Osmosis

10 Key excerpts on "Osmosis"

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  Cambridge O Level Biology 5090

  • Azhar ul Haque Sario(Author)
  • 2023(Publication Date)
  • tredition

    (Publisher)

  Factors Influencing Diffusion: Apart from kinetic energy, several factors affect the rate of diffusion. These include the concentration gradient (the difference in concentration between two areas), the temperature, the size of the particles, and the nature of the medium through which diffusion occurs.

  Understanding Osmosis

  Defining Osmosis: Osmosis is a special type of diffusion. It specifically refers to the movement of water molecules across a semi-permeable membrane from an area of lower solute concentration (more water) to an area of higher solute concentration (less water).

  Kinetic Energy in Osmosis: Just like in diffusion, the kinetic energy of water molecules is the driving force behind Osmosis. Water molecules move randomly and, when they encounter a semi-permeable membrane, those that can pass through do so, moving toward the area with a higher solute concentration.

  Equilibrium in Osmosis: Osmosis continues until the concentration of the solute is equal on both sides of the membrane, or until the osmotic pressure (the pressure required to prevent the flow of water across the membrane) balances the movement of water.

  The Impact and Importance of These Processes Biological Significance: In biological systems, diffusion and Osmosis are crucial for the transport of substances within and between cells. They play a vital role in nutrient absorption, waste removal, and maintaining the balance of fluids and electrolytes.

  Cellular Functionality: Cells rely on diffusion and Osmosis to transport materials like oxygen, carbon dioxide, and other small molecules. The selective permeability of cell membranes, coupled with these processes, ensures that cells function optimally.

  Homeostasis: Diffusion and Osmosis are integral to maintaining homeostasis – the stable internal conditions necessary for survival. For example, osmoregulation, the control of water balance, is a key aspect of homeostasis in many organisms.

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  The Living Barrier

  A Primer on Transfer across Biological Membranes

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

    (Publisher)

  2. Classical Osmosis. Water will move across biological mem-branes by classical Osmosis wherever there is a difference in solute concentrations in the fluids bathing the membrane. This difference in concentration could come about by active pumps transferring 150 The Living Barrier either ions or non-electrolytes across a membrane. As we know that cells have active pumps for ions and non-electrolytes, it has often been suggested that Osmosis is likely to be the major mechan-ism of transcellular fluid movement, the cell producing a high concentration of solute which then causes water to move from one compartment to another by Osmosis. Under these circumstances water transfer is passive or secondary to net solute transfer, without solute transfer there can be no fluid transfer. One of the practical difficulties in accepting this explanation of fluid transfer has been with epithelia that transfer large quantities of fluid across their membranes from one compartment to another. More often than not the fluid transferred has an osmotic pressure or osmolarity identical to that of blood or plasma. In some situations it has been found that these epithelia (the small intestine is the one commonly used) actually appear to transfer water from a low chemical potential to a higher one. That is, the intestine apparently can undertake active transfer of water! This cannot, of course, occur thermodynamically unless the cells have an active water pump able to transfer pure water molecules across the membrane. Most biologists are loath to accept such a pump and prefer to keep the idea that without net solute transfer there can be no net water transfer. But how then to explain this uphill transfer of water by the intestine? A possible way out of the dilemma was found by Jared Diamond in his experiments on the absorption of fluid across the gall-bladder. He showed that there was no need to postulate active pumps for water.

