Chapter 3:Water and the Fitness of the Environment Wilkie South Fort Myers High School 2015-2016
Water:A Prime Example of Emergent Properties Water is ubiquitous (all around us) and is essential for biological processes. What makes water so important are the physical properties that emerge as a result of its interactions between molecules (hydrogen bonds), interactions between the atoms that make up the molecule (polar covalent bonds) and the electronegativity interactions between the atoms (hydrogen and oxygen) that make up those bonds.
Polar Molecules:A Review As I reviewed in the previous powerpoint/lecture, water is a polar molecule because the oxygen atom and two hydrogen atoms do not share the electrons equally. Oxygen having the high electronegativity pulls the electrons towards it s nucleus from the less electronegative hydrogen atoms. This disparity creates a slight negative charge at the oxygen end of the molecule and a slight positive charge at the hydrogen ends of the molecule.
What are water s physical properties:cohesion and Adhesion In science, opposites often attract. This is true in polar molecules. The positive pole (hydrogen end) of one water molecule will attract the negative pole (oxygen end) of another molecule. The resulting weak bond is a hydrogen bond. Cohesion results from water molecules sticking to other water molecules. Because many surfaces and other chemicals having slightly charged water will stick to those as well. Water sticks to the sides of a water bottle because of the parts of the plastic molecules that contain slight positive or negative areas
Why is cohesion/adhesion important to life? Emergent properties are those that result from characteristics in lower levels of complexity: Oxygen is more electronegative than hydrogen, forming polar covalent bonds. Water is a polar molecule Water forms polar bonds. Water is cohesive and adhesive. Cohesion and adhesion makes it possible for plants to move water up their trunks to their leaves where photosynthesis takes place. This process is known as transpiration which can influence global atmospheric water, O 2 and CO 2 concentrations and therefore the climate.
Water s Physical Properties:Surface Tension The interaction between cohesion and adhesion create a film on the surface of the water. When you jump in a pool you do not notice this film, however some organisms like water striders rely on it. The cohesion of water molecules to themselves and the adhesion of water molecules to the insects legs generates surface tension.
Why is surface tension important to life? Could life on Earth still exist if water striders did not probably. Surface tension in large bodies of water also helps to control the exchange of gas (oxygen and carbon dioxide) between the atmosphere and the water. If there were no surface tension the atmospheric concentrations of those two gasses might be very different and since much of life on Earth is dependent on one or both of those gasses, that makes surface tension important.
Water s Physical Properties:High Specific Heat Capacity. Specific heat is often difficult to understand, so I will emphasize it in the online lecture. Water is polar and therefore forms many hydrogen bonds with surrounding water molecules. To change from one state of matter (solid to liquid, liquid to gas) requires an input of energy and the movement of molecules away from each other. Even though hydrogen bonds are weak, there are so many of them that need energy to in order to break them, that water can absorb a tremendous amount of energy before changing temperature. Specific heat is the amount of energy required to raise 1 gram of something by one degree Celsius. Water s specific heat is 1 calorie of energy. This doesn t sound like a lot, but if you compare it to most other non polar substances it is extremely high.
Why is water s specific heat important to life? Since water can absorb tremendous amounts of heat before it changes temperature, it is less likely to evaporate easily. Think about if water (polar molecule) is spilled on the ground vs. gasoline (non polar molecule) spilled on the ground. The gasoline will evaporate much quicker. If water did not have a high specific heat, much of it would have evaporated long ago leaving no liquid water on the planet. Since liquid water is essential to many biological processes no life. When you are hot, you sweat. That sweat absorbs the heat from your body s surface. As the sweat evaporates, the collected heat required to cause that sweat to evaporate goes with it, leaving you cooler, this is known as evaporative cooling and is essential to regulating internal temperature in many mammals. This will make more sense when you watch the lecture and I explain heat of vaporization.
