Plant Cells and WaterChapter 1 (pp 1-17)

Assessment criteriaDescribe the role of hydrogen bonds in the unique physical and chemical properties of water.Analyse the biological importance of water by referring to the unique properties of water.Explain the mechanisms of water transport by comparing diffusion and osmosis and analysing the effect of water potential on water transport.State the role of aquaporins in facilitating cellular water movement.

The roles for which water is uniquely suited.Thermal propertiesThe thermal properties of water ensures that it is in the liquid state over the range of temperatures at which most biological reactions occur.Most reactions can occur only in an aqueous medium.Contribute to temperature regulation.

Solvent propertiesSuitable medium for the uptake and distribution of mineral nutrients and other solutes required for growth.Water in itself is either a reactant or a product in a large number of reactions.The transparency of water allows light to penetrate for photosynthesis or control development.

CellsThe uptake of water by cells generates a pressure known as turgor.Plants must maintain cell turgor in order to remain erect.The uptake of water by cells is also the driving force for cell enlargement.

Properties of waterWater consist of an oxygen atom covalently bonded to two hydrogen atoms.

ElectronegativeTendency to attract electrons

Polar molecule that forms hydrogen bonds.

Separation of charges creates a strong attraction (electrical) between adjacent water molecules or between water and other polar molecules

Bound waterHydration shells around ProteinsNucleic acidCarbohydrateLayers of tightly bound and highly oriented water moleculesPrevent aggregates that can lead to precipitation

Thermal properties of water are biologically importantThermal capacity of waterSpecific heatSpecific heat is the thermal capacity of a substance or the amount of energy that can be absorbed for a given temperature rise.Specific heat of water is 4.184Jg-1C-1

Thermal conductivityRapidly conducts heat away from point of application.

Water exhibits a high heat of fusion and heat of vaporizationThe energy required to covert a substance from the solid to the liquid state is known as heat fusion.The heat of fusion of water is one of the highest known, second only to ammonia.The high heat of fusion of water is attributable to the large amount of energy necessary to overcome the strong intermolecular forces associated with hydrogen bonding.

The density of ice is another important property. Water, unlike other substances, reaches its maximum density in the liquid state (near 4C), rather than as a solid.This occurs because molecules in the liquid state are able to pack more tightly than in the highly ordered crystalline state of ice.

Hydrogen bonding increases the amount of energy required to melt ice and also increases the energy required to evaporate water.The heat of vaporisation of water requires energy that is absorbed from its surroundings. The heat of vaporisation accounts for the pronounced cooling effect associated with evaporation.

Water is the universal solventWater has the ability to partially neutralise electrical attractions between charged solute molecules or ions by surrounding the ion or molecule with one or more layers of oriented water molecules, called a hydration shell.Hydration shells encourage solvation by reducing the probability that ions can recombine and form crystal structures.

Dielectric constantThe polarity of molecules can be measured by a quantity known as the dielectric constant. Water has one of the highest dielectric constants. Therefore excellent solvent for charged ions or molecules.Charged solutes important to plants, but do not readily cross cellular membranes due to the low dielectric constants of nonpolar molecules.

Polarity of water molecules results in cohesion and adhesionThe strong mutual attraction between water molecules resulting from hydrogen bonding is also known as cohesion. One consequence of cohesion is that water has an exceptionally high surface tension.Surface tension is the reason water drops tend to be spherical or that water surface will support the weight of small insects.

Cohesion is directly responsible for the high tensile strength of water. Tensile strength is the maximum tension that an uninterrupted column of any material can withstand without breaking.The same forces that attract water molecules to each other will also attract water to solid surfaces, a process known as adhesion. Adhesion is important in capillary rise of water in small-diameter conduits.

Water movement may be governed by diffusion or by bulk flowThe movement of water is passive process, but is indirectly dependent upon metabolic energy.Passive movement of water can be accounted for by bulk flow or diffusion.

Bulk flow is driven by hydrostatic pressureBulk flow occurs when an external force, such as gravity or pressure is applied. As a result all of the molecules of the substance move in a mass.Bulk flow also accounts for some water movement in plants.

Ficks first law describes the process of diffusionDiffusion is a directed movement from a region of a high concentration to a region of lower concentration.Bulk flow= pressure drivenDiffusion= driven by concentration difference.Ficks first law: F = -D.A.C.l-1

Ficks first law: F = -D.A.C.l-1F is the flux or amount of material crossing a unit area per unit time.D is the diffusion coefficient, the medium through which the diffusing molecule travels.A cross-sectional areaL length of the diffusion pathC is the concentration gradient (difference in concentration)- sign, diffusion is toward lower concentration

DIFFUSIONThere is no change in volume in either chamberWhat would happen if the dotted line was a selectively permeable memembrane

OSMOSISFree movement of solvent (water) Restricted movement of solute moleculeschange in volumeh

Plant cells contain an array of selectively permeable membranesCan you think of any?

