Water, the most abundant molecule in living organisms, is critical in various biological processes due to its unique structure and properties. It facilitates life-sustaining functions such as the transportation of substances, temperature regulation, and biochemical reactions.
Structure of Water Molecules
Water (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The geometry of water molecules is V-shaped or bent, with an angle of approximately 104.5 degrees between the two hydrogen atoms.
The defining characteristic of water is its polarity. Oxygen, being more electronegative, pulls the shared electrons in the covalent bond towards itself, creating a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms. This unequal distribution of charges across the water molecule results in its polarity.
The polarity of water is integral to its ability to form hydrogen bonds, a type of dipole-dipole attraction between the slightly negatively charged oxygen atom of one water molecule and the slightly positively charged hydrogen atoms of another water molecule. These hydrogen bonds are key to many of water's distinct properties.
Properties of Water
Cohesion and Adhesion
Cohesion refers to the attractive force between similar molecules. Due to hydrogen bonding, water molecules tend to stick together, a property known as cohesion.
- High cohesion results in a high surface tension, enabling water droplets to form spheres and small organisms like water striders to 'walk on water.'
- Cohesion also leads to the formation of a meniscus in a graduated cylinder.
Adhesion is the attractive force between different types of molecules.
- Water's ability to adhere to other substances, when combined with cohesion, results in capillary action. Capillary action allows water to move up narrow spaces against gravity, a process critical for the transportation of water and dissolved nutrients from the roots to the rest of the plant in plants' xylem tissue. The cohesion-tension theory further explains how water moves through plants.
Thermal Properties
Water has unique thermal properties, including a high specific heat capacity, high heat of vaporisation, and high heat of fusion, mainly due to the hydrogen bonding between its molecules.
- High specific heat capacity: The specific heat capacity of a substance is the amount of energy required to raise the temperature of one gram of the substance by one degree Celsius. Water's high specific heat capacity means that it can absorb or release a substantial amount of heat with only a slight change in its own temperature. This thermal stability allows water bodies to provide a stable living environment and helps organisms maintain a constant internal temperature, critical for biochemical reactions to proceed at appropriate rates.
- High heat of vaporisation: Heat of vaporisation refers to the amount of energy needed to change one gram of a substance from a liquid to a gas. Water's high heat of vaporisation means that it absorbs a significant amount of heat energy when it evaporates. This property is used by organisms in cooling mechanisms such as perspiration in mammals and transpiration in plants. This property is used by organisms in cooling mechanisms such as perspiration in mammals and transpiration in plants.
- High heat of fusion: Heat of fusion is the amount of energy needed to change one gram of a substance from a solid to a liquid at its melting point. Water has a relatively high heat of fusion, which means that it absorbs a large amount of heat when melting and releases a large amount of heat when freezing. This property prevents bodies of water from freezing solid quickly, allowing aquatic life to survive in cold conditions.
Solvent Properties
Water's polarity enables it to effectively dissolve other polar substances and ionic compounds. This makes water an excellent solvent, often referred to as the 'universal solvent'.
- As a solvent, water dissolves a wide range of substances, including salts, sugars, acids, bases, and gases, making these molecules available for biochemical reactions in cells. The role of water as a solvent is crucial for the metabolism of carbohydrates and lipids, as well as the activation of enzymes that catalyse these reactions.
- For example, the transportation of glucose and oxygen in the blood is facilitated by water's solvent properties.
Significance of Water in Homeostasis
Homeostasis involves maintaining a stable internal environment, and water plays a central role in various aspects of homeostasis.
- Water’s thermal properties help organisms maintain a stable internal body temperature, a critical aspect of homeostasis. For example, the high heat of vaporisation of water allows mammals to cool down via sweating.
- Additionally, the solvent properties of water allow for the transportation of nutrients and waste materials in the body, which are crucial for maintaining homeostasis. For instance, water facilitates the transport of glucose, amino acids, and oxygen in the blood. Understanding osmosis is key to comprehending how water balance is maintained across cell membranes.
