The plasma membrane is only permeable to small nonpolar molecules. Charged ions, polar solutes, and large solutes can excluded, and require protein mediators to cross the membrane.

Oxygen and carbon dioxide are small molecules with no net dipole; though the bonds themselves may be polar, the symmetry of these molecules causes them to carry no net polarity. Oxygen and carbon dioxide diffuse across the membranes of alveoli in the lungs during reoxygenation. Progesterone is a steroid hormone, indicating that it is a small cholesterol derivative. Steroid hormones are capable of diffusing across the membrane due to their lipophilic nature.

Ions do not undergo simple diffusion. Instead, they are carried across the cell membrane by alternative means, such as active transport. This is essential for maintaining electrochemical gradients, such as the proton gradient in mitochondria and the sodium-potassium gradient in neurons.

The fluid mosaic model is used to explain the fluidity of the plasma membrane. This allows proteins to move along and within the membrane structure and promotes cell motility and flexibility. The fluid mosaic model is based on the idea of the hydrophilic heads facing the exterior of the membrane and hydrophobic tails facing the interior. Transmembrane proteins contain hydrophilic and hydrophobic regions in the same orientation, while peripheral proteins are able to adhere to the membrane exterior. Cholesterol is found within the hydrophobic region of the membrane and promotes fluidity and movement by preventing fatty acid tails from adhering to one another.

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.

The cell membrane is composed of a phospholipid bilayer. Each phospholipid molecule contains two fatty acid tails. Saturated fatty acid tails will be straight, encouraging the molecules to pack together. The unsaturated hydrocarbon tails contain kinks that prevent packing, allowing enhanced membrane fluidity. The other main component of membrane fluidity is cholesterol molecules, which also help to prevent packing in the hydrophobic region of the membrane.

Peripheral membrane proteins are situated outside of the membrane, and do not influence fluidity. Integral membrane proteins span the entire bilayer, but do not impact the overall fluidity.

The cell membrane is a lipid bilayer that is hydrophilic on the outside and hydrophobic on the inside. Proteins are embedded in the membrane to assist in regulating the passage of various compounds into and out of the cell. Only certain molecules can cross the membrane without protein assistance. The cell tightly regulates the production of membrane proteins to maintain the environment of the cell, but cannot do anything to regulate the molecules that cross the membrane without protein assistance.

These molecules that can freely cross the membrane must be both small and nonpolar. Carbon dioxide is small (only three atoms) and is nonpolar due to its linear symmetry. Carbon dioxide is thus able to diffuse across the membrane, and cannot be regulated by membrane proteins.

Sodium and chloride ions are small enough to cross the membrane, but carry a charge than makes it impossible for them to cross the hydrophobic region. Water is also quite small, but is barred from the membrane due to its polarity. Glucose is both large and polar and requires proteins for membrane transport.

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.

A hypertonic solution implies that there is a higher concentration of solute outside of the cell than inside. To balance this difference in concentration, water from inside the cell will diffuse across the membrane and the cell will shrivel. If the solution were hypotonic to the cell, the opposite would happen: the cell would take in water from the solution, causing it to swell and potentially burst (lyse).

Though red blood cells do lack a nucleus, this is not related to the question at hand.

In order for the bag to gain weight, water needs to enter the tubing. The dialysis tubing must be placed in a hypotonic solution—a solution containing a higher percentage of water (lower concentration of ions) than the dialysis solution. This will cause water to enter the more concentrated solution within the dialysis tubing, in an attempt to reach equilibrium

Sodium ions and glucose are both monoatomic, so the composition of the solute is irrelevant. We simply need to find the greatest difference in concentration between the tubing and the beaker, in which the greater concentration is found in the beaker. Essentially, we want the maximum value of .

The greatest difference, in this case, occurs when a 50% sodium solution is present in the tubing and pure water is present in the beaker.

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.

The plasma membrane acts as a selective barrier and defines the boundaries of the cell. It allows for the passage of oxygen, nutrients, and wastes. The nucleus is enclosed by the nuclear envelope, not the plasma membrane.