Cell wall hydrostatic pressure

Because of their rigid cell walls, large hydrostatic pressures can exist in plant cells, whereas hydrostatic pressures in animal cells generally are relatively small. Hydrostatic pressures are involved in plant support and also are important for the movement of water and solutes in the xylem and in the phloem. The effect of pressure on the chemical potential of water is expressed by the term VWP (see Eq. 2.4), where Vw is the partial molal volume of water and P is the hydrostatic pressure in the aqueous solution in excess of the ambient atmospheric pressure. The density of water is about 1000 kg m-3 (1 g cm-3) therefore, when 1 mol or 18.0 x 10-3 kg of water is added to water, the volume increases by 18.0 x 10-6 m3. Using the definition ofV,., as a partial derivative (see Eq. 2.6), we need to add only an infinitesimally small amount of water (dnw) and then observe the infinitesimal change in volume of the system (dV). We thus find that Vw for pure water is 18.0 x 10-6 m3 mol-1 (18.0 cm3 mol-1). Although Vw can be influenced by the solutes present, it is generally close to 18.0 x 10-6 m3 mol-1 for a dilute solution, a value that we will use for calculations in this book. 

Figure 2-12. Responses of an initially turgid cell (internal hydrostatic pressure P1 > 0) placed in pure water (n° = 0) to changes in the external osmotic pressure (IT0). As IT0 increases, P decreases. At the point of incipient plasmolysis (plasma membrane just beginning to pull away from the cell wall), P is reduced to zero, which is hence also called the turgor loss point. Constancy of cell volume and water equilibrium are assumed at each step as 11° is raised (actually, IT1 increases a few percent as IT° is raised from 0 to n°JasmoJysjs because the cell shrinks slightly accompanying the decrease of P to zero).

This means that ij/ cell is affected by 3 factors (1) this is the osmotic or solute concentration effect. The concentration of dissolved solutes in a cell will influence water uptake, the greater the concentration, the greater the attractive force to water (2) the matric (or hydrational) potential is contributed by the ability of matrices (e.g. cell walls protein bodies) to be hydrated and bind water (3) the turgor (hydrostatic) pressure occurs because as water enters a cell the contents swell and exert a force upon each unit area of the cell wall. Turgor pressure is in fact the amount by which pressure inside the cell exceeds the atmospheric pressure outside. 

In this context it is interesting to note that archaea, which possess S-layers as exclusive cell wall components outside the cytoplasmic membrane (Fig. 14), exist under extreme environmental conditions (e.g., high temperatures, hydrostatic pressure, and salt concentrations, low pH values). Thus, it is obvious one should study the effect of proteinaceous S-layer lattices on the fluidity, integrity, structure, and stability of lipid membranes. This section focuses on the generation and characterization of composite structures that mimic the supramolecular assembly of archaeal cell envelope structures composed of a cytoplasmic membrane and a closely associated S-layer. In this biomimetic structure, either a tetraether... 

High hydrostatic pressure (HHP) processes have been used mainly for sauces or seafood and proven effective at reducing microbial populations without adverse effects on product quality (Considine et al., 2008 Brinez et al., 2006). HHP treatment causes bacterial inactivation by damaging the cell membrane, which affects membrane permeability and intracellular enzyme inactivation and possibly ruptures the plant cell wall (Kniel et al.,... 

High turgor (hydrostatic) pressures within cells lead to tensions in the cell wall, which maintain the cellular shape and function. Turgor pressure measurements can provide useful information about the physiology of cells. 

Plant cell vacuole Plant cells usually contain one or more membrane-bounded vacuoles. These are used to store nutrients (e.g. sucrose), water, ions and waste products (especially excess nitrogen-containing compounds). Like lysosomes in animal cells, vacuoles have an acidic pH maintained by H+ pumps in the membrane and contain a variety of degradative enzymes. Entry of water into the vacuole causes it to expand, creating hydrostatic pressure (turgor) inside the cell which is balanced by the mechanical resistance of the cell wall. 

Figure 12.1 illustrates the main structure of a nephron [1], The measurements to be reported in this chapter were performed on rats. A rat kidney contains approximately 30000 nephrons as compared to the one million nephrons in a human kidney. The process of urine formation starts with the filtration of plasma in the glomerulus, a system of 20-40 capillary loops. The presence of a relatively high hydrostatic pressure in this system allows water, salts and small molecules to pass out through the capillary wall and into the proximal tubule. Blood cells and proteins are retained, and the filtration process saturates when the protein osmotic pressure balances the hydrostatic pressure difference between the blood and the filtrate in the tubule. For superficial nephrons, the proximal tubule is visible in the surface of the kidney and easily accessible for pressure measurements by means of a thin glass pipette. 

If U is taken to be the volume V of the cell, the last two terms represent respectively the kinetic and the potential energies associated with motion of the simulation wall whose mass is M. The parameter P is the uniform hydrostatic external pressure acting on the simulation cell wall. 

Figure 1-15. Schematic sections of a hypothetical cylindrical cell resembling the intemodal cells of Nitella or Chora, illustrating various dimensions, the hydrostatic pressure, and the stresses existing in the cell wall (a) section perpendicular to cylinder axis, and (b) section through cylinder axis. The colored region indicates an aqueous solution where the hydrostatic pressure P leads to the longitudinal stress trL, which acts in an annulus of area approximately equal to 2nr x f , and the tangential stress crT, which acts along the two sides each of area l x tcw.