Role of Water in Biochemical Reactions
Water is involved in numerous biochemical reactions. It serves as a reactant in hydrolysis reactions and as a product in condensation reactions.
- In hydrolysis reactions, water is used to break bonds in molecules such as carbohydrates, proteins, and nucleic acids. These reactions are crucial for digestion and metabolism.
- In condensation reactions, also known as dehydration synthesis, water is a by-product when bonds are formed between molecules. This process is essential in the formation of complex molecules like proteins, carbohydrates, and nucleic acids.
- Water also participates in redox reactions, specifically in the process of photosynthesis, where water
FAQ
Water's high specific heat capacity and high heat of fusion contribute to the stability of aquatic environments. Its high specific heat capacity means that water can absorb a significant amount of solar heat during the day without experiencing a substantial rise in temperature. Similarly, at night, it can release heat without rapidly cooling. Moreover, the high heat of fusion means that large bodies of water freeze from the top down, creating an insulating layer of ice that allows aquatic life to survive in lower temperatures beneath the surface.
Water has a high surface tension due to the cohesive forces between its molecules. These cohesive forces are a result of hydrogen bonds formed between the slightly negative oxygen atom of one water molecule and the slightly positive hydrogen atoms of another. At the water's surface, molecules are pulled inward by other water molecules, which creates a sort of 'skin' known as surface tension. This high surface tension allows small organisms to 'walk on water' and gives water droplets their spherical shape.
Water is often referred to as a 'universal solvent' because of its ability to dissolve many different substances. This is a result of water's polarity, which allows it to form hydration shells around ions or polar molecules, separating them and causing them to dissolve. While water can't dissolve everything, its solvent abilities are vast, including salts, sugars, acids, bases, and certain gases, which is crucial for many biological processes, such as nutrient transportation and biochemical reactions.
The angle between the two hydrogen atoms in a water molecule is approximately 104.5 degrees because of the shape of the water molecule. It's a V-shaped or bent molecule, with the oxygen atom at the apex of the V. This shape is a result of the tetrahedral arrangement of the electron pairs around the oxygen atom, which includes two lone pairs and two bonding pairs. The repulsion between the electron pairs pushes the hydrogen atoms closer together, forming the 104.5-degree angle.
Water is vital in photosynthesis as it is a primary reactant in the process. In the light-dependent reactions of photosynthesis, water molecules are split in a process known as photolysis. This process provides the electrons needed to replace those lost by chlorophyll molecules when they absorb light and move to a higher energy state. Additionally, the splitting of water generates protons, contributing to the creation of a proton gradient across the thylakoid membrane, driving ATP synthesis. Oxygen, another product of photolysis, is released as a byproduct of photosynthesis.
Practice Questions
Water's polarity is due to the uneven distribution of charges across the molecule. The oxygen atom is more electronegative and pulls the shared electrons in the covalent bond closer to itself, creating a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms. This polarity allows water to form hydrogen bonds, which are responsible for its cohesion and adhesion. Cohesion is the attraction between like molecules, allowing water molecules to stick together, and adhesion is the attraction between different molecules, allowing water to adhere to other substances. These properties, in conjunction with capillary action, are integral to processes such as the movement of water in plants. Additionally, the polarity of water enables it to dissolve other polar substances and ionic compounds, contributing to its function as a solvent.
Water plays a significant role in homeostasis and biochemical reactions. Its high specific heat capacity and high heat of vaporisation help organisms maintain a stable internal body temperature, a critical aspect of homeostasis. For instance, humans cool down through the evaporation of sweat, a process that involves the high heat of vaporisation of water. The solvent properties of water facilitate the transportation of essential substances like nutrients and waste materials in the body. In terms of biochemical reactions, water is both a reactant and a product. It's used in hydrolysis reactions to break bonds in molecules like proteins and carbohydrates during digestion. Conversely, water is produced during condensation reactions when bonds are formed between molecules to create complex compounds such as proteins and nucleic acids.
