Turgor pressure

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Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. [1]

Contents

It is also called hydrostatic pressure, and is defined as the pressure in a fluid measured at a certain point within itself when at equilibrium. [2] Generally, turgor pressure is caused by the osmotic flow of water and occurs in plants, fungi, and bacteria. The phenomenon is also observed in protists that have cell walls. [3] This system is not seen in animal cells, as the absence of a cell wall would cause the cell to lyse when under too much pressure. [4] The pressure exerted by the osmotic flow of water is called turgidity. It is caused by the osmotic flow of water through a selectively permeable membrane. Movement of water through a semipermeable membrane from a volume with a low solute concentration to one with a higher solute concentration is called osmotic flow. In plants, this entails the water moving from the low concentration solute outside the cell into the cell's vacuole.[ citation needed ]

Mechanism

Turgor pressure on plant cells diagram.svg

Osmosis is the process in which water flows from a volume with a low solute concentration (osmolarity), [5] to an adjacent region with a higher solute concentration until equilibrium between the two areas is reached. [6] It is usually accompanied by a favorable increase in the entropy of the solvent. All cells are surrounded by a lipid bi-layer cell membrane which permits the flow of water into and out of the cell while limiting the flow of solutes. When the cell is in a hypertonic solution, water flows out of the cell, which decreases the cell's volume. When in a hypotonic solution, water flows into the membrane and increases the cell's volume, while in an isotonic solution, water flows in and out of the cell at an equal rate. [4]

Turgidity is the point at which the cell's membrane pushes against the cell wall, which is when turgor pressure is high. When the cell has low turgor pressure, it is flaccid. In plants, this is shown as wilted anatomical structures. This is more specifically known as plasmolysis. [7]

A turgid and flaccid cell Turgid.svg
A turgid and flaccid cell

The volume and geometry of the cell affects the value of turgor pressure and how it can affect the cell wall's plasticity. Studies have shown that smaller cells experience a stronger elastic change when compared to larger cells. [3]

Turgor pressure also plays a key role in plant cell growth when the cell wall undergoes irreversible expansion due to the force of turgor pressure as well as structural changes in the cell wall that alter its extensibility. [8]

Turgor pressure in plants

Turgor pressure within cells is regulated by osmosis and this also causes the cell wall to expand during growth. Along with size, rigidity of the cell is also caused by turgor pressure; a lower pressure results in a wilted cell or plant structure (i.e. leaf, stalk). One mechanism in plants that regulate turgor pressure is the cell's semipermeable membrane, which allows only some solutes to travel in and out of the cell, maintaining a minimum pressure. Other mechanisms include transpiration, which results in water loss and decreases turgidity in cells. [9] Turgor pressure is also a large factor for nutrient transport throughout the plant. Cells of the same organism can have differing turgor pressures throughout the organism's structure. In vascular plants, turgor pressure is responsible for apical growth of features such as root tips [10] and pollen tubes. [11]

Dispersal

Transport proteins that pump solutes into the cell can be regulated by cell turgor pressure. Lower values allow for an increase in the pumping of solutes, which in turn increases osmotic pressure. This function is important as a plant response under drought conditions [12] (seeing as turgor pressure is maintained), and for cells which need to accumulate solutes (i.e. developing fruits). [13]

Flowering and reproductive organs

It has been recorded that the petals of Gentiana kochiana and Kalanchoe blossfeldiana bloom via volatile turgor pressure of cells on the plant's adaxial surface. [11] During processes like anther dehiscence, it has been observed that drying endothecium cells cause an outward bending force which leads to the release of pollen. This means that lower turgor pressures are observed in these structures due to the fact that they are dehydrated. Pollen tubes are cells which elongate when pollen lands on the stigma, at the carpal tip. These cells undergo tip growth rather quickly due to increases in turgor pressure. The pollen tube of lilies have a mean turgor pressure of 0.21 MPa when growing during this process. [14]

Seed dispersal

Mature squirting cucumber fruit Mature squirting cucumber.jpg
Mature squirting cucumber fruit

In fruits such as Impatiens parviflora , Oxalia acetosella and Ecballium elaterium , turgor pressure is the method by which seeds are dispersed. [15] In Ecballium elaterium, or squirting cucumber, turgor pressure builds up in the fruit to the point that it aggressively detaches from the stalk, and seeds and water are squirted everywhere as the fruit falls to the ground. Turgor pressure within the fruit ranges from .003 to 1.0 MPa. [16]

