Types of diffusion

Antony Rickhuss

Types of Diffusion and Their uses

Diffusion is the phenomenon of random motion causing a system to decay towards uniform conditions. Diffusion causes particles to disperse throughout the area it is released in, it can be the diffusion of heat, momentum or concentration. Diffusion usually takes place when there are two mediums of different concentations, in which case diffusion will occur and the two mediums will become isotonic – reaching a state of equilibrium.

Osmosis is the movement of water across a partially permeable membrane up a concentration gradient which separates the two solutions at different concentrations. In two volumes of water at different concentrations the solutions will each attempt to pass through the semi-permeable membrane but the solution with a lower concentration will only allow water to pass through as the solute molecules are to big to get through. Osmosis will continue to happen until the 2 solutions reach equilibrium and the flow of water molecules from one side equals the flow of water molecules from the other side through the partially permeable membrane, this is also called isotonic.

Plants use osmosis to take up water into their roots because they have a large surface area and the water coming through the permeable membrane generates the pressure needed to send the water up the plant to the vital areas.

Simple diffusion is a process of diffusion which does not require a special protein channel, facilitated diffusion (or facilitated transport) is a process of diffusion, a form of passive transport, where molecules diffuse across membranes, with the assistance of transport proteins. Ionic diffusion is the diffusion through ion channels.

Uses of Diffusion

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Uses of Diffusion

Gaseous exchange in alvioli in the lungs is a type of diifusion in which oxygen is transported into the blood stream through the alvioli. The alvioli have a thin wall (partially permeable) which allows some particles to pass through in this case – oxygen. Carbon dioxide from the bloodstream can also pass back through so it can be breathed out. Due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blooad and carbon dioxide diffuses out.

Plants use diffusion in the uptake of nutrients and water from the soil. Xylem cells move water and nutrient solutions upwards towards other plant organs from the roots and fine root hairs. Living root cells actively absorb water in the absence of transpiration pull via osmosis creating root pressure (Which allows the plant to transport the nutrients). There are times when plants do not have transpiration pull, usually due to lack of light or other environmental elements. Water in the plant tissues may move to the roots to assist in passive absorption.

Different Types of Diffusion

Atomic diffusion
Light diffusion
Brownian motion, for example of a single particle in a solvent
Collective diffusion, the diffusion of a large number of (possibly interacting) particles
Effusion of a gas through small holes.
Electron diffusion, resulting in electric current
Heat flow (thermal diffusion)
Itō diffusion
Knudsen diffusion
Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
Osmosis
Photon diffusion
Reverse diffusion
Self-diffusion
The decision about which side of the membrane to call "high" osmotic pressure is a troublesome one. The choice made here is the opposite of that made in many biology texts, which attribute "high" osmotic pressure to the solution and zero osmotic pressure to pure water. The rationale for the choice is that the energy which drives the fluid transfer is the thermal energy of the water molecules, and that energy density is higher in the pure solvent since there are more water molecules. The thermal energy of the solute molecules does not contribute to transport, presuming that the membrane is impermeable to them. The choice is also influenced by the observed direction of fluid movement, since under this choice the fluid transport is from high "pressure" to low, congruent with normal fluid flow through pipes from high pressure to low. The final rationale has to do with the measurement of osmotic pressure by determining how much hydrostatic pressure on the solution is required to prevent the transport of water from a pure source across a semi-permeable membrane into the soluton. A positive pressure must be exerted on the solution to prevent osmotic transport, again congruent with the concept that the osmotic pressure of the pure solvent is relatively "high".
Nevertheless, the dialog continues on

Fick’s law is used in steady stae diffusion, i.e when the concentration within the diffusion volume does not change with respect to time In one spatial dimension, this is

Where:

J is the diffusion flux in dimensions of [(amount of substance) length−2 time-1]

D is the diffusion coefficient of diffusivity in dimensions of [length2 time−1]

is the concentration in dimensions of [(amount of substance) length−3

Peer Reviews

Here's what a star student thought of this essay

skatealexia

6th of Mar 2012

Quality of writing

Spelling okay most of the time, alveoli and blood spelt wrong. Punctuation good. Grammar is okay most of the time, but in some parts the wording order or the use of words makes the meaning overcomplicated or unclear.

Level of analysis

Level of analysis is good for this level, but could define the different factors affecting diffusion more clearly, for example the diffusion distance, and the concept of moving from a high to low concentration gradient. Misses out active transport and secondary active transport which is another vital method of diffusion. Could also explain the concept of uptake by osmosis in the roots of plants a bit clearer. Uses of diffusion attempts some good examples, but the concept of exchange of CO2 and O2 in the alveoli again could be explained a bit clearer. The different types of diffusion listed are obviously copied and pasted from somewhere with non relevance to biology or the different types of diffusion and their uses explained. No adequate conclusion and random use of Fick's law.

Response to question

Overall okay as a means of explaining some of the aspects and uses of diffusion, but not brilliant. The introduction is quite complicated in the wording, so could be worded slightly simpler with more flow to be a little be clearer to the reader. Explanation of osmosis is good, but the candidate could have expanded on the meanings of hypertonic and hypotonic and explored these. Sometimes the explanations are not the clearest as the wording can be slightly awkward in places. Main body of the text is okay in most places, but I would like to see the science explained more and the different factors set out clearer. No conclusion. Some sections of text randomly attached with no explanation to their meaning or relevance.

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