Passive transport
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Passive transport is a type of
Passive transport follows Fick's first law.
Diffusion
Diffusion is the net movement of material from an area of high concentration to an area with lower concentration. The difference of concentration between the two areas is often termed as the
Simple diffusion and osmosis are in some ways similar. Simple diffusion is the passive movement of solute from a high concentration to a lower concentration until the concentration of the solute is uniform throughout and reaches equilibrium. Osmosis is much like simple diffusion but it specifically describes the movement of water (not the solute) across a selectively permeable membrane until there is an equal concentration of water and solute on both sides of the membrane. Simple diffusion and osmosis are both forms of passive transport and require none of the cell's ATP energy.
Speed of diffusion
For passive diffusion, the law of diffusion states that the mean squared displacement is with d being the number of dimensions and D the
This can be seen in material transport within the cell. Prokaryotes typically have small bodies, allowing diffusion to suffice for material transport within the cell. Larger cells like eukaryotes would either have very low metabolic rate to accommodate the slowness of diffusion, or invest in complex cellular machinery to allow active transport within the cell, such as kinesin walking along microtubules.
Example of diffusion: Gas Exchange
A biological example of diffusion is the gas exchange that occurs during respiration within the human body.[7] Upon inhalation, oxygen is brought into the lungs and quickly diffuses across the membrane of alveoli and enters the circulatory system by diffusing across the membrane of the pulmonary capillaries.[8] Simultaneously, carbon dioxide moves in the opposite direction, diffusing across the membrane of the capillaries and entering into the alveoli, where it can be exhaled. The process of moving oxygen into the cells, and carbon dioxide out, occurs because of the concentration gradient of these substances, each moving away from their respective areas of higher concentration toward areas of lower concentration.[7][8] Cellular respiration is the cause of the low concentration of oxygen and high concentration of carbon dioxide within the blood which creates the concentration gradient. Because the gasses are small and uncharged, they are able to pass directly through the cell membrane without any special membrane proteins.[9] No energy is required because the movement of the gasses follows Fick's first law and the second law of thermodynamics.
Facilitated diffusion
Facilitated diffusion, also called carrier-mediated osmosis, is the movement of molecules across the cell membrane via special transport proteins that are embedded in the plasma membrane by actively taking up or excluding ions [14]. Through facilitated diffusion, energy is not required in order for molecules to pass through the cell membrane.
Example of facilitated diffusion: GLUT2
An example of facilitated diffusion is when glucose is absorbed into cells through Glucose transporter 2 (GLUT2) in the human body.[12][13] There are many other types of glucose transport proteins, some that do require energy, and are therefore not examples of passive transport.[13] Since glucose is a large molecule, it requires a specific channel to facilitate its entry across plasma membranes and into cells.[13] When diffusing into a cell through GLUT2, the driving force that moves glucose into the cell is the concentration gradient.[12] The main difference between simple diffusion and facilitated diffusion is that facilitated diffusion requires a transport protein to 'facilitate' or assist the substance through the membrane.[14] After a meal, the cell is signaled to move GLUT2 into membranes of the cells lining the intestines called enterocytes.[12] With GLUT2 in place after a meal and the relative high concentration of glucose outside of these cells as compared to within them, the concentration gradient drives glucose across the cell membrane through GLUT2.[12][13]
Filtration
Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic
Osmosis
Osmosis is the net movement of water
See also
References
- ^ a b c "5.2 Passive Transport - Biology 2e | OpenStax". openstax.org. 28 March 2018. Retrieved 2020-12-06.
- ^ a b "5.2A: The Role of Passive Transport". Biology LibreTexts. 2018-07-10. Retrieved 2020-12-06.
- ^ "5.3 Active Transport - Biology 2e | OpenStax". openstax.org. 28 March 2018. Retrieved 2020-12-06.
- ^ .
- ^ "12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes - College Physics for AP® Courses | OpenStax". openstax.org. 12 August 2015. Retrieved 2020-12-06.
- PMID 27611057.
- ^ PMID 25323225.
- ^ a b "22.4 Gas Exchange - Anatomy and Physiology | OpenStax". openstax.org. 25 April 2013. Retrieved 2020-12-06.
- ^ "3.1 The Cell Membrane - Anatomy and Physiology | OpenStax". openstax.org. 25 April 2013. Retrieved 2020-12-06.
- PMID 11337417.
- S2CID 12814450.
- ^ PMID 18393659.
- ^ PMID 26784222.
- ^ Cooper, Geoffrey M. (2000). "Transport of Small Molecules". The Cell: A Molecular Approach. 2nd Edition.
- Alcamo, I. Edward (1997). "Chapter 2–5: Passive transport". Biology coloring workbook. Illustrations by John Bergdahl. New York: Random House. pp. 24–25. ISBN 9780679778844.
- Sadava, David; H. Craig Heller; Gordon H. Orians; William K. Purves; David M. Hillis (2007). "What are the passive processes of membrane transport?". Life : the science of biology (8th ed.). Sunderland, MA: Sinauer Associates. pp. 105–110. ISBN 9780716776710.
- Srivastava, P. K. (2005). Elementary biophysics : an introduction. Harrow: Alpha Science Internat. pp. 140–148. ISBN 9781842651933.