Growth

Tree roots penetrating rock Tree growing out of rock in Coire Earb - geograph.org.uk - 853941.jpg
Tree roots penetrating rock

The action of turgor pressure on extensible cell walls is usually said to be the driving force of growth within the cell. [17] An increase of turgor pressure causes expansion of cells and extension of apical cells, pollen tubes, and other plant structures such as root tips. Cell expansion and an increase in turgor pressure is due to inward diffusion of water into the cell, and turgor pressure increases due to the increasing volume of vacuolar sap. A growing root cell's turgor pressure can be up to 0.6 MPa, which is over three times that of a car tire. Epidermal cells in a leaf can have pressures ranging from 1.5 to 2.0 MPa. [18] These high pressures can explain why plants can grow through asphalt and other hard surfaces. [17]

Turgidity

Turgidity is observed in a cell where the cell membrane is pushed against the cell wall. In some plants, cell walls loosen at a faster rate than water can cross the membrane, which results in cells with lower turgor pressure. [3]

Stomata

Open stomata on the left and closed stomata on the right Stomata opened and closed unlabelled.svg
Open stomata on the left and closed stomata on the right

Turgor pressure within the stomata regulates when the stomata can open and close, which plays a role in transpiration rates of the plant. This is also important because this function regulates water loss within the plant. Lower turgor pressure can mean that the cell has a low water concentration and closing the stomata would help to preserve water. High turgor pressure keeps the stomata open for gas exchanges necessary for photosynthesis. [9]

Mimosa pudica

Mimosa pudica Sismonastia de la Mimosa pudica.jpg
Mimosa pudica

It has been concluded that loss of turgor pressure within the leaves of Mimosa pudica is responsible for the plant's reaction when touched. Other factors such as changes in osmotic pressure, protoplasmic contraction and increase in cellular permeability have been observed to affect this response. It has also been recorded that turgor pressure is different in the upper and lower pulvinar cells of the plant, and the movement of potassium and calcium ions throughout the cells cause the increase in turgor pressure. When touched, the pulvinus is activated and exudes contractile proteins, which in turn increases turgor pressure and closes the leaves of the plant. [19]

Function in other taxa

As earlier stated, turgor pressure can be found in other organisms besides plants and can play a large role in the development, movement, and nature of said organisms.

Fungi

Shaggy ink caps bursting through asphalt due to high turgor pressure Shaggy Ink Caps busting through asphalt.jpg
Shaggy ink caps bursting through asphalt due to high turgor pressure

In fungi, turgor pressure has been observed as a large factor in substrate penetration. In species such as Saprolegnia ferax, Magnaporthe grisea and Aspergillus oryzae, immense turgor pressures have been observed in their hyphae. The study showed that they could penetrate substances like plant cells, and synthetic materials such as polyvinyl chloride. [20] In observations of this phenomenon, it is noted that invasive hyphal growth is due to turgor pressure, along with the coenzymes secreted by the fungi to invade said substrates. [21] Hyphal growth is directly related to turgor pressure, and growth slows as turgor pressure decreases. In Magnaporthe grisea , pressures of up to 8 MPa have been observed. [22]

Protists

Some protists do not have cell walls and cannot experience turgor pressure. These few protists use their contractile vacuole to regulate the quantity of water within the cell. Protist cells avoid lysing in hypotonic solution by utilizing a vacuole which pumps water out of the cells to maintain osmotic equilibrium. [23]

Animals

Turgor pressure is not observed in animal cells because they lack a cell wall. In organisms with cell walls, the cell wall prevents the cell from being lysed by high turgor pressure. [1]

Diatoms

In diatoms, the Heterokontophyta have polyphyletic turgor-resistant cell walls. Throughout these organisms' life cycle, carefully controlled turgor pressure is responsible for cell expansion and for the release of sperm, but not for processes such as seta growth. [24]

Cyanobacteria

Gas-vaculate[ check spelling ] cyanobacterium are the ones generally responsible for water-blooms. They have the ability to float due to the accumulation of gases within their vacuole, and the role of turgor pressure and its effect on the capacity of these vacuoles has been reported in varying scientific papers. [25] [26] It is noted that the higher the turgor pressure, the lower the capacity of the gas-vacuoles in different cyanobacteria. Experiments used to correlate osmosis and turgor pressure in prokaryotes have been used to show how diffusion of solutes into the cell affects turgor pressure within the cell. [27]