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  The Movement Of Molecules Across Cell Membranes

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

    (Publisher)

  C H A P T E R 7 The Movement of Water Water is, of course, the major constituent of living matter, and the movement of water across cell membranes is quantitatively the major phenomenon in biological transport. W e have considered in Chapter 3 the molecular basis of water movement and especially the view that water movement occurs by diffusion through pores in the cell membrane (Section 3 . 7 ) . In the present chapter we shall emphasize the physio-logical, as opposed to the molecular, aspect of water movement. W e shall consider the factors which determine the rate of movement of water across cell membranes and into tissues, and also the factors which de-termine the steady state distribution of water in both cells and tissues. 7.1 T h e Volume of a Cell at the Steady State If the tonicity, that is, the concentration of osmotically active material of the medium surrounding an animal cell, is maintained constant and if the cell remains metabolically active, the cell volume will remain constant over a long period of time. Yet if red cells are placed in media containing different concentrations of impermeable substances, the cells respond to the change of external osmotic pressure as if they were, to a very good first approximation, perfect osmometers (LeFevre, 1 9 6 4 ) . The cell vol-ume is, therefore, capable of varying. Thus the cell is not enclosed by a rigid framework. Yet its volume is maintained constant under physio-logical conditions, and we can show that this constancy requires the continued performance of metabolic activity. If a tissue is removed from the body into an incubation medium and prevented from metabolizing, it will soon swell through absorbing water (and salts) from the medium ( T a b l e 7.1) (Heckman and Parsons, 1 9 5 9 ) . If metabolism recommences, the accumulated water and salts can be returned to the medium and the initial cell volume regained.

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  Netter's Essential Physiology E-Book

  Netter's Essential Physiology E-Book

  • Susan Mulroney, Adam Myers(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  The movement of water across the membrane by diffusion is termed Osmosis, and the permeability of the membrane determines whether diffusion of solute or Osmosis (water movement) occurs. The concentration of the impermeable solute will determine how much water will move through the membrane to achieve osmolar equilibration between ECF and ICF. Whereas osmolarity of a solution describes the concentration of dissolved particles and a solution can be described as hypo-osmotic, isosmotic, or hyperosmotic relative to another solution, whether fluid shifts will occur between two isosmotic solutions across a membrane depends on whether the solutes are permeant. The monosaccharide sucrose is impermeant to cells, and if infused into the plasma (ECF), it will stay in the ECF compartment. Thus a 300 mOsm/L solution of sucrose is isotonic relative to cells with normal osmolarity of 300 mOsm/L, and no fluid shift occurs. A sucrose solution of more than 300 mOsm/L is hypertonic, and a sucrose solution of less than 300 mOsm/L is hypotonic relative to normal cellular osmolarity. In contrast to impermeant solutes, a permeant solute, such as urea, will diffuse freely into cells until it reaches equilibrium. Thus a 300 mOsm/L solution of urea will be hypotonic even though the solution is isosmotic. When this solution is infused into the ECF, it will cause expansion of the ICF compartment. Osmosis occurs when osmotic pressure is present. Osmotic pressure is equivalent to the hydrostatic pressure necessary to prevent movement of fluid through a semipermeable membrane by Osmosis. The idea can be illustrated by using a U-shaped tube with different concentrations of solute on either side of an ideal semipermeable membrane (i.e., the membrane is permeable to water but is impermeable to solute; Fig

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  Osmotic and Ionic Regulation

  Cells and Animals

  • David H. Evans(Author)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  22

  II. WATER PERMEATION

  An understanding of the movement of water across cell membranes is essential to an understanding of the physical basis for osmoregulation. The volume of fluid compartments (intracellular or extracellular) is for, practical purposes, equal to the volume of water that resides therein, and the (passive) distribution of water is determined entirely by the distribution of solutes.

  A.  DRIVING FORCES FOR WATER MOVEMENT

  In one sense, water movements are incredibly simple; water permeation is a passive process. There is no evidence for active water transport; water movement is driven entirely by the gradient of the chemical potential of water. For biological membranes separating two aqueous solutions (for which standard chemical potential of water will be the same), the transmembrane difference in the chemical potential of water given by Equation 1.14 is:

  Δ μ w     =   ν w   ( Δ P −   R T   Δ C s )

  (1.15)

  which we can express in the practical units of pressure by dividing by the partial molar volume of water (vw ) to yield:

  Δ μ w ν w     =   Δ P −   R T   Δ C s

  (1.16)

