Cell Diffusion

11 Key excerpts on "Cell Diffusion"

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  eBook - PDF

  Color Atlas of Physiology

  • Stefan Silbernagl, Agamemnon Despopoulos(Authors)
  • 2011(Publication Date)
  • Thieme

    (Publisher)

  Anionic macromolecules like albumin, which must remain in the bloodstream because of its colloid osmotic action ( p. 210), are held back by the wall charges at the intercellular spaces and, in some cases, at the fenestra. Long-distance transport between the various organs of the body and between the body and the outside world is also necessary. Convection is the most important transport mechanism involved in long-distance trans-port ( p. 24). Transport In, Through and Between Cells (continued) Inflammation and irritation of skin and mucosa, meningitis 19 1 Fundamentals and Cell Physiology Plate 1.9 Transport In, Through and Between Cells II 20 1 Fundamentals and Cell Physiology Diffusion is movement of a substance owing to the random thermal motion (brownian move-ment) of its molecules or ions ( A1 ) in all directions throughout a solvent. Net diffusion or selective transport can occur only when the solute concentration at the starting point is higher than at the target site. ( Note: uni-directional fluxes also occur in absence of a concentration gradient—i.e., at equilibrium— but net diffusion is zero because there is equal flux in both directions.) The driving force, “force” not to be taken in a physical sense, of diffusion is, therefore, a concentration gra-dient . Hence, diffusion equalizes concentra-tion differences and requires a driving force: passive transport (= downhill transport). Example: When a layer of O 2 gas is placed on water, the O 2 quickly diffuses into the water along the initially high gas pressure gradient ( A2 ). As a result, the partial pressure of O 2 ( P o 2 ) rises, and O 2 can diffuse further downward into the next O 2 -poor layer of water ( A1 ).

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  Physicochemical and Environmental Plant Physiology

  • Park S. Nobel(Author)
  • 2020(Publication Date)
  • Academic Press

    (Publisher)

  1.2. Diffusion

  Diffusion leads to the net movement of a substance from a region of higher concentration to an adjacent region of lower concentration of that substance (Fig. 1-5 ). It is a spontaneous process; that is, no energy input is required.

  Diffusion takes place in both the liquid and the gas phases associated with plants and is a result of the random thermal motion of the molecules—the solute(s) and the solvent in the case of a solution or of gases in the case of air. The net movement caused by diffusion is a statistical phenomenon—a greater probability exists for molecules to move from the concentrated region to the dilute region than vice versa (Fig. 1-5 ). In other words, more molecules per unit volume are present in the concentrated region than in the dilute one, so more are available for diffusing toward the dilute region than are available for movement in the opposite direction. If left isolated from external influences, diffusion of a neutral species tends to even out concentration differences originally present in adjoining regions of a liquid or a gas. In fact, the randomizing tendency of the generally small, irregular motion of particles by diffusion is a good example of the increase in entropy or disorder that accompanies all spontaneous processes. In 1905, Albert Einstein, the German-born theoretical physicist and later American citizen (who received the Nobel Prize in Physics in 1921), described such diffusion as a case of Brownian motion or movement, which was first observed microscopically by the Scottish botanist Robert Brown in 1827 for colloidal particles.

  Figure 1-5  The random thermal motion of uncharged molecules of species j produces a net movement from a region of higher concentration (left-hand side) to a region of lower concentration (right-hand side).

  Diffusion is involved in many plant processes, such as gas exchange and the movement of nutrients toward root surfaces. For instance, diffusion is the mechanism for most, if not all, steps by which CO2 from the air reaches the sites of photosynthesis in chloroplasts. CO2 diffuses from the atmosphere up to the leaf surface and then diffuses through the stomatal pores. After entering a leaf, CO2 diffuses within intercellular air spaces (Fig. 1-2 ). Next, CO2 diffuses across the cell wall, crosses the plasma membrane of a leaf mesophyll cell, and then diffuses through the cytosol to reach the chloroplasts (Fig. 1-1 ). Finally, CO2 enters a chloroplast and diffuses up to the enzymes that are involved in carbohydrate formation. If the enzymes were to fix all of the CO2 in their vicinity, and no other CO2 were to diffuse in from the atmosphere surrounding the plant, photosynthetic processes would stop (in solution, “CO2 ” can also occur in the form of bicarbonate, HCO3 – , and the crossing of membranes does not have to be by diffusion, refinements that we will return to in Chapter 8 , Section 8.3D

