A1.1 Water

State where the first cells originated

For life to form, the molecular ingredients of life need to react with each other in a liquid solvent, so cells originated in water.

List reasons why water is the substance on which life depends

• Water is a solvent, and it acts as a medium in which chemical reactions of life can occur.

• Water is a metabolite in many reactions

• Water is a temperature buffer, allowing for enzyme activity

• Water maintains biological structures

Describe the structure of an atom

A positively charged nucleus containing protons and neutrons surrounded by negatively charged electrons.

Outline the formation of ionic and covalent bonds between atoms

• Transfer of electrons between a metal and a non-metal with electrostatic attraction.

• Sharing of pairs of electrons between two non-metals with similar electronegativity.

Explain the sharing of electrons between atoms in a polar covalent bond

Electrons are shared unequally because one atom is more electronegative than the other resulting in a polar molecule

State the location of the polar covalent bond within a water molecule

The polar covalent bond forms between the central slightly negative O atom and the 2 H atoms slightly positive.

Explain the partial charges of the oxygen and hydrogen atoms within a water molecule

Because the oxygen atom has more protons it attracts the shared electrons more, creating an unequal distribution of charge

Draw hydrogen bonding between water molecules

Outline the cause of the formation of hydrogen bonds between water molecules

The partially positive hydrogen atoms of a water molecule are attracted to the partially negative oxygen atom of another water molecule forming a hydrogen bond

Outline the consequences of the collective strength of hydrogen bonds between water molecules

• H bonds are made and broken quickly as the molecules move, but the large number of bonds contributes to water’s stability and retention on Earth.

• Water’s ability to attract charged and polar molecules gives it many physical properties

Define cohesion

Water’s ability to make hydrogen bonds with itself allows it to stick with other water molecules

Describe how water moves through the xylem of a vascular plant

As transpiration pull occurs through the stomata, it creates an upward suction that pulls water up in the xylem tissue, and the cohesion of water molecules pulls it up to the stomata in a chain-like manner.

Outline the cause of surface tension

The cohesive properties of water molecules create an inward force, allowing the liquid to resist external forces and behave as an elastic membrane

State a benefit to living things that results from surface tension

Water striders can walk on water due to the surface tension

Define adhesion

Water molecules are attracted to hydrophilic molecules, which are polar or charged

Define polar

Polar molecules result from an unequal sharing of electrons between atoms of a molecule

Define ion

Charged ions result form when when atoms have different numbers of electrons and protons

Compare cation and anion

• A cation has a positive charge with more protons than electrons

• An anion has a negative charge with more electrons than protons

Explain why water is attracted to molecules that are polar or charged. 

Water is a polar molecule, so it forms electrostatic or hydrogen bonds with opposite charges on other particles

Outline the cause of capillary action

Adhesion of water to the walls of a vessel will cause an upward force on the liquid against external forces like gravity

Describe capillary action in plant tissue

Cohesive forces between water molecules and adhesive forces between water molecules and the cellulose in xylem cell walls facilitate the upward transport of water to the stomata.

Outline the cause and effect of capillary action in soil

Adhesion of water molecules to soil particles and cohesion between water molecules allow water to move upwards through soil pores allowing plants to take in the water through osmosis.

Identify solvent and solutes of a solution

• Solvent is the liquid in which a solute dissolves

• Solute is the substance dissolved in a solvent

Define solvation

Interaction of a solvent with a dissolved solute

Explain why water is able to dissolve charged and polar molecules

• Polar molecules are hydrophilic and can form hydrogen bonds with water

• Charged ions are electrostatically attracted to water and are hydrophilic

Outline the solvation of hydrophilic and hydrophobic substances

• Water forms hydrogen bonds with hydrophilic or polar molecules, forming a hydration shell around the solute, causing them to separate from each other

• Non-polar molecules or hydrophobic molecules are insoluble and are attracted to other hydrophobic molecules, so they will clump together when exposed to water

State an example of a molecule with a function that depends on it being hydrophobic and insoluble

Phospholipids have a hydrophobic tail, causing them to form lipid bilayers which forms the cell membrane

State an example of a molecule with a function that depends on it being hydrophilic and soluble

The outer surface of the hemoglobin protein has a high concentration of hydrophilic amino acids allowing it to dissolve in the cytoplasm of red blood cells

Outline the role of water as a medium for metabolism

Cytosol is the liquid part of the cytoplasm that dissolves reactants and enzymes needed for cellular anabolic and catabolic reactions

Describe the role of water as a medium for transport in vascular plants

Dissolved mineral ions and sugars can be transported in an aqueous solution in the xylem and phloem of vascular plants.

