Unit Biomolecules - Water

WATER WATER H Water is made up of two hydrogen atoms covalently bonded to an oxygen atom (molecular formula = H2O). Liquid water is essenCal for life to exist. The cytosol of a cell is predominantly composed of water (>70%), which allows for solutes to be dissolved and able to interact. Water is also an important metabolite within cells and is involved as either a substrate or a product in essenCal processes such as hydrolysis reacCons, photosynthesis and cell respiraCon. Water also funcCons as a temperature buffer, maintaining a homeostaCc temperature range in cells. H O Water (H2O) POLARITY While covalent bonding involves the sharing of electrons, they are not shared equally between the atoms in water, resulCng in a slight difference in charge between the poles of the water molecule (creaCng polarity). Oxygen has a comparaCvely high electronegaCvity (greater affinity for electrons) and aVracts the electrons more strongly than hydrogen, so oxygen becomes slightly negaCve (d–) and hydrogen slightly posiCve (d+). COVALENT – NO CHARGE (electrons shared equally) POLARITY – SLIGHT CHARGE (unequal electron sharing) IONIC – FULLY CHARGED (gain or loss of electrons) HYDROGEN BONDING Hydrogen bonds are relaCvely strong polar associaCons that form between a d+ hydrogen atom and a d– fluorine, oxygen or nitrogen atom on. In water, hydrogen bonding will occur between the oxygen and hydrogen dipoles of different water molecules. Hydrogen bonds are constantly forming, breaking and reforming based on the moCon of the water molecules. O O H H H O H bond H H H PROPERTIES OF WATER Water has several unique properCes which contribute to its biologically significant role as the medium for life. These properCes arise primarily due to the polarity of water and its capacity to form hydrogen bonds. • Water has solvent properCes (it is the universal solvent and can dissolve charged and polar substances) • Water has cohesive and adhesive properCes (water can form associaCons with other water molecules) • Water has several unique thermal properCes (water has the highest specific heat capacity of any liquid) • Water has certain physical properCes (its overall density contributes towards its buoyancy and viscosity) 1. SOLVENT PROPERTIES H δ– Na+ H O O H H δ– δ– O H H O H H δ+ δ+ δ+ Cl H O H A solvent is a substance that dissolves another substance (the solute) to form a soluCon. Water is commonly referred to as the universal solvent due to its capacity to dissolve a large number of substances. Due to its polarity, water can associate with any substance that is charged (ions) or has electronegaCve atoms (polar). The polar regions of a water molecule associate with molecular surfaces that have an opposing charge, forming dispersive hydraBon shells. Even though polar associaCons are significantly weaker than ionic bonds, sufficient quanCCes of water molecules can dissociate ionic compounds. A substance that is soluble in water is described as hydrophilic (‘water-loving’), while substances that do not dissolve in water (non-polar substances – such as lipids) are consequently described as being hydrophobic (‘water-haCng’). δ+ – δ+ δ+ H H O HydraCon Shells The capacity of water to dissolve a large variety of substances makes it an important medium for metabolic reacBons. Solutes that are dissolved to form aqueous soluCons are more likely to collide with enzymes and undergo necessary chemical reacCons. Water is also a reagent and a product for many metabolic processes that occur within a cell (condensaCon reacCons produce water, while hydrolysis reacCons consume water). The solvent properCes of water make it an important transport medium within mulCcellular organisms. In animals, the blood plasma transports dissolved solutes. In vascular plants, mineral ions are transported via xylem vessels while dissolved nutrients are transported via the phloem. Substances that are insoluble in water cannot be transported freely and are typically complexed to soluble components (in animals, lipids are packaged with proteins to form water-soluble lipoproteins that can then be transported via the blood). 2. COHESIVE PROPERTIES Water is cohesive as water molecules can sCck together by forming intermolecular hydrogen bonds. These cohesive properCes are biologically significance as they contribute towards water having one of the highest surface tensions of all liquids. At the fluid surface, water molecules can only form hydrogen bonds with the internal molecules. This creates an inward force that gives the fluid surface an elasCc tendency and causes the surface molecules to cohere more strongly, allowing small insects to glide along the surface of water. Surface molecules only aVracted to one side creaCng surface tension Internal molecules equally aVracted in all direcCons 3. ADHESIVE PROPERTIES Water is adhesive as water molecules can sCck to other molecules with similar properCes (ionic or polar). The adhesive properCes of water are what causes it to form a meniscus when placed in a glass measuring cylinder. The water adheres to the sides of the cylinder, creaCng a central depression. This does not occur in plasCc measuring cylinders as the plasCc is non-polar (hydrophobic) – the water will form a flat surface. Water a spherical Waterforms forms a spherical shape a hydrophobic shapeonon a hydrophobic surface (reduce contact) surface (reduce contact) Water forms a fla:er Water forms a fla+er shape hydrophilic shape on on hydrophilic surfaces adhesion) surfaces (h (h adhesion) The aVracCon of water to charged or polar surfaces allows it to flow in opposiCon to gravitaConal forces when under tension. This is known as capillary acBon. The strength of the capillary acCon is dependent on the diameter of the pore the water is moving through (smaller diameter = more capillary acCon). Capillary acCon allows water to be transported up the stems of plants via xylem vessels. The evaporaCon of water from the leaves creates a negaCve pressure gradient that pulls water molecules towards the leaf due to the cohesive properCes of water molecules. The water molecules are able to adhere to the walls of the xylem because the cell walls of the xylem vessels are made of cellulose – which is a polysaccharide that is polar. 