All proteins have at least a primary, secondary, and tertiary structure, but only some, such as hemoglobin, have a quaternary structure. Secondary structures are determined by hydrogen bonding between different amino acids in the polypeptide chain that form the primary structure. There may be multiple and different secondary structures in a single protein.

Enzymes are polypeptides. Polypeptides are created from proteins/amino acids. They contain a nitrogen, and have a nitrogen-carbon-carbon backbone.

Carbohydrates are referred to as polysaccharides when they form large molecules. They function as energy storage, and are responsible for the structure of plant cell walls. They are made of carbon, hydrogen, and oxygen.

Lipids in large molecules make up fats, oils, waxes, and phospholipids. They provide membrane structure, energy storage, and insulation. They are also made of carbon, hydrogen, and oxygen, and contain numerous carbon-hydrogen bonds.

Nucleic acids (DNA and RNA) are made of chains of nucleotides, bound together by phosphodiester bonds.

The cysteine-cysteine disulfide bond is an example of a tertiary bond. Tertiary bonds are bonds between R-side groups. Other examples include non-polar associated bonds, polar associations with a polar aqueous environment, and ionic bonds. Primary level bonds are described as the sequence of amino acids. Secondary bonds consist of local folding due to bonds between an oxygen on a carboxyl group with a hydrogen from an amino group. This bonding includes alpha helixes and beta sheets. Quaternary bonds are defined as the association between polypeptides.

Amino acids link together by peptide bonds to form proteins. Nucleotides link together to form nucleic acids like DNA and RNA. Monosaccharides are sugars that form links to form carbohydrates. Fatty acids attach to a glycerol backbone to form lipids, except those that are derived from cholesterol.

Phosphorous is found in nucleotides, but not amino acids. In certain reactions, proteins can be modified by kinases to contain phosphate groups. Kinases are enzymes that catalyze the transfer of phosphate groups onto substrates. They function in signal transduction pathways.

There are only a few residues (amino acids in proteins) which can be phosphorylated: Serine, threonine, tyrosine, histidine, arginine, and lysine.

All amino acids have a carboxyl end, and an amino end, both of which contain the same respective atoms. The main differences in amino acids come from the different side chains contained by each amino acid.

The structure of a given amino acid consists of an alpha carbon, amino group, carboxyl group, hydrogen, and a R-side group. R-side groups can have a variety of characteristics. They can be non-polar, polar, acidic, or basic. Amino acids can bond together through a peptide bond via dehydration synthesis—the loss of a single oxygen from one amino acid’s carboxyl group and two hydrogens from the other amino acid’s amine group.

Polypeptides are polymers of amino acids formed by this process. Glycerol is a component of a phospholipid.

DNA is composed of three key components. The backbone of the molecule is made of deoxyribose sugars and phosphate groups. The coding region of DNA is composed of the nitrogenous bases: adenine, thymine, cytosine, and guanine.

A single subunit of DNA is composed of one deoxyribose, one phosphate, and one nitrogenous base. This subunit is called a nucleotide.

A nucleoside is a nucleotide without a phosphate group: only a deoxyribose sugar and a nitrogenous base.

Amino acids are the subunit for proteins, and are not found in DNA.

Proteins have six main functions: 1) movement (e.g. actin and myosin), 2) structure (e.g. keratin), 3) transport (e.g. hemoglobin), 4) protection (e.g. antibodies), 5) communication (e.g. hormones), 6) and catalyzation of chemical reactions (e.g. enzymes).

Primary protein structure is the sequence of the amino acids, linked by peptide bonds. Secondary protein structure involves helices and pleated sheets formed by hydrogen bonds between backbone amino and carboxyl groups. Tertiary protein structure involves electrostatic interactions between the R groups of the amino acids in the polypeptide. The tertiary structure of a protein may be globular or filamentous, and may include disulfide bonds and/or salt bridges. Quaternary protein structure involves interactions between two or more polypeptide chains.