Water and Inorganic Ions

• A water molecule ($H_2O$) consists of one oxygen atom covalently bonded to two hydrogen atoms
• The bond angle is approximately $104.5°$
• Oxygen is more electronegative than hydrogen, so the shared electrons are pulled closer to the oxygen
• This creates a polar molecule: the oxygen end is slightly negative ($\delta^-$) and the hydrogen ends are slightly positive ($\delta^+$)
• Water is described as a dipolar molecule

• The $\delta^+$ hydrogen of one water molecule is attracted to the $\delta^-$ oxygen of a neighbouring water molecule
• This attraction is a hydrogen bond — a type of weak intermolecular force
• Each water molecule can form up to four hydrogen bonds with surrounding molecules
• Individual hydrogen bonds are weak, but collectively they give water its remarkable properties
• Hydrogen bonds are constantly forming and breaking (dynamic)

a) Solvent Properties

• Water is an excellent solvent for polar and ionic substances
• The slightly charged regions of water molecules surround and separate ions (e.g., $Na^+$, $Cl^-$) and interact with polar molecules
• Biological importance: Most biochemical reactions occur in aqueous solution; transport of substances in blood, lymph, and plant sap; metabolic reactions require dissolved reactants

b) High Specific Heat Capacity

• Water requires a large amount of energy to raise its temperature ($4.18 \text{ J g}^{-1}\text{°C}^{-1}$)
• Due to the large number of hydrogen bonds that must be broken
• Biological importance: Bodies of water (oceans, lakes) resist rapid temperature changes, providing stable aquatic habitats; organisms can maintain relatively stable internal temperatures; important for thermoregulation

c) High Latent Heat of Evaporation

• A lot of energy is needed to evaporate water (to break hydrogen bonds)
• Biological importance: Sweating and transpiration are effective cooling mechanisms — as water evaporates from the skin or leaf surface, it absorbs heat energy, cooling the organism

d) Cohesion and Surface Tension

• Cohesion: Water molecules attract each other (via hydrogen bonds), creating a strong pulling force
• Biological importance: Cohesion helps draw water up through xylem vessels in plants (transpiration stream); water column doesn't break easily
• Surface tension: Cohesion at the surface creates a "skin" that small organisms can walk on (e.g., pond skaters)

e) Adhesion

• Water molecules are attracted to other polar surfaces (e.g., glass, cellulose in xylem walls)
• Biological importance: Helps water move up narrow tubes by capillary action (important in xylem transport)

f) High Density of Ice (Less Dense Than Liquid Water)

• Water is most dense at 4°C
• When water freezes, hydrogen bonds hold molecules in a fixed lattice that is less dense than liquid water
• Ice floats on the surface
• Biological importance: Ice forms an insulating layer on lakes and ponds, allowing organisms beneath to survive in the liquid water below; prevents entire water bodies from freezing solid

g) Metabolic Roles

• Water is a reactant in many reactions:
  • Hydrolysis reactions: water breaks bonds (e.g., digestion of polymers)
  • Photosynthesis: water is split in the light-dependent reactions (photolysis)
  • Condensation reactions: water is produced when monomers join (e.g., forming peptide bonds, glycosidic bonds)

Inorganic ions play essential roles in biological processes:

Water potential ($\Psi$, psi) is the tendency of water to move from one area to another.

• Pure water has a water potential of 0 kPa (the highest possible)
• Adding solutes lowers water potential (makes it more negative)
• Water always moves from higher (less negative) to lower (more negative) water potential by osmosis

$\Psi = \Psi_s + \Psi_p$

Where:

• $\Psi_s$ = solute potential (always negative or zero)
• $\Psi_p$ = pressure potential (usually positive in plant cells due to turgor)

Question: Explain how the properties of water make it important for transport in organisms. (5 marks)

Solution:

Water is an excellent solvent for polar and ionic substances, allowing it to transport dissolved molecules such as glucose, amino acids, oxygen, and hormones in the blood and in plant sap. Its cohesive properties (due to hydrogen bonding between water molecules) create a continuous column of water in xylem vessels, enabling the transpiration stream to pull water from roots to leaves without the column breaking. Water's adhesion to the walls of narrow xylem vessels aids movement by capillary action. Its high specific heat capacity means blood can transport heat around the body without large temperature changes, helping to maintain stable conditions for enzyme activity.

• Water is dipolar due to uneven electron distribution; this allows hydrogen bonding between molecules.
• Key properties: excellent solvent, high specific heat capacity, high latent heat of evaporation, cohesion, adhesion, ice is less dense than liquid water.
• Water is a reactant in hydrolysis and photolysis and a product of condensation reactions.
• Inorganic ions have diverse roles: $Fe^{2+}$ in haemoglobin, $Mg^{2+}$ in chlorophyll, $PO_4^{3-}$ in ATP/DNA, $H^+$ in chemiosmosis.
• Water potential $\Psi = \Psi_s + \Psi_p$; water moves from high to low water potential.