Water: Properties and Biological Importance

Polarity of Water

Water (H2O) is a polar molecule, meaning it has an uneven distribution of charge. The oxygen atom is more electronegative than hydrogen, resulting in a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• Polarity: Allows water to form hydrogen bonds with other polar molecules.

• Example: Water dissolves ionic compounds like NaCl due to its polarity.

Hydrogen Bonding

Hydrogen bonds are weak attractions between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another.

• Key Point: Hydrogen bonding is responsible for many of water's unique properties.

• Example: Ice floats on water because hydrogen bonds stabilize the solid structure, making it less dense.

Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. Responsible for surface tension.

• Adhesion: The attraction between water molecules and other substances (e.g., water climbing up plant cell walls).

• Example: Cohesion and adhesion enable capillary action in plants.

Surface Tension

Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension due to hydrogen bonding.

• Example: Some insects can walk on water because of surface tension.

High Specific Heat

Water can absorb or release a large amount of heat with only a slight change in its own temperature.

• Formula:

• Biological Importance: Helps moderate Earth's climate and maintain stable temperatures in organisms.

Evaporative Cooling

As water evaporates, the surface of the remaining liquid cools down. This is because the molecules with the highest kinetic energy leave first.

• Example: Sweating cools the body through evaporative cooling.

Density of Ice

Ice is less dense than liquid water because hydrogen bonds form a crystalline structure that spaces molecules apart.

• Biological Importance: Ice floats, insulating aquatic life in winter.

Water as the Solvent of Life

Water's polarity makes it an excellent solvent, especially for ionic and polar substances.

• Hydrophilic: Substances that dissolve in water.

• Hydrophobic: Substances that do not dissolve in water.

pH and Buffers

pH measures the concentration of hydrogen ions () in a solution.

• Formula:

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.

• Example: Blood contains bicarbonate buffer to maintain pH.

Carbon and the Molecular Diversity of Life

Carbon’s Versatility

Carbon can form four covalent bonds, allowing for a variety of stable, complex molecules.

• Key Point: Carbon forms the backbone of organic molecules.

Hydrocarbons

Hydrocarbons are organic molecules consisting only of carbon and hydrogen.

• Properties: Nonpolar, hydrophobic, can release energy when oxidized.

• Example: Methane (CH4), ethane (C2H6).

Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

• Structural Isomers: Differ in covalent arrangement.

• Cis-trans Isomers: Differ in spatial arrangement around double bonds.

• Enantiomers: Mirror images of each other.

Key Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties.

• Hydroxyl (-OH): Alcohols, polar.

• Carbonyl (C=O): Aldehydes and ketones.

• Carboxyl (-COOH): Carboxylic acids, acidic.

• Amino (-NH2): Amines, basic.

• Sulfhydryl (-SH): Thiols, can form disulfide bonds.

• Phosphate (-PO42-): Organic phosphates, energy transfer.

• Methyl (-CH3): Affects gene expression.

Macromolecules: Structure and Function

Dehydration and Hydrolysis

• Dehydration Reaction: Joins monomers by removing a water molecule.

• Hydrolysis: Breaks polymers into monomers by adding water.

Carbohydrates

Carbohydrates are sugars and their polymers, serving as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

• Formula for Glucose:

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.

• Example: Formation of lipid bilayers in membranes.

Proteins

Proteins are polymers of amino acids with diverse functions (enzymes, structure, transport, etc.).

• Levels of Structure:

  • Primary: Sequence of amino acids.

  • Secondary: Alpha helices and beta sheets (hydrogen bonding).

  • Tertiary: 3D folding due to side chain interactions.

  • Quaternary: Association of multiple polypeptides.

• Function Depends on Shape: Denaturation (loss of structure) leads to loss of function.

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information.

• DNA: Double helix, stores genetic code.

• RNA: Single-stranded, involved in protein synthesis.

Cell Structure and Function

Vacuoles: Structure and Function

• Plant Cells: Large central vacuole stores water, ions, and waste; maintains turgor pressure.

• Animal Cells: Smaller vacuoles, mainly for storage and transport.

Cell Organelles: Structure and Function

• Nucleus: Contains DNA, controls cell activities.

• Mitochondria: Site of cellular respiration, produces ATP.

• Chloroplasts (plants only): Site of photosynthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes (mainly animal cells): Digestive enzymes for breakdown of macromolecules.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Plasma Membrane: Selectively permeable barrier.

• Cell Wall (plants only): Provides structure and protection.

Animal Cells vs. Plant Cells: Similarities and Differences

Labeling Structures/Organelles in Animal and Plant Cells

• Common Organelles: Nucleus, mitochondria, ER, Golgi apparatus, plasma membrane, ribosomes.

• Plant-Specific: Cell wall, chloroplasts, large central vacuole.

• Animal-Specific: Lysosomes, centrioles.

Additional info: This guide covers foundational concepts from Chapters 3-6 of a typical General Biology course, focusing on water's properties, carbon chemistry, macromolecules, and cell structure/function.