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Diffuse

Diffuse

:This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation). Diffusion, being the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. It is readily observed for example when dried foodstuff like spaghetti is cooked; water molecules diffuse into the spaghetti strings, making them thicker and more flexible. It is a physical process rather than a chemical reaction, which requires no net energy expenditure. In cell biology, diffusion is often described as a form of passive transport, by which substances cross membranes.

Examples of diffusion

- A balloon filled with helium will deflate a little bit every day, because helium atoms diffuse out of the balloon through its wall
- When spaghetti is cooked, water molecules diffuse into the spaghetti strings, making them thicker and more flexible. Adding salt to the water reduces diffusion by reducing the osmotic pressure.
- Carbon dioxide bubbles in soft drinks start as small nuclei and grow because of the diffusion of carbon dioxide molecules towards them
- Heat diffuses through the walls of a mug filled with hot coffee
- A gas distributes itself over a room by diffusion
- A sugar cube in a glass of water that is not stirred will dissolve slowly and the sugar molecules will distribute over the water by diffusion

The nature of diffusion

The different forms of diffusion can be modelled quantitatively using the diffusion equation, which goes by different names depending on the physical situation. For instance - steady-state bi-molecular diffusion is governed by Fick's first law, steady-state thermal diffusion is governed by Fourier's law. The diffusion of electrons in an electrical field leads essentially to Ohm's law (see Einstein relation). The generic diffusion equation is time dependent, i.e., applies to non-steady-state situations as well. In all cases of diffusion, the net flux of the transported quantity (atoms, energy, or electrons) is equal to a physical property (diffusivity, thermal conductivity, electrical conductivity) multiplied by a gradient (a concentration, thermal, electric field gradient). Noticeable transport occurs only if there is a gradient - for example in thermal diffusion, if the temperature is constant, heat will move as quickly in one direction as in the other, producing no heat transport and change in temperature. Diffusion occurs as a result of the Second Law of Thermodynamics, which states that the entropy or disorder of any system must always increase with time. Because substances diffuse from regions of higher concentration to regions of lower concentration, they are going from a state of higher order to a state of lower order, in accordance with the Second Law of Thermodynamics. Therefore, diffusion is a spontaneous, natural process, and to reverse diffusion would require the expenditure of energy to counteract the higher order of the system and prevent a violation of the laws of entropy.

Types of diffusion

Diffusion does not only refer to diffusion of particles, it refers to all transport phenomena occurring within thermodynamic systems under the influence of thermal fluctuations (i.e under the influence of disorder; this excludes transport through an hydrodynamic flow, which is a macroscopic, ordered phenomena). Diffusion is the process through which an inhomogeneous thermodynamic system at local thermodynamic equilibrium returns to global thermodynamic equilibrium, through the homogeneisation of the values of its intensive parameters.
- Atomic diffusion
- Brownian motion, for example of a single particle in a solvant
- Collective diffusion, the diffusion of a large number of (possibly interacting) particles
- Electron diffusion, resulting in electric current
- Heat flow (thermal diffusion)
- Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
- Osmosis
- Photon diffusion
- Reverse diffusion

Diffusion across biological membranes

- Facilitated diffusion
- Ion diffusion through ion channels
- Simple diffusion, not requiring a special protein channel
- Diffusion in the respiratory system - in the alveoli of mammalian lungs, due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out

External links

- [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html Some pictures that display diffusion and osmosis] ja:拡散

Diffusion (disambiguation)

Diffusion has several meanings:
- Diffusion is the spontaneous spreading of something such as particles, heat, or momentum.
- Diffusion (anthropology), the flow of an idea or artifact from one culture to another.
- Diffusion of responsibility
- Diffusion (cryptography), the spreading of influence of bits in a cipher.
- Diffusion (business) is the process by which a new idea or new product is accepted by the market.

Particle

A particle is
- in common use, a very small to insignificant amount. See also: grain.
- in particle physics, an elementary particle (a basic unit of matter or energy). See also: (in wave physics) Particle displacement (in accoustics) Sound particle velocity
- in colloid chemistry, a colloidal particle (phase colloid or molecular colloid)
- in ecology, a small nonbiological object of nonbiological.
- in linguistics, a grammatical particle.
- in music, the band Particle
- in computer graphics, an element of a particle system (simulation). ja:粒子

Heat

---- Heat (also improperly called heat change) is the transfer of thermal energy due to a temperature gradient. The SI unit for heat is the joule. Heat is a process quantity, and is to thermal energy as work is to mechanical energy. Heat flows between regions that are not in thermal equilibrium with each other; it spontaneously flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy --a state quantity-- that is related to the random motion of their atoms or molecules. When two bodies of different temperature come into thermal contact, they will exchange internal energy until the temperature is equalized (that is, until they reach thermal equilibrium). The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy: heat is related to the change in internal energy and the work performed by the system. The term heat is used to describe the flow of energy, while the term internal energy is used to describe the energy itself. Understanding this difference is a necessary part of understanding the first law of thermodynamics. Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body).

