Water|water]] or [[Hydrophilic|hydrophilic]] molecules across a membrane with the help of carrier [[Proteins|proteins]]. Within carrier proteins there are three types: [[Transporter|transporters]] ([[Uniporter|uniporters]]), counter transporters ([[Antiporter|antiporters]]) and co-transporters ([[Symporters|symporters]]). Some counter transporters and co-transporters use ion gradients across the membrane to drive the movement of [[Glucose|glucose]], [[Amino acid|amino acids]], [[Proton|protons]] and [[Calcium|calcium ions]] by [[Active transport|active transport]]. This process is known as [[Secondary active transport|secondary active tran]][[Secondary active transport|sport]] and is not diffusion. | |||
Molecules which are transported via facilitated diffusion include: | Molecules which are transported via facilitated diffusion include<ref>LODISH, H et al. (2008) Molecular Cell Biology, 6th ed. p439-440. New York: W.H. Freeman and Company</ref>: | ||
*[[ | *[[Glucose|Glucose]] | ||
*[[ | *[[Amino acid|Amino acids]] | ||
*[[ | *[[Water|Water]] | ||
* | *Selected [[Ions|Ions]] | ||
=== References === | === References === | ||
<references /> | <references /> | ||
Latest revision as of 09:49, 8 December 2018
Diffusion is a mechanism used to transport materials across cell membranes, using no energy (ATP).
There are three types of diffusion[1]:
- simple diffusion
- facilitated diffusion
- co-transport (eg. Coupling of Sodium + Glucose)
Simple Diffusion
The movement of ions and molecules from an area of high concentration to a low concentration across a cell membrane (down their concentration gradient). This agrees with the Second Law of Thermodynamics which says that molecules will spontaneously move towards an area of lower concentration, from an area of higher concentration. Molecules that can travel across cell membranes are called lipophilic molecules as they dissolve in the lipid bilayer.
Molecules which are transported by simple diffusion include:
- Oxygen
- Carbon dioxide
- Steroid Hormones - a good physiological example of simple diffusion. Steroid hormones (similar to cholesterol) will move freely across membranes - provided it is down their concentration gradient.
Facilitated Diffusion
Facilitated diffusion is the diffusion of ions, water or hydrophilic molecules across a membrane with the help of carrier proteins. Within carrier proteins there are three types: transporters (uniporters), counter transporters (antiporters) and co-transporters (symporters). Some counter transporters and co-transporters use ion gradients across the membrane to drive the movement of glucose, amino acids, protons and calcium ions by active transport. This process is known as secondary active transport and is not diffusion.
Molecules which are transported via facilitated diffusion include[2]:
- Glucose
- Amino acids
- Water
- Selected Ions