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  The Living Barrier

  A Primer on Transfer across Biological Membranes

  • Roy J. Levin(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  CHAPTER 5 The Movement of Molecules Across Cell Membranes The previous chapters developed the idea that the outer plasma membrane acts as a diffusion barrier between the external environ-ment and the internal world of the cell. The advantages of such a barrier are clear. The delicate molecular structures of life are not only protected from a changing and possibly harmful external environment but they are also maintained at conditions optimal for their biological function. The other side of the coin, however, is that the cell membrane barrier will delay or prevent not only the inward passage of nutrients essential to the welfare of the cell but also the outward passage of waste products from metabolism. Furthermore, in cells that manufacture substances for export, such as hormones or enzymes, mechanisms must be present to allow passage of these without damaging the protective barrier function. Numerous cellular mechanisms have in fact developed to overcome these problems of transfer. It will be useful at this point to make a general survey of the transfer systems that occur across biological membranes. We can then examine each in greater detail and discuss its importance to cellular physiology. Rather than list the many cellular transfer systems, an attempt has been made to classify them under various headings in Fig. 22. This has been done to avoid the devastatingly barbed comment of the French novelist, Albert Camus that The absurd man multiplies what he cannot unify! There is, however, the danger of absurdity in making a classification of transfer systems at our present state of knowledge, for some systems can be made to overlap into different categories. This is not a real problem as long as we remember that the classification is only for convenience. It can and should be altered as soon as further work makes such changes advisable.

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  Life's Devices

  The Physical World of Animals and Plants

  • Steven Vogel(Author)
  • 2020(Publication Date)
  • Princeton University Press

    (Publisher)

  And for diffusion within individ-ual cells compared with water, data collected by Mastro and Keith (1984) show that large molecules have disproportionately low diffusion coeffi-cients—coefficients for protein molecules may be more than a hundred-fold lower in cells than in water. The internal architecture of cells seems to slow macromolecular movement disproportionately. Indeed, the loss of much of this structure on the death of a cell is marked by an increase in Brownian motion that is quite conspicuous in time-lapse movies. Diffusion versus convection What must be emphasized in any discussion of diffusion is how strongly its effectiveness depends on the size of the system in question. Purcell (1977) gave an amusing but important illustration of the phe-nomenon by asking whether a bacterium ought to swim as it feeds on dissolved small molecules in the broth in which it lives. His answer is that swimming isn't worth the bother—food diffuses to the bacterium faster than the microorganism can possibly move. It lives in a world analogous to that of a cow in a bovine nirvana—a pasture of grass growing so fast that the cow has no need to walk as it grazes. Bacteria do swim, though, and Purcell noted that there is some profit in explorations that sample more concentrated or salubrious broth—new and perhaps greener pastures. Closer to home, the delay in transmission of a nerve impulse from nerve cell to nerve cell is only about a millisecond—transmission depends on diffusion of a so-called transmitter substance across the synaptic gap, but the gap is only about 20 nanometers across. But for long distances the times involved in diffusive processes get devastatingly long. Schmidt-Nielsen (1979) did the following sobering calculation. If it takes a hun-dredth of a second for diffusion of a quantity of a substance such as ox-ygen between a typical cell and a capillary, then diffusion of the same quantity over a distance of 1 millimeter will take 100 seconds, and diffu-165

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  Biophysics

  A Physiological Approach

  • Patrick F. Dillon(Author)
  • 2012(Publication Date)
  • Cambridge University Press

    (Publisher)