Notation

Total heat is traditionally abbreviated as Q, and is measured in joules in SI units. Total heat, heat transfer rate, and heat flux are often abbreviated with different cases of the letter Q. They are often switched in different contexts. Sign Convention: When a body releases heat into its surroundings, Q < 0 (-). When a body absorbs heat from its surroundings, Q > 0 (+). Heat transfer rate, or heat flow per unit time, is labeled :

\dot =  \,\!

to indicate a change per unit time. In Unicode, this is Q̇, though it may not display correctly in all browsers. It is often shown as ˙Q, .Q, Q·, or as a Q with no dot, where it is not easy to produce a dotted Q. Some form of dotted Q, such as .Q, is preferable, since undotted Q is used for total heat. It is measured in watts. Heat flux is defined as amount of heat per unit time per unit cross-sectional area, is abbreviated q, and is measured in watts per meter squared. It is also sometimes notated as Q″ or q″ or

\dot

.
Changes of temperature
The amount of heat energy, $\Delta Q$ , required to change the temperature of a material from an initial temperature, T₀, to a final temperature, T_f depends on the heat capacity of that material according to the relationship: : $\Delta Q = \int_^C_p\,dT \,\!$ The heat capacity is dependent on both the amount of material that is exchanging heat and its properties. The heat capacity can be broken up in several different ways. First of all, it can be represented as a product of mass and specific heat capacity (more commonly called specific heat): : $C_p = mc_s \,\!$ or the number of moles and the molar heat capacity: : $C_p = nc_n \,\!$ Both the molar and specific heat capacities only depend upon the physical properties of the substance being heated, not on any specific properties of the sample. The above definitions of heat capacity only work approximately for solids and liquids, but for gases they don't work at all most of the time. The molar heat capacity can be "patched up" if the changes of temperature occur at either a constant volume or constant pressure. Otherwise, it's generally easiest to use the first law of thermodynamics in combination with an equation relating the internal energy of the gas to its temperature.
Changes of phase
A boiling pot of water, at sea level and normal atmospheric pressure, will always be at 100 °C no matter how much heat is added. The extra heat changes the phase of the water from liquid into water vapor. The heat added to change the phase of a substance in this way is said to be "hidden," and thus it is called latent heat (from a Latin word for hidden). Latent heat is heat per unit mass necessary to change the state of a given substance. Thus: : $L = \frac \,\!$ and : $Q = \int_^ L\,dm \,\!$ that is to say that turning 1 pound of water into one pound of steam at 100 °C and at normal atmospheric pressure would look like 1000 BTU = (1000 BTU/lb)(1 lb). Note that as pressure increases, the L rises slightly. where $M_o$ is the amount of mass initially in the new phase, and M is the amount of mass that ends up in the new phase. L generally doesn't depend on the amount of mass that changes phase, so the equation can normally be written: : $Q = L\Delta m \,\!$ Sometimes L can be time-dependent if pressure and volume are time-varying, so that the integral can be handled: : $Q = \int L\fracdt \,\!$ someone check the above, please, to see if the latent heat really depends on where on the (P, V, T) curve the transition is taking place.
Heat transfer mechanisms
As mentioned previously, heat tends to move from a high temperature region to a low temperature region. This heat transfer may occur by the mechanisms conduction, and radiation. The term convection is used to describe the combined effects of conduction and fluid flow. In the past, this has been regarded as a third mechanism of heat transfer, but, logically, it is not a mechanism of its own.
Conduction
Conduction is the most common means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms. In insulators the heat current is carried almost entirely by phonon vibrations. The "electron fluid" of a conductive metallic solid conducts nearly all of the heat current through the solid. (Phonon currents are still there, but carry less than 1% of the energy.) Electrons also conduct electric current through conductive solids, and the thermal and electrical conductivities of most metals have about the same ratio. A good electrical conductor, such as copper, usually also conducts heat well. The Peltier-Seebeck effect exhibits the propensity of electrons to conduct heat through an electrically conductive solid. Thermoelectricity is caused by the relationship between electrons, heat currents and electrical currents.
Convection
Convection is usually the dominant form of heat transfer in liquids and gases. This is a term used to characterize the combined effects of conduction and fluid flow. In convection, enthalpy transfer occurs by the movement of hot or cold portions of the fluid together with heat transfer by conduction. For example, when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan. Two types of convection are commonly distinguished, free convection, in which gravity and buoyancy forces drive the fluid movement, and forced convection, where a fan, stirrer, or other means is used to move the fluid. Buoyant convection is due to the effects of gravity, and absent in microgravity environments. An example of convection is water heated up in a pot warms throughout the pot- not just the bottom.
Radiation
Radiation is the only form of heat transfer that can occur in the absence of any form of medium and as such is the only means of heat transfer through a vacuum. Thermal radiation is a direct result of the movements of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface. Since the amount of emitted radiation increases with increasing temperature, a net transfer of energy from higher temperatures to lower temperatures results. For room temperature objects (~300 K), the majority of photons emitted (and involved in radiative heat transfer) are in the infrared spectrum, but this is by no means the only frequency range involved in radiation. The frequencies emitted are partially related to black-body radiation. Hotter objects—a campfire is around 700 K, for instance—transfer heat in the visible spectrum or beyond. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.
Heat transfer features