4. THERMAL PROPERTIES Water has the capacity to absorb significant amounts of thermal energy (heat) before changing state. This is due to the extensive hydrogen bonding between water molecules – these bonds must be broken before water changes state and this requires heat absorpCon. Consequently, water has the following properCes: • Water has a relaCvely high mel/ng point (0ºC) and boiling point (100ºC) compared to similar liquids • Water has a very high specific heat capacity (the energy required to heat 1 gram of a substance by 1ºC) • Water has a high heat of vaporisa/on (the energy absorbed per gram to change from a liquid to a gas) • Water has a high heat of fusion (amount of energy lost when 1g of liquid becomes 1g of solid at 0ºC) ⇒ Hot skin produces sweat ⇒ ⇒ ⇒ Consequently, water is an excellent medium for living organisms as it is slow to change temperature and thus supports the maintenance of constant condiCons (both internal and external). One specific benefit of the thermal properCes of water is that it makes an excellent coolant. Many animals employ sweaCng as a means of cooling down. The evaporaCon of sweat requires significant inputs of thermal energy (due to the high specific heat capacity). As body heat is used to convert sweat into vapour, the organism begins to cool. ⇒ Heat causes evapora5on Heat absorbed, skin cool Another property of water is that it has high thermal conducBvity. Thermal conducCvity is a measure of a medium’s ability to move heat across a temperature gradient. Water will absorb and transfers heat more readily than air because the water parCcles are packed more Cghtly together (due to polarity and hydrogen bonding). The thermal conducCvity of water allows a body to transfer heat effecCvely via the blood plasma, allowing for thermoregulaCon. However, aquaCc organisms will lose heat more quickly in cold water as the water will rapidly transfer body heat into the surrounding water (this may potenCally lead to hypothermia). 5. PHYSICAL PROPERTIES The capacity for water to form polar associaCons results in both buoyancy and viscosity. Buoyancy is the upward force applied to an object in a medium and is determined by the density of the medium (higher density results in greater buoyancy). As water forms hydrogen bonds, it is denser than other substances (such as organic solvents and air). The buoyancy of water can be further increased by forming a soluCon (salt water is more buoyant than fresh water). The buoyancy of water allows certain organisms to float. As hydrogen bonding makes water denser, it also makes it more viscous than other substances. Viscosity is a measure of a fluid’s tendency to flow. Hydrogen bonding between water molecules will generate fricCon and reduce flow rates. The presence of cells and dissolved solutes within blood plasma increases viscosity. WATER VERSUS AIR The physical and thermal properCes of water have significant consequences for how organisms survive in terrestrial or marine habitats. The different properCes of water and air present disCnct survival challenges. Thermal ConducCvity Specific Heat Capacity Buoyancy Viscosity Air Low (0.024WmK) Low (1.005 J/g.ºC) Low (less dense) Low (less viscous) Water High (0.58WmK) High (4.186 J/g.ºC) High (1000´ denser) High (50´ viscous) Biological significance Heat is lost more rapidly in water Water temperature is harder to change Objects more likely to float in water Water is more resistant to flow AQUATIC ADAPTATIONS The black-throated loon (Gavia arc/ca) and the ringed seal (Pusa hispida) both live in the ArcCc where they interact with both water and air. Both of these organisms have adapted to the physical properCes of water. Black-Throated Loon While most birds have hollow bones (for flight), the loon has solid bones that are less dense than the bones found in most other terrestrial organisms. This allows the loon to dive under water in search of prey, while sCll retaining the capacity to float (buoyancy). The loon’s legs are located at its rear, making it hard to walk on land but very effecCve at propulsion in water (more viscous). The loon’s feathers form an interlocking structure that funcCons as a barrier to water, so as to prevent heat loss (water has a high thermal conducCvity). Ringed Seal The ringed seal has denser bones than the loon, allowing it to stay submerged for longer periods of Cme (less buoyancy). The seals have a streamlined body shape to move efficiently through water (higher viscosity than air). The ringed seal has an outer coat of fur that traps air for waterproofing and also has a thick layer of blubber to prevent heat loss while in water. They do not possess many effecCve physiological cooling mechanisms as water temperatures are generally stable (water has the highest specific heat capacity of any liquid). This makes ringed seals parCcular vulnerable to the effects of climate change.