  3 Diffusion and directed transport The movement of material within the cell and across membranes always requires a driving force. For diffusive processes, the driving force is the electrochemical gradient. The electrical component of this force requires a separation of charge: the negative and positive charges must be kept from one another until a conductive channel opens and the charged species can fl ow down their electrical gradient. Intact membranes, whether the cell membrane or those of organelles, are needed to provide the voltage buildup that will allow current to fl ow when conduction becomes possible. Within the cytoplasm or in the extracellular fl uid, the charges are not kept separate, and without a voltage there will be no electrical gradient. In these cases, diffusion is entirely driven by concentration gradients. The generation of these gradients is an active process: functions linked to ATP hydrolysis are ultimately responsible for all diffusion gradients. Once the gradients are generated, however, they will produce the movement of material without further ATP input. Across the membrane, of course, there can be both concentration and electrical gradients for charged moieties. In some cases, like Na + across the resting cell membrane, these gradients will be in the same direction, inward in this case, with a higher Na + concentration on the outside and a negative charge on the inside attracting the positive sodium. In others, like K + across the resting cell membrane, the electrical and concen-tration gradients are in opposite directions, with the higher K + concentration on the inside driving K + out countered by the negative charge on the inside drawing K + in. If diffusion down a gradient does not require ATP directly, other intracellular transport processes do require ATP. The movement of vesicles, organelles, and other cargo by kinesin, dynein and non-polymerized forms of myosin requires the direct hydrolysis of ATP to power each step.

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  Berne and Levy Physiology E-Book

  Berne and Levy Physiology E-Book

  • Bruce M. Koeppen, Bruce A. Stanton, Julianne M Hall, Agnieszka Swiatecka-Urban(Authors)
  • 2023(Publication Date)
  • Elsevier

    (Publisher)

  Fick’s first law of diffusion quantifies the rate at which a molecule diffuses from point A to point B:

  J = − DA

  Δ C

  Δ X

  (Equation 1.1)

  where

  • J = the flux or rate of diffusion per unit time
  • D = the diffusion coefficient
  • A = the area across which the diffusion is occurring
  • ΔC = the concentration difference between points A and B
  • ΔX = the distance along which diffusion is occurring

  The diffusion coefficient takes into account the thermal energy of the molecule, its size, and the viscosity of the medium through which diffusion is taking place. For spherical molecules, D is approximated by the Stokes-Einstein equation:

  D =

  kT

  6 π r η

  (Equation 1.2)

  where

  • k = Boltzmann’s constant
  • T = temperature in degrees Kelvin
  • r = radius of the molecule
  • η = viscosity of the medium

  According to Eqs. 1.1 and 1.2 , the rate of diffusion will be faster for small molecules than for large molecules. In addition, diffusion rates are high at elevated temperatures, in the presence of large concentration gradients, and when diffusion occurs in a low-viscosity medium. With all other variables held constant, the rate of diffusion is linearly related to the concentration gradient.

  Fick’s equation can also be applied to the diffusion of molecules across a barrier, such as a lipid bilayer. When applied to the diffusion of a molecule across a bilayer, the diffusion coefficient (D) incorporates the properties of the bilayer and especially the ability of the molecule to diffuse through the bilayer. To quantify the interaction of the molecule with the bilayer, the term partition coefficient

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  Ion Transport Across Membranes

  Incorporating Papers Presented at a Symposium Held at the College of Physicians & Surgeons, Columbia University, October, 1953

  • Hans T. Clarke(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Ion Transport Across Biological Membranes HANS H. USSING DIFFUSION T H R O U G H BIOLOGICAL M E M B R A N E S At the outset it might be worth while to define briefly what we ex-perimental biologists mean by a biological membrane. I think most of us can agree upon a formulation like this: Whenever we meet, in a liv-ing organism or part thereof, a boundary that presents a diffusion re-sistance to solutes higher than that of the phases separated by the boundary, it is called a membrane. The membrane is often, but not always, anatomically discernible. The objects we study under the name of biological membranes are extremely diverse. Thus.we have membranes on the multicellular level like the gastric mucosa or the frog skin epithelium. Then there are the cell membranes like, for instance, the membranes of the nerve fiber. Finally, the work of the last few years tends to show that even membranes on the subcellular level are highly important. Notably, the surface of the mitochondria shows membrane-like properties, such as the ability to maintain, and under certain circumstances to create, within the mitochondrium concentrations of a number of substances which differ from those of the surroundings. As an example we may take a table from a recent paper by Bartley and Davies (1952) (Table I). It is seen that the Na ion undergoes a conspicuous concentration in the mitochondria as compared to the surrounding medium. At first sight there seems very little in common between the nerve fiber membrane and the skin of a frog, or between the tip of a plant root and the gill of a crab. Nevertheless, these different structures show many similarities in the way they handle inorganic ions. Formerly it was generally assumed that the similarities stemmed from the fact that the element determining the behavior of ions was in all cases a cell membrane, or possibly a number of cell membranes placed in series.

