All of the examples listed are considered second messengers except for protein kinase C, which interacts with second messenger pathways as an effector; however, it is not a second messenger itself.

Recall that second messengers are used to amplify signals within the cell. A ligand may bind to a receptor on the cell surface in order to activate a signaling cascade. Second messagers will help propagate this cascade throughout the cytosol. The messengers essentially help transfer the signal from the receptor on the cell membrane to the proteins in the cytosol that will ultimately be affected.

Chemotaxis refers to the movement of an organism in response to a chemical stimulus. Single or multicellular organisms may direct their movements according to certain chemicals in their environment. This is important because these organisms need to find food, flee from harmful substances, and chemotaxis also aids in development. Positive chemotaxis is movement towards a higher concentration of the chemical, whereas negative chemotaxis is movement away from the chemical.

Chemotaxis refers to the movement of an organism in response to a chemical stimulus. Single or multicellular organisms may direct their movements according to certain chemicals in their environment. This is important because these organisms need to find food, flee from harmful substances, and chemotaxis also aids in development. Positive chemotaxis is movement towards a higher concentration of the chemical, whereas negative chemotaxis is movement away from the chemical.

All of the examples listed are considered second messengers except for protein kinase C, which interacts with second messenger pathways as an effector; however, it is not a second messenger itself.

Recall that second messengers are used to amplify signals within the cell. A ligand may bind to a receptor on the cell surface in order to activate a signaling cascade. Second messagers will help propagate this cascade throughout the cytosol. The messengers essentially help transfer the signal from the receptor on the cell membrane to the proteins in the cytosol that will ultimately be affected.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

G-protein coupled receptors begin the signal transduction pathway by interacting with intracellular G-proteins. This interaction isn't possible until a ligand forces a conformational change in the GPCR, thereby freeing up a site for the G-protein to bind. This interaction permits the G-protein to exchange a GDP for a GTP, thereby activating the G-protein and continuing signal transduction.

G-protein coupled receptors begin the signal transduction pathway by interacting with intracellular G-proteins. This interaction isn't possible until a ligand forces a conformational change in the GPCR, thereby freeing up a site for the G-protein to bind. This interaction permits the G-protein to exchange a GDP for a GTP, thereby activating the G-protein and continuing signal transduction.

Exocytosis is a process by which the cell packages content and secretes it from the cell in a vesicle. Hormones, which act on cells far away from where they are produced, will travel out of the cell to reach their target tissues and organs. Vesicles of hormones will fuse with the membrane of the cell and release the hormone into the blood for transport.

DNA, RNA, and cytoplasmic constituents do not leave the cell and would not be exocytosed. Integral membrane proteins are placed in the membrane via vesicle fusion, but are not exocytosed in the process.

The ligand binds the receptor on its extracellular terminus; therefore the ligand itself never enters the cell or passes through the membrane. Second messengers let the cell 'know' what is happening on the outside, but these extracellular molecules do not directly enter the cell.

All of the other answers describe benefits of the second messenger system.