Biology Chapter: Chemical Basis of Life

Chemical Basis of Life: Macromolecules

• Macromolecules are large, complex organic molecules abundant in living organisms.
• Organic molecules contain carbon.
• Carbon has four electrons in its outer shell, allowing it to form up to four covalent bonds (octet rule).
• Carbon forms nonpolar or polar bonds.
• Molecules with polar bonds are water-soluble; those with nonpolar bonds (like hydrocarbons) are not.

Functional Groups

• Functional groups are groups of atoms with special chemical features.
• Each type of functional group shows the same properties in all molecules where it occurs.

Isomers

• Isomers are molecules with the same molecular formula but different structures.
• Structural isomers contain the same atoms but in different bonding relationships.
• Stereoisomers have identical bonding relationships but different spatial positioning of atoms.
  • Cis-trans isomers have different positioning around a double bond.
  • Enantiomers are mirror image molecules.

Polymer Formation

• Polymers are formed when monomers are linked together via dehydration reactions.
• A molecule of water is removed each time a new monomer is added.
• The process repeats to form long polymers.
• Polymer formation is catalyzed by enzymes.
• Polymers are broken down via hydrolysis reactions, where a water molecule is added to break a bond.

Carbohydrates

• Carbohydrates are composed of carbon, hydrogen, and oxygen atoms.
• The general formula is C_n(H₂O)_n.
• Most carbon atoms are linked to a hydrogen atom and a hydroxyl group.
• Monosaccharides are the simplest sugars.
  • Pentoses have 5 carbon atoms (e.g., ribose, deoxyribose).
  • Hexoses have 6 carbon atoms (e.g., glucose).
• Glucose isomers exist (structural and stereoisomers, D and L).
• Disaccharides are formed from two monosaccharides linked by a glycosidic bond.
  • Examples include sucrose, maltose, and lactose.
• Polysaccharides are long polymers of monosaccharides.
  • Examples include starch (energy storage) and cellulose (structural).

Lipids

• Lipids are composed predominantly of hydrogen and carbon atoms.
• A defining feature of lipids is their insolubility in water due to nonpolar properties.
• Lipids include fats, phospholipids, steroids, and waxes.
• Lipids comprise about 40% of the organic matter in the average human body.
• Fats (triglycerides or triacylglycerols) are formed by bonding glycerol to three fatty acids via dehydration reactions.
• Fatty acids can be saturated (all single bonds) or unsaturated (one or more double bonds).
  • Saturated fats tend to be solids at room temperature; unsaturated fats tend to be liquids (oils).
  • Cis forms are natural; trans forms are artificial and linked to disease.
• Phospholipids are amphipathic molecules consisting of a hydrophilic phosphate head and hydrophobic fatty acid tails.
• Steroids are characterized by four interconnected rings of carbon atoms.
  • Cholesterol is a common example.
  • Tiny structural differences can alter biological properties dramatically (e.g., estrogen vs. testosterone).