- Latent heat: Transfer of heat through a physical change in the medium such as water-to-ice or water-to-steam involves significant energy and is exploited in many ways: steam engine, refrigerator etc. (see latent heat of fusion)
- Heat pipe: Using latent heat and capilliary action to move heat, it can carry many times as much heat as a similar sized copper rod and is starting to have applications in laptop personal computers.
Heat dissipation
In cold climates, houses with their heating systems form dissipative systems. In spite of efforts to insulate such houses, to reduce heat losses to their exteriors, considerable heat is lost, or dissipated, from them which would make their interiors uncomfortably cool or cold. The house is an open system in as much as it is incapable of preventing heat from escaping. Furthermore, the interior of the house must be maintained out of thermal equilibrium with its exterior for the sake of its inhabitants. In such a house, a thermostat is a device capable of starting the heating system when the house's interior falls to a set temperature, and of stopping that same system when another set temperature has been achieved. Thus the thermostat controls the flow of energy into the house, that energy eventually being dissipated to the exterior.
See also

- Heat death of the Universe
- Heat pump
- Heat equation
- Heat exchanger
- Heat transfer coefficient
- Internal energy
- Shock heating ko:열 ja:熱 simple:Heat

Transport phenomena

In physics and engineering, transport phenomena comprise the various mechanisms by which particles or quantities move from one place to another. Three common examples of transport phenomena are diffusion, convection, and radiation. There are three main types of transport phenomena:
- Heat transfer,
- Mass transfer, and
- Fluid dynamics (or momentum transfer) An important principle in the study of transport phenomena is analogy between phenomena. For example, mass, energy, and momentum can all be transported by diffusion:
- The spreading and dissipation of odors in air is an example of mass diffusion.
- The conduction of heat in a solid material is an example of heat diffusion.
- The drag experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates). The transport of mass, energy, and momentum can also be affected by the presence of external sources:
- An odor dissipates more slowly when the source of the odor remains present.
- The rate of cooling of a solid that is conducting heat depends on whether a heat source is applied.
- The gravitational force acting on a rain drop counteracts the drag imparted by the surrounding air. The same equations governing convection in heat transfer can be applied to convection in mass transfer.

References

- R. Byron Bird, Warren E. Stewart, Edwin N. Lightfoot, Transport Phenomena (1960) John Wiley & Sons, New York, ISBN 0-471-07392-X

Energy

Energy is a measure of being able to do work. This is a fundamental concept pertaining to the ability for action. In physics, it is a quantity that every physical system possesses. This quantity is not absolute but relative to a state of the system known as its reference state or reference level. The energy of a physical system is defined as the amount of mechanical work that the system can produce if it changes its state to its reference state; for example if a liter of water cools down to 0°C or if a car hits a tree and decelerates from 120 km/h to 0 km/h. Energy of an object can be in several forms, potential—due to the position of the object relative to other objects; kinetic—energy because of its motion; chemical—due to chemical bonds between atoms that make up the substance; electrical—due to its charge; thermal—due to its heat; and nuclear—due to the instability of the nuclei of its atoms. In the case where the "object" is an electromagnetic wave or light, then radiant energy can also be defined. One form of energy can be readily transformed into another; for instance, a battery converts chemical energy into electrical energy, which can be converted into thermal energy. Similarly, potential energy is converted into kinetic energy of moving water and turbine in a dam, which in turn transforms into electric energy by generator. The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from translational symmetry of time (that is, independence of any physical process on the moment it started). Some works (thus some forms of energy) are not easily measured by the unaided observer. The term 'energy' is also used in a spiritual or non-scientific way that cannot be quantified, to make certain prepositions look like they are more plausible, by imitating the scientific terminology. Usually this has something to do with mystical and/or healing type references such as acupuncture and reiki.

Forms of Energy

Below is a list of different energy forms. Lotka (1956, p. 5) asked an interesting question about what defines an energy form. :"We are equipped with two separate and distinct senses, the one responding to electromagnetic waves ranging from about 4×10^-4 to 8×10^-4 mm., light waves; the other to somewhat longer waves otherwise of the same character, heat waves. Accordingly we have two separate terms in our language light and heat, to denote two phenomena which, objectively considered, are not separated by any line of division, but merge into one another by gradual transition. Here the question might be raised whether an electromagnetic wave of a length of 9×10^-4 mm. is a light wave or a heat wave." That is to ask, if all forms of energy are defined in terms of infinitesimal increments of the wave spectrum, what makes one form of energy different to another?
- Kinetic energy: the energy of moving objects
- Thermal energy: the energy associated with heat
- Sound energy: the energy of compression waves
- Electrical energy: the energy of moving charged particles
- Potential Energy: the energy that an object has due to position; also known as stored energy
- Chemical energy: the stored energy of chemical substances
- Nuclear energy: the stored energy of the atomic nucleus
- Radiant energy: the energy of electromagnetic waves, including light

Units

SI

The SI unit for both energy and work is the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton-metre and, in terms of SI base units:

1\ \mathrm = 1\ \mathrm \left( \frac \right ) ^ 2 = 1\ \frac

An energy unit that is used in particle physics is the electronvolt (eV). One eV is equivalent to 1.60217653×10⁻¹⁹ J. In spectroscopy the unit cm^-1 = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation

E = h \nu = h c/\lambda

. (Note that torque, which is typically expressed in newton-metres, has the same dimension and this is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian requires exactly 1 newton-metre=joule of energy.)

