this video will be covering key Concepts in the IB Biology topic of water we'll take a look at the molecular structure of water then discuss how this impacts its solvent ability and physical properties these give rise to its use as an aquatic habitat and the processes of cohesion and adhesion which lead to capillary action at the end of this video you should be able to draw a molecule of water and explain how it contributes to biological processes to understand why water is such an important molecule for life we first need to understand its molecular structure a water molecule is composed of two hydrogen atoms each coal bonded to the oxygen atom a calent bond is a type of bond formed when atoms share electrons here the oxygen and hydrogens share their electrons within the molecule because oxygen is more electr negative than hydrogen oxygen pulls the shared electrons in these calent bonds closer to itself this unequal distribution of electrons results in the oxygen atom having a partial negative charge while the hydrogen atoms acquire partial positive charges we represent this with this Greek lowercase Delta symbol here Additionally the water molecule has a bent shape with an angle of approximately 105° between the hydrogen oxygen hydrogen bonds this bent shape combined with the uneven electron distribution creates an overall dipole moment in the molecule making it polar since water is polar water molecules are attracted to each other through the force of hydrogen bonding hydrogen bonds form between a negative oxygen atom of one water molecule and a positive hydrogen atom of another molecule hydrogen bonds are strong intermolecular forces and help us understand how water behaves it gives water its unique properties such as its high boiling point and its role as a versatile solvent one of the important properties of water emerging from this polar structure is this solvent ability because water is polar other polar molecules are attracted to it so hydrophilic substances can dissolve in water as they form hydrogen bonds with the water molecules this gives water a very powerful solvent ability this is one of the reasons why water is such a useful medium for Life while the mechanism for the origin of the first cells is unclear it occurred 3.8 billion years ago at a time when most of the Earth's surface was covered in primordial oceans in these oceans critical compounds came together to form polymers needed to build cells water was the perfect medium for this because the enzymes required to catalyze these reactions function in aquous environments and the molecules can move freely in water to collide with one another and start the reactions we still see this in modern-day cells the cytool is the fluid encased by the cell membrane it's composed of around 70% water and is the site of most chemical reactions conducted by the cell however this solvent ability is complemented by the fact that hydrophobic substances cannot dissolve into water instead they repel water giving rise to new properties I for instance lipids are hydrophobic and insoluble cholesterol is an example of a key lipid that's required for cells as it affects the fluidity of the membrane because of its insolubility it can be embedded into the equally hydrophobic core of the plasma membrane instead of being repelled however when it comes to transport cholesterol needs to be packaged into lipoproteins like LDL and HDL cholesterols as a hydrophobic molecule it won't dissolve into blood the main component of blood is plasma which is about 90% water meaning that it can dissolve hydrophilic molecules for transport but requires hydrophobic molecules to be repackaged into a hydrophilic container water has physical properties too aquatic habitats like Marine environments benefit from several of these physical properties let's take for example the habitat of the Arctic sea the ringed seal lives within the water whereas the black throated Lon lives in the coastal environment four physical properties of water that impact this habitat are its specific heat capacity thermal conductivity buoyancy and viscosity the specific heat capacity of a substance is the amount of heat required to change the temperature of a substance by 1° C the Practical effect this has is is that it reflects the amount of energy a substance can absorb water for example has a high specific heat capacity it can absorb a significant amount of heat from the surrounding environment then release it slowly as the environmental temperature decreases the ability to absorb and release heat is a very useful property to Marine environments over the warmer summer months heat from the Sun and air is absorbed by the ocean stored in the Water by breaking the hydrogen bond that hold it together then as the temperature cools in the winter the hydrogen bonds reform slowly and release that energy and heat over time this coolant property allows bodies of water to create warmer microclimates as it buffers the surrounding area from the cold allowing the ringed seal and black throated loon to live comfortably thermal conductivity is similar to specific heat capacity in that it's one of the thermal properties of water however thermal conductivity is the ability of a substance to conduct or transfer heat relative to air water also has a high thermal conductivity in water the strong hydrogen bonding Network facilitates rapid energy transfer leading to higher thermal conductivity heat transfers into and out of water quickly so as a warm-blooded mammal the ringed seal needs to insulate itself well against heat loss while swimming resulting in its thick layer of blubber air is primarily composed of non-polar molecules like nitrogen and oxygen which have weaker intermolecular forces heat transfer in air occurs through collisions between relatively few and weakly interacting gas molecules resulting in lower thermal conductivity the black throated loon living predominantly in the land and Air does not lose as much heat to its environment relative to a seal living in water buoyancy is the upward force exerted by a fluid which acts against the downward force of gravity buoyancy is determined by the density of a medium water is more dense than air so this upward force allows objects and organisms to float in the ocean this uplifting Force therefore allows the ringed seal to swim more easily and spends less energy on keeping itself afloat