 In this video, I will describe the unique properties of water, and their importance to the human body, and define pH, and describe the role of pH buffers in the body. Polar covalent bonds hold the hydrogen atoms to the oxygen atom in a water molecule. Each molecule of water has one oxygen bonded to two hydrogen atoms through these polar covalent bonds. This makes a water molecule a polar molecule. Polar molecules can participate in a type of chemical bonding known as hydrogen bonds, where the partial positive charges and partial negative charges are attracted. The partial positive charges and partial negative charges in water molecules can also create hydrogen bonds with ions that have complete positive charges and negative charges. However, non-polar molecules that do not have any partial charges or complete charges cannot participate in hydrogen bonding with water molecules. It is this polarity of water that gives water many essential vital properties. The picture here shows us oil droplets that float on water do not mix with the water. Now the oil droplets float to the top if they are less dense than water, but they do not mix with the water. They are not dissolved in the water because they cannot participate in hydrogen bonding. Whereas polar molecules and ions will dissolve readily in a solution of water. Non-polar molecules will not. Cohesion is one of the vital properties of water that helps to contribute to the exclusion of non-polar molecules as the water molecules are attracted to one another through hydrogen bonding. Water molecules are holding together through this hydrogen bonding and this creates a force known as the surface tension on the surface of a beaker of water. As shown in the picture, the pin here does not sink through the water and can float right on top. Although once the surface tension is broken if you were to push the pin through the surface, it is more dense than the water and would sink to the bottom of the beaker. So this surface tension is what prevents the pin from sinking and surface tension results from the cohesion of water as a result of the polarity of water. Water has a high specific heat capacity. That means that it requires a large amount of heat energy to change the temperature of water. And so water is very good for stabilizing the temperature of our body and also for stabilizing the temperature of our environment. The majority of the liquid in the oceans is water and it's liquid water in part as a result of the high specific heat capacity of water that as water cools it releases a large amount of heat. And so it takes a long time for the large amount of water in the oceans to freeze to form solid water or ice. And an interesting characteristic of water when it freezes, the hydrogen bonding between water molecules creates a crystalline structure that is less densely packed than liquid water. For this reason ice floats on liquid water and this helps to stabilize the temperature of the oceans and maintain liquid water on our planet. Because as water freezes and the ice floats to the top, that ice is then exposed to the warmth of the sunlight that can help to melt the ice maintaining liquid water on the planet. Water also has a high heat of vaporization meaning that it requires a large amount of heat energy to evaporate liquid water. This helps us stabilize our body temperature as we secrete sweat onto the surface of our body. When that sweat evaporates it can remove a large amount of heat from the body helping to stabilize our body temperature. Water participates in some very important chemical reactions as either the product or the reagent in those chemical reactions. So on the top here we see the example of a dehydration synthesis reaction in which water is a product of the chemical reaction. The reagents are known as monomers. We'll see lots of examples of this type of reaction where two monomers are joined together to form a larger molecule we call a dimer. This process may repeat to add a third, fourth, fifth monomer and so on forming a long chain of monomers we call a polymer. The opposite of dehydration synthesis is known as hydrolysis. The ending of that word lysis refers to breaking apart and the prefix hydro refers to water. In this chemical reaction water is a reagent that participates in a chemical reaction to break apart monomers. If we have a dimer as is illustrated in the bottom left that dimer can be split apart producing two monomers and a molecule of water is consumed in that chemical reaction. Let's apply these terms hydrolysis and dehydration synthesis to an example. In our food we consume polymers of the glucose monomer, which are commonly referred to as starch or carbohydrates. Another term for these polymers of carbohydrates is polysaccharide. The starch that's found in bread or pasta for example, when you eat that it's broken down by hydrolysis. Many of the chemical reactions that form catabolism hydrolysis reactions that break down polymers to release monomers. A starch molecule such as amylose is a long chain of glucose monomers and when amylose is broken down through hydrolysis the products are glucose monomers. Now the opposite dehydration synthesis can take glucose monomers and join them together to form long chains. For example the polysaccharide glycogen is a long chain of glucose molecules that's formed by animal cells like our own. Water is an excellent solvent meaning water is excellent at dissolving other molecules, specifically molecules which are polar or molecules that are ions. So