Villenas-FA-Biochem-Lecture-Module-4-Properties-of-Water PDF

Module 4: Water, its properties and as a reaction medium in the cell TITLE: THE WONDERS OF WATER OVERVIEW: This module will discuss about the different properties of water that are important for maintaining life. Among these properties, the cohesive and adhesive property will be furth...

Module 4: Water, its properties and as a reaction medium in the cell TITLE: THE WONDERS OF WATER OVERVIEW: This module will discuss about the different properties of water that are important for maintaining life. Among these properties, the cohesive and adhesive property will be further discussed in the aspect of its biological importance. This will also give emphasis on the ability of water to maintain the temperature of the environment making earth conducive for the existence of life. LEARNING OBJECTIVES: After putting your heart and mind in studying this module, you will be able to: 1. Identify the physical properties of water; 2. Relate cohesion with surface tension 3. Compare and contrast adhesion and cohesion 4. Justify or invalidate that water is a universal solvent; 5. Describe how water regulates the temperature of the environment making the earth habitable for the organisms; and 6. Relate the biological importance of water to its physical properties. LECTURE DISCUSSION: Water covers 75% of the earth’s surface and made up the 70%-90% of each of the cells of the living organisms. Its distinctive physical properties make the outside environment suited for the living organisms and the internal environment of the organisms appropriate for the physical and chemical processes of life. Basically, the physical properties of water are explained Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 1 of 12 by the hydrogen bonding between the individual molecules, that is, the polarity of water molecules results in hydrogen bonding (Figure 4.1). "Water molecules" by Siyavula Education is licensed under "File:210 Hydrogen Bonds Between Water Molecules-01.jpg" by CC BY 2.0. To view a copy of this license, visit OpenStax College is licensed under CC BY 3.0. To view a copy https://creativecommons.org/licenses/by/2.0/ of this license, visit https://creativecommons.org/licenses/by/3.0 Figure 4.1 A water molecule consists of two hydrogen atoms each covalently bonded to a more electronegative oxygen atom. The polar molecule has a right angle shape with a slight positive charge on each hydrogen atom and a slight negative charge associated with the oxygen. Hydrogen bonds form between the hydrogen atoms of one water molecule and the oxygen atom of another molecule, creating a higher level of structural organization and leading to the emergent properties of water. Water can form up to four hydrogen bonds (Figure 4.1, right). 4.1 What makes water a polar molecule? Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 2 of 12 Figure 4.2 A needle's weight pulls the surface downward. At the same time, the surface tension pulls it up, suspending it on the water's surface preventing it from sinking. Notice the indentation in the water around the needle. (credit: Cory Zanker) Figure 4.3 Water’s cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat. (credit: Tim Vickers) The first ones in the concepts are cohesion, adhesion and surface tension(Figures 4.2 and 4.3). These properties are obviously exhibited in the movement of water in plants which made them move against the gravity of the earth. Do you wonder how the water from the soil trace its way towards the roots, the stem and the leaves? That is due to the hydrogen bonding which Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 3 of 12 causes the water molecules to stick together. How is this so? The liquid water is unusually cohesive due to the constant forming and re-forming of hydrogen bonds that hold the molecules together. The cohesion creates a more structurally organized liquid and enable water to move against gravity as exhibited in plants. The adhesion of water molecules (Figure 4.4) to the walls of the plant vessels also contribute to the movement or transport of water. The hydrogen bonding between water molecules produces a high surface tension at the interface between the water and the air. That is, at the surface of a liquid, the inwardly- acting cohesive forces produce a surface tension as the molecules are specifically attracted to one another making it difficult to break or stretch the surface of the liquid. Figure 4.4 The adhesive forces exerted by the glass' internal surface exceeding the cohesive forces between the water molecules themselves causes capillary action in a glass tube. (credit: modification of work by Pearson-Scott Foresman, donated to the Wikimedia Foundation) 4.2 Compare and contrast adhesion and cohesion. 4.3 Explain how cohesion and adhesion play a role in surface tension. The next properties of water are related to heat and temperature. These are high specific heat capacity, high latent heat of vaporization (evaporative cooling) and density and freezing properties. Let us look into these one by one. Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 4 of 12  Specific heat is the amount of heat absorbed or lost when 1 g of a substance changes its temperature by 1°C. water’s specific heat of 1 cal/ g/ °C is unusually high compared with that of other common substances, that is, water must absorb or release a relatively large quantity of heat in order for its temperature to change. Heat must be absorbed to break hydrogen bonds before water molecules can move faster and the temperature can rise, and conversely, heat is released when hydrogen bonds form as the temperature of water drops. The ability of large bodies of water to stabilize air temperature is due to the high specific heat of water. The high proportion of water in the environment and within organisms keeps temperature fluctuations within limits that permit life. 4.4 What is the importance of the high specific heat of water for life on Earth? Vaporization or evaporation, as we all know, is the transformation from liquid to gas. This happens when molecules with sufficient kinetic energy overcome their attraction to other molecules and escape into the air as gas. The addition of heat increases the rate of evaporation by increasing the kinetic energy of molecules. The heat of vaporization is the quantity of heat that must be absorbed for 1g of a liquid to be converted to a gas. Water has a high heat of vaporization (540 cal/g) because a large amount of heat is needed to break the hydrogen bonds holding water molecules together. As the substance vaporizes, the liquid left behind loses the kinetic energy of the escaping molecules and cools down. Evaporative cooling helps to protect terrestrial organisms from overheating and contributes to the stability of temperatures in lakes and ponds. In addition, this property of water helps moderate the climate on earth as solar heat is dissipated from tropical seas during evaporation and heat is released when moist tropical air condenses to form rain. Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 5 of 12 Figure 4.5 Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice makes it less dense than the liquid water's freely flowing molecules, enabling it to (b) float on water. (credit a: modification of work by Jane Whitney, image created using Visual Molecular Dynamics (VMD) software1; credit b: modification of work by Carlos Ponte) Then we have the density and freezing properties. As water cools towards its freezing point, the individual molecules slow down sufficiently for each one to form its maximum number of hydrogen bonds. To come up with this, the water molecules in liquid water must move farther apart to give enough space for all four hydrogen bonds to fit into. As a result, the water expands as it freezes, making the ice less dense than liquid water (Figure 4.5) and therefore floats upon its surface. The floating ice, in return, insulates the liquid water below. Figure 4.6 When we mix table salt (NaCl) in water, it forms spheres of hydration around the ions. Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 6 of 12  4.5 What is evaporative cooling? 4.6 What is the importance of floating ice in ocean? And lastly, we have the solvent properties of water. Do you believe that water is a universal solvent? When we say “universal” does it mean to say that water can really dissolve any kind of substances? Let us try to look into this property of water for you to determine whether it is really a universal solvent or can we say it is just a “versatile solvent”? Or we have to stick with the very common quote in chemistry which is “like dissolves like”. You are familiar with the solution which is composed of two or more substances: the solute/s which is the substance that is being dissolved, for example the table salt, and the solvent (water) which is considered as the dissolving agent(Figure 4.6). If you have an aqueous solution it means to say that water is the solvent. The polarity of water makes it an excellent solvent for other polar molecules. That is, the water has its partially positive (cation) and partially negative (anion)regions which become attracted to oppositely charged ions or partially charged regions of polar molecules. Hence, the solute molecules then become surrounded by water molecules resulting to a formation of solution. The ionic and polar substances, which dissolve in water are said to be hydrophilic or water-loving, meaning they have an affinity for water due to electrical attractions and hydrogen bonding. While the non-ionic and nonpolar compounds are hydrophobic or water-hating, meaning they will not mix with or dissolve in water. 4.7 Why is water known as the universal solvent? 4.8 Do you agree with this statement? Justify your answer. Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 7 of 12 Figure 4.7 The pH scale measures hydrogen ions' (H+) concentration in a solution. (credit: modification of work by Edward Stevens) In addition to the ones previously mentioned properties of water molecules, is its ability to dissociate resulting to formation of hydrogen ion (H+ which consequently binds to another water molecule to form a hydronium ion, H3O +)and a hydroxide ion OH-. Although reversible and rare, this dissociation into highly reactive hydrogen and hydroxide ions has significant biological consequences. In pure water, the concentrations of H + and OH- ions are both 10-7M. But when either basic or acidic substances are dissolve in water, the H + and OH- balance changes. An acid adds H+ to a solution, whereas a base reduces H+ in a solution by accepting hydrogen ions or by adding hydroxide ions, which then combine with H+ and thus remove hydrogen ions (Figure 4.7). A strong acid or strong base dissociates completely when mixed with water. A weak acid or base reversibly dissociates, releasing or binding H+. a solution with higher concentration of H+ than of OH- is considered acidic. A basic solution has a higher concentration of OH - than H+. As the H+ increases in an acidic solution, the pH value decreases. The difference between each unit of the Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 8 of 12 pH scale represents a tenfold difference in the concentration of H + and OH-. From here, you can imagine or wonder what pH should the cells of an organism be? Well, most cells have an internal pH close to 7. It is made possible by the presence of buffers which maintain a constant pH by either accepting excess H+ ions or donating H+ ions when H+ concentration decreases. Weak acid- base pairs that reversibly bind hydrogen ions are typical of most buffering systems. Buffers basically minimize the changes in concentration of H+ and OH- in a solution. This is the substance (carbonic acid-bicarbonate buffer system) that keeps blood at pH ~7.4. If the pH of the blood drops at 7 or rises at 7.8, it can kill the person. 