  The driving force for passive water transport, as it is often described, is the difference between the hydrostatic pressure gradient (ΔΡ) and the gradient of “osmotic pressure” (Δπ) where Δπ = RTΔCs . This conventional usage can be confusing because π is not a pressure; rather, π is a so-called colligative property of the solution, a measure of composition. Likewise, Δπ is not a pressure difference; it is an expression of the difference in water concentration across the membrane. The association of Δπ with a pressure arises from an analysis of the equilibrium distribution of water across a membrane that is permeable to water but impermeable to solute and is configured such that a hydrostatic pressure can be applied to one side as indicated in Figure 1.3 . If a single, impermeant solute (s) is present on both sides of the membrane such that Cs (2) > Cs (1), then the resulting concentration gradient of water will drive water from side 1 to side 2—that is, from high water concentration to low water concentration. An equilibrium can be established by applying a pressure to the piston on side 2 such that the water flow, denoted here as the volume flow Jv (see below) is reduced to zero. In this condition, Δμ

  w

  equals zero, and from Equation 1.16 we obtain the classic van’t Hoff equation22

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  Physiology, Biophysics, and Biomedical Engineering

  • Andrew Wood(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Because a current of ions across a membrane will in general be due both to concentration and electri-cal gradients, the conductance is actually the electrical current divided by the electrochemical rather than electrical potential difference (this will be expanded in Section 5.5.1). 5.1.3.4 Osmotic Flow This is just a specialized form of simple diffusion, but in this case the water is doing the diffusing across the membrane and the solute (or dissolved particle species), which is larger in diameter than water, is unable to permeate the membrane. The number of water molecules per unit volume (which is another way of expressing concentration) on the side where there is less solute will be greater, so water flows down its concentration gradient 79 Membrane Biophysics into the region with the higher solute concentration (which incidentally will make the solute concentration slowly less because of the diluting effect). For a rigid container this water flow can be prevented by applying an opposing hydrostatic pressure. It is this pres-sure that is known as osmotic pressure, and an appropriate unit is the pascal (Pa). In cell systems, most of the solutes that make up the osmotic pressure of the intracellular and extracellular fluids actually do get through the membrane, but relatively slowly. 5.1.3.5 Hydraulic Flow or Filtration This is perhaps more important in plant rather than animal cells, but particularly in cell layers such as capillary endothelium, where there is distension of cells because of elevated fluid pressure, flow across the membrane can occur because of this. In general, the flow is of both solute and solvent species (i.e., NaCl and water) and is termed bulk flow. The membrane acts like a sieve, and the larger species do not permeate, but for the smaller species, the rate of flow depends on the pressure difference rather than the concentration difference.

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  Porous Media

  Applications in Biological Systems and Biotechnology

  • Kambiz Vafai(Author)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  Physical foundations of mechanisms of water and solute transport across cell membranes have been subjects of numerous research and review papers (Disalvo et al. 1989; Zeuthen and MacAulay 2002; Walsh et al. 2004; Kargol 2007, Elmoazzen et al. 2008). Initially, the processes of transport of non-electrolytic substances across cell membranes have been described using the Fick’s law of diffusion. In 1932, Jacobs and Stewart (Elmoazzen et al. 2008) gave quantitative estimates of membrane permeability and developed a dif-ferential equation describing the rate of water and solute permeation as a function of concentration, cell size, and the membrane permeation coeffi-cient. They have made certain simplifying assumptions, such as the constancy of membrane thickness and the extracellular concentration. Also, assuming that the osmotic pressure for a given substance is proportional to its con-centration, they de facto introduced an assumption that solutions are dilute. This is also an assumption made in a vast majority of papers on membrane transport. Another assumption was that the transport is passive, that is, it is driven by thermodynamic forces, such as concentration or pressure difference. This is in contrast to active transport, which requires energy input from some source, typically from the adenosine triphosphate (ATP) hydrolysis. Kedem and Katchalsky (1958) (Katchalsky and Curran 1965) developed a formal-ism describing transport properties of membranes using three parameters: the coefficients of filtration L p , permeation ω , and reflection σ . Starting from the laws of linear thermodynamics of irreversible processes, they derived equations for the volume flux and the solute flux induced by the osmotic pressure and hydraulic pressure gradients. These equations, known as the Kedem-Katchalsky (KK) equations, have been widely used in studies of pas-sive membrane transport processes.

