so no review of biochemistry would be complete without spending some time talking about water water is a really interesting molecule it's inorganic we tend to think of the chemistry of living things be about cart molecules that contain carbon so organic molecules the water is actually an inorganic molecule and what water is is it's a solvent so basically many of the chemical reactions that occur in living systems happen in water so water is actually there sometimes it's participating as an intermediate sometimes the other molecules are just dissolved in it a water molecule is really unique so water molecule has an oxygen and two hydrogen's if you remember from taking your chemistry class oxygen is electronegative that means it really likes electrons it tends to hold them really tightly to it hydrogen on the other hand is simply a proton and an electron it's just those two particles and because it only has one electron oxygen tends to hold those electrons it tends to pull hydrogens electrons really close to it and that results in a charged differential so you have sort of a negative charge on one end of the molecule and a positive charge on the other end it's not a full negative and a full positive charge it's because it a Delta minus and a Delta plus so they're sort of they're partial positive and negative charges and this allows water to do something really unique so because of the partial positive charge on hydrogen and the partial negative charge on oxygen water molecules are attracted to each other and so you have the oxygen one water molecule attracted to the hydrogen's of other water molecules and even though this bond isn't as strong as a covalent bond or an ionic bond it's still a strong enough bond to have it's a special name it's called a hydrogen bond hydrogen bonds are caught in liquid water they're constantly being formed and breaking and formed and breaking but they're enough to cause these molecules to be very sticky so water molecules tend to stick to each other a little bit like velcro and it's because of this property of hydrogen bonding another picture of hydrogen bonding this shows five water molecules and you can see that there's a partial positive charge on the hydrogen and it's always facing the partial negative charge on the oxygen so here we see it here let's see the hydrogen's are always oriented towards the oxygens for all of these water molecules now think about a little bit of vegetable oil that maybe you've spilled on a countertop that oil is going to spread and form a really thin film and it does this because oil molecules the interactions between those molecules are nonpolar interactions that we call them Vander Waals forces and even though they exist they're weak and so there isn't a strong attraction between those molecules they're going to spread out and they're not going to stick to each other particularly well water on the other hand water molecules are very attracted to each other so if you smell a drop of water on a countertop it's gonna form a bead and that's because the water molecules are actually holding that hydrogen bonding is holding the water molecules together and resulting in this sort of formation of surface tension that's probably familiar with on the surface how you drop bonding also gives water some other characteristics so it takes a lot of energy because of the hydrogen bonds it takes a lot of energy to actually cause water molecules to separate from each other and when we go from liquid to a gas it's a phase change right but it takes way more energy to cause water to go from a liquid to a gas the similarly has a molecule with a similar molecular weight like methane so for example methane and water have almost the same molecular weight you would expect all other things being equal that if methane is a gas at room temperature water would be a gas at room temperature but in fact they're not in all that way methane is a gas and water is a liquid and that has that directly to do with the fact that these water molecules really want to stick to each other and therefore it takes a lot more energy a lot more heat to cause them to form a gas and so water doesn't become a gas until 100 degrees Celsius so this characteristic is is called the specific heat capacity and a mystery and this actually is really important to life on earth because it means that water molecules and the oceans which cover a tremendous proportion of our planet they actually the oceans themselves can absorb a tremendous amount of heat and they can release a tremendous amount of heat to keep the planets overall temperature fairly stable so the water on our planet acts as a big temperature buffer other planets have very extreme temperatures they're they're incredibly cold at night and incredibly hot during the day they go through these you know several hundred degrees of temperature change in a in a in a single day not necessarily 24 hours on another planet but in a single day on that planet and yet on earth you know our temperature stay we think it gets cold and and warm but but really compared to other planets is just nothing we'd have a very temperate planet some of that is the atmosphere a lot of it has to do with the amount of water that's on the earth and the fact that water has this specific heat capacity and keeps our planet comfortable and very temperate the temperature is very stable life likes consistency so it's going to be it's a lot easier for living things to survive and