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Plant Physiology - Lecture 2.docx

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University of Toronto Scarborough
Biological Sciences
Connie Soros

Plant Physiology – Lecture 2  Water is essential in the life cycle of plants  Since photosynthesis utilizes CO2 that enters the leaves via open stomates on the lower leaf surface, the leaves are also exposed to water loss and the threat of dehydration through the common diffusional pathway  Water must be absorbed by the roots and transported through the plant body to prevent leaf desiccation  Balancing the uptake, transport, and loss of water to the atmosphere is an important challenge for plants  Most (approx. 97%) of the water absorbed by plant roots evaporates from leaf surfaces – this process is called transpiration  Of the remaining water, about 2% is used for growth and the last 1% is consumed in the biological reactions of photosynthesis and other metabolic processes TURGOR PRESSURE  Remember that plant cells have cell walls which animal cells do not  The presence of cell walls has an important impact on water balance within the plant  Cell walls allow plant cells to build up large internal hydrostatic pressures called turgor pressure  In a plant cell, the turgor pressure pushes the plasma membrane against the rigid cell wall and provides a force for cell expansion THE STRUCTURE AND PROPERTIES OF WATER  The polarity and tetrahedral shape of water molecules give them hydrogen bonding ability which in turn gives water its unique physical properties  For example, it is an excellent solvent, has a high specific heat, an unusually high latent heat of vaporization and a high tensile strength HYDROGEN BONDING  The one O2 and two H2 atoms of water are connected by covalent bonds (a chemical bond that involves sharing of pairs of electrons between atoms)  The oxygen atom has a higher electronegativity than the hydrogen atom and attracts the electrons of the covalent bond resulting in a partial negative charge at the oxygen end of the molecule and a slightly positive charge at each hydrogen  As a result, water is a polar molecule (negative in the centre and positive at the ends, with an unequal sharing of electrons causing the “bent” structure)  Because of the charge and structure, water also can form an unusually large number of intermolecular hydrogen bonds (four) for a molecule of its size. One at each hydrogen atom and two at the oxygen atom.  These strong attractive forces between molecules of water give rise to water’s potency as a solvent, high surface tension and capillary forces THERMAL PROPERTIES OF WATER  The extensive hydrogen bonding between water molecules results in water having both a high specific heat capacity and a high latent heat of vaporization  Specific heat capacity is the heat energy required to raise the temperature of a substance by a set amount  When the temperature of water is raised, the water molecules vibrate faster and stronger (increased molecular kinetic E) and the hydrogen bonds act like rubber bands to absorb some of the heat energy, leaving less energy available to increase motion  By doing this, water requires a much larger heat input than many other liquids to raise its temperature. This is important for plants because it helps to buffer temperature fluctuations.  The latent heat of vaporization is the energy required to separate molecules from the liquid phase and move them to the gas phase, without a change of temperature, a process that occurs during transpiration  Transpiration is the process of water evaporation from plant foliage, mostly from stomata. This process prevents the plant tissue from overheating and provides the moisture necessary to diffuse carbon dioxide into plant cells and enable the emission of oxygen WATER AS A SOLVENT  Water is an excellent solvent and is often referred to as the universal solvent. It can dissolve greater amounts of a wider variety of substances than other related solvents.  Ionic substances and polar molecules dissolve particularly well in water, e.g., salts, sugars, (eg. Those that contain polar –OH or –NH2 groups) acids, alkalis, and some gases – especially oxygen and carbon dioxide, these are known as hydrophilic (water-loving) substances, while those that are immiscible with water (e.g., fats and oils), are known as hydrophobic (water-repelling) substances.  Most of the major components in cells (proteins DNA and polysaccharides) are easily dissolved in water.  This property is also due to the hydrogen bonding ability and polar structure of water, especially for such a small molecule ­ Example: Dissolving a salt in water- Table salt (NaCI) – is a strong ionic crystal. The ionic bond is formed because of the electrostatic attraction between oppositely charged ions. In ions, electrons of one atom are transferred to another atom. The atom that loses the electrons becomes a positively charged ion, or a cation, while the one that gains electrons becomes a negatively charged ion, or an anion. When an ionic compound is dissolved by stirring it in water, the positive poles of the water molecules are attracted to the anions. While the negative poles of other water molecules are attracted to the cations, so the polar water molecules “pull” the ions out of the crystal. As a result, the ionic bonds eventually break and ions are released into the water When the salt is dissolved, every if hydration of the salt, the crystal breaks apart (dissolves), releasing the ions into the water Therefore, when table salt (NaCI) is dissolved, two hydrated ions appear in the water: a positively cation Na+ and a negative charged anion CI-. Each is surrounded with a shell of closely attracted water molecules that prevent the ions from forming into a crystal again COHESIVE CHARACTERISTICS OF WATER  Cohesion is the mutual attraction between water molecules as a result of extensive hydrogen bonding  A related property is adhesion, which is the attraction of water to a solid phase such as a cell wall or glass surface, again due to the formation of hydrogen bonds  Surface tension is a force exerted by water molecules at the air-water interface, resulting from the cohesion and adhesion properties of water molecules. This force minimizes the surface area of the air-water interface ­ Surface tension is a property that allows the surface of liquid to resist external forces. ­ The cohesion forces in liquid molecules cause surface tension because each water molecule is pulled in every direction by neighbouring water molecules with a net force of 0. ­ On the
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