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.

Osmosis is defined as a form of passive transport since it does not require an expenditure of energy. Osmosis follows a concentration gradient while active transport acts against the concentration gradient. Also, active transport requires energy and passive transport does not.

Diffusion is defined as the spread of molecules into available space. The random nature of the movement of molecules in an open area means that molecules will seek to spread out—diffuse—evenly within a given area. This means that molecules will evenly distribute themselves when random assortment is permitted within a space.

The plasma membrane functions as a selective barrier to allow the flow of oxygen, nutrients, and wastes to and from the cell. The selectively permeable plasma membrane encloses the cytoplasm. The cytoplasm in turn contains the nucleus, mitochondria, and other organelles.

Transport proteins span the exterior of the cell and allow substances to pass through the membrane. Some transport proteins are involved in facilitated diffusion, which is a type of passive transport. Other transport proteins are involved in active transport such as the sodium-potassium pump.

Osmosis is an example of passive transport. Osmosis does not use energy. Instead, it acts using the concentration gradient. The sodium-potassium pump, proton pump, and electrogenic pump are all examples of active transport. This means that they can pump solutes against their concentration gradients using cellular energy.

The fluid mosaic model describes the mosaic structure of the cell membrane. According to this model, the cell membrane is composed of a variety of protein molecules randomly bobbing around in a sea of phospholipids. Due to phospholipid polarity, they naturally form a bilayer in accordance with their dual hydrophobic (tails) and hydrophilic (heads) natures.