Osmosis in plant cells is indirectly energy dependentw=w* + RT ln XwIncreasing the solute concentration in an aqueous solution decreases the mole fraction of water in the solution.As the mole fraction of water decreases, the chemical potential and hence the molar free energy of water decreases.

Plant cells control the movement of water in and out of cells by altering the solute concentration of the cytosol relative to the solution external to the cell.Root cells take up nitrate ions NO3 from the soil by active transport to create a NO3 gradient across the cell membrane.The uptake of NO3 ions is an active transport process and requires an input of energy.This decreases the mole fraction of water in the cytosol compared to the soil. Water will diffuse spontaneously.

Activity 1Use the diagram to explain why osmosis is, indirectly, an energy dependent process in plants.

The chemical potential of water has an osmotic as well as a pressure componentsolutessolutespressureDiffusion will continue until the force tending to drive water into the tube by the force generated by the hydrostatic head or the applied pressure.

The chemical potential of water may also be influenced by electrical potential and gravitational field.In spite of its strong dipole nature, the net electrical charge for water is zero and so the electrical term can be ignored.Where water movement involves heights of 5 to 10 metres or less, the gravitational term is commonly omitted.

Hydrostatic pressure and osmotic pressure are two components of water potentialWater potential is proportional to w - w* and can be defined as = P- .P is the hydrostatic pressure and is the osmotic pressure.The value for pure water is zero.

Water potential is the sum of its component potentials = p + sp pressure potential ( identical to P and represents the hydrostatic pressure)s osmotic potential equal to osmotic pressure also called the solute potential.Third component matric potential (M), is a result of the absorption of water to solid surfaces.

The osmotic potential of most plant cells is due primarily to the contents of the large central vacuole.Cell vacuoles contain on the order of 50 to 80 percent of the cellular water.Most of the remaining cellular water is located in the cell wall spaces, while the cytoplasm accounts for as little as 5 to 10%.

In a laboratory pressure (p) can be estimated as the difference between atmospheric pressure and the hydrostatic pressure generated by the height of the water column.The pressure component arises from the force exerted outwardly against the cell walls by the expanding protoplast. This is known as turgor pressure.

An equal but opposite inward pressure, called wall pressure, is exerted by the cell wall. A cell experiencing turgor pressure is said to be turgid.A cell that experiences water loss to the point where turgor pressure is reduced to zero is said to be flaccid.

Dynamic flux of H2O is associated with changes in water potentialIncipient plasmolysis is the condition in which the protoplast just fill the cell volume. Turgor pressure is zero and the water potential is equal to osmotic potential.

Hypotonic solution: Water will enter the cell as it moves down the water potential gradient. Net movement of water into the cell will cease when the osmotic potential of the cell is balanced by its turgor pressure and the water potential of the cell is zero.Hypertonic: More negative than the cell and favours loss of water from the cell. The protoplast then shrinks away from the cell wall, a condition known as plasmolysis.

Plasmolysis vs WiltingPlasmolysis can be studied in the laboratory simply by subjecting tissues to hypertonic solutions and observing protoplast volume changes under the microscope. Protoplast that pulls away from the cell wall leaves a void that is filled with external solution. Plasmolysis therefore does not give rise to negative pressure.

Wilting is the response to dehydration in air under natural conditions. Because of surface tension, water in the small pores of the cell wall resists the entry of air and the collapsing protoplast maintains contact with the cell wall. This tends to pull the wall inward and substantial negative pressure may develop.

Aquaporins facilitate the cellular movement of waterPorins are a class of membrane proteins that belong to a large family of proteins called major intrinsic proteins (MIPs) that are found in the cell membranes.Porin-type channels are nonselective cation channels.In plants porins are generally restricted to the outer membranes of mitochondria and chloroplasts.

Aquaporins are membrane protein channels or pores controlling the selective movement of water primarily.The presence of aquaporins do not affect the electrical conductance of a membrane which indicates that small ions such as H+ are not conducted by these membrane channels.

Aquaporins

The hydrophobic amino acids are on the outer side of the pore and interact with the hydrophobic fatty acids of the lipid bilayer whereas the hydrophilic amino acids are in the inner side of the pore and interact with water molecules as they move through the pore from one side of the membrane to the other.

Gating is the term used to describe this regulated opening and closing of these protein channels. Gating through PIPs can be controlled by cytoplasmic pH, the concentration of divalent cations such as Ca2+ as well as by aquaporin protein phosphorylation.The presence of aquaporins provides a low resistance pathway for the movement of water across a membrane.

Since aquaporins are gated, this provides greater control for the movement of water.The permeability of the tonoplast is two orders of magnitude greater that of the plasma membrane. Thus, this allows the vacuole to replenish or buffer the cytoplasm with water when the cell is exposed to hypertonic conditions. Aquaporins are important in regulating the osmotic properties of plant cells. This process is called OSMOREGULATION.

Two-component sensing/signalling systems are involved in osmoregulationUse the diagram to explain step-by-step the two-component sensory signalling of the enzyme histidine kinase.

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