Water Properties and Functions PDF

Water Molecular structure of water Water is made up of two hydrogen atoms covalently bonded to an oxygen atom Oxygen has a high electronegativity and attracts shared electrons more strongly This results in electrons orbiting closer to the oxygen atom – creating polarity Hydrogen bonds The dip...

Water Molecular structure of water Water is made up of two hydrogen atoms covalently bonded to an oxygen atom Oxygen has a high electronegativity and attracts shared electrons more strongly This results in electrons orbiting closer to the oxygen atom – creating polarity Hydrogen bonds The dipolarity of a water molecule allows it to form polar associations with other charged molecules (polar or ionic) Hydrogen bonds are particularly strong polar associations that form between hydrogen and either fluorine, oxygen or nitrogen This is because H atoms have a very weak electronegativity, while F, O & N atoms have a very strong electronegativity Water molecules will collectively form hydrogen bonds (H + O) Electronegativity 1 Hydrogen bonds in different states of water Hydrogen bonds in DNA 2 Cohesion & adhesion Water has the ability to form intermolecular associations with other molecules that share common properties Cohesion: Water interacts with other water molecules (it can form intermolecular hydrogen bonds) Adhesion: Water interacts with polar / ionic molecules (it will not interact with non-polar molecules) This is important for surface tension and capillary action Water surfaces as habitats 3 Transport of water in xylem Capillary action in soil and plant cell walls 4 Water as a solvent Water is considered the universal solvent – it can dissolve many substances that contain electronegative atoms (all polar and charged / ionic substances) This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken electrostatic forces and result in the dissociation of atoms Substances that freely associate and readily dissolve in water are hydrophili Hydrophilic substances include all polar molecules and ions Substances that do not freely associate or dissolve in water are hydrophobic Hydrophobic substances include non-polar molecules (such as fats and oils) 5 Amphipathic molecules are chemical compounds that have both polar and nonpolar regions, giving them both hydrophilic and hydrophobic properties. Water as the medium for transport The transport of materials within the blood will depend upon their solubility Water Soluble Substances: Sodium chloride (ionic) and glucose (polar) are freely transported in the blood Amino acids are transported in an ionized state Oxygen is soluble in low amounts – generally transported in red blood cells Water Insoluble Substances: Lipids (cholesterol, fat) are packaged with proteins (lipoproteins) for transport In plants, water is the basis of the fluid (sap) found in two transport systems: Xylem transports mineral ions Phloem transports sucrose and other products of photosynthesis 6 Physical properties of water Physical properties of water (as compared e.g. to air) have major consequences for aquatic organisms. These properties include: Buoyancy an upward force exerted by a fluid that opposes the weight of an immersed object Viscosity the measure of the fluid’s resistance to deformation Thermal conductivity the measure of how easily heat flows through a material Specific heat capacity the amount of energy to raise the temperature of 1 kg of a substance by 1 °C 7 Water as coolant Water has the ability to absorb significant amounts of heat before changing state This is due to the extensive hydrogen bonding between water molecules – these all need to be broken (via heat energy) before a change in state occurs Thus water is a highly effective coolant, making it a principal component of sweat Other physical properties of water Water is transparent, allowing light to pass right through it Important for photosynthesis to occur in aquatic plan Water is less dense as a solid (ice) than in its liquid form Water adopts a lattice formation (due to hydrogen bonds) This results in more space existing between water molecules Consequently, ice is lighter than water (i.e. it will float) This is important for life on earth as floating ice will insulate bodies of water from freezing air temperatures (>0°C) 8 The origin of water on Earth Earth formed too close to the Sun to harbour liquid water, hence it must have extraplanetary origins Its most likely source were asteroids Some meteorites (e.g. carbonaceous chondrites) have the hydrogen isotope composition similar to Earth’s seawater Chemical properties of carbon In organic molecules, a carbon atom can form up to four covalent bonds (or two single and one double bond) with other carbon atoms or atoms of other non-metallic elements A covalent bond is formed by sharing a pair of electrons between two atoms The strength of the covalent bonds ensures the stability of organic molecules covalent bond is the strongest bond between atoms. Chemical properties of carbon 9 The carbon-based metabolism Metabolism is the totality of chemical reactions that occur in a cell or organism Metabolic processes Metabolic reactions are controlled by enzymes. Metabolic reactions serve three key functions: They provide a source of energy for cellular processes (growth, reproduction, homeostasis, etc.) They enable