Biological Importance of Water: MDCAT Biology notes
Biological importance of water MDCAT notes: polarity, hydrogen bonds, specific heat, heat of vaporization, cohesion, adhesion, ice density, solvent role.
Water in living things
Water is the most abundant compound in living organisms. It makes up roughly 70–90% of most cells, and the human body is about three-quarters water by weight. The amount differs between tissues. Human bone cells contain about 20% water, while brain cells contain about 85%. Most metabolic reactions happen in water, and water itself takes part in reactions such as hydrolysis and photosynthesis.
Structure: polarity and hydrogen bonds
In a water molecule, oxygen is more electronegative than hydrogen. The O–H bonds are polar covalent: the oxygen end carries a partial negative charge and the hydrogen ends carry partial positive charges. Neighbouring molecules attract one another through hydrogen bonds. Almost every special property of water comes from polarity and hydrogen bonding.
Properties and their biological importance
| Property | Cause | Importance |
|---|---|---|
| Universal solvent | High polarity | Dissolves polar and ionic substances; the medium for cell reactions and transport |
| High specific heat capacity | Hydrogen bonds absorb heat before molecules speed up | Water heats and cools slowly, which gives thermal stability to organisms, lakes, underground water and Earth's climate |
| High heat of vaporization | Hydrogen bonds must be broken to evaporate water | Evaporating a small amount of water removes a large amount of heat, which gives cooling by sweating and transpiration |
| Cohesion | Attraction between water molecules | Keeps water columns unbroken in xylem; causes surface tension |
| Adhesion | Attraction between water and other polar surfaces | Water sticks to xylem walls, wood and cell walls |
| Low density of ice | Hydrogen bonds hold molecules in an open lattice, so water expands when it freezes | Ice floats and insulates the water below, so aquatic life survives winter |
Cohesion, adhesion and capillarity
Capillary action, the rise of water in narrow tubes, depends on cohesion, adhesion and surface tension. Density plays no part in it. Cohesion and adhesion together let water move through living bodies, for example up xylem vessels, and act as a transport medium.
Water as a temperature stabilizer
Because of its high specific heat (heat capacity), water absorbs or loses a lot of heat with only a small change in its own temperature. This keeps body temperature steady and stops large temperature swings in aquatic habitats and underground water. Its high heat of vaporization makes it an effective cooling agent. These are two different properties, so read the question carefully:
- "Stable temperature" or "thermal stability" points to high specific heat capacity.
- "Cooling by evaporation" points to high heat of vaporization.
Other roles
- Protection: water forms a cushion around organs, as in cerebrospinal fluid and amniotic fluid.
- Ionization: a small fraction of water ionizes into $\mathrm{H^+}$ and $\mathrm{OH^-}$, which matters for pH and many reactions.
Common MDCAT traps
- Bone cells contain about 20% water, not 70% or more.
- Water is a universal solvent because of its polarity. Hydrogen bonding explains its thermal properties.
- Vaporization breaks hydrogen bonds between molecules, not the covalent O–H bonds.
- Water sticking to xylem walls or wood is adhesion; water sticking to water is cohesion.
- Density is the odd one out in capillary action.
Quick revision
- Water is polar, and its molecules are linked by hydrogen bonds.
- High specific heat makes water a temperature stabilizer.
- High heat of vaporization makes water a cooling agent.
- Ice is less dense than water because hydrogen bonds form an open lattice.
- Circulation of water depends on cohesion and adhesion.