Hydrogen Bonding in Water (College Board AP Biology)

Hydrogen Bonding in Water

• Water's polarity means that one pole of a water molecule is attracted to the oppositely charged pole of the neighboring molecule
• This is a force of attraction
  • And is therefore a form of chemical bond, called a hydrogen bond
• The bonding requires energy to separate molecules from each other
• Which has important implications for how water behaves and how it can support life

Diagram of Hydrogen Bonding Between Four Water Molecules

The polarity of water molecules allows hydrogen bonds to form between neighbouring water molecules

Solvent Properties of Water

• As water is a polar molecule many ions (eg. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it
  
  • This allows chemical reactions to occur within the cytosol (as the dissolved solutes are more chemically reactive when their individual molecules are free to move about)
  • Metabolites can be transported efficiently (except nonpolar molecules which are hydrophobic)
• Water molecules 'surround' individual solute particles to ensure each solute particle is isolated from others
  
  • This explains why solutions are clear - we can't see individual molecules that are separated from their crystal structures
  • This is also why concentrated solutions have a lower water potential or a higher osmolarity
    
    • Because many water particles are 'occupied' in keeping a solute molecule in solution, fewer water molecules are free to diffuse across selectively permeable membranes

Focus on Water as a Solvent

• Different solutes behave differently with water as a solvent
• Even though water is a universal solvent, different metabolites have different solubilities in water
• Different solutes have different hydrophobic and hydrophilic properties which affect their solubility in water

Highly soluble metabolites

• Some are highly soluble (eg. sodium chloride, urea), some are insoluble (eg. fats) and some have intermediate solubility (eg. oxygen and certain amino acids with a large R group)
• Highly soluble metabolites simply travel dissolved in the blood plasma
  
  • For example, salts, glucose, amino acids
  • Even the amino acids with hydrophobic R groups (the variable group that distinguishes each amino acid) are soluble enough to be freely transported in water
• Different transport mechanisms have evolved to assist in the transportation of the less soluble metabolites

Less soluble metabolites

• Oxygen requires assistance through combining with hemoglobin, to allow more oxygen to be carried than directly in blood plasma
  
  • Oxygen, a low solubility metabolite, is less soluble at body temperature (37ºC) than at 20ºC
  • Oxygen has a low solubility but is soluble enough to allow enough to dissolve in oceans, rivers and lakes for aquatic animals to breathe
  • Hemoglobin can bind oxygen to allow sufficient oxygen to be transported to all body cells
• Insoluble metabolites like fats require emulsification, and transport in lacteals, or by being converted to soluble phospholipids
• Cholesterol, which is insoluble, is converted to lipoproteins by combining with proteins

Water Has a High Specific Heat 

• Specific heat is a measure of the energy required to raise the temperature of 1 kg of a substance by 1 °C
• Water has a high specific heat, meaning a relatively large amount of energy is required to raise its temperature
• The high specific heat is due to the many hydrogen bonds present in water
  
  • It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly
• The advantage for living organisms is that it:
  
  • Provides suitable, stable habitats
  • Is able to maintain a constant temperature as water is able to absorb a lot of heat without wide temperature fluctuations
    
    • This is vital in maintaining temperatures that are optimal for enzyme activity
    • The presence of water stabilizes the climate in areas where there are large bodies of water such as oceans and lakes
• Water in blood plasma is also essential in transferring heat around the body, helping to maintain a fairly constant temperature, especially at body extremities eg. finger tips
  
  • As blood passes through more metabolically active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant
  • Water in tissue fluid also plays an important regulatory role in maintaining a constant body temperature

Water has a High Heat of Vaporization

• In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and allow individual vapor particles to escape (evaporate)
  
  • This explains water's high boiling point (100 °C) relative to comparable small compounds such as methane (CH₄: -161.6 °C) and hydrogen sulfide (H₂S: -60 °C)
• Water is present on Earth in all three physical states (solid, liquid and gas) thanks to this characteristic
• Ice, liquid water and water vapor all play a vital role in the biosphere
• This is an advantage for living organisms as only a little water is required to evaporate for the organism to dissipate a great amount of heat
  
  • This provides a cooling effect for living organisms, for example, the transpiration from leaves or evaporation of water in sweat from the skin

Solvent and Heat Properties of Water Summary Table

Cohesion & Adhesion

• Hydrogen bonds between water molecules allows for strong cohesion between water molecules
  
  • Allowing columns of water to move (called mass transport) through the xylem of plants and through blood vessels in animals
  • Enabling surface tension where a body of water meets the air, these hydrogen bonds occur between the top layer of water molecules to create a film on the body of water
  • This layer is what allows insects such as pond skaters (Gerris lacustris) to move across the surface of water without breaking the surface and sinking

A pond skater (Gerrus lacustris) on water. Note the indentations in the surface of the water at the animal's feet.

CC BY-SA 4.0 by AfroBrazilian, via Wikimedia Commons

How surface tension makes a water droplet form

Cohesion causes surface tension around a water droplet due to water molecules pulling together

• Cohesion causes surface tension around a water droplet due to water molecules pulling together
• Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as adhesion
• This also enables water to move up the xylem during transpiration
• Cohesion and adhesion both contribute to water forming a meniscus in glassware, where water molecules adhere to polar molecules in the glass
• Water adheres to the xylem walls (made of lignin)
  • This is known as capillary action, and can contribute to the forces that pull water up a plant's stem