Other units of energy

In cgs units, one erg is 1 g cm² s⁻², equal to 1.0×10⁻⁷ J. Another obsolete metric unit is the litre-atmosphere (101.325 J). The imperial/US units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepower-hour (2.6845 MJ). The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×10⁶ J (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use). The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one kilogram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to 4.1868 kJ. The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)

Transfer of energy

Work

Main article: mechanical work. Work is a defined as a path integral of force F over distance s: :

W = \int \mathbf \cdot \mathrm\mathbf

The equation above says that the work (

W

) is equal to the integral of the dot product of the force (

\mathbf

) on a body and the infinitesimal of the body's position (

\mathbf

Heat

Main article: Heat. Heat is the common name for thermal energy of an object that is due to the motion of the atoms and molecules that constitute the object. This motion can be translational (motion of molecules or atoms as a whole; vibrational - relative motion of atoms within molecules or rotational motion. It is the form of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released when atoms are rearranged between various molecules by a chemical reaction. The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: conduction, convection and/or radiation.

Conservation of energy

The first law of thermodynamics says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see off shell). Noether's theorem relates the conservation of energy to the time invariance of physical laws. An example of the conversion and conservation of energy is a pendulum. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever. (In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.) Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.

Types of energy

All forms of energy: thermal, chemical, electrical, radiant, nuclear etc. can be in fact reduced to kinetic energy or potential energy. For example thermal energy is essentially kinetic energy of atoms and molecules; chemical energy can be visualized to be the potential energy of atoms within molecules; electrical energy can be visualized to be the potential and kinetic energy of electrons; similarly nuclear energy is the potential energy of nucleons in atomic nucleii.

Kinetic energy

Main article: Kinetic energy. Kinetic energy is the portion of energy related to the motion. :

E_k = \int \mathbf \cdot \mathrm\mathbf

The equation above says that the kinetic energy (

E_k

) is equal to the integral of the dot product of the velocity (

\mathbf

) of a body and the infinitesimal of the body's momentum (

\mathbf

). For non-relativistic velocities, that is velocities much smaller than the speed of light, we can use the Newtonian approximation :

E_k = \begin \frac \end mv^2

where E_k is kinetic energy m is mass of the body v is velocity of the body At near-light velocities, we use the correct relativistic formula: :

E_k = m c^2 (\gamma - 1) = \gamma m c^2 - m c^2 \;\!

\gamma = \frac

where v is the velocity of the body m is its rest mass c is the speed of light in a vacuum, which is approximately 300,000 kilometers per second

\gamma m c^2 \,

is the total energy of the body

m c^2 \,

is again the rest mass energy. See also, E=mc². In the form of a Taylor series, the relativistic formula for can be written as: :

E_k = \frac mv^2 - \frac \frac  + \cdots

Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the relativistic formula. However, the phrase "conservation of energy" is often confusing to a non scientist. This is so, because of the common usage of the terms "save energy" or conserve energy" used in campaigns for conservation of energy resources like electricity or fossil fuels.

Potential energy

Main article: Potential energy. In contrast to kinetic energy, which is the energy of a system due to its motion, or the internal motion of its particles, the potential energy of a system is the energy associated with the spatial configuration of its components and their interaction with each other. Any number of particles which exert forces on each other automatically constitute a system with potential energy. Such forces, for example, may arise from electrostatic interaction (see Coulomb's law), or gravity. In an isolated system consisting of two stationary objects that exert a force

f(x)

on each other and lay on the x-axis, their potential energy is most generally defined as :

E_p = -\int f(x) \, dx

where the force between the objects varies only with distance

x

and is integrated along the line connecting the two objects. To further illustrate the relationship between force and potential energy, consider the same system of two objects situated along the x-axis. If the potential energy due to one of the objects at any point

x

U(x)

, then the force on the that object

x

is :

f(x) = -\frac

This mathematical relationship demonstrates the direct connection between force and potential energy: the force between two objects is in the direction of decreasing potential energy, and the magnitude of the force is proportional to the extent to which potential energy decreases. A large force is associated with a large decrease in potential energy, while a small force is associated with a small decrease in potential energy. Notice how, in this case, the force on an object depends entirely on its potential energy. These two relationships – the definition of potential energy based on force, and the dependence of force on potential energy – show how the concepts of force and potential energy are intimately linked: if two objects do not exert forces on each other, there is no potential energy between them. If two objects do exert forces on each other, then potential energy naturally arises in the system as part of the system's total energy. Since potential energy arises from forces, any change in the system's spatial configuration will either increase or decrease the system's potential energy as the objects are repositioned. When a system moves to a lower potential energy state, energy is either released in some form or converted into another form of energy, such as kinetic energy. The potential energy can be "stored" as gravitational energy, elastic energy, chemical energy, rest mass energy or electrical energy, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike kinetic energy, which exists in any moving body, potential energy exists in any body which is interacting with another object. For example a mass released above the Earth initially has potential energy resulting from the gravitational attraction of the Earth, which is transferred to kinetic energy as the gravitational force acts on the object and its potential energy is decreased as it falls. Equation: :

E_p = mgh \;

where m is the mass, h is the height and g is the value of acceleration due to gravity at the Earth's surface (see gee).