a solvent is the dissolving agent in the solution and the solute is what is dissolved in that solution. Here we have the example of table salt sodium chloride being dissolved by water. Sodium chloride is an ionic molecule so the sodium ions have a positive charge which are attracted to the partial negative charges of the oxygen atoms and water molecules. And the chloride anions have negative charges that are attracted to the partial positive charges of the hydrogen atoms in water molecules. Molarity is a common measurement of the concentration of solute that's dissolved in a solution. Molarity equals the number of moles per liter of solution. Mole is a number of molecules, it's a very large number known as Avogadro's number. So one mole is equal to 6.022 times 10 raised to the 23rd molecules. Let's look at the example for copper nitrate. The molecular weight of copper nitrate is 187.56 grams per mole. So if we had two moles it would weigh twice that it would be 375.12 grams whereas if we had a tenth of a mole the weight would be a tenth of the molecular weight or 18.756 grams. Let's take an example looking at cylinder 1 if we place 18.756 grams of copper nitrate or one tenth of a mole of copper nitrate into cylinder A and filled it with water to bring the final solution to a volume of 25 milliliters. We can calculate the concentration of that solution in cylinder A. It's 0.1 moles divided by the 25 milliliters and then we'll need to use the conversion factor 1000 milliliters per liter in order to give us the final answer in molarity or moles per liter. That is 4 moles per liter. Now if we look at cylinder B, cylinder B has the same amount of solute dissolved in twice as much liquid. And so for cylinder B the same quantity of copper nitrate 18.756 grams or one tenth of a mole of copper nitrate was placed into cylinder B and then water was poured in to fill cylinder B up to 50 milliliters of total solution. And we can calculate the concentration as 0.1 divided by 50 milliliters and then convert milliliters to liters by multiplying by 1000. This gives us 2 moles per liter in cylinder B. The mass volume unit is another common measurement of the concentration of a solution. And this is one of the most common units seen in the medical setting. For example, a saline solution that's used as an IV, an intravenous saline solution, is typically 0.9% sodium chloride dissolved in water. 0.9% means there's 0.9 grams of sodium chloride dissolved in every 100 milliliters of solution. Another example from the medical setting is the concentration of blood glucose. Blood glucose concentrations are typically reported in milligrams per deciliter. While a milligram is one one thousandth of a gram, a deciliter is equal to one tenth of a liter or 100 milliliters. If a molecule dissolves well in water, it's what we call a hydrophilic molecule. Hydrophilic literally translates to water loving. The examples we see here are glucose and sodium chloride. Glucose is a polar molecule that is not an ion, but the partial positive and partial negative charges in the glucose molecules are able to form hydrogen bonds with water molecules. And that makes glucose a hydrophilic molecule that dissolves easily in water. Similarly, sodium chloride is hydrophilic because it's an ionic molecule. Hydrophobic molecules are non-polar molecules. For example, the triglyceride or neutral fat is a non-polar molecule that cannot form hydrogen bonds with water molecules and therefore is not easily dissolved in water. Some molecules have a region that is a polar portion of the molecule and another region that is a non-polar portion of the molecule. The example we see here is a phospholipid. These phospholipids are important to form the plasma membrane that is the majority of the barrier surrounding a cell. The phospholipids have a polar head region that faces the water of the extracellular fluid and the water of the intracellular fluid. And phospholipids have a non-polar tail region that faces away from the watery solutions that are in and outside of a cell. In order to create that arrangement, these phospholipids will organize into a bilayer so that the hydrophobic fatty acid tails of the phospholipid will face each other and hide away from the watery solutions. Whereas the phosphorus containing polar head, the hydrophilic region of the molecule, will face towards the watery solution. So any molecule that has a polar region and a non-polar region could be considered an amphiphilic molecule. The phospholipid is just a major example that we'll study as we learn about the structure of cells. In the Bronsted-Lowry acid base theory, an acid is a chemical that can release a hydrogen ion into solution. Whereas a base is a chemical that can accept a hydrogen ion from a solution. The first example shown here in the illustration is water functioning as an acid. As water loses a hydrogen ion, it becomes the hydroxide ion OH with a negative charge. Now the hydroxide ion is the conjugate base of water because the hydroxide ion can accept a hydrogen ion to produce water. So whenever a Bronsted-Lowry acid releases a hydrogen ion, the molecule becomes its conjugate base. Similarly, when a base accepts a hydrogen ion, it becomes a conjugate acid. On the right here we see the example of the base ammonia. When ammonia accepts a hydrogen ion, it becomes the conjugate acid ammonium. At the bottom here we have another example. The acid carbonic acid can release a hydrogen ion to solution, and the base sodium hydroxide can accept a hydrogen