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. 

Second messengers are intracellular signaling molecules. Epinephrine is a hormone that is released into the bloodstream and is thus never inside the cell. cAMP, Ca²⁺ and IP3 are all examples of second messengers. They respond to primary messengers—which are often hormones—by amplifying their effects and/or turning on downstream effectors.

The phases of mitosis can be described as follows:

During prophase chromatin condenses to form discrete chromosomes.

During metaphase microtubules attach to the kinetochores and chromatids begin segregating.

During anaphase sister chromatids have been separated and reside at opposite poles of the cell.

During telophase both sets of chromatids are surround by new nuclear membranes and chromosomes decondense into chromatin.

Cytokinesis (the dividing of the cytoplasm into two cells) follows telophase.

If the cell were arrested during telophase, distinct chromatids would no longer be visible.

Metaphase is the stage of cell division during which the sister chromatids line up along the metaphase plate in the center of the cell.  

Interphase is the stage of the cell cycle during which the cell prepares for cell division. The cell spends most of its life cycle in interphase, during which is transcribes genes, synthesizes proteins, and performs most of its intended functions. Interphase is not generally considered a phase of mitosis.

Prophase is the stage of the cell cycle during which the chromatin condenses into chromosomes and the nuclear envelope dissolves.

Anaphase occurs when the sister chromatids that have lined up along the metaphase plate separate and are pulled to opposite poles of the cell by the spindle fibers.  

Telophase occurs when two daughter nuclei form in the two new cells at the conclusion of mitosis.

The order of mitosis is: prophase, metaphase, anaphase, telophase, (interphase).