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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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  Biosimulation

  Simulation of Living Systems

  • Daniel A. Beard(Author)
  • 2012(Publication Date)
  • Cambridge University Press

    (Publisher)

  2 Transport and reaction of solutes in biological systems Overview Transport of mass, into, out of, and within biological systems (including single cells, multicellular organisms, and even ecological systems) is fundamental to their operation. The subject of transport phenomena is treated in great depth in classic texts [10], as well as in books focused on biological systems [62]. Here we explore a number of examples that allow us to see how fundamental transport phenomena are accounted for in a wide range of biological systems. Specifically, we develop and apply basic frameworks for simulating transport in the following sorts of systems:  Well-mixed systems. 1 The defining characteristic of these systems is that they are fluid systems (often aqueous solutions in biological application) with the solutes of interest distributed homogeneously (i.e., well mixed) over the timescales of interest. An example of a well-mixed system is the aquarium studied in the previous chapter. Other examples are chemical reaction sys- tems inside cells or compartments within cells when spatial gradients of the intracellular reactants do not significantly influence the behaviors that are sim- ulated. Models of well-mixed systems (or models that adopt the well-mixed assumption) do not explicitly account for the spatial distribution of the vari- ables simulated. For biochemical systems this means that, at any given time, concentrations are constant throughout a compartment. The kinetics of such systems are typically described by ordinary differential equations, as in the examples of Section 2.1 of this chapter and in Chapter 3. Note that different physical mechanisms may justify the well-mixed assump- tion in different systems. In cells, molecular diffusion can effectively drive mixing on timescales on the order of seconds or less. In the previous chapter’s aquarium, mixing is driven by a mechanical pump that circulates the water.

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  Principles of Medical Physiology, 2/E

  • Sabyasachi Sircar(Author)
  • 2016(Publication Date)
  • Thieme

    (Publisher)

  Chemical equilibrium All chemical reactions are potentially reversible but they tend to proceed unidirectionally if one of the reactants or products is removed from the field of reaction. Consider the reaction shown below: CO + H O H CO H + HCO 2 Carbonic anhydrase 2 3 + 3 – 2 will be Osmosis from the hypoosmolar to the hyper- osmolar solution. This is made clear by the example illustrated in Fig. 2.4 which explains why the mag- nitude and direction of Osmosis cannot be predicted without knowing the pore size of the membrane. In real-life situations, the complexity of biological membranes makes it impossible to accurately pre- dict whether a solute particle will pass through it. Moreover, body fluids contain innumerable types of solutes, which is another reason why their tonicity cannot be predicted. Therefore, the only way of esti- mating the tonicity is to determine it experimentally. In clinical parlance, the word tonicity always refers to the tonicity of a solution with respect to an erythrocyte. In other words, it is the erythrocytic cell membrane across which the tonicity is tested. If the erythrocyte shrinks in a solution by losing water through Osmosis, the solution is hypertonic. If the erythrocyte swells up in a solution by gaining water through Osmosis, the solution is hypotonic. If the erythrocyte neither shrinks nor swells in the solution, the solution is called isotonic. The fluid used in clinics for intravenous transfu- sion is both isotonic and isoosmolar to the plasma. A 0.9% saline solution is most often used for the pur- pose. The 0.9% NaCl solution is roughly isoosmolar (308 mOsm/L) to the body fluids (290 mOsm/L) as can be calculated easily. 2 A 5% glucose solution is also isotonic initially when infused intravenously, but as the glucose is metabolized, the solution gradually becomes hypotonic. An isoosmolar solu- tion of urea will not be isotonic since urea rapidly diffuses into the erythrocytes.

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