Water s Physical Properties: Density All matter has a specific density. Density is a measurement of how much space a specific mass of a substance has. It can be calculated using the equation Density=Mass/Volume The units are often g/ml or g/cm 3 Like most substances as water cools down its molecules move closer together. Most substances are the most dense in their solid state. As water reaches a critical temperature 3.78 degrees Celsius, the molecules get as close as they can. At this temperature the positive ends of the water molecules repel the positive ends of adjacent water molecules, and the negative ends repel the adjacent negative ends of other water molecules. This forces water molecules to spread/expand out as the water continues to cool, each water molecule forms hydrogen bonds in a regular lattice pattern. Since the molecules are now expanding the volume actually increases What will happen to the density calculation if you have a larger number in the denominator?
Why is water s density important to life? Since water has the unusual property of being less dense as a solid, it actually floats in liquid water. If water did not do this, lakes, rivers and even oceans might freeze from the bottom up eventually freezing solid. Instead ice floats on the surface and actually insulates the water underneath making a suitable habitat for aquatic and marine organisms.
Water s Physical Properties:Solvent Ability First some basic vocabulary Solution-A mixture of two substances where distinguishing between the solute and solvent is difficult (salt water is a solution) Solute-The substance being dissolved in the solution (salt) Solvent-The substance doing the dissolving in the solution (water) Water is very good a dissolving other polar substances. Salt is an Ionic Compound which means it has a positive ion and negative ion for all intensive purposes, it is polar. Water s polar nature allows it to essentially pick the salt crystal apart into its cation and anion. Once ions are separate the water molecules keep them separate by forming a hydration shell. The positive hydrogen end of the water molecules surround the negative chlorine ion. The negative oxygen end of the water molecules surround the positive sodium ion. The result is the ions are kept separate from each other and stay in solution.
Why is water s solvent ability important to life? Water s ability to dissolve most substances makes it very important in chemical reactions that occur in the body. Although not all molecules are as easily dissolved as an ionic compound like NACl, large proteins have areas that are polar (as we will learn later on), this means that water can dissolve very large complex molecules like proteins. The cell membrane is made up of a phospholipid bilayer. The heads of the bilayer contain (negatively charged) phosphate ions.this makes the heads polar and hydrophyllic (like water). The tails on the other hand are long chains of fatty acids. Fats don t mix well with water so these chains are hydrophobic (dislike water). Since the cell is surroundend by water.the combined hydrophyllic/hydrophobic regions of the phosphate head and fatty acid tails can help to regulate the materials that can travel into and out of the cell.
Water and Acids and Bases We will shift gears for the last few slides away from the concept of water and its emergent properties and discuss the nature of ph. Although water by definition is H 2 O, it actually exists as a reversible reaction (seen to the right). In this reaction a H+ ion (that is a hydrogen atom that has left it s electron behind) is lost from one water molecule (creating an OH- ion) That H+ ion is picked up by another water molecule creating a hydronium ion H 3 O+. In pure water the balance of this reaction is almost exact, which means just as many hydronium ions will exist as do hydroxide ions. The ph scale that measures the concentration of Hydronium ions (often simplified to simply H+) will be 7. What this means is that there is one ten millionth of a mole (more about moles later in the semester) of H+ and OH- for every liter of water molecules.
What is the ph scale? The ph scale is an indication of the relative concentration of hydrogen ions (H+) and hydroxide ions (OH-). When there are more H+ ions than OHions the solution is acidic. When there are more OH ions than H+ ions, the solution is basic or alkaline. Substances when mixed with water that dissolve and release H+ ions are contributing H+ ions to the solution and thus make the solution more acidic. HCL (Hydrochloric acid) H2SO4 (Sufuric acid) are both strong acids because they donate H+ to water easily. Substance when mixed with water that dissolve and relase OH- ions are contributing OH- ions to the solution and thus make the solution more basic. NaOH (Sodium hydroxide) is a strong base because it releases OH- ions that absorb the H+ ions.
How is ph important to biological processes? The ph of the environment in which an organism lives can have profound impacts on the way the organism functions. ph (as we will learn later) has a tremendous impact on chemical reactions controlled by enzymes. ph in your blood must be regulated in a process known as homestasis. Buffers work to counter the decrease in ph (making your blood more acidic) associated with the production of CO2 that forms carbonic acid in the blood stream. An environment that has an altered ph can have impacts at the ecosystem level, as can be seen in the picture to the right, which is the result of acid rain associated with human atmospheric pollution.