Metabolic Medium

The capacity of water to dissolve a large variety of substances makes it an important medium for metabolic reactions

• Solutes dissolved into aqueous solutions are more likely to collide with enzymes and undergo necessary chemical reactions
  

• Water can also promote enzyme activity by absorbing heat from exothermic reactions and maintain acid-base neutrality
  

Substances that are involved in metabolic reactions can be described as being either hydrophilic or hydrophobic

• Substances that freely associate and readily dissolve in water are characterised as hydrophilic (‘water loving’) – this includes all polar molecules and ions
  

• Substances that do not freely associate or dissolve in water are characterised as hydrophobic (‘water-hating’) – this includes all large non-polar molecules (lipids)
  

Transport Medium

The movement of water-soluble substances in plants and animals involves different transport systems

• In animals, the blood plasma transports dissolved solutes – including amino acids, simple sugars, wastes (urea) and a small amount of gases (O2 and CO2)
  

• In vascular plants, mineral ions are transported via xylem vessels while dissolved nutrients are transported via the phloem
  

Certain substances are not water-soluble and cannot be freely transported within an aqueous environment

• In animals, lipids are packaged with proteins to form water-soluble lipoproteins that can be transported via the blood

• The mechanism of lipid transport is not well understood in plants, but may involve conjugation to amino acids and transport via the phloem