Lipids can be used as an energy source and are often used by cells to store energy for future use. Phosphate groups are not found in all lipids. For example most steroids, which are lipids, do not have phosphate groups. Lipids are hydrophobic and are repelled by water.

In saturated fats, the carbons have single bonds between them and the maximum number of hydrogen atoms.  Saturated fats do not have nitrogen, and they are solid at room temperature.

Unsaturated fats have at least one double bond between two carbons in the fatty acid and do not have the maximum number of hydrogens.  They do not contain nitrogen and are usually liquid at room temperature.

Lipids are what make up fats. Fats store energy and help insulate animals against the cold. They also form lipid bilayers that surround cells and organelles. All steroid hormones are derived from cholesterol, a lipid. Thus, lipids play a significant role in signal transduction. Folding proteins is a function left to other proteins.

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.