Explain how carbohydrates, lipids, proteins, and nucleic acids are constructed from smaller subunits, and relate the structure of each macromolecule to its function (MA STE HS-LS1, structure and function).

What this topic is asking

The Massachusetts STE framework (HS-LS1) wants you to understand that living things are built from a small set of carbon-based molecules, and to explain how the structure of each class of biological molecule fits its function. On the High School Biology MCAS this content rarely appears as bare recall. Instead, a question 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. The standard HS-LS1-6, which asks you to explain how carbon, hydrogen, and oxygen from sugar molecules can combine to form amino acids and other large carbon-based molecules, sits right at the center of this topic.

Monomers, polymers, 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 the backbone element because each carbon atom can form four covalent bonds, allowing long chains, branches, and rings. This versatility is why such a huge variety of molecules can be built from so few elements. The framework standard HS-LS1-6 captures this: sugars supply the carbon, hydrogen, and oxygen that cells rearrange to build amino acids, fats, and other large carbon-based molecules.

The four classes of biological molecule

Carbohydrates

The subunit 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, usually in a ratio close to 1:2:1.

Lipids

Lipids (fats, oils, and phospholipids) are not built from one repeating monomer, but many are assembled from glycerol and fatty acids. They store energy at high density, cushion and insulate the body, and, as phospholipids, form the cell membrane. Lipids are largely nonpolar, so they do not mix with water, which is exactly why a phospholipid bilayer makes a good barrier.

Proteins

The subunit 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 and keratin.

Nucleic acids

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

Why structure determines function

The MCAS keeps returning to one big idea: structure determines function. 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. You will meet this idea again in enzymes and biochemical reactions.

Reading the molecules in a model or data table

Because the test is computer-based and stimulus-driven, you are often shown a model of a molecule rather than asked to recall a definition. The fastest way to identify a molecule is to look for the subunit: rings joined in a chain suggest a polysaccharide, a chain of differently shaped units suggests a protein (the units are amino acids), a long glycerol-and-tail shape suggests a lipid, and a ladder or a phosphate-sugar-base unit suggests a nucleic acid. A data table might give the elements present: only carbon, hydrogen, and oxygen rules out proteins and nucleic acids (which contain nitrogen), helping you narrow the class.

Try this

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

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

Q2. Explain why carbon is the central element of biological molecules. [2]

• Cue. Each carbon atom can form four covalent bonds, so carbon can build long chains, branches, and rings, allowing a huge variety of molecules.