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.

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.

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.

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.

Peripheral proteins are located either on the inside or outside of the cell, but do not pass all the way through. Glycoproteins are located on the outside of the cell membrane. Mitochondria are located within the cell and make ATP. Golgi apparatus is also located within the cell, thus does not function for transport through the membrane. Integral proteins act as carrier proteins and allow substances to pass through the cell membrane from the inside to the outside of the cell (or outside to the inside).

The cell membrane is the most permeable to small, nonpolar molecules. Sodium ions (Na+) and water (H2O) are both small, but Na+ is charged and H2O is highly polarized. Both will therefore have a difficult time passing freely through the cell membrane. Water has some ability to cross the membrane, but it generally requires facilitated diffusion. Glucose (C6H12O6) is a large, polar molecule, and requires a protein channel in order to cross the membrane. Insulin is a peptide hormone, meaning that it is composed of amino acids. As such, it is both large and polar. Generally, insulin will bind to a receptor on the cell surface in order to elicit an effect; it will rarely cross the membrane at all.

Oxygen (O2), which is small and nonpolar, can easily diffuse across the membrane. This is essential for loading hemoglobin in the lungs and releasing oxygen in capillaries.

If a solution is hypotonic to a cell, then it contains fewer solutes than the cell. If a solution is hypertonic to a cell, then it contains more solutes than the cell.

In a hypertonic environment, the external environment of the cell has a higher concentration of non-penetrating solutes. The solutes are unable to cross the cell membrane to create equilibrium. Instead, water will try to balance the concentrations by moving toward the region of greater solutes. This process of osmosis will cause water to leave the cell, and the cell will shrivel.

If there was no expenditure of energy when determining the voltage across the plasma membrane, there would be equal electrical charge on both sides of the bilayer as the ions travel to reach equilibrium. This means that ATP must be used in order to establish a resting potential, keeping the ions away from electrical equilibrium.

The sodium-potassium pump is an example of how ions can be pumped against their electrochemical gradients in order to establish a negative voltage inside the cell. The cell membrane is not permeable to sodium or potassium.

Cellular membranes are considered semipermeable, and allow certain substances to pass through without assistance from proteins. We typically follow the rule of thumb that substances that are small or nonpolar will be able to pass through the membrane. Water and hydrogen gas are both very small and can pass through the membrane relatively easily. Steroid hormones are large, but nonpolar, so they can pass through. Glucose is both large and polar, so it requires a carrier protein in order to cross.