Hydrogen bonds are constantly forming and breaking, with each bond lasting for just a fraction of a second. The hydrogen bonds hold water together by the property of cohesion (the molecular force between particles within a substance that acts to unite them).
Cohesion, due to hydrogen bonding contributes to wave formation and all other water movements that take place in lakes. Water movements play a vital part in the distribution of temperature, nutrients and dissolved gases. The movements also determine the distribution of plankton and micro-organisms.
Cohesion also holds together the transpiration stream, as well as adhesion (water molecules attach themselves to the side of the xylem vessel). Water is therefore able to flow easily through tubes such as the xylem vessels, due to its low viscosity.
Surface Tension
Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than all other liquids, except mercury. At the point where water and air meets, is an ordered arrangement of water molecules which are hydrogen bonded to one another and the water below, resulting in an interface surface under tension. Molecules that are dissolved in water lower it’s surface tension, playing an important part in molecules that move across a plasma membrane, for example. Surface tension is also important in the movement of water up the capillary-like vessels and tracheids in the stems of plants. It enables the surface film to support, and provide a suitable habitat for aquatic organisms. Many organisms have evolved to allow them to spread their body over a large surface area to prevent breaking water’s surface tension.
Turgidity, Support and Movement
Water helps plant cells to remain turgid, which is essential for maintaining maximum leaf surface area for absorption and photosynthesis to take place. It also helps to maximise pollination and seed dispersal. Leaves will also wilt if they do not have sufficient supplies water, which will decrease their surface area. This would result in light absorption, temperature and water loss.
Water-filled tissues also provide skeletal support in animals. Water is the major component found in the fluid situated in the coelum, where muscles can act against it in organisms such as annelids, which have a hydrostatic skeleton. Therefore, water also contributes to movement within many organisms.
Transport
The uptake of minerals from soil through the root hairs by plants takes place in a solution. A further example of transport, involving water is the transpiration stream and the water-based movements of hormones, sugars and amino acids in the phloem. The cytoplasm, blood, tissue fluid and plasma in animals, which are all involved with transport, are all water-based.
Water as the Universal Solvent
Water can dissolve almost anything, making it an ideal environment for chemical reactions to occur. When sodium chloride is placed in water, for example, the attraction of the ions that are held together is greatly reduced. This is because water weakens the attraction of ions of opposite charge, and once they have separated, water molecules surround the ions, preventing them from rejoining. Due to this, water is a very good solvent, which is of great biological importance, as chemical reactions that take place in cells do so in an aqueous solution. For example, enzyme reactions of photosynthesis, respiration and excretion occur in solution. Water also acts as a reactant in the light dependent stage of photosynthesis, meaning that carbon dioxide is also solvent in water. A further example is hydrolytic reactions, such as digestive enzymes, where water also acts as a reactant.
Evaporation and Cooling
Water has a high heat of vaporisation (the energy required to convert liquid water to a gas). Due to the great amount of energy needed to break the hydrogen bonds that hold water molecules together, more energy is necessary to evaporate liquid water than most other substances.
Water’s high heat of vaporisation helps moderate the earth’s climate. A large amount of energy from the sun is absorbed by lakes, for example during the evaporation of its surface waters. As water evaporates, the surface that is remaining cools down. This is because the warmest molecules are those with the greatest kinetic energy and are most likely to leave in the gaseous state. The evaporative cooling of water helps to stabilise temperature.
Thermal Properties
Hydrogen bonding is responsible for the thermal properties of water. Water has a high heat capacity, which is crucial for living organisms, as they need to maintain certain temperatures in order for enzyme activity to take place. The high water content of cells and tissues ensures that a constant temperature is maintained. This is known as water acting as a temperature buffer.
Water is one of the very few substances that is less dense when in the form of ice. Most substances contract when they solidify, but water expands, due to the hydrogen bonding. As the temperature of water decreases, the kinetic energy of the individual molecule decreases and the
molecules slow down, allowing the maximum number of four hydrogen bonds to form with neighbouring water molecules.
The structure that is formed by hydrogen bonds in ice:
For this structure to form, the water molecules spread out and expand to allow maximum bond formation. Water freezes from the top and progresses down, causing ice to float and act as an insulator, meaning that many organisms can survive below the ice surface.
Organisms maintain a constant body temperature, due to evaporation, which allows sweating or panting to take place. This is because when water molecules manage to escape during evaporation, a lot of energy is also released, providing a good cooling procedure.
Water has a number of unique properties, making it vital for organisms to survive. Without it, life would not ever have evolved. It provides surface tension, allowing many organisms to support themselves and move efficiently. It provides the perfect environment for chemical reactions to take place, due to the fact that it is a universal solvent. Water also has a very high heat capacity, allowing organisms to maintain the appropriate body temperature, which is vital for their survival. Finally, water has a unique structure, involving polarity and hydrogen bonding. By considering these examples and the many others that have also been discussed, it is clear to see how unique and important the chemical and physical properties of water are to all living organisms.