Measurements

When measuring turgor pressure in plants, many factors have to be taken into account. It is generally stated that fully turgid cells have a turgor pressure that is equal to that of the cell and that flaccid cells have a value at or near zero. Other cellular mechanisms to be taken into consideration include the protoplast, solutes within the protoplast (solute potential), transpiration rates of the cell and the tension of cell walls. Measurement is limited depending on the method used, some of which are explored and explained below. Not all methods can be used for all organisms, due to size or other properties. For example, a diatom does not have the same properties as a plant, which would place limitations on methods that could be used to infer turgor pressure. [28]

Units

Units used to measure turgor pressure are independent from the measures used to infer its values. Common units include bars, MPa, or newtons per square meter. 1 bar is equal to 0.1 MPa. [29]

Methods

Water potential equation

Turgor pressure can be deduced when the total water potential, Ψw, and the osmotic potential, Ψs, are known in a water potential equation. [30] These equations are used to measure the total water potential of a plant by using variables such as matric potential, osmotic potential, pressure potential, gravitational effects and turgor pressure. [31] After taking the difference between Ψs and Ψw, the value for turgor pressure is obtained. When using this method, gravity and matric potential are considered to be negligible, since their values are generally either negative or close to zero. [30]

Pressure-bomb technique

Diagram of a pressure bomb Pressurebomb.svg
Diagram of a pressure bomb

The pressure bomb technique was developed by Scholander et al., reviewed by Tyree and Hammel in their 1972 publication, in order to test water movement through plants. The instrument is used to measure turgor pressure by placing a leaf (with stem attached) into a closed chamber where pressurized gas is added in increments. Measurements are taken when xylem sap appears out of the cut surface and at the point which it doesn't accumulate or retreat back into the cut surface. [32]

Atomic force microscope

Atomic force microscopes use a type of scanning probe microscopy (SPM). Small probes are introduced to the area of interest, and a spring within the probe measures values via displacement. [33] This method can be used to measure turgor pressure of organisms. When using this method, supplemental information such as continuum mechanic equations, single force depth curves and cell geometries can be used to quantify turgor pressures within a given area (usually a cell).

Pressure probe

This machine was originally used to measure individual algal cells, but can now be used on larger-celled specimens. It is usually used on higher plant tissues but was not used to measure turgor pressure until Hüsken and Zimmerman improved the method. [34] Pressure probes measure turgor pressure via displacement. A glass micro-capillary tube is inserted into the cell and whatever the cell exudes into the tube is observed through a microscope. An attached device then measures how much pressure is required to push the emission back into the cell. [32]

Micro-manipulation probe

These are used to accurately quantify measurements of smaller cells. In an experiment by Weber, Smith and colleagues, single tomato cells were compressed between a micro-manipulation probe and glass to allow the pressure probe's micro-capillary to find the cell's turgor pressure. [35]

Theoretical speculations

Negative turgor pressure

It has been observed that the value of Ψw decreases as the cell becomes more dehydrated, [30] but scientists have speculated whether this value will continue to decrease but never fall to zero, or if the value can be less than zero. There have been studies [36] [37] which show that negative cell pressures can exist in xerophytic plants, but a paper by M. T. Tyree explores whether this is possible, or a conclusion based on misinterpreted data. He concludes that claims of negative turgor pressure values were incorrect and resulted from mis-categorization of "bound" and "free" water in a cell. By analyzing the isotherms of apoplastic and symplastic water, he shows that negative turgor pressures cannot be present within arid plants due to net water loss of the specimen during droughts. Despite this analysis and interpretation of data, negative turgor pressure values are still used within the scientific community. [38]

Tip growth in higher plants

A hypothesis presented by M. Harold and colleagues suggests that tip growth in higher plants is amoebic in nature, and is not caused by turgor pressure as is widely believed, meaning that extension is caused by the actin cytoskeleton in these plant cells. Regulation of cell growth is implied to be caused by cytoplasmic micro-tubules which control the orientation of cellulose fibrils, which are deposited into the adjacent cell wall and results in growth. In plants, the cells are surrounded by cell walls and filamentous proteins which retain and adjust the plant cell's growth and shape. It is concluded that lower plants grow through apical growth, which differs since the cell wall only expands on one end of the cell. [39]

Related Research Articles

<span class="mw-page-title-main">Cell wall</span> Outermost layer of some cells

A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier. Another vital role of the cell wall is to help the cell withstand osmotic pressure and mechanical stress. While absent in many eukaryotes, including animals, cell walls are prevalent in other organisms such as fungi, algae and plants, and are commonly found in most prokaryotes, with the exception of mollicute bacteria.