Internal energy

Main article: Internal energy. Internal energy is the kinetic energy associated with the motion of molecules, and the potential energy associated with the rotational, vibrational and electric energy of atoms within molecules. Internal energy, like energy, is a quantifiable state function of a system.

History

In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world. The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither Galileo nor Newton made any contributions to the theoretical model of energy, and it was not until the middle of the 19th century that these concepts were introduced. The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contibuted to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum. William Thomson (Lord Kelvin) amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy. For further information, see the Timeline of thermodynamics.

Energy and Economy

Main articles: energy development, energy policy The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to steam power, then the internal combustion engine and electricity, are key elements in the development of modern civilization. Future energy development, for example of renewable energy, may be key to avoiding the effects of global warming.

Notes

This definition is one of the most common; e.g. [http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_6.html Glossary at the NASA homepage]

External links

- [http://www.unitconversion.org/unit_converter/energy.html Online Energy and Work Converter] - convert between various units of energy and work, such as joule, erg, gigawatt-hour, newton meter, calorie, Btu, and so on
- [http://www.unitconversion.org/unit_converter/energy-v.html Interactive Energy and Work Conversion Table] - convert selected unit to all other units of energy and work
- [http://jumk.de/calc/energy.shtml Conversions of energy units]
- [http://www.physicsweb.org/article/world/15/7/2 What does energy really mean? From Physics World]
- [http://www.energy.ca.gov/glossary/ Glossary of Energy Terms]
- [http://www.iea.org International Energy Agency IEA - OECD] Category:Introductory physics Category:Fundamental physics concepts Category:Physical quantity ko:에너지 ms:Tenaga ja:エネルギー simple:Energy th:พลังงาน

Cell (biology)

(red) and DNA (green)]] The cell is the structural and functional unit of all living organisms, and are sometimes called the "building blocks of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion or 10¹⁴ cells). The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word cell comes from the Latin cella, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to small rooms monks lived in.

Overview

Properties of cells

cork Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities. All cells share several abilities:
- Reproduction by cell division.
- Metabolism, including taking in raw materials, building cell components, creating energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
- Synthesis of proteins, the functional workhorses of cells, such as enzymes. A typical mammalian cell contains up to 10,000 different proteins.
- Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
- Traffic of vesicles.

Types of cells

vesicle One way to classify cells is whether they live alone or in groups. Organisms vary from single cells (called single-celled or unicellular organisms) that function and survive more or less independently, through colonial forms with cells living together, to multicellular forms in which cells are specialized. 220 types of cells and tissues make up the multicellular human body. Cells can also be classified into two categories based on their internal structure.
- Prokaryotic cells are structurally simple. They are found only in single-celled and colonial organisms. In the three-domain system of scientific classification, prokaryotic cells are placed in the domains Archaea and Eubacteria.
- Eukaryotic cells have organelles with their own membranes. Single-celled eukaryotic organisms such as amoebae and some fungi are very diverse, but many colonial and multicellular forms such as plants, animals, and brown algae also exist.

Subcellular components

brown alga (2) nucleus (3) ribosome (4) vesicle,(5) rough endoplasmic reticulum (ER), (6) Golgi apparatus, (7) Cytoskeleton, (8) smooth ER, (9) mitochondria, (10) vacuole, (11) cytoplasm, (12) lysosome, (13) centrioles]] centriole All cells whether prokaryotic or eukaryotic have a membrane, which envelopes the cell, separates its interior from its environment, controls what moves in and out, and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell then briefly describe their function.

Cell membrane - a cell's protective coat

Main article: Cell membrane The cytoplasm of a eukaryotic cell is surrounded by a plasma membrane. A form of plasma membrane is also found in prokaryotes, but is usually referred to as the cell membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (fat-like molecules) and proteins. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell.

Cytoskeleton - a cell's scaffold

Main article: Cytoskeleton The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell's structure by directing, bundling, and aligning filaments.

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long term information storage, but some viruses (retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g. mRNA) and enzymatic functions (e.g. ribosomal RNA) in organisms that use RNA for the genetic code itself. Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory). A human cell, e.g. has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). The nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is.

Organelles

Main article: Organelle The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are only found in eukaryotes.
- Cell nucleus - a cell's information center: The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.
- Ribosomes - the protein production machine: Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.
- Mitochondria and chloroplasts - the power generators: Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex metabolic pathways. Chloroplasts are larger than mitochondria, and convert solar energy into a chemical energy ("food") via photosynthesis. Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes like plants and algae. There are a number of plant organelles that are modified chloroplasts; they are broadly called plastids and are often involved in storage.
- Endoplasmic reticulum and Golgi apparatus - macromolecule managers:: The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be exported from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid synthesis, detoxification and as a calcium reservoir. The Golgi apparatus, sometimes called a Golgi body or Golgi complex is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily folded membranes.
- Lysosomes and peroxisomes - the cellular digestive system: Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
- Centrioles - They help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells.
- Vacuoles-They store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane.

Anatomy of cells

Prokaryotic cells

Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
- The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
- Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like fungi, but has a different chemical composition
- A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.

Eukaryotic cells

There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are usally found in multi-cellular organisms, while prokaryotic cells are usually on their own. Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (i.e. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.
- Eukaryotes can move using cilia or flagella. The flagella are more complex than those of prokaryotes.

