Water
The electrons in the covalent bond between hydrogen and oxygen in water are not shared equally. The oxygen atom is more electronegative so it attracts the electrons more giving it a partial negative charge (δ-), leaving the hydrogen atoms with a partial positive charge (δ+). This makes water a polar molecule. This gives water the ability to form hydrogen bonds. This is a weak electrostatic attraction between the δ- on an oxygen atom of 1 water molecule and the δ+ on a hydrogen atom of another water molecule. Each water molecule can form up to 4 hydrogen bonds. Individually, hydrogen bonds are weak, however collectively they are strong.
Properties of water that are important to living organisms:
Liquid at room temperature: Water is a liquid at room temperature and atmospheric pressure. Water molecules constantly move around breaking and re-forming hydrogen bonds. It takes energy to break the hydrogen bonds to form a gas. Despite hydrogen bonds, water has low viscosity and flows easily. This provides a habitat for living organisms in rivers, lakes and oceans. Water is a major component of cells and is a medium for metabolic reactions. Water is also an important transport medium.
Density: Water is densest at 4°c. Solid water (ice) is less dense so floats on the surface of liquid water. When water freezes the water molecules form a regular lattice and are held in place by hydrogen bonds, so the molecules are more spread out than in a liquid. The density of water allows large aquatic organisms such as whales to float. Water freezes from the surface down. Ice insulates the surface of water so large bodies of water do not freeze solid allowing aquatic organisms to survive the winter underneath the ice. Ice is a habitat for some organisms such as polar bears (require ice to hunt from). Warmer water is less dense and rises. Changes in density with temperature, is important in circulating nutrients in oceans.
Polar solvent: Water is a polar molecule and dissolves other polar molecules and ions. Water is a polar molecule and is attracted to other polar or changed molecules and ions. Water surrounds the particles forming a hydration shell and separates them, so they dissolve. Most metabolic reactions take place in aqueous solution. Substances are transported in aqueous solution e.g. glucose in blood plasma.
Cohesion and adhesion: Water is viscous as its molecules cohere to each other and adhere to other molecules. Water molecules form hydrogen bonds with each other (cohesion) so tend to stick to each other. Water molecules also form hydrogen bonds with other polar molecules (adhesion). Cohesion allows water to form continuous columns of water, which is important for transport of water in xylem vessels in plants. This is helped by adhesion to lignin in the xylem vessel walls. Surface tension allows insects such as pond skaters to move across the surface of water.
High specific heat capacity: A large amount of energy is required to make bodies of water change temperature. Hydrogen bonds restrict the movement of water molecules. Energy is required to break the hydrogen bonds and raise the temperatures of water. Energy is also released when hydrogen bonds form. Internal temperature changes are minimised which helps organisms (contain 70%-90% water) to maintain a stable body temperature for enzyme activity. Temperature changes of large bodies of water are minimised, so they provide a thermally stable environment for aquatic organisms.
High latent heat of vaporisation: A large amount of heat energy is required to separate a water molecule from other water molecules in a liquid to become a vapour. A lot of energy is required to break the many hydrogen bonds between the water molecules so water molecules can separate to become a vapour. Evaporation of water cools organisms (e.g. sweating and panting in animals/evaporation from the mesophyll cells as part of transpiration in plants).
(Description/explanation/biological importance)
Water
The electrons in the covalent bond between hydrogen and oxygen in water are not shared equally. The oxygen atom is more electronegative so it attracts the electrons more giving it a partial negative charge (δ-), leaving the hydrogen atoms with a partial positive charge (δ+). This makes water a polar molecule. This gives water the ability to form hydrogen bonds. This is a weak electrostatic attraction between the δ- on an oxygen atom of 1 water molecule and the δ+ on a hydrogen atom of another water molecule. Each water molecule can form up to 4 hydrogen bonds. Individually, hydrogen bonds are weak, however collectively they are strong.
Properties of water that are important to living organisms:
Liquid at room temperature: Water is a liquid at room temperature and atmospheric pressure. Water molecules constantly move around breaking and re-forming hydrogen bonds. It takes energy to break the hydrogen bonds to form a gas. Despite hydrogen bonds, water has low viscosity and flows easily. This provides a habitat for living organisms in rivers, lakes and oceans. Water is a major component of cells and is a medium for metabolic reactions. Water is also an important transport medium.
Density: Water is densest at 4°c. Solid water (ice) is less dense so floats on the surface of liquid water. When water freezes the water molecules form a regular lattice and are held in place by hydrogen bonds, so the molecules are more spread out than in a liquid. The density of water allows large aquatic organisms such as whales to float. Water freezes from the surface down. Ice insulates the surface of water so large bodies of water do not freeze solid allowing aquatic organisms to survive the winter underneath the ice. Ice is a habitat for some organisms such as polar bears (require ice to hunt from). Warmer water is less dense and rises. Changes in density with temperature, is important in circulating nutrients in oceans.
Polar solvent: Water is a polar molecule and dissolves other polar molecules and ions. Water is a polar molecule and is attracted to other polar or changed molecules and ions. Water surrounds the particles forming a hydration shell and separates them, so they dissolve. Most metabolic reactions take place in aqueous solution. Substances are transported in aqueous solution e.g. glucose in blood plasma.
Cohesion and adhesion: Water is viscous as its molecules cohere to each other and adhere to other molecules. Water molecules form hydrogen bonds with each other (cohesion) so tend to stick to each other. Water molecules also form hydrogen bonds with other polar molecules (adhesion). Cohesion allows water to form continuous columns of water, which is important for transport of water in xylem vessels in plants. This is helped by adhesion to lignin in the xylem vessel walls. Surface tension allows insects such as pond skaters to move across the surface of water.
High specific heat capacity: A large amount of energy is required to make bodies of water change temperature. Hydrogen bonds restrict the movement of water molecules. Energy is required to break the hydrogen bonds and raise the temperatures of water. Energy is also released when hydrogen bonds form. Internal temperature changes are minimised which helps organisms (contain 70%-90% water) to maintain a stable body temperature for enzyme activity. Temperature changes of large bodies of water are minimised, so they provide a thermally stable environment for aquatic organisms.
High latent heat of vaporisation: A large amount of heat energy is required to separate a water molecule from other water molecules in a liquid to become a vapour. A lot of energy is required to break the many hydrogen bonds between the water molecules so water molecules can separate to become a vapour. Evaporation of water cools organisms (e.g. sweating and panting in animals/evaporation from the mesophyll cells as part of transpiration in plants).
(Description/explanation/biological importance)