Describe the role of water as a medium for transport in animal blood

Dissolved salt ions, amino acids, gases, proteins, glucose, and waste products can be transported in the animal blood plasma

Explain how lipids are transported in blood

Lipids are hydrophobic; therefore, they are coated in phospholipids to form a lipoproteins which are amphipathic

Define physical property

A characteristic of matter that exists without the matter needing to react with other things

List physical properties of water that are consequential for animals in aquatic habitats

• Buoyancy

• Viscosity

• Thermal conductivity

• Specific heat capacity

Outline the cause and effect of buoyancy

Buoyancy is the upward force applied to an object in a fluid. If the density of the object is less than the density of the fluid, then the buoyant force will be greater than gravity, causing the object to float

Outline the cause and effect of viscosity

Viscosity is the measure of the fluid’s tendency to flow, and it depends on the amount of friction the molecules of a liquid experience as they flow over each other.

Compare viscosity of air to water to blood

• Air is the least viscous because its molecules are far apart and have weak attraction forces

• Water is more viscous because it forms hydrogen bonds with itself

• Blood is more viscous because it includes cells and dissolved solutes

Define thermal conductivity

It is the measure of a substance’s ability to move temperature across a gradient, and it depends on how easily energy travels through the substance

Outline a consequence to life of the thermal conductivity of air and water

A thick layer of fat called blubber is used in seals to protect from cold temperatures.

Define specific heat capacity

The amount of heat needed to raise the temperature of a substance per unit mass

Compare less conductive to more conductive materials

• Less conductive: Heat moves slowly, better at insulation and preventing heat loss

• More conductive: Heat moves quickly, better for absorbing and transferring heat

Describe why water has a high specific heat capacity

Water has numerous hydrogen bonds which are weak, but due to their large number, a lot of energy is required to break them all.

State two benefits to life of the high specific heat capacity of water

• Water can help the body’s temperature rise or lower according to external temperature changes

• The temperature of aquatic habitats changes slowly despite surrounding air temperatures

Outline a benefit to life of water's high specific heat capacity.

Provides a stable environment for the habitat of living things

Compare the physical properties of water to those of air.

• Water has a higher buoyant force than air

• Water has higher viscosity than air

• Water has higher thermal conductivity than air

• Water has higher specific heat capacity than air

Describe how the  black-throated loon (Gavia arctica) and/or the ringed seal (Pusa hispida) interact with the physical properties of water in their habitat

• Ringed seal: Buoyant force allows the ringed seal to stay afloat, water is viscous so the seal has a streamline body to help swim through it, the seal has blubber to protect from water’s high thermal conductivity, and water has high specific heat capacity allowing for a stable environment for the seal

• Black throated loon: Due to lower buoyant force of air, the loon has to expend energy to fly, air is not as viscous so it can fly through it with a streamline body, it doesn’t lose as much body heat due to low thermal conductivity of air, but it has feather insulation to protect from air’s low specific heat capacity.

Explain the hypothesis that asteroids are responsible for the origin of water on Earth

Water came on Earth from extra-planetary objects like asteroids and comets because at present, asteroids near Earth seem to be the most likely sources of water.

State two reasons why water was retained on early Earth.

• The distance from the Earth to the Sun keeps the temperature of the Earth low enough to condense water.

• Earth’s gravity keeps water from escaping the planet

Outline the evidence for extraplanetary origin of water

The deuterium-to-protium ratio of Earth’s ocean water matches that of asteroids that crashed on Earth

Explain why the presence of water is considered fundamental to the search for extraterrestrial life

Water functions as a solvent allow key chemical reactions to take place, and its physical properties provide benefits to all life on Earth

Define “Goldilocks zone” in relation to the search for extraterrestrial life

Objects that fall in the right distance from a star so that water remains in its liquid form on the surface.