The plasma membrane, also called cell membrane, is composed of a phospholipid bilayer that separates the outside of the cell from the inside of the cell. It is selectively permeable, and contains many proteins that facilitate the transduction of signals in and out of the cell, and allow for passage of specific substances. The nucleus, endoplasmic reticulum, nucleus, and mitochondria are membrane-bound organelles that exhist in the cytoplasm of eukaryotic cells.

Amphipathic molecules have both hydrophobic and hydrophilic regions (polar and non-polar). Phospholipids are amphipathic because they have a polar head and a non-polar tail. Since integral proteins are embedded within the phospholipid bilayer, the parts that are on either side of the plasma membrane are hydrophilic, and the parts that are in contact with the tails of the phospholipids are hydrophobic.

Phospholipids are the major component of cell membranes (the lipid bilayer), composed of two fatty acid tails attached to a glycerol head. The third hydroxyl group that is joined to a phosphate gives the cell a negative charge. The heads are hydrophilic, or “water loving” and the tails are hydrophobic, or “water fearing.” The hydrophilic heads create a selectively permeable “gate” into and out of the cell. Between the hydrophilic heads are the hydrophobic tails. Thus, small, uncharged, lipophilic molecules can diffuse into and out of the cell.

The fluid mosaic model states that amphipathic proteins are embedded in the phospholipid bilayer. The cell membrane is a fluid mosaic of phospholipids and proteins. Phospholipids and proteins move laterally within the membrane. The unsaturated hydrocarbon tails of the phospholipids keep membranes fluid at lower temperatures and cholesterol helps them resist changes in fluidity in the face of temperature changes. Like a mosaic, integral proteins are embedded in the lipid bilayer while peripheral proteins are attached to the membrane surface. Glycoproteins and glycolipids are also embedded on the exterior side of the plasma membrane and interact with surface molecules of other cells. This membrane structure results in selective permeability of the cell membrane, allows for cell-cell adhesion, and cell signaling.

Passive transport occurs when a molecule or ion moves down its concentration gradient and therefore requires no energy. More specifically, in facilitated diffusion, a type of passive transport, a transport protein speeds the movement of water or a solute across a membrane and down its concentration gradient. Active transport, on the other hand, uses energy or ATP to move solutes against their concentration gradients.

Phospholipids are the most abundant because their structure makes it possible for them to form membranes since they have hydrophilic and hydrophobic regions. In the phospholipid bilayer of a cell, the hydrophilic heads are exposed to the intracellular and extracellular compartments, which contain cytosol, and extracellular fluid, both of which are aqueous. The tails of the phospholipids are hydrophobic, and thus "hide" from the aqueous environments on either side of the cell membrane. This structure allows small, uncharged, and fat-soluble molecules to diffuse across the membrane. 

When the blood pressure is low or when the body’s osmolarity is high, the posterior pituitary releases ADH. The reabsorption of water will increase the blood’s volume and result in an increase in blood pressure. With the reabsorption of water, the osmolarity is lowered. Recall that osmolarity is the amount of solutes (e.g. sodium, potassium, chloride, etc.) over the amount of solvent (e.g. water). When water is reabsorbed, the water’s volume increases; therefore, the osmolarity is decreased. Without ADH, blood pressure becomes low and without enough water the osmolarity of the blood increases—hyperosmolarity.

Hypertonic is the correct answer for the solution with more solute in it. The root "hyper" means more; therefore, a hypertonic solution will have more solute. A hypotonic solution will have less solute in it. Isotonic solutions will have equal concentrations of solutes between the two solutions.

Hypotonic is the correct answer for the solution with less solute in it. The root "hypo" means less; therefore, a hypotonic solution will have less solute. A hypertonic solution will have more solute in it. Isotonic solutions will have equal concentrations of solutes between the two solutions.

Osmosis is the process by which water will diffuse from the hypotonic side of the membrane to the hypertonic side. Water will naturally travel to areas with higher solute concentration in order to lower the concentration and make it equal to the concentration of its surroundings.