Episode 4 is all about WATER! In the first of the content episodes, we zoom in to the elemental level and discuss water structure and its correlating properties.
Episode 4 is all about WATER! In the first of the content episodes, we zoom in to the elemental level and discuss water structure and its correlating properties. Melanie tells you to think of a familiar Disney character (1:48) and has you sketching water polarity (2:40). She shares some word reminders to tell the difference between cohesion and adhesion (4:30) with some real world examples. Have you ever seen an insect walk on water? (6:18) Melanie shares a fun fact for the rock collectors (6:37) and describes water’s role in monomer/polymer formations (7:10)
The Question of the Day (9:13) asks “Describe how the properties of water contribute to maintaining homeostasis.”
Thank you for listening to The APsolute RecAP: Biology Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Water Chemistry
Lets Zoom Out
Unit 1 - Chemistry of life
Topic 1.1 - Structure of Water and Hydrogen Bonding
Big idea - Systems interaction
Zoom in:
Like really far in. We are talking elements. What influences the chemical behavior of an atom? Your understanding of this question is going to connect to many concepts in molecular structure and function. So take a moment. Draw and label a water molecule on your notebook, a whiteboard, chalkboard. What does it look like? Why is it shaped like that? How might that shape influence the way that water behaves? Alright, so what influences the chemical behavior of an atom, and consequently the chemical behavior or water. Good ‘ol H2O - two elements of hydrogen for every element of oxygen. If you look at the structure of water, its bent. Not quite 90 degree bent. Oxygen has eight electrons and they are organized into two rings. Remember that electrons are the subatomic particle that influences how elements interact with other elements. Hydrogen has one electron, so it will covalently bond with oxygen. When hydrogen bonds to oxygen it does so in a way that forms a Mickey Mouse shape. It really is the best analogy. What happens why hydrogen bonds to oxygen in this Mickey Mouse shape concept is it causes four of oxygen’s electrons to huddle off to one side. This gives water polarity - well what's polarity? If you think about the way the Earth has two sides or two poles - that same concept is true for water. Water has an uneven distribution of charge. Water itself is not positive or negative, but the oxygen side of the molecule is what we call electronegative whereas the hydrogen side of water is electropositive. Because water is polar, it's going to interact with other polar or charged molecules in a very specific way.
So, you have your drawn water molecule. Now lets label its electronegativity. We have delta negative on the oxygen side and delta positive on the positive side. So draw another water molecule, right next to the first. How are they likely going to interact? Well if you have familiarity with your chemistry background, you know that opposite charges attract. So we are going to form a hydrogen bond between the hydrogen of one water molecule and the oxygen of another. The most common hydrogen bond is with nitrogen - which means that if water interacts with let's say, ammonia, or NH3, it's also going to form a hydrogen bond between the positive hydrogen and the delta negative nitrogen. Hydrogen bonding is the foundation by which water molecules interact with other things. When we look at water molecules as a collective, we’ll find hydrogen bonds forming between two water molecules and polar covalent bonds forming within water molecules. So let's predict. How is it advantageous for different biological functions to have molecular bonds of different strengths? Hydrogen bonds are really an intermolecular force, whereas covalent and ionic bonding are significantly stronger. Hydrogen bonds are weak, but that means that we can easily form and break them.
One of the big buzz words with water are its properties. Which essentially means - what some behaviors that water has based upon its chemical structure. The three big properties of water you need to know for the exam are cohesion, adhesion and surface tension. Lets look at cohesion and adhesion - those words sound so much alike and are really easy to confuse with each other. They both begin with different prefixes. Co meaning “with” and Ad meaning “to.” So to adhere to something means you’re sticking to something else. To have cohesion means you're sticking to yourself. So if we take that basic concept and apply it to water, the same will hold true. Water likes to stick to itself - cohesion. It also likes to stick to other things - adhesion. Both cohesion and adhesion give water its remarkable ability to defy gravity. Let's think of some examples you might know. Plants are growing, in the ground. Water has to go from the roots to the shoots against the force of gravity. How is this possible? Water has adhesion to the walls of that vascular structure and cohesion to itself. In that way, it has a barrel of monkey’s quality in which each water molecule pulls up against the water molecule below it, allowing it to defy gravity. You’ve also seen this effect in a graduated cylinder in class. If you look at the meniscus at the bottom curve in a graduated cylinder, water almost appears to jump up the sides of the cylinder, because forces of adhesion are stronger than cohesion. If you’ve ever seen a droplet of water on a waxy surface like a leaf or even on the top of a penny - water forms a bubble. Water cohesion gives it this remarkable surface tension. Boiling water has to break those hydrogen bonds in order to cause the water molecules to move more rapidly. It's also what causes certain insects to have the ability to walk on water. The interaction they have with the water is not stronger than the surface tension or interaction of the water molecules themselves.
So water exists in its three states: we have our gas, our liquid and our solid. But because of its hydrogen bonding, it has different properties in each of these states of matter. In liquid water, hydrogen bonds are continuously breaking and reforming. However when water freezes, the hydrogen bonds become more stable and the water molecules get pushed farther apart. So this is why ice floats on water. The water molecules are more spread out and are therefore less dense. Here's a fun fact - ice is technically a mineral because it has all of the properties that other minerals have. Minerals are natural inorganic solids that have a definite chemical composition and crystalline structure.
Water is a key player in two of our basic chemical reactions. The first is dehydration reactions. Dehydration reactions are used to join monomers together and form a larger polymer. Dehydration, to dehydrate, to take out water. So if you look at two monomers adjacent and you want to link them tougher you can pull a hydroxyl, or OH off of one monomer and a hydrogen off of the other. By pulling off HOH you get H2O. The electrons left over on those monomers are now available to covalently bond together. Dehydration reactions have another name. They are often called condensation synthesis. Condensation synthesis, aso named because there is condensation, or the collection of water, when something is being synthesized or put together. The other type of reaction that heavily involves water is hydrolysis. Once again we have a name that describes the function. Hydro- lysis, hydrolysis. Hydro involving water and lysis to break or cut. So in this instance, we are breaking a polymer into smaller monomers. We are using water to cut or break this apart. When this occurs we stick a hydroxyl onto one monomer and a hydrogen onto the other. And in that way the bond has been broken. We are going to the suffix lysis come back in other units, for example cytolysis will be the bursting or cutting up of cells.
To recap
Water - it's pretty great. Its polarity has a direct correlation to the different functions that it has. Those functions are cohesion, adhesion and surface tension. Water is going to have a really big role in later units involved in both breaking and forming bonds, carrying electrons from one process to another. We are as living things, water based life forms.
Today’s question of the day is in FRQ format. Describe how the properties of water contribute to maintaining homeostasis.