ion from solution. When carbonic acid reacts with sodium hydroxide, the products are sodium bicarbonate, and bicarbonate is the conjugate base of carbonic acid. And water is the other product, and in this case water is the conjugate acid of the hydroxide ion, which was the base in the chemical reaction. Acids are solutes that when dissolved in a watery solution will release hydrogen ions, increasing the concentration of hydrogen ions that are dissolved in the solution. In contrast, a base is a solute that when dissolved in water will decrease the concentration of hydrogen ions and increase the concentration of hydroxyl ions, which are the OH anions shown in the illustration. The pH scale is a measurement of the concentration of hydrogen ions in a solution, or the acidity of a solution. The higher the concentration of hydrogen ions, the more acidic a solution, and the lower the pH of that solution. A solution that has a pH equal to 7 is considered neutral, whereas a pH greater than 7 is considered a basic or alkaline solution, and a pH lower than 7 is considered an acidic solution. To calculate the pH, we can take the negative log base 10 of the hydrogen ion concentration measured in molarity, or moles per liter. Therefore, a pH of 14 has a hydrogen ion concentration of 1 times 10 to the negative 14th moles per liter. Similarly, a solution with a pH of 4 has a hydrogen ion concentration of 1 times 10 to the negative fourth moles per liter. You'll notice that moving one unit on the pH scale is equivalent to a tenfold change in hydrogen ion concentration, so that a pH of 1 has 10 times more hydrogen ions than a pH of 2, and 100 times more hydrogen ions than a pH of 3, or 1,000 times more hydrogen ions than a pH of 4. The figure here shows us several examples of familiar liquids in their pH. So an extremely alkaline solution with a pH of 14 is liquid drain cleaner, or other heavy-ud cleaners like bleaches and oven cleaners and lye are very alkaline solutions. Ammonia is also fairly alkaline with a pH of 10.5 to 11.5. And notice that seawater and blood are both slightly alkaline solutions. Blood has a pH of about 7.4, making it just slightly alkaline. In contrast, milk, saliva, and urine commonly have just slightly acidic pH in the range of 6.3 to 6.6. Coffee is typically slightly acidic with a pH around 5, whereas lemon juice and vinegar are much more acidic with a pH near 2. And a very strong acid such as the hydrochloric acid in a car battery has a pH near 0. The pH of human blood is typically about 7.4. And it's essential that we maintain a pH in the optimal range between 7.38 and 7.42 in order to maintain homeostasis. If the blood pH falls below 7.38, this will lead to disruption of the system, organ systems functions, and we call a pH below 7.38 acidosis. Some of the symptoms of acidosis include disruption of the functions of the central nervous system, can lead to confusion, headache, drowsiness, and eventually loss of consciousness and coma. If the blood pH falls further and below 7, ultimately organ dysfunction will cause death. Similarly, if the blood pH is too high above 7.42, it will also disrupt the functions of organ systems and is a condition known as alkalosis. So alkalosis will also disrupt the functions of the nervous system and can lead to twitching or trembling. Hand tremor is a difficulty keeping the hands still, resting movement of the hand from lots of twitching. Similarly, the sensory nerves can become overactive leading to tingling sensations or can disrupt sensation leading to numbness. Alkalosis can disrupt the functions of the brain leading to anxiety, confusion, lightheadedness, and eventually loss of consciousness and a coma. And if the pH of the blood pH becomes above 7.8, then organ systems will shut down leading to death. Because it is essential to maintain the blood pH of about 7.4, there are homeostatic mechanisms that will help to stabilize the blood pH. We'll discuss in more detail the mechanisms that maintain blood homeostasis as we discuss the respiratory system and the urinary system. But one basic concept that I would like you to learn now is the idea of a pH buffer. So pH buffers are a mixture of chemicals that help to stabilize the pH of a solution. Typically a pH buffer is an acid at its conjugate base or a base at its conjugate acid. The example we're seeing here is the bicarbonate buffering system of the blood. Bicarbonate is the conjugate base of carbonic acid. And so bicarbonate can accept a hydrogen ion from solution raising the pH of that solution. Then we have a mechanism to remove carbonic acid from the body. Carbonic acid can be converted to water and carbon dioxide. And then the respiratory system is able to help us remove carbon dioxide from the body as a way to help stabilize the blood pH. If there's too low of a pH, if it's starting to fall below that homeostatic set point of 7.4, then the respiratory drive will increase and you'll start to breathe deeper or faster to remove more carbon dioxide from the blood. Now carbon dioxide is constantly being generated from metabolism of cells throughout our body. And that carbon dioxide can be converted to carbonic acid that will have the effect of lowering the pH of the blood. If our blood pH becomes too high above 7.4, then the respiratory system can respond by decreasing the rate at which carbon dioxide is removed from the blood and help to lower the blood pH. Restoring the homeostatic set point of around 7.4 that enables the optimal functions of our organ systems.