2 Biological Molecules

Structure connects molecules to function

A molecule’s structure and chemical properties help determine what it does in a cell. The four major classes of biological molecules are carbohydrates, lipids, proteins, and nucleic acids. Water is not a macromolecule, but its properties make it an essential medium for cellular chemistry.

Water and its interactions

A water molecule has a bent shape. Oxygen attracts shared electrons more strongly than hydrogen, so the oxygen end has a slight negative charge while the hydrogen ends have slight positive charges. This uneven distribution makes water polar. The partial charges let neighboring water molecules form hydrogen bonds.

Water’s polarity gives it several important properties:

• Solvent action: Water surrounds and disperses many ions and polar molecules, allowing dissolved substances to move in blood and cell fluid. Nonpolar substances, such as oils, do not dissolve readily.

• Temperature moderation: Hydrogen bonds absorb or release heat as they form or break, so water changes temperature relatively slowly.

• Cohesion: Attraction among water molecules helps maintain continuous water columns in plants and contributes to surface tension.

Nonpolar molecules tend to cluster together in water. This behavior helps organize cell membranes and contributes to the folding of some proteins.

Takeaway: Water’s polarity shapes its interactions with dissolved substances and helps create conditions for cellular chemistry.

Carbohydrates: fuel, storage, and support

Carbohydrates contain carbon, hydrogen, and oxygen. Their simplest units are monosaccharides, including glucose, an important cellular fuel. Two sugar units can join to form a disaccharide, while many joined units form a polysaccharide. Covalent glycosidic bonds connect the units, and differences in how they are linked affect the carbohydrate’s properties.

Carbohydrates serve several roles:

• Fuel: Cells break down glucose in cellular respiration to transfer energy into ATP.

• Energy storage: Plants store glucose units as starch; animals and fungi store them as glycogen. Their branched structures allow stored glucose to be released when needed.

• Structural support: Cellulose, a major component of plant cell walls, is made of glucose units joined in a different pattern from starch. Humans cannot digest cellulose, but it provides dietary fiber.

Takeaway: The same basic sugar units can form carbohydrates with different properties and functions because their bonds and arrangements differ.

Lipids: energy and membranes

Lipids are a diverse group of molecules that are largely nonpolar and do not mix well with water. Unlike proteins and nucleic acids, most lipids are not polymers built from repeating monomer units.

• Fats and oils commonly consist of glycerol joined to three fatty acids. Their many energy-rich carbon–hydrogen bonds make them effective long-term energy stores. Unsaturated fatty acids have one or more carbon–carbon double bonds, which often create bends that affect how tightly fat molecules pack.

• Phospholipids have a water-attracting head and water-repelling tails. In water, they form bilayers: heads face the watery environments, while tails tuck inward. This arrangement forms the basic boundary of cell membranes.

• Steroids have four fused carbon rings. Cholesterol is part of animal-cell membranes and is a starting material for some hormones.

Takeaway: Lipids store energy and help create cell boundaries; their interactions with water help explain how membranes form.

Proteins: shape enables specific work

Proteins are chains of amino acids linked by peptide bonds. Each amino acid has a variable side chain, or R group, with its own chemical properties. The order of amino acids—the protein’s primary structure—affects how the chain folds. Interactions among amino acids and with the surroundings help produce a specific three-dimensional shape.

That shape enables a protein’s function. Enzymes bind particular reactants, called substrates, and accelerate chemical reactions. Antibodies recognize foreign molecules, transport proteins carry substances, and structural proteins support cells and tissues. An enzyme’s binding region has a shape and chemical properties suited to particular substrates.

Changes in temperature or pH can disrupt a protein’s shape and impair its function. This change is called denaturation.

Takeaway: A protein’s amino-acid sequence influences its shape, and its shape enables its particular work.

Nucleic acids: storing and using information

Nucleic acids are polymers of nucleotides, each containing a sugar, a phosphate group, and a nitrogen-containing base. The order of bases stores biological information.

DNA usually consists of two strands forming a double helix. Its bases pair specifically: A with T, and C with G. Because of this complementary pairing, each strand can serve as a template for copying the information.

RNA usually consists of one strand and uses U instead of T. Different RNAs help use DNA’s instructions to make proteins: messenger RNA carries a copy of information, transfer RNA delivers amino acids, and ribosomal RNA is part of the protein-making machinery.

Takeaway: Nucleotide sequences store information, and complementary base pairing supports its copying and use.

Putting the molecular roles together

Across all four major classes of biological molecules, composition, bonding, and shape connect chemistry to life. Carbohydrates provide fuel, energy storage, and structural materials. Lipids store energy and form membrane barriers. Proteins fold into shapes suited to particular tasks. Nucleic acids store and help express biological information. Water’s polarity supports many of the interactions that organize these molecules and enable cellular chemistry.

• phospholipid