Water

Give one example of the biological importance of water as a *solvent* (1 mark)

Transport of nutrients, in plant

4. Organisms that live on land usually experience greater ranges of environmental temperature than organisms living in water. Explain why (2 marks)

Water *higher specific heat capacity* than air (1) So *more* heat needed to raise temperature of water than air (1)

1. Give *two* properties of water that are important in biology. Explain the importance of each property you identify (4 marks)

*Any two* 1. *Polar*; dissolves in charged particles (ions)/ acts as universal solvents (2) 2. A *metabolite*; involved in metabolic reactions/ condensation/ hydrolysis (2) 3. *Cohesion* between water molecules; provides surface tension/ prevents columns of water breaking (2) 4. *High latent heat of evaporation*; evaporation of small amount of water cools organisms (2)

6. Scientists looking for extra-terrestrial life are looking for planets with evidence of free water. Explain why water is considered so important for life to occur (6 marks)

*Life* thought to have evolved in water (1) It provides *support* for bodies of organisms (1) Water is a *major component* of cytoplasm (1) Water is a *universal solvent*/metabolic reactions occur in aqueous solution (1) Water is a *metabolite* (1) Water *stabilises* external temperature (1) Water important in *cooling*/stabilising internal temperature of organisms (1)

Explain why it is a benefit to organisms that water has a high specific heat capacity (2 marks)

1. Requires lots of heat energy to increase temperature of water 2. Maintains optimum temperature for enzyme

Describe how water is moved through a plant according to the cohesion tension hypothesis (4 marks)

1. water evaporates/ transpires from leaves 2. reduces water potential in cell 3. water drawn out of xylem 4. creates tension 5. cohesive forces between water molecules 6. water pulled up as a column

5.1. Most substances contract when they cool. Water is unusual. As it freezes, it expands, becoming lighter. Suggest why water becomes lighter as it expands (2 marks)

Density= mass/volume (1) Ice same mass water but greater volume (1)

Explain why it is a benefit to organisms that a large amount of heat energy is required to evaporate water (1 mark)

Cooling effect helps body maintain temperature

2.2. Explain *one* other effect on the plants caused by the evaporation of water from its leaves (2 marks)

Cools leaves (1) *Evaporation* uses heat from leaves (1)

3.2. Give *one* example of a hydrophilic substance and *one* example of a hydrophobic substance (2 marks)

Hydrophilic: oxygen, carbon dioxide, amino acid, glucose, sucrose... Hydrophobic: lipid

3.1. Explain the meaning of the terms hydrophilic and hydrophobic (2 marks)

Hydrophilic; *will* mix with water (1) Hydrophobic; will *not* mix with water (1)

5.2. Suggest *one* biological advantage of this property of water (2 marks)

Ice colder than water (1) Being lighter than water, ice floats on water (1) Reduces freezing of water below ice (1)

Explain why it is a benefit to organisms that when water freezes to ice, it becomes less dense (1 marks)

Provides a habitat

2.1. The evaporation of water from the leaves of a plant results in columns of water being pulled up through its narrow xylem vessels. Explain why this column of water does *not* break (3 marks)

Water molecules are *polar* (1) Form *hydrogen bonds* between water molecules (1) *Cohesion* prevents breakage (1)