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  Differential Equations and Mathematical Biology

  • D.S. Jones, Michael Plank, B.D. Sleeman(Authors)
  • 2009(Publication Date)
  • Chapman and Hall/CRC

    (Publisher)

  (12.1.5) This is the classic diffusion equation. In our context it is an expression of Fick’s second law and in physics it is the basic equation of heat conduction (cf. Chapter 11). In this case C is interpreted as temperature and D heat conductivity. Let us now consider the application of Fick’s laws to the problem of diffusion of a solute into a cell. The diffusion here depends critically on the transport of solution through the membrane surrounding the protoplasm of the cell, namely the plasma membrane . This membrane consists of a double layer of lipid molecules called the bilipid layer . This layer contains a structure of globular proteins on both its surfaces, some of which penetrate across the entire width of the bilipid layer. However, the process by which these proteins control the diffusion of the solute is not fully understood. The simplest theory of transport into or out of a cell by diffusion is derived on the assumption that the intracellular diffusion is so rapid that the solute concentration there is uniform in space. In most experiments of diffusion into cells, the volume of extracellular space is made so large that the solute concen-tration there is not affected to any significant extent by any loss in cells and may be assumed constant. Consider then an idealised cell membrane model to consist of a homogeneous lipid layer separating two aqueous phases, the cell interior and the cell exterior (see Figure 12.1.2a). At each of the two lipid interfaces, a discontinuity in solute concentration exists at equilibrium. This discontinuity is a consequence of the molecular barrier that exists for a solute molecule entering the lipid from the aqueous phase. Suppose C o represents 296 Differential Equations and Mathematical Biology V n j d s S FIGURE 12.1.1 : The conservation of mass. the concentration on the outside of the cell and ¯ C o the concentration inside the lipid layer and adjacent to the outer interface.

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  Transport And Diffusion Across Cell Membranes

  • Wilfred Stein(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  How the phospholipids act as barrier and passage will be the subject of Chapter 2 of this volume. How some transmembrane proteins can act as channels will be the content of Chapter 3, while the more complicated, highly specific transport proteins will be dealt with in Chapters 4-6. 1.2. THERMODYNAMICS AND KINETICS OF THE DIFFUSION PROCESS In this section I develop some physical concepts about diffusion in general—how molecules and ions move from place to place within a system composed of a single phase or of a number of phases. The con- 36 1. Physical Basis of Movement across Cell Membranes cepts to be developed here will be needed for a full understanding of the processes of movement across biological membranes. An excellent ac-count of the physical basis of such processes is given in Nobel (1974), and another fine treatment that carries the discussion beyond that needed for our present study is that by Finkelstein and Mauro (1959). Readers who have a firm grounding in such physical principles might care to move directly on to Section 1.3. 1.2.1. Diffusion as a Random Walk Figure 1.12 is an attempt to convey visually the concept of diffusion as a random walk process. The horizontal line is a planar barrier and separates two regions containing different concentrations of a diffusing molecule (the small circles). There are many more of these molecules above the line than below it. In both regions molecules will, in their ceaseless motions, be jumping from place to place, each jump being at random. During the course of such wanderings, the diffusing molecules will jump across the line dividing the two regions. Simply because more molecules are present in the upper region than in the lower region, more molecules will cross from above to below than from below to above. In other words, there will be a net diffusive movement of molecules from above to below, in the direction of diminishing concentration gradient.

