2.2.2 Properties of water

Question 1

Name some of waters key roles

Answer 1

-Solvent
-Transport medium
-Medium in which chemical reactions/metabolism take place
-Coolant

Question 2

Explain the importance of water as a solvent with an example

Answer 2

Water molecules are attracted to the charged part of solute molecules.

Respiratory and photosynthetic gases dissolve before entering cells; digestion produces soluble products; metabolism takes place in solution.

Question 3

Explain the importance of water as a liquid at biological temperatures with an example

Answer 3

Hydrogen bonding between the water molecules means it is liquid at biologically important temperatures (0-100°C).

Transport in plants and animals require water

Question 4

Explain the importance of water to do with cohesion and adhesion with an example

Answer 4

Water molecules stick together and are attracted to surfaces.

Surface tension on water; transport of water in xylem relies upon cohesion (and adhesion).

Question 5

Explain the importance of water to do with freezing with an example.

Answer 5

Density changes as water freezes – less dense, so ice floats (layer of ice also insulates).

Water bodies freeze from the top down. Lakes seldom freeze completely; organisms survive in the cold, deeper water.

Question 6

Explain the importance of water to do with thermal with an example.

Answer 6

High specific heat capacity; high latent heat of vaporization.

Water temperature quite stable ; evaporation has a cooling effect.

Question 7

Explain the importance of water to do with metabolism with an example.

Answer 7

Metabolism takes place in aqueous solution; reactant/product in some key reactions.

Hydrolysis; condensation; H 2O used in photosynthesis.

Question 8

Why is water called a universal solvent?

Answer 8

Water is called this as it’s highly effective at separating molecules of other substances, which can then move freely.

Question 9

What is adhesion to do with water molecules?

Answer 9

Water molecules are attracted to other hydrophilic molecules.

Question 10

What is polarity?

Answer 10

This characteristic describes molecules which have a different charge on opposite sides of the molecule.

Question 11

What is cohesion to do with water?

Answer 11

Water molecules are attracted to each other due to their hydrogen bonds.

Question 12

High specific heat capacity of water

Answer 12

A body of water can absorb a lot of energy whilst its temperature remains relatively constant.

Question 13

Why is sweating a good temperature regulator?

Answer 13

The water in sweat evaporates from the skin and takes a lot of heat with it which has a cooling effect. This is due to its high latent heat of vaporization.

Question 14

High specific heat capacity

Answer 14

The specific heat capacity of water (the amount of heat, measured in joules, required to raise 1kg of water 1°C) is very high: much of the heat absorbed is used to break the hydrogen bonds which hold the water molecules together.

Question 15

High latent heat of vaporization of water

Answer 15

hydrogen bonds attract molecules of liquid water to one another and make it difficult for the molecules to escape as vapour: thus a relatively high energy input is necessary to vaporize water and water has a much higher boiling point than other molecules of the same size.

Question 16

Molecular mobility of water

Answer 16

The weakness of individual hydrogen bonds means that individual water molecules continually jostle one another when in the liquid phase.

Question 17

Density and freezing properties

Answer 17

As water cools from towards its freezing point the individual molecules slow down sufficiently for each one to form its maximum number of hydrogen bonds. To do this the water molecules in liquid water must move further apart to give enough space for all four hydrogen bonds to fit into it. As a result water expands as it freezes, so that ice is less dense than liquid water and therefore floats upon its surface.

Question 18

Colloid formation in water

Answer 18

Some molecules have strong intermolecular forces which prevent their solution in water, but have charged surfaces which attract a covering of water molecules. This covering ensures that the molecules remain dispersed throughout the water, rather than forming large aggregates which could settle out. The dispersed particles and the liquid around them collectively form a colloid.

Question 19

What is the biological importance of waters solvent properties?

Answer 19

Allow water to act as a transport medium for polar solutes. For example, movements of minerals to lakes and seas; transport via blood and lymph in multicellular animals; removal of metabolic wastes such as urea and ammonia in urine.

Question 20

What’s the biological importance of water as a transportation stream?

Answer 20

The continuous column of water is able to move up the xylem because of cohesion between water molecules and adhesion between water and the walls of the xylem cell tissue.

Question 21

What’s the biological importance of molecular mobility?

Answer 21

The rather weak nature of individual hydrogen bonds means that water molecules can move easily relative to one another - this allows osmosis ( vital for uptake and movement of water) to take place.

Question 22

What is the biological importance of water expand on freezing?

Answer 22

Since ice floats it forms at the surface of ponds and lakes - it therefore insulates organisms in the water below it, and allows the ice to thaw rapidly when temperatures rise. Changes in density also maintain circulation in large bodies of water, thus helping nutrient cycling.

Question 23

What is the biological importance of metabolic functions of water?

Answer 23

Water is used directly…
1. as a reagent (source of reducing power) in photosynthesis
2. to hydrolyse macromolecules to their subunits, in digestion for example
…and is also the medium in which all biochemical reactions take place.

Question 24

What is the biological importance of volatility/stability of water?

Answer 24

Is balanced at Earths temperatures so that a water cycle of evaporation, transpiration and precipitation is maintained.

Question 25

What is the biological importance of water as a supporting role?

Answer 25

The cohesive forces between water molecules mean that it is not easily compressed, and thus it is an excellent medium for support.

Question 26

What’s the biological importance of water’s transparency?

Answer 26

Water permits the passage of visable light. This means that photosynthesis (and associated food chains) is possible in relatively shallow aquatic environments.

Question 27

What is the biological importance of water in lubrication?

Answer 27

Waters cohesive properties means that it is viscous, making it a useful lubricant in biological systems.

For example: Synovial fluid - lubricates many vertebrae joints
Pleural fluid - minimizes friction between lungs and thoracic cage (ribs) during breathing.
Mucus - permits easy passage of faeces down the colon, and lubricantes the penis and vagina during intercourse.

Question 28

What is the biological importance of water in thermoregulation?

Answer 28

The high specific heat capacity of water means that bodies composed largely of water (cells are typically 70-80% water) are very thermostable, and thus less prone to heat damage by changes in environmental temperatures.

The high latent heat of vaporization of water means that a body can be considerably cooled with a minimal loss of water - this phenomenon is used extensively by mammals (sweating) and reptiles (gaping) and may be important in cooling transpiring leaves.

Question 29

Water to do with metabolism

Answer 29

• Hydrolysis of bonds during digestion
• Source of H+ ions in photosynthesis
• Produced in respiration

Question 30

Water to do with transport

Answer 30

• Many water-soluble molecules are transported in solution
• Mamallian transport system contains blood: blood plasma is principally water
• Plants use mass flow and transport materials in their vascular system.