A1.1 Water

Water as the medium for life

The first cells evolved in a watery environment. Water in its liquid state allows dissolved molecules to move around, so they are easily able to collide and react with each other. Most life processes occur in water

Cohesion

Attraction between molecules of the same type, how water molecules form hydrogen bonds between one another and stick together

Adhesion

The force of attraction between unlike molecules

Structure of water

Water is composed of one atom of oxygen and two atoms of hydrogen, which are held together by covalent bonds. The oxygen atom attracts electrons more strongly than the hydrogen atoms, creating an uneven sharing of electrons. This results in a weak negatively charged region on the oxygen atom and a weak positively charged region on the hydrogen atoms, creating an asymmetrical shape

Hydrogen bonds in water

The polarity of water enables the formation of hydrogen bonds between the positive and negatively charged regions of adjacent water molecules

Properties of water: cohesion

Hydrogen bonds between water molecules allows for strong cohesion between water molecules. This allows columns of water to move under tension through the xylem of plants. Surface tension occurs due to hydrogen bonds between the top layer of water molecules, creating a film on the body of water. This allows insects to move across the surface of water

Properties of water: adhesion

Water is able to bond to other polar or charged molecules such as cellulose. This enables water to move up the xylem during transpiration, as it is drawn up capillary tubes by capillary action. The spaces between cellulose fibres in plant cell walls can draw water from xylem vessels to allow water to flow through plant tissue

Capillary action

The movement of water molecules through a narrow space where the force of gravity is exceeded and water can flow upwards

Transpiration

The loss of water from leaves by evaporation through the stomata

Properties of water: solvent properties

Polar molecules with positive or negative charges can form hydrogen bonds with water, so they are hydrophilic. Non-polar molecules cannot form hydrogen bonds with water, so they are hydrophobic. Most biological molecules are hydrophilic and can be dissolved, thus water is regarded as the universal solvent

Highly soluble molecules

Sodium chloride, urea, salts, glucose, amino acids

Insoluble molecules

Phospholipids

Less soluble molecules

Oxygen is sparingly soluble. Haemoglobin binds to oxygen to allow sufficient oxygen to be transported to body cells

Physical properties of water: specific heat capacity

Specific heat capacity is a measure of the energy required to raise the temperature of 1kg of a substance by 1ºC. Water has a higher SHC compared to air, which is due to the hydrogen bonds present in water, which require a lot of energy to break. Thus, water temperature does not fluctuate greatly, providing a stable aquatic habitat.

Ringed seal

The ringed seal is able to survive throughout the year due to the stable sea temperatures caused by the high SHC of water. The density of ice is lower than liquid water, which means that ice floats on water, forming a habitat for seals. The seal relies on a layer of fat called blubber to insulate it from the outside air

Physical properties of water: thermal conductivity

Thermal conductivity refers to the ability of a substance to conduct heat. The thermal conductivity of water is almost 30x higher than air, which makes air a good insulator for cold climates

Black-throated loon

A species of diving bird which spends much time underwater catching its prey. Their feathers trap an insulating layer or air, which helps them regulate body temperature

Physical properties of water: buoyancy

The ability of an object to float in water. The black-throated loon has solid bones which increase their weight and compresses air out of the lungs and feathers during a dive. The layer of blubber on the seal improves its buoyancy

Physical properties of water: viscosity

The resistance of a fluid to flow. The viscosity of water is much higher than air, enabling the loon to fly through the air without much friction. The seal has flippers to propel itself, the loon uses its webbed feet to push against the water and reduce drag

Extraplanetary origin of water

It was hypothesised that asteroids, and the meteorites that break off from them, may be the origin of Earth’s water, as many contain ice and organic materials. During impact, these meteorites may have released water vapour trapped by the Earth’s gravity

Goldilocks zone

The area around a star where temperatures are favourable for water to exist in liquid form

For an exoplanet to support life, it must have the following characteristics:

A water signature, be located in the Goldilocks zone of its solar system, and be large enough to support an atmosphere