evolve and where the environment is more stable water is so important that when you hear scientists talking about life on other planets they almost always ask the first question they ask we have found that discovered a new planet the first question they ask is you know did they find water on that planet and then they look at other features of the planet to see whether or not it might be conducive to life but water at least the way we understand life water is a requirement and that doesn't mean we won't find life on out in the universe at some point that looks completely different and doesn't rely on water but the way we understand it water is certainly necessary let's go back a slide there so I mentioned phase changes a little bit ago and another really interesting property of water is the fact that ice floats generally as things that colder molecules or the solution of molecules would get denser so for example you would expect with warm-water that there's more energy the molecules are vibrating more they're going to be a little further apart in warm water and then when that you get to cold water you can see that the there's less kinetic energy in those molecules and they're a little bit closer together so when you have warm water and cold water you tend to see the warm water on top but this would argue that there should be ice at the very bottom of the water and that's actually not what happens when water organizes from and changes phase from a liquid to a solid and forms crystals the crystal pattern actually forces the water molecules to be further apart and interestingly this feature is what you see if you pour you know ice if you pour water into a cup of ice the ice will float and this is what you're saying the oceans as well you see if in places where the water is cold enough to be frozen you have a layer of ice on the top but underneath the ice is all these living things that have not been frozen and this turns out to be really important for life on Earth as well we talked about this idea that maybe life evolves and hydrothermal vents under the ocean there's a lot of really important life that occurs and if water froze from the bottom up that may have prevented the evolution of life as we know or many of the ecosystems that exist on life on Earth so this idea that ice floats is also another feature of water that makes it conducive to life all right I said on the first slide that water is a solvent so what do we mean by solvent this is a sodium chloride crystal so if you put a little salt on your french fries this is what you're this is what you're adding to your fries so this is sodium chloride these are the sodium atoms they have a positive charge these are the chloride ions they have a negative charge and so they form this beautiful little crystal and you can see like any electrostatic attraction you have the negative charges is always aligned to face the positive charges so every chloride ion is surrounded by sodium ions and every sodium ion is surrounded by chloride ions if you take a crystal instead of chlorine you put in water you can shake it and make it dissolve quickly but if you do not think it will still dissolve so what's happening well remember that we said that water it has is polar right it has negative charges and positive charges well it turns out that a sodium atom can be surrounded by water molecules with the oxygen facing the positive charge so this is called a shell of hydration and you can see what happens is and it does this spontaneously the water molecules will surround the sodium atoms and then they'll also surround the chloride and you can see here now we have the positive hydrogen's at the Delta plus on the hydrogen's surrounding the chloride and so water is actually able to gun back to my slide water is actually able to form these shells of hydration around ions and pull them into solution and so very quickly water will take this and all of these sodium and chloride ions will be dissolved in the water so again water is the solvent because these ions are dissolved in the water and it's not just sodium chloride that's dissolved in water many things dissolve in in water so this is actually hydrochloric acid here and this is sodium hydroxide and here's sodium chloride but it's not just these molecules it's many proteins its carbohydrates things like glucose are soluble in water so there there are many molecules in biology that are soluble in water and chemical reactions that are happening in your body in your cells on all the time are mean your cell is basically a bag of water with many many things dissolved in it all right so this is just a cute cartoon this is Beatrice the biologist and here we have a sodium ion and a chloride ion and there they're bonded together in an ionic bond and sodium says I just love hanging out with you chloride and chloride says he he thought 'i'm and then sodium says oh no is that water and chloride says what where and then you know they get these shells of hydration right the water molecules and you see how they're oriented properly the negatively charged oxygen is surrounding the positively charged sodium and the positively-charged hydrogen's here are surrounding the negatively charged chloride and sodium says I'll never forget you so this is a cute little cartoon but it's illustrating these shells of hydration and the process of how water acts as a solvent okay so that's it for water and next we're going to be talking about organic molecules