# 2.5 Water as a Medium of Life Water is an essential constituent for all life on Earth, with its distribution being directly proportional to the abundance of life. Organisms are composed of approximately 70-90% water, with concentrations varying from 20% in seeds and bones to over 99% in jellyfish. Its unique properties make it the "medium of life," where most biochemical reactions occur. ## Properties of Water Water's vital role in biology is due to its unique chemical and physical properties, which stem from its polarity and the resulting hydrogen bonds. ### 1. High Polarity A water molecule ($\text{H}_2\text{O}$) consists of one oxygen atom joined to two hydrogen atoms by *polar covalent bonds*. In these bonds, electrons are shared unequally. The more electronegative oxygen atom pulls the shared electrons closer, giving it a slight negative charge ($\delta^-$), while the hydrogen atoms are left with slight positive charges ($\delta^+$). This makes water a **polar molecule**. * **Universal Solvent:** Because of its polarity, water is an excellent solvent for other polar and ionic substances (electrolytes). When dissolved, these substances dissociate into ions, which is a favourable state for chemical reactions. * **Biological Significance:** This property ensures that nearly all biochemical reactions in cells occur in an aqueous medium. ### 2. Hydrogen Bonding The polarity of water molecules allows them to form weak attractions with each other. A **hydrogen bond** forms between the slightly positive hydrogen atom of one molecule and the slightly negative oxygen atom of a neighbouring molecule. * **State of Matter:** These numerous but weak bonds are why water is a liquid at temperatures suitable for life. * **Biological Significance:** Hydrogen bonding is responsible for many of water's other crucial properties, including cohesion, adhesion, and high specific heat capacity. ### 3. Cohesion and Adhesion * ***Cohesion*** is the attraction between water molecules, causing them to stick together. This creates high surface tension. * ***Adhesion*** is the attraction of water molecules to other polar surfaces. * **Biological Significance:** Together, cohesion and adhesion allow water to be an effective **transport medium**, enabling it to move through the vascular tissues of plants (xylem) and the circulatory systems of animals. ### 4. High Specific Heat Capacity **Specific heat capacity** is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water has a very high specific heat capacity (1 Calorie/g/°C or 4.18 J/g/°C). * **Reason:** A large amount of heat energy is first used to break the hydrogen bonds between water molecules before the temperature of the water itself can increase. * **Biological Significance:** This property allows water to act as a **temperature stabilizer (regulator)**. It protects organisms from sudden thermal changes, helping them maintain a stable internal temperature (homeostasis). ### 5. High Heat of Vaporization **Heat of vaporization** is the amount of heat needed to convert a unit mass of a liquid into a gas. Water has a high heat of vaporization (574 calories/gram). * **Reason:** Significant energy is required to break the hydrogen bonds and allow water molecules to escape as vapor. * **Biological Significance:** This is a highly efficient cooling mechanism for organisms. When sweat or water evaporates from a surface (e.g., skin, leaves), it carries away a large amount of body heat, causing a cooling effect with minimal water loss. For example, the evaporation of just 2 mL of water can lower the temperature of 998 mL of remaining water by 1°C. ### 6. Hydrophobic Exclusion **Hydrophobic exclusion** is the tendency of non-polar (hydrophobic) substances to aggregate and minimize their contact with water. * **Example:** Oil droplets in water will merge to form a single large drop. * **Biological Significance:** This principle is crucial for the formation and integrity of **lipid bilayer membranes** (cell membranes), as the hydrophobic tails of phospholipids spontaneously orient themselves away from the aqueous environment. ### 7. Ionization Water molecules can reversibly dissociate (**ionize**) into a hydrogen ion ($\text{H}^+$) and a hydroxyl ion ($\text{OH}^-$). $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$ * **Biological Significance:** These ions are highly reactive and participate in many cellular reactions. Their concentration determines the **pH** of a solution, which is critical for enzyme function and other biochemical processes. ### 8. Lower Density of Ice Unlike most substances, the solid form of water (ice) is **less dense** than its liquid form, which is why ice floats. * **Reason:** As water freezes, its molecules form a crystal **lattice** structure. The hydrogen bonds hold the molecules a fixed distance apart, creating a more open, and therefore less dense, arrangement than in liquid water. * **Biological Significance:** In freezing weather, a layer of ice forms on the surface of ponds and lakes. This ice acts as an **insulating layer**, protecting the aquatic life in the liquid water below from freezing. ## Possible Questions/Answers - **Q:** Why is water considered a "polar molecule"? **A:** Because of the unequal sharing of electrons in its covalent bonds, the oxygen atom has a slight negative charge, and the hydrogen atoms have slight positive charges, creating two poles. - **Q:** What is the biological importance of water's high specific heat capacity? **A:** It acts as a temperature stabilizer for organisms and environments, protecting against sudden temperature fluctuations and helping maintain homeostasis. - **Q:** How does sweating cool the body? **A:** Sweating utilizes water's high heat of vaporization. As sweat evaporates from the skin, it absorbs a large amount of body heat, thus cooling the organism. - **Q:** Why is it significant that ice floats on water? **A:** The floating ice layer on ponds and lakes in winter insulates the water below, preventing it from freezing solid and allowing aquatic organisms to survive.