The plasma membrane is able to maintain the appropriate level of fluidity by manipulating a variety of factors. More cholesterol in the membrane reduces its permeability, which is useful in hot conditions. Desert animals would also be expected to have very few unstaurated fatty acids, and the fatty acid tails would be longer. All of these factors would help the plasma membrane not be too fluid in the hot sun.

The natural flow of molecules is from areas of high concentration to areas of low concentration. To act against this gradient, energy must be input. Active transport requires the use of energy to move a molecule up its concentration gradient. Diffusion, osmosis, and facilitated diffusion are forms of passive transport that move a molecule down its concentration gradient.

Endocytosis involves the use of vesicles to transport large molecules into the cell. Facilitated diffusion, osmosis, and active transport use diffusion or protein channels to transport molecules.

Note that exocytosis also uses vesicles, but uses them to export large molecules out of the cell or to incorporate them into the cell membrane.

Exocytosis allows the membrane of secretory vesicles to be incorporated into the cellular membrane. This expands the membrane surface, while including the desired proteins into the membrane. Due to fluidity and the mosaic model of the membrane, these proteins can then distribute to other areas on the cell surface.

Endocytosis does the opposite process, and involves a pinching off of the cell membrane in order to transport incoming materials. Ribosomes translate the proteins, however, processing of membrane proteins occurs in the endoplasmic reticulum and Golgi body, ultimately packaging membrane proteins in secratory vesicles for exocytosis.

Any amino acid with a charged side chain will be polar. Membranes have hydrophobic tails on the interior, and hydrophilic heads facing the outside and inside of the cell. Since polar molecules are charged, they will interact with the hydrophilic parts of the environment, and therefore they will not be found within the membrane interior.

This question is tricky because we need to remember that if there are 2.0 moles of NaCl in the water, then there are 4.0 moles of solute because it will dissociate to one  and one ; therefore, this is an isotonic environment and there will be no net movement of water. When a compound dissociates in solution, it is the ion concentration that will affect the movement of water, rather than the amount of initial solid.

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.

Passive transport is simple diffusion, without the aid of any protein channels. Since the plasma membrane is composed of a lipid bilayer, the molecules that most readily cross it are small and hydrophobic.

Remember that water is polar; hydrophobic molecules are nonpolar, like lipids and oils. Steroids are derived from cholesterol and are extremely lipid-soluble, so they can cross the membrane unassisted. Large, polar molecules (such as glucose or peptides) and charged ions need channel proteins to facilitate their crossing. Water can diffuse across the membrane, but only when it is moving down its concentration gradient. The answer choice given has water moving from a hypertonic (high-solute, or low-water) environment into a cell, which cannot occur passively.

While the availability of nutrients is a tempting answer, it is important to remember that even in incredibly nutrient-rich environments cells reach a maximum size. The concentration of water in the cytoplasm and the amount of membrane bound organelles will not really have any effect on the size of the cells.

The correct answer is the surface area to volume ratio because it dictates the amount of chemical activity carried out per unit of time. The volume of the cell determines its metabolic needs: larger cells hold more biological material and require more energy and maintenance. The surface area of the cell determines its ability to transport nutrients and absorb nutrients, providing the tools to maintain the cell's volume. A large surface area to volume ratio is essential for the cell.

Phospholipids found in the plasma membrane are comprised of a polar head group, two fatty acids, and a glycerol backbone. The phosphate group carries a negative charge, allowing it to interact with polar aqueous environments. The two fatty acid tails form the hydrophobic region of the bilayer interior. The glycerol backbone forms the structural component for linking the polar and non-polar regions.

Glycogen is a polymer of glucose sugars, and is not found in phospholipids. Triglycerides are composed of three fatty acids and a glycerol backbone.