- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
A1.1.1 Water as the medium for life
Students should appreciate that the first cells originated in water and that water remains the medium in which most processes of life occur.
In the Water unit students are introduced to the structure and function of water, as the medium of life. Water has many useful properties, and so it is ubiquitous in life on earth. The useful properties of water arise from its structure.
The unit is planned to take 2 school days
- Water is the medium of life.
- Use theories to explain natural phenomena—the theory that hydrogen bonds form between water molecules explains the properties of water.
- State why scientists cannot prove without a doubt that hydrogen bonds exist between water molecules.
List reasons why water is a substance on which life depends.
- The first cells originated in water.
- Water is the “universal solvent” allowing it to dissolve and transport molecules around a body.
- Water is a metabolite in condensation and hydrolysis reactions.
- Water is a temperature buffer in bodies and ecosystems.
- Water maintains biological structures (such as phospholipid bilayer, proteins and DNA).
A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
Water molecules are polar and hydrogen bonds form between them
- Describe the structure of an atom (in terms of protons, neutrons and electrons).
- Contrast ion with atom.
- Define anion and cation.
- Contrast covalent, ionic and hydrogen bonds.
- Write the molecular formula for water and draw the atomic structure of the molecule.
- Describe the cause and effect of the polar nature of water.
- Describe where and how water is able to form hydrogen bonds.
Hydrogen bonds are attractions of electrostatic force caused by the difference in charge between slightly positive hydrogen ions and other, slightly negative ions. In the case of water, hydrogen bonds form between neighboring hydrogen and oxygen atoms of adjacent water molecules. The attraction between individual water molecules creates a bond known as a hydrogen bond
Water molecule formed by hydrogen bonds between oxygen and hydrogen. However sharing of electrons is unequal and are attracted to the oxygen more. Partial negative charge develops on the oxygen and partial positive on the hydrogen. Attraction between water molecules forms a hydrogen bond and forms when a hydrogen atom in one polar molecule is attracted to the slightly negative oxygen atom in the other molecule.
A water molecule consists of an oxygen atom covalently bound to two hydrogen atoms. Since O is more electronegative than H, an unequal sharing of electrons occurs. This creates a polar covalent bond with H having a partial positive charge and O having a partial negative charge.
Water is also bent so the positive charge exists more or less on one side and the negative charge from the O exists on the opposite side. The partial +ve charge is attracted to the partial –ve charge creating an intermolecular attraction between the water molecules called a “Hydrogen bond.”. H-bonds are the strongest of the intermolecular bonding, but is still considered a weak bond; however since there are so many H2O molecules they give water its unique properties and make it essential to life on this planet
State the location of the polar covalent bond within a water molecule.
- There are two polar covalent bonds within a water molecule; one between the oxygen atom and each hydrogen atom
