Remember that depolarization is the first step of an action potential, during which the membrane potential increases rapidly. This rapid increase is attributed to the movement of sodium ions into the cell. Since sodium ions are positively charged, the inside of the cell becomes more positive and the outside of the cell becomes more negative. This change in polarity causes an increase in membrane potential. When a neuron is excited past the threshold stimulus, an action potential ensues. An action potential causes huge alterations in membrane potential due to the movement of ions during depolarization, repolarization, and hyperpolarization. The sodium-potassium pump constantly moves sodium and potassium ions against their concentration gradients, which helps maintain the negative resting membrane potential. 

During resting membrane potential, a sodium ion inside the cell will not have a higher electrical potential energy than a sodium ion outside the cell. Recall that the electrical potential energy is higher when the electrical potential is higher. The resting membrane potential is negative; therefore, the inside of the cell has a lower potential than the outside of the cell. Since the potential inside the cell is lower, a sodium ion inside the cell will have a lower potential than a sodium ion outside the cell.

Two main factors are required for a membrane potential. First, there must be a concentration or electrochemical gradient of ions. A potential difference occurs between two regions in space that have an uneven distribution of charges; therefore, for a membrane potential there must be a concentration gradient of ions (a difference between the ion concentrations) between the outside and the inside of the cell. Second, there must be a semipermeable membrane that separates the interior of the cell from the exterior. If a semipermeable membrane isn’t present, then the ions can undergo simple diffusion, which will equilibrate the concentration of ions. This would diminish the concentration gradient and the potential difference would become zero. 

Neurotransmitters are not required for establishing a membrane potential. They are chemicals that bind to receptors on membranes and initiate a cellular response.

Students should know the main players in establishing action potentials are K+ and Na+.  Further, Na+ inward flow through open channels brings an action potential to a peak depolarization of about +60 mV, which is sodium's equilibrium potential

Albumin, glucose, and potassium ions are all examples of NON-permeable solutes. To be permeable, a solute must be small and nonpolar. Albumin, the main osmoregulatory protein, is bulky and too large to cross the membrane freely. Glucose is also large, as well as polar, and cannot cross the membrane. Potassium cannot freely cross due to the positive charge on the ion. All of these will require facilitated means of entering the cell.

Testosterone is a steroid hormone, and as such is small and nonpolar. All steroid hormones have intracellular receptors, and are able to enter a cell freely. Of the four choices, it is the only solute that can permeate the cell membrane.

Integral membrane proteins are proteins that span the entire membrane, whereas peripheral membrane proteins are proteins that associate only with only one side of the membrane (the "periphery").

Plasma membrane channels are proteins that facilitate the exchange of ions and other molecules between the extracellular and intracellular sides of a cell. To accomplish this task, a channel must span through the membrane (phospholipid bilayer); therefore, membrane channels are classified as integral membrane proteins. Recall that the inside of a phospholipid bilayer is extremely hydrophobic. Since like dissolves like, a membrane channel must contain hydrophobic regions that can interact with the interior of the phospholipid bilayer.

Amphipathic molecules contain both polar and nonpolar regions. Integral proteins (and most peripheral proteins) are amphipathic molecules. Phospholipids are also amphipathic molecules because they contain a polar head and a nonpolar tail.

Water molecules are small and can travel through the membrane via simple diffusion. The rate of simple diffusion, however, is extremely slow due to the polarity of water molecules and their reluctance to enter the hydrophobic core of the membrane. Faster transportation of water molecules occurs via facilitated diffusion by specialized membrane channels called aquaporins; therefore, water can be transported via simple diffusion and facilitated diffusion.

Hydrophobic regions on membrane channels are essential to span the hydrophobic portions of the phospholipid bilayer. Although they are hydrophobic, lipids are not the main components of the hydrophobic regions. The hydrophobic regions in a membrane channel consist of hydrophobic amino acids that contain nonpolar side chains. 

Remember that facilitated diffusion and simple diffusion are both forms of passive transport; therefore, these processes do not require energy and transport molecules from a region of high concentration to low concentration. They don’t move molecules against their electrochemical gradient. 

Facilitated diffusion occurs at a much higher rate than simple diffusion. Facilitated diffusion uses membrane channels to transport molecules; therefore, it is much easier for molecules to traverse through a channel (facilitated diffusion) than the phospholipid bilayer (simple diffusion).

Each phospholipid consists of a polar phosphate head and two non-polar lipid tails. The phospholipid bilayer consists of polar heads facing the inside and outside of the cell, which interact with polar, aqueous environments. The non-polar hydrocarbon tails are packed on the inside of the bilayer, as far away from the polar environments as possible.

Both prokaryotic and eukaryotic cells have plasma membranes that separates the cell's contents from the external environment. Prokaryotic cells frequently have more external layers (cell wall and cell capsule), since they are generally more exposed and require additional protection from the environment, but these layers will always lie one top of the fundamental cell membrane.

Prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotes lack such organelles. Prokaryotic organisms are always unicellular, while eukaryotic organisms can be either unicellular (protists) or multicellular.

Cell containment, as the name suggests, is the process by which the contents of a cell (organelles, cytoskeleton, etc.) are contained within a region of space inside the cell. This means that there must be a barrier that prevents the contents from leaking out of the cell. This barrier is the phospholipid bilayer, or cell membrane. Phospholipid bilayers can be found in plasma membranes or in the membranes that cover the organelles. Recall that most organelles in an eukaryotic cell contain a phospholipid bilayer that separates the contents of the organelle from the cytosol.

Proper cell containment depends heavily on the properties of the phospholipid bilayer. The cell membrane is semipermeable; this means that the cell membrane only permits certain molecules to pass through. If the cell membrane were permeable to everything, then the contents inside the cell can easily traverse the cell membrane and exit the cell. The semipermeability of the cell membrane is very important to maintain cell containment and regulate the homeostatic environment of the cell interior.

The acidity of lysosomes and presence of mitochondrial DNA are irrelevant to cell containment. The acidity of lysosomes is important for eliminating biological waste products inside the cell, and the mitochondrial DNA is important to produce unique mitochondrial proteins; however, they do not play a role in maintaining the contents of the cell or an organelle.