<span class="mw-page-title-main">Osmotic pressure</span> Measure of the tendency of a solution to take in pure solvent by osmosis

Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in its pure solvent by osmosis. Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from its pure solvent by a semipermeable membrane.

<span class="mw-page-title-main">Vacuole</span> Membrane-bound organelle in cells containing fluid

A vacuole is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the requirements of the cell.

Halotolerance is the adaptation of living organisms to conditions of high salinity. Halotolerant species tend to live in areas such as hypersaline lakes, coastal dunes, saline deserts, salt marshes, and inland salt seas and springs. Halophiles are organisms that live in highly saline environments, and require the salinity to survive, while halotolerant organisms can grow under saline conditions, but do not require elevated concentrations of salt for growth. Halophytes are salt-tolerant higher plants. Halotolerant microorganisms are of considerable biotechnological interest.

<span class="mw-page-title-main">Stoma</span> In plants, a variable pore between paired guard cells

In botany, a stoma, also called a stomate, is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange between the internal air spaces of the leaf and the atmosphere. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that regulate the size of the stomatal opening.

<span class="mw-page-title-main">Plasmolysis</span> Process by which cells lose water in a hypertonic solution

Plasmolysis is the process in which cells lose water in a hypertonic solution. The reverse process, deplasmolysis or cytolysis, can occur if the cell is in a hypotonic solution resulting in a lower external osmotic pressure and a net flow of water into the cell. Through observation of plasmolysis and deplasmolysis, it is possible to determine the tonicity of the cell's environment as well as the rate solute molecules cross the cellular membrane.

Lysis is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a lysate. In molecular biology, biochemistry, and cell biology laboratories, cell cultures may be subjected to lysis in the process of purifying their components, as in protein purification, DNA extraction, RNA extraction, or in purifying organelles.

Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and matrix effects such as capillary action. The concept of water potential has proved useful in understanding and computing water movement within plants, animals, and soil. Water potential is typically expressed in potential energy per unit volume and very often is represented by the Greek letter ψ.

<span class="mw-page-title-main">Cytorrhysis</span>

Cytorrhysis is the permanent and irreparable damage to the cell wall after the complete collapse of a plant cell due to the loss of internal positive pressure. Positive pressure within a plant cell is required to maintain the upright structure of the cell wall. Desiccation resulting in cellular collapse occurs when the ability of the plant cell to regulate turgor pressure is compromised by environmental stress. Water continues to diffuse out of the cell after the point of zero turgor pressure, where internal cellular pressure is equal to the external atmospheric pressure, has been reached, generating negative pressure within the cell. That negative pressure pulls the center of the cell inward until the cell wall can no longer withstand the strain. The inward pressure causes the majority of the collapse to occur in the central region of the cell, pushing the organelles within the remaining cytoplasm against the cell walls. Unlike in plasmolysis, the plasma membrane maintains its connections with the cell wall both during and after cellular collapse.

<span class="mw-page-title-main">Contractile vacuole</span> Organelle used in regulating osmosis

A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole.

Suction pressure is also called Diffusion Pressure Deficit. If some solute is dissolved in solvent, its diffusion pressure decreases. The difference between diffusion pressure of pure solvent and solution is called diffusion pressure deficit (DPD). It is a reduction in the diffusion pressure of solvent in the solution over its pure state due to the presence of solutes in it and forces opposing diffusion.

<span class="mw-page-title-main">Tonicity</span> Measure of water potential across a semi-permeable cell membrane

In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane-impermeable solutes across a cell membrane which determine the direction and extent of osmotic flux. It is commonly used when describing the swelling-versus-shrinking response of cells immersed in an external solution.

<span class="mw-page-title-main">Guard cell</span> Paired cells that control the stomatal aperture

Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells become turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.

