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arianna g. per.4


 * Students know **** cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.

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Osmosis is the movement of **water** molecules across a partially-permeable membrane down a water potential gradient..[|[1]] More specifically, it is the movement of water across a partially permeable membrane from an area of high [|water potential] (low [|solute] concentration) to an area of low water potential (high solute concentration). It is a physical process in which a solvent moves, without input of energy, across a semipermeable membrane (permeable to the [|solvent], but not the solute) separating two solutions of different concentrations.[|[2]] Osmosis releases energy, and can be made to do work[|[3]]. Osmosis is a passive process, like [|diffusion].Net movement of solvent is from the less-concentrated (//[|hypotonic]//) to the more-concentrated (//[|hypertonic]//) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic. The [|osmotic pressure] is defined to be the [|pressure] required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a [|colligative property], meaning that the osmotic pressure depends on the[|molar concentration] of the solute but not on its identity. Osmosis is important in [|biological systems], as many [|biological membranes] are semipermeable. In general, these membranes are impermeable to [|organic] solutes with large molecules, such as [|polysaccharides], while permeable to water and small, uncharged solutes. Permeability may depend on solubility properties, charge, or chemistry, as well as solute size. Water molecules travel through the plasma cell wall, tonoplast (vacuole) or protoplast in two ways, either by diffusing across the phospholipid bilayer directly, or via[|aquaporins] (small transmembrane proteins similar to those in facilitated diffusion and in creating ion channels). Osmosis provides the primary means by which [|water] is transported into and out of [|cells]. The [|turgor] pressure of a cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its relatively hypotonic environment.


 * Diffusion** is a time-dependent process, constituted by random motion of given entities and causing the statistical distribution of these entities to spread in space. The concept of diffusion is tied to notion of [|mass transfer], driven by a concentration gradient, but diffusion can still occur when there is no concentration gradient (but there will be no net[|flux]).

The concept of diffusion emerged from physical sciences. The paradigmatic examples were [|heat diffusion], [|molecular diffusion] and [|Brownian motion]. Their mathematical description was elaborated by [|Joseph Fourier] in 1822, [|Adolf Fick] in 1855 and by [|Albert Einstein] in 1905. Applications outside physics were pioneered by [|Louis Bachelier] who in 1900 used a [|random walk] model to describe price fluctuations on financial markets. In a less quantitative way, the concept of diffusion is invoked in the social sciences to describe the spread of ideas ([|Diffusion of innovations], [|Lexical diffusion], [|Trans-cultural diffusion]).

**Diffusion in physics**
[|molecular diffusion], the moving entities are small molecules. They move at random because they frequently collide. Diffusion is this thermal motion of all (liquid and gas) molecules at temperatures above [|absolute zero]. Diffusion rate is a function of only temperature, and is not affected by concentration. [|Brownian motion] is observed in molecules that are so large that they are not driven by their own thermal energy but by collisions with solvent particles. [[File:diffusion_center.gif|thumb|350px|right|The following image shows change in excess carriers being generated (green:electrons and purple:holes) with increasing light intensity (Generation rate /cm3) at the center of an intrinsic semiconductor bar. Electrons have higher diffusion constant than holes leading to fewer excess electrons at the center as compared to holes.While Brownian motion of large molecules is observable under a microscope, small-molecule diffusion can only be probed in carefully controlled experimental conditions. Under normal conditions, molecular diffusion is relevant only on length scales between nanometer and millimeter. On larger length scales, transport in liquids and gases is normally due to another [|transport phenomenon],[|convection].