Cell functions

Cell growth and metabolism

Main articles: Cell growth, Cell metabolism Between successive cell divisions cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, where the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways. The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

Making new cells

Main article: Cell division Cell divisions (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.]] Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells. DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

Protein synthesis

Main article: Protein biosynthesis Protein synthesis is the process in which the cell builds proteins. DNA transcription refers to the synthesis of a messenger RNA (mRNA) molecule from a DNA template. This process is very similar to DNA replication. Once the mRNA has been generated, a new protein molecule is synthesized via the process of translation. The cellular machinery responsible for synthesizing proteins is the ribosome. The ribosome consists of structural RNA and about 80 different proteins. When the ribosome encounters an mRNA, the process of translating an mRNA to a protein begins. The ribosome accepts a new transfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an amino acid, the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein.

Origins of cells

Main article: Origin of life The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.

Origin of first cell

If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example, by producing nucleotides) will automatically advantage the neighbouring DNA molecules. This might be viewed as "parasitism by default". Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged. If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor given for simplicity than complete accuracy, since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules, acting both as replicators and enzymes: see RNA world hypothesis . But the core of the reasoning is the same.) Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth. Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in ones mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phospholipids.html] can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-celluar organisms could be achieved. This theory is expanded upon in the book, "The Cell: Evolution of the First Organism" by Joseph Panno Ph.D.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.

History

- 1632-1723: Antony van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
- 1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope. ::...I could exceedingly plainly perceive it to be all perforated and porous, much like honeycomb...these pores or cells, were not very deep, but consisted of a great many little boxes... – Hooke describing his observations on a thin slice of cork.
- 1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.
- The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
- Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935 he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

References

- Category:Cell biology Category:Biology ko:세포 ms:Sel ja:細胞 simple:Cell th:เซลล์ (ชีววิทยา)

Passive transport

Passive transport is a means of moving biochemicals, and other atomic or molecular substances, across membranes. Unlike active transport, this process does not involve chemical energy. Passive transport is dependent on the permeability of the cell membrane, which, in turn, is dependent on the organization and characteristics of the membrane lipids and proteins. The four main kind of passive transport are diffusion, facilitated diffusion, filtration and osmosis.

Diffusion

Main article: Diffusion Diffusion is the net movement of material from an area of high concentration of that material 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 material from area of higher concentration to the lower, 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"). If and when the concentration gradient have been eliminated, no net exchange of material occurs. Although material may move forth from one area to the other, it will be balanced by movement of the same amount of material to the opposite direction. Diffusion is biologically important because it enables the abolishment of concentration gradients in the body. For example, metabolic activity will consume oxygen, which will reduce its concentration in the bloodstream; diffusion of oxygen in the alveoli of the lungs allows it to be replenished.

Facilitated diffusion

Main article: Facilitated diffusion Facilitated diffusion is movement of molecules across the cell membrane via special carrier proteins that are embedded within the cellular membrane. A lot of large molecules, such as glucose, are insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane. Bear in mind, however, that facilitated diffusion is a passive process, and the solutes still move down the concentration gradient.

Filtration

Main article: Filtration Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumin, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, to allow a veriety of solutes to pass through and be metabolized.

Osmosis

Main article: Osmosis Osmosis is basically diffusion of water molecules. Most cell membranes are permeable to water, and since the diffusion of water plays such an important role in the biological functioning of any living being, a special term has been coined for it -- osmosis. Water molecules "stick" together via weak hydrogen b Category:Transport phenomena Category:Biochemistry Category:Physiology

Cell membrane

A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. It separates a cell's interior from its surroundings and controls what moves in and out. Cell surface membranes often contain receptor proteins and cell adhesion proteins. There are also other proteins with a variety of functions. These membrane proteins are important for the regulation of cell behavior and the organization of cells in tissues. In animal cells, the cell membrane establishes this separation alone, whereas in yeast, bacteria and plants an additional cell wall forms the outermost boundary, providing primarily mechanical support. The plasma membrane is only about 10 nm thick and may be discerned only faintly with a transmission electron microscope. One of the key roles of the membrane is to maintain the cell potential.

A fluid mosaic

The basic composition and structure of the plasma membrane is the same as that of the membranes that surround organelles and other subcellular compartments. The foundation is a phospholipid bilayer, and the membrane as a whole is often described as a fluid mosaic – a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins, which may function as channels or transporters across the membrane, or as receptors. The model was first proposed by S.J. Singer (1971) as a lipid protein model and extended to include the fluid character in a publication with G.L. Nicolson in "Science" (1972). Some of these proteins simply adhere to the membrane (extrinsic or peripheral proteins), whereas others might be said to reside within it or to span it (intrinsic proteins – more at integral membrane protein). Glycoproteins have carbohydrates attached to their extracellular domains. Cells may vary the variety and the relative amounts of different lipids to maintain the fluidity of their membranes despite changes in temperature. Cholesterol molecules (in case of eukaryotes) or hopanoids (in case of prokaryotes) in the bilayer assist in regulating fluidity.

