Water

• Water


  Theme: Unity and Diversity
• IB Guiding Questions


  • What physical and chemical properties of water make it essential for life?
  • What are the challenges and opportunities of water as a habitat?
• SL and HL Content

  • A1.1.1: Water as the medium for life
  • A1.1.2: Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
  • A1.1.3: Cohesion of water molecules due to hydrogen bonding and consequences for organisms
  • A1.1.4: Adhesion of water to materials that are polar or charged and impacts for organisms
  • A1.1.5: Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals
  • A1.1.6: Physical properties of water and the consequences for animals in aquatic habitats
• SL and HL Key Terms

  • Hydrogen bonds
  • Polar covalent bonds
  • Cohesion
  • Adhesion
  • Solvent
  • Metabolism
  • Xylem
  • Surface tension
  • Capillary action
  • Hydrophobic
  • Hydrophilic
  • Buoyancy
  • Viscosity
  • Thermal conductivity
  • Specific heat capacity
  • Apoplast pathway
• The first cells originated in water
• Reasons why water is a substance on which life depends

  • It is the medium for metabolic reactions
  • It is a transport medium
  • It is involved in many chemical reactions (e.g. hydrolysis)
• Life probably originated in water 3.8 billion years ago
• Polar covalent bonds


  Unequal sharing of electrons within water molecules
• Hydrogen bonding occurs between water molecules due to the polarity of covalent bonding
• Nonpolar covalent bonds are common within the many carbon compounds upon which life is based, such as proteins and DNA
• Polar covalent bonds can occur in amino acid R-groups which impacts the tertiary structure of a protein
• Hydrogen bond


  Attraction between slightly positive (δ+) and slightly negative (δ-) regions of two different polar molecules
• Each water molecule can form up to 4 hydrogen bonds with other water molecules
• Water's ability to attract polar and charged molecules grants it a number of emergent properties such as cohesion, adhesion, solvency
• Collectively these properties make water the primary medium of life
• Xylem vessels transport water (and dissolved minerals) from the roots to the leaves of plants
• Capillary action


  Ability of water to flow against gravity in a narrow space
• Capillary action is greater in fine clay soils, and weaker in porous sandy soils
• The apoplast pathway allows water and dissolved nutrients to move through the cell walls of plants
• Cellulose in the plant cell walls is porous and polar, allowing water to move by capillary action
• Surface tension

  Property of a substance to resist an external force
• Cohesion results in surface tension
• Surface tension allows some animals to walk on water, and some seeds to disperse away from the parent plant
• Solvent

  A substance that dissolves other substances
• Water is often called the universal solvent, as it allows a wide variety of hydrophilic substances to dissolve in it, because of its polarity
• Most metabolic reactions will only occur when the reactants are dissolved in water
• Polar molecules and ions dissolve in water
• Ionic minerals are dissolved in water as it is transported up the xylem of a plant
• Organic molecules such as glucose and amino acids dissolve in water and are transported through the phloem of plants
• Substances transported in animal blood

  • Salt ions such as Na+ and Cl-
  • Amino acids
  • Proteins such as antibodies and those used in blood clotting
  • Glucose
  • Waste products of metabolism such as urea
  • A small amount of dissolved gasses such as CO2 and O2
• Metabolism

  The complex network of interdependent and interacting chemical reactions occurring in living organisms
• Hydrophobic


  Not charged and do not readily mix with water
• Lipids are hydrophobic, allowing them to be used as an energy store
• Lipids are transported in blood as lipoprotein complexes, with proteins and phospholipids coating the fat droplets
• Physical properties of water

  • Buoyancy
  • Viscosity
  • Thermal conductivity
  • Specific heat capacity
• Bony fish can change their density by changing the size of a swim bladder, allowing them to move up and down in water
• Birds' hollow, lightweight limb bones allow the buoyant force of water to hold them up
• Hydrophobic substances

  Not charged and do not readily mix with water
• Hydrophobic substances

  • Lipids
• Because they are hydrophobic, lipids can not be directly transported in solution around the body of an organism