One way to calculate the stresses is to imagine that the cellular contents are removed, leaving only the cell wall, which has a uniform hydrostatic pressure acting perpendicular to its inside surface. The projection of this P over the appropriate area gives the force acting in a certain direction, and the reaction to this force is an equal force in the opposite direction in the cell wall. By dividing the force in the cell wall by the area over which it occurs, we can determine the cell wall stress. 

Before concluding this discussion of cell walls, we note that the case of elasticity or reversible deformability is only one extreme of stress-strain behavior. At the opposite extreme is plastic (irreversible) extension. If the amount of strain is directly proportional to the time that a certain stress is applied, and if the strain persists when the stress is removed, we have viscous flow. The cell wall exhibits intermediate properties and is said to be viscoelastic. When a stress is applied to a viscoelastic material, the resulting strain is approximately proportional to the logarithm of time. Such extension is partly elastic (reversible) and partly plastic (irreversible). Underlying the viscoelastic behavior of the cell wall are the crosslinks between the various polymers. For example, if a bond from one cellulose microfibril to another is broken while the cell wall is under tension, a new bond may form in a less strained configuration, leading to an irreversible or plastic extension of the cell wall. The quantity responsible for the tension in the cell wall — which in turn leads to such viscoelastic extension — is the hydrostatic pressure within the cell. 

The tonoplast does not have an appreciable difference in hydrostatic pressure across it. A higher internal hydrostatic pressure would cause an otherwise slack (folded) tonoplast to be mechanically pushed outward. The observed lack of such motion indicates that AP is close to zero across a typical tonoplast. If the tonoplast were taut, AP would cause a stress in the membrane, analogous to the cell wall stresses discussed in Chapter 1 (Section 1.5C) namely, the stress would be rAPI2t for a spherical vacuole (see Eq. 1.15). However, the tensile strength of biological membranes is low—membranes can rupture when a stress of 0.2 to 1.0 MPa develops in them. For a tonoplast 7 nm thick with a maximum stress before rupturing of 0.5 MPa surrounding a spherical vacuole 14 pm in radius, the maximum hydrostatic pressure difference across the tonoplast is... 

A loss of water from plant shoots—indeed, sometimes even an uptake — occurs at cell-air interfaces. As we would expect, the chemical potential of water in cells compared with that in the adjacent air determines the direction for net water movement at such locations. Thus we must obtain an expression for the water potential in a vapor phase and then relate this P to for the liquid phases in a cell. We will specifically consider the factors influencing the water potential at the plant cell-air interface, namely, in the cell wall. We will find that vFcel1 wal1 is dominated by a negative hydrostatic pressure resulting from surface tension effects in the cell wall pores. 

Plant cells come into contact with air where the cell walls are adjacent to the intercellular air spaces (see Fig. 1-2). Thus, the water potential in the cell walls must be considered with respect to T 1W in the adjacent gas phase. The main contributing term for T in cell wall water is usually the negative hydrostatic pressure arising from surface tension at the numerous ail-liquid interfaces of the cell wall interstices near the cell surface. In turn, Z 11 wal1 can be related to the geometry of the cell wall pores and the contact angles. 

The magnitude of the negative hydrostatic pressure that develops in cell wall water can be estimated by considering the pressure that occurs in a liquid within a cylindrical pore — the argument is similar to the one presented... 

The strong water-wall adhesive forces, which are transmitted throughout the cell wall interstices by water-water hydrogen bonding, can lead to very negative hydrostatic pressures in the cell wall. At 20°C the surface tension of water is 7.28 x 1CT8 MPa m (Appendix I), the voids between the microfibrils in the cell wall are often about 10 nm across (r = 5 nm), and cos a can equal 1 for wettable walls. For water in such cylindrical pores, Equation 2.25 indicates that when the contact angle is zero P would be... 

Fig. 2-l7a). This is an estimate of the negative hydrostatic pressure or tension that could develop in the aqueous solution within cell wall interstices of typical dimensions, supporting the contention that q cellwa11 can be markedly less than zero. Moreover, in such fine pores with hydrophilic... 

We will now show that the availability of water adjacent to that in wettable cell walls affects P0611 wal1 and the contact angle in the interstices. Suppose that pure water in mesophyll cell wall interstices 10 nm across is in equilibrium with water vapor in the intercellular air spaces where the relative humidity is 99%. As we calculated previously, VFWV for this relative humidity is -1.36 MPa at 20°C. Hence, at equilibrium the (pure) water in the cell wall interstices has a hydrostatic pressure of —1.36 MPa ( F = P - n -I- pwgh Eq. 2.13a). Using Equation 2.25 we can calculate the contact angle for which P can be -1.36 MPa for pores 5 nm in radius ... 

At the end of Chapter 1 (Section 1.5C) we indicated that cell enlargement requires yielding of the cell wall, an irreversible or plastic process that occurs when the internal hydrostatic pressure exceeds some critical or threshold value, />J,u.esh0id- This leads to another growth equation of the following form ... 

Water is conducted to and across the leaves in the xylem. It then moves to the individual leaf cells by flowing partly apoplastically in the cell walls and partly symplastically (only short distances are involved, because the xylem ramifies extensively in a leaf). The water potential is usually about the same in the vacuole, the cytosol, and the cell wall of a particular mesophyll cell (see values in Table 9-3). If this were not the case, water would redistribute by flowing energetically downhill toward lower water potentials. The water in the cell wall pores is in contact with air, where evaporation can take place, leading to a flow along the cell wall interstices to replace the lost water. This flow can be approximately described by Poiseuille s law (Eq. 9.11), which indicates that a (very small) hydrostatic pressure decrease exists across such cell walls. 

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