Detailed structure

In fact, not all lipid molecules in the cell membrane are "fluid," in the sense of free to diffuse. Lipid rafts and caveolae are examples of more-cohesive membrane regions. Many proteins are not free to diffuse. The cytoskeleton undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface – for example, the "apical" surface of epithelial cells that line the vertebrate gut – and limits how far they may diffuse within the bilayer. Rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called microvilli, increase cell surface area and facilitate the absorption of molecules from the outside. Synapses are another example of highly-structured membrane.

Transport across membranes

As a lipid bilayer, the cell membrane is semi-permeable. This means that only some molecules can pass unhindered in or out of the cell. These molecules are either small or lipophilic. Other molecules can pass in or out of the cell, if there are specific transport molecules. Depending on the molecule, transport occurs by different mechanisms, which can be separated into those that do not consume energy in the form of ATP (passive transport) and those that do (active transport).

Passive transport

Passive transport is a means of moving different chemical substances across membranes through diffusion of hydrophobic (non-polar) and small polar molecules, or facilitated diffusion of polar and ionic molecules, which relies on a transport protein to provide a channel or bind to specific molecules. This spontaneous process decreases free energy, and increases entropy in a system. Unlike active transport, this process does not involve any chemical energy (ATP).

Active transport

Main article: active transport. Typically moves molecules against their electrochemical gradient, a process that would be entropically unfavorable were it not stoichiometrically coupled with the hydrolysis of ATP. This coupling can be either primary or secondary. In the primary active transport, transporters that move molecules against their electrical/chemical gradient, hydrolyze ATP. In the secondary active transport, transporters use energy derived from transport of another molecule in the direction of their gradient, to move other molecules in the direction against their gradient. This can be either symport (in the same direction) or antiport (in the opposite direction). Examples include: #The usual cases of molecular exchangers, transporters and pumps #endocytosis and exocytosis, where molecules packaged in membrane vesicles are either imported or exported respectively, can be thought of as active transport.

External links

- [http://www.biochemweb.org/lipids_membranes.shtml Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology]
- [http://www.westernblotting.org/protocol%20membrane%20extraction.htm Cell membrane protein extraction protocol] Category:Membrane biology ms:Membran sel ja:細胞膜

Osmotic pressure

Osmotic pressure or turgor (also called turgor pressure) is the pressure produced by a solution in a space that is enclosed by a differentially permeable membrane. When a biological cell is in a hypotonic environment (the cell interior contains a lower concentration of water than its exterior), water flows across the cell membrane into the cell, causing it to expand. The membrane (or, in plant cells, the cell wall) restricts the expansion, which causes an increase in pressure. The resulting pressure is called turgor. This pressure is what prevents more water from flowing into the cell, thus creating a pressure equilibrium between water flowing down the concentration gradient and the taut membrane pushing back. In this example, the equilibrium prevents the cell from ever becoming isotonic to its environment. Cells not adapted to hypotonic environments, with the flow of water into them but no strong membrane or cell wall, will burst. The osmotic pressure π of a dilute solution can be calculated using the formula :

\pi = MRT \,

where :M is the molarity :R is the gas constant :T is the absolute temperature Note the similarity of the above formula to the ideal gas law, and also that osmotic pressure is not dependant on particle charge

Applications

Osmotic pressure is the basis of reverse osmosis, a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinators use pressures around 50 atm to produce fresh water from ocean salt water. Osmotic pressure is necessary for most plants. It is the resulting turgor that allows herbaceous plants to stand upright, and how plants regulate the aperture of their stomata

External links

- [http://www.microscopy-uk.org.uk/mag/artoct99/plantupright.html How do non woody plants stay upright?] Category:Cell biology Category:Membrane biology ja:浸透圧

Fick's law of diffusion

Fick's laws of diffusion describe diffusion, and define the diffusion coefficient D.

History

Fick's laws of diffusion were derived by Adolf Fick in the year 1855.

Fick's First Law

Fick's First Law is used in steady state diffusion, i.e., when the concentration within the diffusion volume does not change with respect to time (J_in=J_out).

J = - D \frac

Where
-

J

is the diffusion flux in dimensions of [parts length^-2 time^-1], [mol m^-2 s^-1]
-

D

is the diffusion coefficient in dimensions of [length² time^-1], [m² s^-1]
-

\Phi

is the concentration in dimensions of [parts length^-3], [mol dm^-3]
-

x

is the position [length], [m]

Fick's Second Law

Fick's Second Law is used in non-steady state diffusion, i.e., when the concentration within the diffusion volume changes with respect to time.

\frac = D \frac

Where
-

\Phi

is the concentration in dimensions of [parts length^-3], [mol dm^-3]
-

t

is time [s]
-

D

is the diffusion coefficient in dimensions of [length² time^-1], [m² s^-1]
-

x

is the position [length], [m]

Applicability

Fick's law based equations have been commonly used to model transport processes in foods, biopolymers, pharmaceuticals, porous soils, semiconductor doping process, etc. A large amount of experimental research in polymer science and food science has shown that more general approach is required to describe transport of components in materials undergoing glass transition. In the vicinity of glass transition the flow behavior becomes non-Fickian'. See also non-diagonal coupled transport processes (Onsager relationship).