<span class="mw-page-title-main">Pulvinus</span> Swollen or thickened leaf base

A pulvinus may refer to a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent movement. Pulvinus is also a botanical term for the persistent peg-like bases of the leaves in the coniferous genera Picea and Tsuga. Pulvinar movement is common, for example, in members of the bean family Fabaceae (Leguminosae) and the prayer plant family Marantaceae.

<span class="mw-page-title-main">Osmosis</span> Migration of molecules to a region of lower solute concentration

Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential to a region of low water potential, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a selectively permeable membrane separating two solutions of different concentrations. Osmosis can be made to do work. Osmotic pressure is defined as the external pressure required to prevent net movement of solvent across the membrane. Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.

Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of electrolytes to keep the body fluids from becoming too diluted or concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.

Stomatal conductance, usually measured in mmol m−2 s−1 by a porometer, estimates the rate of gas exchange and transpiration through the leaf stomata as determined by the degree of stomatal aperture.

Pavement cells are a cell type found in the outmost epidermal layer of plants. The main purpose of these cells is to form a protective layer for the more specialized cells below. The arrangement and undulating geometry of these cells are demonstrated to enhance the epidermal tear resistance by extending the path of cracks and hindering their progression along cell interfaces, thereby preserving the plant epidermal integrity. This layer helps decrease water loss, maintain an internal temperature, keep the inner cells in place, and resist the intrusion of any outside material. They also separate stomata apart from each other as stomata have at least one pavement cell between each other.

Leaf expansion is a process by which plants make efficient use of the space around them by causing their leaves to enlarge, or wither. This process enables a plant to maximize its own biomass, whether it be due to increased surface area; which enables more sunlight to be absorbed by chloroplasts, driving the rate of photosynthesis upward, or it enables more stomata to be created on the leaf surface, allowing the plant to increase its carbon dioxide intake.

Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.

References

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  33. ↑ Beauzamy, Lena (May 2015). "Quantifying Hydrostatic Pressure in Plant Cells by Using Indentation with an Atomic Force Microscope". Biophysical Journal . 108 (10): 2448–2456. Bibcode:2015BpJ...108.2448B. doi:10.1016/j.bpj.2015.03.035. PMC   4457008 . PMID   25992723.
  34. ↑ Hüsken, Dieter; Steudle, Ernst; Zimmermann, Ulrich (1 February 1978). "Pressure Probe Technique for Measuring Water Relations of Cells in Higher Plants". Plant Physiology. 61 (2): 158–163. doi:10.1104/pp.61.2.158. PMC   1091824 . PMID   16660252.
  35. ↑ Weber, Alain; Braybrook, Siobhan; Huflejt, Michal; Mosca, Gabriella; Routier-Kierzkowska, Anne-Lise; Smith, Richard S. (1 June 2015). "Measuring the mechanical properties of plant cells by combining micro-indentation with osmotic treatments". Journal of Experimental Botany . 66 (11): 3229–3241. doi:10.1093/jxb/erv135. PMC   4449541 . PMID   25873663.
  36. ↑ Yang, Dongmei; Li, Junhui; Ding, Yiting; Tyree, Melvin T. (1 March 2017). "Experimental evidence for negative turgor pressure in small leaf cells of Robinia pseudoacacia L versus large cells of Metasequoia glyptostroboides Hu et W.C. Cheng. 2. Höfler diagrams below the volume of zero turgor and the theoretical implication for pressure-volume curves of living cells". Plant, Cell & Environment . 40 (3): 340–350. doi: 10.1111/pce.12860 . PMID   27861986.
  37. ↑ Oertli, J.J. (July 1986). "The Effect of Cell Size on Cell Collapse under Negative Turgor Pressure". Journal of Plant Physiology. 124 (3–4): 365–370. Bibcode:1986JPPhy.124..365O. doi:10.1016/S0176-1617(86)80048-7.
  38. ↑ Tyree, M. (January 1976). "Negative Turgor Pressure in Plant Cells: Fact or Fallacy?". Canadian Journal of Botany . 54 (23): 2738–2746. doi:10.1139/b76-294.
  39. ↑ Pickett-Heaps, J.D.; Klein, A.G. (1998). "Tip growth in plant cells may be amoeboid and not generated by turgor pressure". Proceedings: Biological Sciences. 265 (1404): 1453–1459. doi:10.1098/rspb.1998.0457. PMC   1689221 .
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