Water: The Medium for Life

Water as the Medium for Life

• The first cells originated in water, as oceans blocked harmful ultraviolet radiation, allowing early life to evolve.
• Approximately 71% of the Earth's surface is water.
• 97% of the Earth's water is found in oceans, while only 3% is fresh water.
• Water forms a large proportion of living organisms: 65%-95% by mass of most multicellular plants and animals.
• About 80% of a human cell consists of water.
• Water is the medium in which most life processes occur.

Hydrogen Bonds and Polar Covalent Bonds

• Water (H₂O) consists of two hydrogen atoms covalently bound to an oxygen atom.
• In water, electrons are shared unequally between hydrogen and oxygen atoms.
• Oxygen is more electronegative and has a stronger pull on shared electrons than hydrogen atoms.
• The oxygen end of the water molecule becomes slightly negative, while the hydrogen ends become slightly positive.
• Covalently bonded molecules with a slight charge are polar covalent.
• Slightly charged regions of the water molecule attract other polar or charged compounds.
• Water molecules associate via weak hydrogen bonds.
• Hydrogen bonds are transitory and constantly form, break, and reform.

Cohesion and Surface Tension

• Cohesion is the force by which individual molecules of the same type attract and stick together.
• Surface tension is a liquid's property that allows it to resist an external force due to cohesion between water molecules.
• Cohesive forces result in water droplets forming into a sphere, which has the smallest possible volume.

Adhesion and Viscosity

• Adhesion is the force by which individual molecules stick to surrounding materials and surfaces when they are polar or charged.
• Water molecules stick to other molecules that are charged or polar due to the similarity in why they stick to each other.
• A large number of bonds provides adhesive forces.
• Water droplets stick to surfaces and seem to defy gravity because of adhesive forces.
• Viscosity is a measure of a fluid's resistance to flow.

Capillary Action

• Capillary tubes are channels with a very small internal diameter.
• Capillary action is the tendency of a liquid to move up against gravity when confined in a narrow tube (capillary).
• Water is drawn up the tree trunk by force generated in transpiration.

Solvent Properties

• A solute is a dissolved molecule or ion in a solution. A solvent is a liquid in which another substance can be dissolved.
• Water can dissolve polar regions.
• The polar attraction of large quantities of water molecules can interrupt intra-molecular forces (like ionic bonds) which results in the dissociation of atoms.
• Hydrophilic substances are chemically attracted to water.
• Hydrophobic substances are insoluble in water.
• Due to cells mostly being water, diffusion into and out of them happens easily when a substance is in solution.

Physical Properties

• Buoyancy is the ability of any fluid to provide a vertical upwards force on an object placed in or on it.
• Thermal conductivity (k) measures how easily heat flows through a specific material.
• Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1°C.
• Density, thermal conductivity, specific heat capacity, and dynamic viscosity are all physical properties of water.