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  Physicochemical and Environmental Plant Physiology

  • Park S. Nobel(Author)
  • 2009(Publication Date)
  • Academic Press

    (Publisher)

  Fig. 1-5 ). In other words, more molecules per unit volume are present in the concentrated region than in the dilute one, so more are available for diffusing toward the dilute region than are available for movement in the opposite direction. If left isolated from external influences, diffusion of a neutral species tends to even out concentration differences originally present in adjoining regions of a liquid or a gas. In fact, the randomizing tendency of the generally small, irregular motion of particles by diffusion is a good example of the increase in entropy, or disorder, that accompanies all spontaneous processes. In 1905, Albert Einstein described such diffusion as a case of Brownian motion or movement, which was first observed microscopically by Robert Brown in 1827 for colloidal particles.

  Figure 1-5. The random thermal motion of uncharged molecules of species j produces a net movement from a region of higher concentration (left-hand side) to a region of lower concentration (right-hand side).

  Diffusion is involved in many plant processes, such as gas exchange and the movement of nutrients toward root surfaces. For instance, diffusion is the mechanism for most, if not all, steps by which CO2 from the air reaches the sites of photosynthesis in chloroplasts. CO2 diffuses from the atmosphere up to the leaf surface and then diffuses through the stomatal pores. After entering a leaf, CO2 diffuses within intercellular air spaces (Fig. 1-2 ). Next, CO2 diffuses across the cell wall, crosses the plasma membrane of a leaf mesophyll cell, and then diffuses through the cytosol to reach the chloroplasts (Fig. 1-1 ). Finally, CO2 enters a chloroplast and diffuses up to the enzymes that are involved in carbohydrate formation. If the enzymes were to fix all of the CO2 in their vicinity, and no other CO2 were to diffuse in from the atmosphere surrounding the plant, photosynthetic processes would stop (in solution, “CO2 ” can also occur in the form of bicarbonate, HCO3 − , and the crossing of membranes does not have to be by diffusion, refinements that we will return to in Chapter 8 , Section 8.3D). In this chapter we develop the mathematical formulation necessary for understanding both diffusion across a membrane and diffusion in a solution.

  1.2A. Fick's First Law

  In 1855 Adolph Fick was one of the first to examine diffusion quantitatively. For such an analysis, we need to consider the concentration (c

  j

  ) of some solute species j in a solution or gaseous species j in air; the subscript j indicates that we are considering only one species of the many that could be present. We will assume that the concentration of species j in some region is less than in a neighboring one. A net migration of molecules occurs by diffusion from the concentrated to the dilute region (Fig. 1-5 ; strictly speaking, this applies to neutral molecules or in the absence of electrical potential differences, an aspect that we will return to in Chapter 3

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  Inside the Photon

  A Journey to Health

  • Tony Fleming, Elizabeth Bauer(Authors)
  • 2014(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  Protein Diffusion within the Plasma Membrane 183 Figure 5.4 Growth of a cell tip. how cells work. In the following we look at how cells use membrane proteins to act cooperatively to achieve some of the basic steps of division. Mitosis, for instance, contains many steps, including stages of the cell cycle. Genetic signals are responsible for these stages that are kept within the complete genome, which for Homo sapiens is 3.4 × 10 9 base pairs (bp) long. However, E and H fields play an important role in adjunct (Fig. 5.4). A numerical study of protein diffusion within the plasma membrane of a cell exposed to a static E field demonstrates the mechanism for a single cell. A finite-difference method was used to mathematically model the equations of membrane diffusion proposed by Jaffe and Nuccitelli (1977) and Poo (1981). A computer program was developed and used to compare theory with experiment. Several runs were performed varying the cell radius (10–100 μ m), exposure level (100–1,000 V m − 1 ) and various other parameters, including the diffusion constant. Next, the diffusion constant was matched to the time to reach electrophoretic equilib-rium observed by Poo after exposure of Xenopus myotomal cells. The numerical results compare reasonably well with experimental time-sequence asymmetry data for concanavalin receptors reported 184 Cell Division and Membrane Diffusion Figure 5.5 Diffusion of proteins within the membrane surface (a) before an external E field is applied, (b) after an external E field is applied and there is a net negative charge on the surface and (c) after an external E field is applied where a dipolar charge is induced within proteins. by Poo. A final run was made at weak exposure levels to model the experimental exposure of Mougeotia protoplasts conducted by White et al. (1990). Electric fields are known to cause proteins to diffuse within plasma membranes.

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