Explain how carbohydrates, lipids, proteins and nucleic acids are constructed from monomers and how the structure of each macromolecule relates to its function (NYSSLS LS1, structure and function).

What this topic is asking

The New York State Science Learning Standards (NYSSLS LS1) want you to understand that living things are built from a small set of molecules, and to explain how the structure of each class of biological molecule fits its function. On the Life Science: Biology Regents this content rarely appears as bare recall. Instead, a cluster gives you a diagram, a data table, or a short passage, and asks you to identify a molecule, state a function, or explain why structure determines function.

Water: the medium of life

Most of a cell is water. A water molecule is polar: the oxygen end is slightly negative and the hydrogen ends slightly positive, so water molecules attract one another and many other substances. This makes water an excellent solvent, lets it move in columns (cohesion), and lets it resist temperature change, which helps cells maintain a stable internal environment. Because so many molecules dissolve in water, water is where the chemistry of life happens.

The four classes of biological molecule

Carbohydrates

The monomer is a monosaccharide (a simple sugar such as glucose). Two joined make a disaccharide; many joined make a polysaccharide. Carbohydrates store readily available energy (starch in plants, glycogen in animals) and provide structure (cellulose in plant cell walls). They contain carbon, hydrogen and oxygen.

Lipids

Lipids (fats, oils, phospholipids) are not built from a single repeating monomer, but many are assembled from glycerol and fatty acids. They store energy at high density, cushion and insulate, and, as phospholipids, form the cell membrane. Lipids are largely nonpolar, so they do not mix with water.

Proteins

The monomer is the amino acid; there are about 20 kinds. A chain of amino acids (a polypeptide) folds into a specific three-dimensional shape. Proteins are the workhorses of the cell: enzymes that speed reactions, antibodies that defend the body, receptors and transport proteins in membranes, and structural fibers such as collagen.

Nucleic acids

The monomer is the nucleotide (a phosphate, a sugar, and a nitrogenous base). DNA stores the genetic information; RNA helps carry it out. These are covered in detail in DNA structure and replication.

Why structure determines function

The exam keeps returning to one big idea: structure determines function (a crosscutting concept). The clearest example is the protein. The order of amino acids (set by a gene) determines how the chain folds, and the folded shape determines what the protein can do. An enzyme's active site is a precise shape that fits its substrate; an antibody's binding site is a precise shape that fits an antigen. Change the amino-acid sequence and you can change the shape, which can change or destroy the function. The same logic explains why high temperature or extreme pH, which unfold (denature) a protein, stop it working.

Elements and the role of carbon

Biological molecules are built mostly from a few elements: carbon, hydrogen, oxygen, nitrogen, and (in proteins and nucleic acids) phosphorus and sulfur. Carbon is central because each carbon atom can bond to four others, allowing long chains, branches and rings. This versatility is why such a huge variety of molecules can be built from so few elements.

Try this

Q1. Identify the monomer (building block) of a protein and of a carbohydrate. [2]

• Cue. Protein: amino acid. Carbohydrate: monosaccharide (simple sugar such as glucose).

Q2. Explain why water is described as the solvent of life. [2]

• Cue. Water is polar, so it attracts and dissolves many substances, allowing the reactions of metabolism to take place in solution inside cells.