4.9 Explain the importance of buffer in human body? These physical properties are where all biological importance of water depend. So let us look into these importance of water to the living system, we can also include the environment of the organisms. First is the solvent property of water which allows water to act as a transport medium for polar solutes. For example, the movements of minerals to lakes and seas; the transport via blood and lymph in multicellular animals and the removal of the metabolic wastes such as urea and ammonia in urine. In connection to this solvent property is its molecular mobility wherein the rather weak nature of individual hydrogen bonds means that the water molecules can move easily relative to one another, which also allows osmosis to take place. Next is the water’s lubricant properties. This include the cohesive and adhesive characteristics of water which make it viscous and serve as useful lubricant in biological systems. In animals for example, the presence of synovial fluid lubricates the joints of many vertebrates; the pleural fluid minimizes friction between the lungs and the ribs during breathing; and the fluid/mucus permits easy passage of feces down to the colon to the anus. This property can also be observed in plants in the process of transpiration. The continuous column of water is able to Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 9 of 12 move up the xylem because of cohesion between water molecules and adhesion between water and the walls of the xylem vessels. Moreover, the cohesive forces between the water molecules mean that it is not easily compressed which make it an excellent medium for support. For example, is the hydrostatic skeleton in earthworm, turgor pressure in herbaceous part of plants, the amniotic fluid in mammals which supports and protects its fetus, and as a supporting medium for large, aquatic mammals such as whales. Then we have the importance of water in thermoregulation. On one hand, the high specific heat capacity of water shows that bodies of the organisms which is composed largely of water (in its cells themselves which are usually composed of 70-80% water each) are very thermostable and thus less prone to heat damage by changes in temperature of the environment. On the other hand, the high latent heat of vaporization of water shows that a body can be considerably cooled with a minimal loss of water just what can be exhibited when we are sweating, when the reptiles are gaping and may also be important in regulating the transpiration process in plants. So, this also leads to another property which is volatility/stability which helps in the balancing of the earth’s temperature leading to the natural movement of evaporation, transpiration and precipitation in the water cycle. Then we have the very obvious, yet slightly overlooked property of water which is transparency. As we all know water is basically clear and transparent. This permits the passage of visible light which means that photosynthesis is possible in relatively shallow aquatic environments. So, if photosynthesis is present in an environment, it will lead to the continuity of life in that area. Water also has the ability to expand at freezing temperature. This is observed in some parts of the earth where ice floats and forms at the surface of ponds, lakes or oceans. This formation of ice insulates the organisms in the water below it and it also allows penguins and polar bears having somewhere to stand on. But of course when the temperature rises, it allows Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 10 of 12 the ice to thaw rapidly. The changes in density also maintain the circulation of nutrients in large bodies of water. The last but not the least is the water’s metabolic functions. Water is used directly as a reactant (source of reducing power) in photosynthesis; it hydrolyses the macromolecules to their subunits (e.g. digestion); and most importantly water serves as the medium in which all the biochemical reactions take place in all kinds of organisms. So based on all the mentioned circumstances and examples, we can say that “water is (indeed) life” as if you’re just saying that “kilay is life” meaning for most girls/ladies, they cannot survive without making up their eyebrows, first and foremost, and they cannot function properly without fixing it first. That is, we cannot survive for a longer period of time without water because everything starts on the presence and movement of water in every living organism and its presence in the environment. 4.10 Discuss at least three importance of water to the living system. For additional information and better understanding of the topic, please check and watch any (better if all) of the following sites: Keep Calm https://www.youtube.com/watch?v=3jwAGWky98c its…. https://www.youtube.com/watch?v=pjClihDmfeA https://www.youtube.com/watch?v=ZScEgE55XTM https://www.youtube.com/watch?v=TohtP4GeV70 https://www.youtube.com/watch?v=hjgjYmlK35M Advance: https://www.youtube.com/watch?v=2HeIIT_fGdA Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 11 of 12 REFERENCES: Campbell Neil., et al (2014). Biology. 10th Edition. Pearson Education Inc. Clark Mary Ann, Jung Choi and Matthew Douglas. 2020. Biology 2e Rice University ©2020 Rice University. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). Biochem Lecture Module 4: Properties of Water Prepared by FAVillenas Page 12 of 12