LEC 3 Water and Carbon: Properties, Structure, and Biological Importance

Water

Hydrogen Bonds in Water

Water molecules are polar, resulting in hydrogen bonding between molecules. This property is fundamental to water's unique physical and chemical characteristics, which are essential for life.

• Polarity: Water has a partial positive charge on hydrogen atoms and a partial negative charge on oxygen atoms, leading to strong intermolecular attractions.

• Hydrogen Bonding: Each water molecule can form hydrogen bonds with up to four other water molecules, resulting in a tetrahedral arrangement in ice.

• Example: Ice floats on water due to its lower density, a result of the tetrahedral hydrogen bond structure.

Special Characteristics of Water

Cohesion

Cohesion refers to the attraction between water molecules due to hydrogen bonding, resulting in high surface tension and the ability of water to form droplets.

• Surface Tension: Water molecules at the surface are pulled inward, creating a measurable tension that allows small objects to rest on the surface without sinking.

• Example: Water strider insects walking on water.

Temperature Stabilization

Water stabilizes temperature due to its high specific heat and heat of vaporization, which buffer organisms and environments against rapid temperature changes.

• Specific Heat: The amount of heat required to raise the temperature of 1 g of water by 1°C is high, allowing water to resist temperature changes.

• Heat of Vaporization: Water requires significant energy to change from liquid to gas, facilitating evaporative cooling.

• Evaporative Cooling: As water evaporates, it removes heat from surfaces, helping organisms regulate temperature.

• Equation: $q = m c \, \Delta T$ (where $q$ is heat energy, $m$ is mass, $c$ is specific heat, and $\Delta T$ is temperature change)

Water and Ice Density

Ice is less dense than liquid water due to the hexagonal arrangement of water molecules in ice, causing it to float.

• Example: Lakes and oceans freeze from the top down, providing insulation for aquatic life.

Water as the Solvent of Life

Water's polarity allows it to dissolve many substances, making it an excellent solvent for biological reactions.

• Hydrophilic Substances: Have an affinity for water and dissolve easily.

• Hydrophobic Substances: Repel water and do not dissolve.

• Example: Salt (NaCl) dissolves in water, while oil does not.

Carbon

Organic Chemistry

Organic chemistry is the study of carbon compounds, which form the basis of all living organisms.

• Versatility: Carbon can form four covalent bonds, allowing for a variety of structures including chains, rings, and branches.

• Bonding: Carbon bonds with hydrogen, oxygen, nitrogen, and other elements to form diverse organic molecules.

• Hydrocarbons: Compounds containing only carbon and hydrogen; they are nonpolar and hydrophobic.

Isomers

Isomers are compounds with the same molecular formula but different structures, resulting in different properties.

• Structural Isomers: Differ in the covalent arrangement of atoms.

• Geometric Isomers: Differ in spatial arrangement due to inflexibility of double bonds.

• Enantiomers: Molecules that are mirror images of each other; important in pharmaceuticals.

Functional Groups

Functional groups are specific groups of atoms attached to carbon skeletons that confer distinctive chemical properties to organic molecules.

Functional Group	Structure	Properties/Examples
Hydroxyl	-OH	Found in alcohols; polar, forms hydrogen bonds
Carbonyl	>C=O	Found in aldehydes (end of chain) and ketones (within chain)
Carboxyl	-COOH	Found in carboxylic acids; acts as an acid
Amino	-NH2	Found in amines; acts as a base
Sulfhydryl	-SH	Found in thiols; helps stabilize protein structure
Phosphate	-OPO32-	Found in organic phosphates; important in energy transfer (ATP)

Example: The phosphate group is essential in ATP, the energy currency of the cell.

Additional info: Organic molecules with different functional groups have distinct chemical reactivities and biological roles, which is crucial for metabolism and cellular function.