Water

• Water


  The medium for life
• The first cells originated in water
• Liquid water is essential for life as we know it
• Water on Earth originated around 4.5 billion years ago and life first emerged at least 3.8 billion years ago
• In order for life to form and maintain itself, the molecular ingredients of life need to react with each other in a liquid solvent - water
• There is scientific debate around whether the water was in a pond, hydrothermal pool or sea - but there is no debate that water was present
• Water as a solvent


  Water can dissolve a variety of molecules
• Water's role as a solvent


  Helps multicellular organisms transport molecules around a body and acts as the medium in which chemical reactions of life occur
• Water as a metabolite


  Water is involved in many chemical reactions, either as a reactant or as a product of reaction, such as photosynthesis, digestion and aerobic respiration
• Condensation reaction


  When water is formed as one of the products when two molecules join together
• Hydrolysis reaction


  When water reacts with a chemical to break it into smaller molecules
• Water as a temperature buffer


  The many intermolecular hydrogen bonds between water maintain its high heat capacity, which helps to buffer temperature changes in cells and allows enzymes to work best at an optimal temperature
• Water


  • Contributes to the formation of cell membranes
  • Impacts the folding of proteins
  • Surrounds DNA to support the double-helix
• Hydrogen bonds


  A consequence of the polar covalent bonds within water molecules
• Atom


  The smallest unit of matter, unique to a particular element, with three constituent parts: protons, neutrons and electrons
• Atoms


  • Are electrically neutral if they have an equal number of protons and electrons
  • Atoms that have either a deficit or a surplus of electrons are called ions
• Atomic bonding


  Electrons may be transferred to other nearby atoms or shared between atoms, enabling the formation of chemical compounds
• Types of bonds in IB Biology


  • Nonpolar covalent bond
  • Polar covalent bond
  • Ionic bond
  • Hydrogen bond (intermolecular force, not a true bond)
• Nonpolar covalent bond


  Electrons are shared equally between atoms
• Polar covalent bond


  Electrons are shared unequally between atoms
• Ionic bond


  An attraction between a positively charged ion and a negatively charged ion
• Hydrogen bond


  An attraction between two polar molecules
• Water molecule


  Consists of 2 hydrogen and 1 oxygen atom, with polar covalent bonds
• Water molecule polarity


  The shared electrons are pulled towards the oxygen atom, creating a slightly negative oxygen and slightly positive hydrogen atoms
• Hydrogen bonds form between water molecules
• Hydrogen bonds between water molecules


  • Maintain water's high heat capacity
  • Contribute to the stability and retainment of water on Earth
  • Grant water the ability to attract polar and charged molecules, leading to properties like cohesion, adhesion, solvency and high specific heat
• Cohesion of water molecules

  The force by which individual water molecules attract and stick together due to hydrogen bonding
• Consequences of water cohesion


  • Allows plants to move water under tension in xylem
  • Retains water on Earth's surfaces to serve as habitats
  • Contributes to the physical properties of water important to living organisms
• Cohesion-tension hypothesis for water transport in plants


  Transpiration creates negative pressure, which "pulls" water up the xylem vessels, with cohesion between water molecules forming a continuous chain
• Surface tension


  A property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules
• Water's high surface tension


  • Allows organisms like water striders to "walk on water" and provides a stable environment for other organisms that live on or near the surface of water
  • Allows some seeds to disperse away from the parent plant
• Adhesion


  The force by which individual molecules cling to surrounding materials and surfaces
• Adhesion of water to polar or charged materials


  • Allows plants to move water using capillary action
  • Permits water to move through soil, even against the force of gravity
• Polar molecules like glucose can form hydrogen bonds with water, allowing them to be transported around the body in the blood
• Adhesion


  Hydrogen bonds form between water and polar molecules
• Polar molecules


  • Result from unequal sharing of electrons between atoms within the molecule
  • Glucose has five hydroxyl (-OH) groups along the carbon skeleton
• Glucose has polar hydroxyl (-OH) groups


  Water can form hydrogen bonds with glucose
• Water's oxygen atom has a slight negative charge (δ-)


  It is attracted to the slight positive charge (δ+) of glucose hydrogen atom
• Glucose's oxygen atom has a slight negative charge (δ-)


  It is attracted to the slight positive charge (δ+) of water hydrogen atom
• Cation


  Has a positive charge because it has more protons than electrons (e.g. Na+)