Temperature dependence of the Diffusion coefficient

The diffusion coefficient at different temperatures is often found to be well predicted by

D = D_0 e^

Where:
- D is the diffusion coefficient
- D₀ is the maximum diffusion coefficient (at infinite temperature)
- Q is the activation energy for diffusion in dimensions of [energy / parts]
- T is the temperature in units of [absolute temperature] (Kelvin or Rankine)
- R is the gas constant in dimensions of [energy temperature^-1 parts^-1] Typically, a compound's diffusion coefficient is 10,000x greater in air than in water. Carbon Dioxide in air has a diffusion coefficient of 0.16 cm^2/s, and in water, it's coefficient is 1.6 X 10^-5 cm^2/s [http://www.cco.caltech.edu/~brokawc/Bi145/Diffusion.html].

A Biological Perspective

The first law gives rise to the formula :

\mathrm = \frac

It states that the rate of diffusion of a gas across a membrane is
- Constant for a given gas at a given temperature by an experimentally determined factor,

K

- Proportional to the surface area over which diffusion is taking place,

A

- Proportional to the difference in partial pressures of the gas across the membrane,

P_2 - P_1

- Inversely proportional to the distance over which diffusion must take place, or in other words the thickness of the membrane,

D

. Fick's first law is also important in radiation transfer equations. However, in this context it becomes inaccurate when the diffusion constant is low and the radiation becomes limited by the speed of light rather than by the resistance of the material the radiation is flowing through. In this situation, one can use a flux limiter. The exchange rate of a gas across a fluid membrane can be determined by using this law together with Graham's law.

External link

- [http://www.diffusion-polymers.com/messages/406.html Diffusion coefficient] - diffusion-polymers.com

References

- A. Fick, Phil. Mag. (1855), 10, 30.
- A. Fick, Poggendorff's Annel. Physik. (1855), 94, 59.
- W.F. Smith, Foundations of Materials Science and Engineering 3^rd ed., McGraw-Hill (2004) Category:Diffusion Category:Statistical mechanics Category:Physical chemistry Category:Eponymous laws ja:フィックの法則

Ohm's law

Ohm's law, named after its discoverer Georg Ohm ^[1], states that the potential difference or voltage drop (V) between the ends of a conductor and the current (I) flowing through the conductor are proportional at a given temperature: :

V = I - R

The equation contains the proportionality constant R, which is the electrical resistance of the device.

Overview

The law is strictly true only for resistors whose resistance does not depend on the applied voltage, which are called ohmic or ideal resistors or ohmic devices. Fortunately, the conditions where Ohm's law holds are very common. However, if R is assumed to be constant, then Ohm's law is never completely accurate for "real world" devices because no real device is an ohmic device for every voltage and current. At some level, the device will open or short, for example, by burning up or arcing. The relation

V / I = R

even holds for non-ohmic devices, but then the resistance R depends on V and is no longer a constant. To check whether a given device is ohmic or not, one plots V versus I and checks that the curve is a straight line or not. The Ohm's law equation is often stated as :

V = I \cdot R

in part because that is the variation very commonly used with resistors.

Physics

Physicists often use the continuum form of Ohm's Law: :

\mathbf = \sigma \cdot \mathbf

where J is the current density (current per unit area), σ is the conductivity (which can be a tensor in anisotropic materials) and E is the electric field. The common form

V = I \cdot R

used in circuit design is the macroscopic, averaged-out version. The equation above is only valid in the reference frame of the conducting material. If the material is moving at velocity v relative to a magnetic field B, a term must be added as follows :

\mathbf = \sigma \cdot \left( \mathbf + \mathbf\times\mathbf \right)

The analogy to the Lorentz force is obvious, and in fact Ohm's law can be derived from the Lorenz force and the assumption that there is a drag on the charge carriers proportional to their velocity. In a sense, Ohm's law is trivial, because resistivity is defined in terms of current and voltage. The actual content of the law is that in many cases of practical interest, the resistance of a component is not a function of the applied voltage. There are exceptions, of course, such as diodes, which have a non-linear current-voltage relationship. A perfect metal lattice would have no resistivity, but a real metal has crystallographic defects, impurities, multiple isotopes, and thermal motion of the atoms. Electrons scatter from all of these, resulting in resistance to their flow.

Temperature effects

When the temperature of the conductor increases, the collisions between electrons and atoms increase. Thus as a substance heats up because of electricity flowing through it (or by any heating process), the resistance will increase. The resistance of an Ohmic substance depends on temperature in the following way: :

R = \frac \cdot \rho = \frac \cdot \rho_0 (\alpha (T - T_0) + 1)

where ρ is the resistivity, L is the length of the conductor, A is its cross-sectional area, T is its temperature,

T_0

is a reference temperature (usually room temperature), and

\rho_0

and

\alpha

are constants specific to the material of interest. In the above expression, we have assumed that L and A remain unchanged within the temperature range. It is worth mentioning that temperature dependence does not make a substance non-ohmic, because at a given temperature R does not vary with voltage or current (

V / I = \mathrm

). Intrinsic semiconductors exhibit the opposite temperature behavior, becoming better conductors as the temperature increases. This occurs because the electrons are bumped to the conduction energy band by the thermal energy, where they can flow freely and in doing so they leave behind holes in the valence band which can also flow freely. Extrinsic semiconductors have much more complex temperature behaviour. First the electrons (or holes) leave the donors (or acceptors) giving a decreasing resistance. Then there is a fairly flat phase in which the semiconductor is normally operated where almo

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