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{{Short description|Transport that does not require energy}}{{More citations|date=November 2022}}File:Blausen 0213 CellularDiffusion.png|thumb|260px|Passive diffusion across a cell membranecell membranePassive transport is a type of membrane transport that does not require energy to move substances across cell membranes.WEB, 5.2 Passive Transport - Biology 2e {{!, OpenStax|url=https://openstax.org/books/biology-2e/pages/5-2-passive-transport|access-date=2020-12-06|website=openstax.org|date=28 March 2018 |language=en}}WEB, 2018-07-10, 5.2A: The Role of Passive Transport,bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/5%3A_Structure_and_Function_of_Plasma_Membranes/5.2%3A_Passive_Transport/5.2A%3A_The_Role_of_Passive_Transport, 2020-12-06, Biology LibreTexts, en, Instead of using cellular energy, like active transport,WEB, 5.3 Active Transport - Biology 2e {{!, OpenStax|url=https://openstax.org/books/biology-2e/pages/5-3-active-transport|access-date=2020-12-06|website=openstax.org|date=28 March 2018 |language=en}} passive transport relies on the second law of thermodynamics to drive the movement of substances across cell membranes.JOURNAL, Skene, Keith R., 2015, Life’s a Gas: A Thermodynamic Theory of Biological Evolution, Entropy, en, 17, 8, 5522–5548, 10.3390/e17085522, 2015Entrp..17.5522S, free, Fundamentally, substances follow Fick’s first law, and move from an area of high concentration to an area of low concentration because this movement increases the entropy of the overall system.WEB, 12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes - College Physics for AP® Courses {{!, OpenStax|url=https://openstax.org/books/college-physics-ap-courses/pages/12-7-molecular-transport-phenomena-diffusion-osmosis-and-related-processes|access-date=2020-12-06|website=openstax.org|date=12 August 2015 |language=en}} The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins.{{citation needed|date=March 2021}} The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.Passive transport follows Fick’s first lawFick’s first lawDiffusion 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 concentration gradient, and diffusion will continue until this gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to an area of lower concentration, it is described as moving solutes “down the concentration gradient” (compared with active transport, which often moves material from area of low concentration to area of higher concentration, and therefore referred to as moving the material “against the concentration gradient“). However, in many cases (e.g. passive drug transport) the driving force of passive transport can not be simplified to the concentration gradient. If there are different solutions at the two sides of the membrane with different equilibrium solubility of the drug, the difference in the degree of saturation is the driving force of passive membrane transport.JOURNAL, Borbas, E., etal, Investigation and Mathematical Description of the Real Driving Force of Passive Transport of Drug Molecules from Supersaturated Solutions, Molecular Pharmaceutics, 2016, 13, 11, 3816–3826, 10.1021/acs.molpharmaceut.6b00613, 27611057, It is also true for supersaturated solutions which are more and more important owing to the spreading of the application of amorphous solid dispersions for drug bioavailability enhancement.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 langle r^{2}rangle =2dDt with d being the number of dimensions and D the diffusion coefficient). So to diffuse a distance of about x takes time sim x^2/2dD , and the “average speed” is sim 2dD / x. This means that in the same physical environment, diffusion is fast when the distance is small, but less when the distance is large.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.JOURNAL, Wagner, Peter D., 2015-01-01, The physiological basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases,erj.ersjournals.com/content/45/1/227, European Respiratory Journal, en, 45, 1, 227–243, 10.1183/09031936.00039214, 0903-1936, 25323225, free, 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.WEB, 22.4 Gas Exchange - Anatomy and Physiology {{!, OpenStax|url=https://openstax.org/books/anatomy-and-physiology/pages/22-4-gas-exchange|access-date=2020-12-06|website=openstax.org|date=25 April 2013 |language=en}} 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. 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.WEB, 3.1 The Cell Membrane - Anatomy and Physiology {{!, OpenStax|url=https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane|access-date=2020-12-06|website=openstax.org|date=25 April 2013 |language=en}} No energy is required because the movement of the gasses follows Fick’s first law and the second law of thermodynamics.

Facilitated diffusion

File:Blausen 0394 Facilitated Diffusion.png -