Glycolysis (the process of breaking down glucose) takes place in the cytoplasm, or cytosol—the aqueous portion of the cytoplasm. It is in the cytoplasm where the enzymes required for glycolysis are found.

The citric acid cycle takes place in the mitochondrial matrix, and the electron transport chain takes place along the inner mitochondrial membrane in order to pump protons into the intermembrane space.

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

A phosphatase removes a phosphate group from 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.

Ubiquitin ligases add ubiquitin to their ligands. The addition of ubiquitin acts as a signal that a protein has become ineffective and is ready for degradation. When multiple ubiquitin residues have been added to a protein molecule, it is transported to the lysosome in the cell to be digested.

A phosphatase removes a phosphate group from its ligand.

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

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.

Several different types of proteins can change the structure of a ligand, such as isomerases.

A cell must exchange molecules and ions with its surroundings.  This process is controlled by the selective permeability of the plasma membrane.  Passive transport requires no energy from the cell; molecules like water can diffuse into and out of the cell through the phospholipid bilayer freely by way of osmosis.  Other molecules and ions, like sodium, are actively transported across the phospholipid bilayer.  This requires ATP created by the cell.  Active transport moves solutes against their concentration gradients, which is why it requires energy. 

Tonofibrils are groups of keratin tonofilaments (intermediate filaments) most commonly found in the epithelial tissues, not endocrine tissues, and which play an important structural role in cell makeup.

The primary ability of secondary messengers is their ability to leave the cell membrane and travel through the phospholipid bilayer by being selectively hydrophilic or -phobic, allowing egress. This enables, for example, a cascade effect that greatly amplifies the strength of the original primary messenger signal.

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.

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.

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.