ultimately air has no buoyancy so in order for the lon to move it needs to power its own lift Force such that it's higher than gravitational force this takes more effort and energy furthermore the SEAL swimming is more efficient because of water's viscosity this is the resistance a liquid has to flow as a liquid water is viscous where air is not so it provides resistance when the ringed seals flippers push against it propelling the seal forward when the Loon uses its webbed feet to swim it uses the same viscosity to its Advantage relative to other liquids however water has a low viscosity which means significant energy is not needed for organisms to move through water when we talked about water's molecular structure we discussed how hydrogen bonds form between the positive hydrogen atoms and the negative oxygen atoms of two different molecules this results in polar molecules having forces of attraction giving rise to a phenomenon called cohesion cohesion occurs when molecules of the same polar substance are attracted to one another as a consequence because water molecules link using hydrogen bonds surface tension occurs surface tension is the elasticity of a liquid surface it can resist external forces like a light insect moving across it or a paper clip being rested on top because of this elasticity see these hydrogen bonds contain energy this energy is what produces the cohesive effect because it takes a greater amount of energy in order to break the bonds and separate the water molecules disrupting the surface of the water so if this insect distributes its small weight equally over the surface of the water it can move across without breaking the bonds this creates a thriving habitat on water surfaces composed of insects algae frog eggs water spiders and many other organisms another Force observable in polar molecules is adhesion where cohesion occurs when two molecules of the same substance are attracted to one another adhesion occurs when molecules of different substances form bonds this attraction can be greater than equal to or less than the cohesive force adhesion also plays an important role in biological processes for example adhesion occurs in the xylem of plants xylem are a type of vascular tissue long thin tubes that extend all the way from The Roots up through the stem and into the leaves xylm transport water that has been absorbed from the soil into the roots and adhesion is key to this process see the walls of the xylm are plant cell walls composed primarily of cellulose cell cellulose has a number of hydroxy groups that readily form hydrogen bonds with water molecules and this attraction causes water to stick to the walls of the xylm in other words cellulose is hydrophilic or attracted to water so even though water is moving up the xylem Against Gravity this adhesion helps prevent it from falling down this force of adhesion is complemented by cohesive forces as well resulting in a phenomenon called capillary action capillary action is when a liquid like water moves upwards in a thin tube against the force of gravity we've explained how adhesion between water molecules and the xylm walls contributes to this process but cohesion also plays a role because the water molecules are attracted to each other when one molecule is pulled upwards the next molecule will follow and so on this is often called the beads on a string model because the negative pressure at the top of the xylem caused by evaporation pulls the water molecules up just like beads on a string you can see this occurring here adhesion causes the water molecules to stick to the xylm walls before cohesion causes them to follow each other eventually sticking to the wall higher up the tube this repeats until the water exits the xylm into the leaves where it evaporates through pores called stomata and moves out into the atmosphere this is also called the cohesion tension Theory and it's the mechanism behind transpiration in Plants capillary action in soil is a critical process also influenced by the adhesive properties of water in the context of soil adhesion refers to the attraction between water molecules and the surfaces of soil particles soil particles which can be composed of sand silt clay and organic matter have various surface properties that can attract and hold water molecules the adhesive forces between the water molecules and the soil particles are responsible for the initial wetting of the soil another way to describe capillary action is the movement of water within the spaces of a poorest material due to the forces of adhesion cohesion and surface tension when water is present in the soil it moves through the tiny spaces between soil particles due to these forces we can think of these spaces as capillaries like the tubes of the xylem adhesion causes the water to cling to the soil particles while cohesion helps pull more water molecules along with it the finer the soil particles the smaller the capillary spaces which enhances capillary action surface tension contributes to the soil's ability to retain water by holding it in the poor spaces against the force of gravity this combination of processes allows water to move upward Against Gravity in the soil ensuring that water can reach plant roots even when the water table is lower than the root Zone this maintains the soil moisture which is vital for seed germination root development plant hydration and nutrient uptake in this video we leared that water is a polar molecule that can form hydrogen bonds with other polar molecules this gives water powerful hydrophilic solvent abilities so aquous substances can conduct many reactions we talked about this in the context of the cytool in cells water has a high specific heat capacity buoyancy viscosity and thermal conductivity making it an ideal habitat for marine organisms two examples of these organisms are the ringed seal and black throated loon since water can form hydrogen bonds with itself this gives it cohesive properties as the water molecules are attracted to each other causing surface tension water also displays adhesion where it's attracted to solid substances this cohesion and adhesion gives rise to capillary action where water will move upwards Against Gravity in a thin tube this is seen in plant syum and in soil together these attributes make water a fundamental and dynamic component of both biological systems and the natural environment