the synthesis and assimilation of new materials for use in the cell They enable the elimination of waste Within metabolism, we may distinguish anabolic and catabolic reactions Anabolism Anabolic reactions build up complex molecules from simple ones Includes the formation of macromolecules (polymers) from monomers The synthesis of macromolecules involves condensation reactions Monomers covalently joined and water produced as by-product Catabolism Catabolic reactions break down complex molecules into simple ones Includes the release of monomers from macromolecules The breakdown of macromolecules involves hydrolysis reactions 10  Covalent bonds are broken via the consumption of water Central metabolic pathways Main classes of biomolecules Most organic macromolecules are polymers composed of monomers 11 Carbohydrates Monosaccharides Carbohydrates function primarily as a short-term energy source Monosaccharides can be joined together via condensation reactions to form more complex carbohydrates such as disaccharides and polysaccharides Disaccharides and polysaccharides can be broken down into monosaccharides via hydrolysis reactions Hexoses and pentoses 12 Monosaccharides differ in the number of carbon atoms Hexoses (e.g. glucose, fructose, galactose) contain 6 C atoms Pentoses (e.g. deoxyribose, ribose) contain 5 C atoms Chains and rings Pentoses and hexoses are unusual in that they can exist in straight-chain form as well as in the ring form.They need to be in the ring form in order to form disaccharides and polysaccharides. 13 Properties and uses of glucose Glucose is one of the most important biomolecules, a substrate of cellular respiration, a product of photosynthesis, and a monomer in various di- and polysaccharides. It has the following crucial properties: Molecular stability thanks to stable covalent bonds High solubility in water as glucose is a polar molecule Easily transportable in blood and in fluids between cells Yields a great deal of chemical energy in catabolic reactions (oxidation) Examples of disaccharides Examples of polysaccharides 14 Polysaccharide branching Polysaccharide as energy storage compounds Branching and coiling of glycogen and amylopectin during polymerization makes them compact and thus efficient as storage compounds At the same time, due to their large molecular size they are relatively insoluble in water Structure of cellulose 15 Cellulose as a structural polysaccharide 16 Comparison of polysaccharides Glycoproteins in cell-cell recognition Carbohydrates can be linked with proteins forming glycoproteins The ABO blood system is based on the presence of glycoprotein antigens in the red blood cell membranes They enable the immune system to distinguish between self and non-self 17 Lipids Lipids are a class of organic molecules that do not dissolve well in polar solvents such as water. This is due to the presence of relatively many non-polar covalent C-C and C-H bonds in their structure Types of lipids Triglycerides – Function as a long-term energy source in animals (fats) and plants (oils) Phospholipids – Structural component of cell membranes Steroids – Act as hormones in plants and animals, and are structural components of animal cell membranes (cholesterol) Waxes – Act as a protective layer against water loss in plant leaves and animal skin Carotenoids – Light-absorbing accessory pigment in plants (involved in photosynthesis) Glycolipids – Complexes of carbohydrate and lipid that function as cell receptor and cell recognition molecules 18 Triglyceride formation Triglycerides (fats and oils) function primarily as a long-term energy source. They are formed via condensation between glycerol and three fatty acid chains 19 Types of fatty acids Fatty acids form hydrocarbon chains found in certain types of lipids such as triglycerides and phospholipids. Saturated fatty acids do not possess any double bonds Linear in structure, often solid at room temperature Unsaturated fatty acids possess double bonds Structure may be bent, liquid at room temperature Single (mono-) or multiple (poly-) double bonds Cis and trans unsaturated fatty acids Unsaturated fatty acids may adopt one of two isomeric forms – cis and trans These distinct structural configurations have different chemical properties 20 Triglycerides in adipose tissue In animals, triglycerides are stored in adipose tissue. They are insoluble in water, forming a droplet within adipose cell cytoplasm They release twice as much energy per gram in cell respiration as carbohydrates They conduct heat poorly, so they can serve as a thermal insulator They are liquid at body temperature, so they can act as a shock absorber (e.g. around the kidneys) 21 Thermal insulation in marine mammals Phospholipid bilayers Due to their amphipathic properties, phospholipids form bilayers in water Hydrophobic tails extend towards each other to minimize contact with the aqueous solutions inside and outside the cell Hydrophilic heads arrange themselves on the outside of the bilayer 22 Transport through the cell membrane Steroid hormones Some steroids act as hormones which trigger various responses in the body via chemical signalling. Thanks to their hydrophobic properties, steroid hormones can pass freely through cell membranes of the target cells. Human sex hormones testosterone and oestradiol belong to this group 23 24