Cholesterol is a steroid and steroids are types of lipids. Cholesterol is common in animal cell membranes and works as a precursor for many other steroids and affects cell membrane fluidity.

A fatty acid is considered unsaturated if it contains one or more double bonds (indicated by the = sign). These double bonds occur between carbon atoms (represented as C). Be able to recognize the general structure of important macromolecules. Lipids rich in saturated fatty acids tend to be solids at room temperature (butter, lard, coconut oil).

A phospholipid has a hydrophobic tail and a hydrophillic head. The hydrocarbon section composes the hydrophobic tail and dislikes water. The phosphate group composes of the hydrophillic head and likes water. The combination makes a semi-permeable membrane that we know as the lipid bilayer. 

Enzymes are not changed or consumed by the reactions they catalyze, but can be altered by environmental conditions. They work in three-dimensional active sites to bind specific substrates and lower the activation of certain reactions, subsequently increasing the reaction rate. Reaction rate can be further increased when enzymes react with cofactors or coenzymes, but decreased when enzymes are blocked from their specified active sites by competitive inhibitors.

Macromolecules, such as proteins, nucleic acids, and polysaccharides, are composed of monomers. Each polymer is made from at least two smaller monomers. Protein monomers are amino acids, nucleic acid monomers are nucleotides, and polysaccharide monomers are monosaccharides. In order to form polymers, the monomers must form covalent bonds with one another.

For proteins, these covalent bonds are peptide bonds, and for saccharides they are glycosidic linkages.

G proteins are second messengers involved in cell signaling and propagation or effects within the cell. G protein receptors in the plasma membrane bind to extracellular ligands, causing them to recruit G proteins. Inactive G proteins carry ADP. Once they bind to G protein receptors in the membrane, this GDP molecule is removed, and a GTP molecule is substituted to activate the G protein.

The activated G protein then binds another protein and hydrolyzes GTP to GDP to activate this protein and stimulate cellular effects.

An amino acid is the monomer unit of the polymer known as a polypeptide. Polypeptide chains form the primary structure of proteins.

A monosaccharide is the simplest unit of a carbohydrate; a monosaccharide dimer is a disaccharide. Triglycerides are a simple form of lipid and nucleic acids are primarily composed of nucleotide monomers.

Peptide bonds form between the amine group of one amino acid and the carboxylic acid of another via a covalent linkage. The formation of a polypeptide chain from amino acid residues constitutes the protein primary structure.

Secondary structure is formed by hydrogen bonding between the amino and carboxyl backbone units of the polypeptide. Tertiary structure is formed by disulfide covalent bonds, hydrophobic interactions, and R-group hydrogen bonding. Quaternary structure is the joining of multiple polypeptide subunits.

Peptide nuerohormones are synthesized in the rough endoplasmic reticulum of the cell body of the neuron. They are then packaged in the Golgi complex and transported along the axon to the nerve endings. Since peptide hormones must be transported out of the cell in vesicles, they are not likely to be synthesized by cytoplasmic ribosomes. These ribosomes are primarily involved in synthesizing cytoplasmic proteins that do not leave the cell.

The nucleus houses DNA and is the site of transcription. The nucleolus is the site of ribosomal submit synthesis. The smooth endoplasmic reticulum is responsible for certain waste disposal and other functions.

Euchromatin is the most active part of the genome and is almost always in active transcriptional mode. mRNA is formed via transcription from DNA in the nucleus, in the form of euchromatin, as opposed to heterochromatin. Heterochromatin is characterized by tight winding of DNA around histones, such that it cannot easily be accessed by RNA polymerase and transcription proteins. Most DNA resembles euchromatin during interphase, when transcription is most active, and condenses to heterochromatin during mitosis.

Note that assembly of eukaryotic ribosomal subunits takes place in the nucleolus, but mRNA is always transcribed directly from DNA within the greater nuclear structure.