There are many factors that determine the fluidity of cell membranes. Membranes that are composed of fully saturated, long fatty acid tails are generally less fluid then the opposite conditions. In addition, lower temperatures result in a less fluid membrane.

Membranes that have fatty acid tails with double bonds are more fluid because the double bonds make it difficult for multiple phospholipids to float next to one another. The shape of the double bond adds a another dimension to the lipid, preventing the tails from packing together. Unsaturated fatty acids are thus more fluid than saturated fatty acids.

In higher temperatures, the cell membrane is going to become increasingly fluid due to the increased movement of the phospholipids. The cell membrane can control its fluidity in high temperatures by both increasing the saturated fatty acid tail content of the phospholipids, as well as making the fatty acid tails longer. Cholesterol can also help by acting as a buffer at high temperatures.

Although cell walls often serve very similar functions for different species, the composition of the cell walls can vary significantly. Plant cell walls employ cellulose, while bacteria use peptidoglycan. Fungal cell walls use the polymer chitin.

The correct answer is peptidoglycan. Cellulose composes the cell walls of plants, whereas chitin composes the cell walls of fungi. Starch and glycogen are stored polymers of glucose in plants and animals, respectively.

A Gram-positive cell has the following basic structural characteristics: stains dark purple in the Gram stain, has a thick peptidoglycan layer, and possesses no outer membrane beyond this layer. Thus, there is also no periplasmic space. Acid-fast stains are only used for specific bacteria that have waxy cell walls.

Because leucine is a hydrophobic amino acid, it would make sense that it would be most stable in a hydrophobic environment. The interior of the phospholipid bilayer is a hydrophobic environment; therefore, leucine and other hydrophobic amino acids are more commonly found in the membrane-spanning portions of transmembrane proteins.

Polar and hydrophilic amino acids are most commonly found in the cytosolic and exoplasmic regions of the membrane, as these regions interact with the aqueous environment outside of the membrane.

Because the molecule is a sugar, it is too large to passively diffuse across the plasma membrane and contains polar regions that would make this impossible.

There are several other mechanisms by which the sugar could enter the cell. One of these is receptor-mediated endocytosis. In this process the sugar would bind to receptors on the plasma membrane, which stimulates a budding event and eventually leads to the formation of a vesicle inside the cell.

Symport is another potential mechanism. In symport, the import of a molecule is coupled with the import of another molecule through the same transmembrane protein. For example, glucose has a symport mechanism with sodium ions.

Of the given choices, only channel and carrier proteins allow substances to cross the membrane. While channel proteins create an open pore through which substances can cross, carrier proteins will change their shape in order to allow substances to cross the membrane.

The question asks about membranes in general, not just the cell plasma membrane; therefore, all of the answers are true. The plasma membrane's most important functions are protecting the internal environment of the cell and selectively allowing nutrients into the cytoplasm (semi-permeability). The final answer describes a function of the inner-membrane of the mitochondria, which houses the electron transport chain.

Energy is not necessary when a compound is being moved down its electrochemical gradient. Diffusion, facilitated diffusion, and osmosis all involve a compound moving from a higher to a lower concentration. Since this is the spontaneous direction of flow, no energy input is required.

In order to move a compound against its electrochemical gradient, energy is needed. This type of transport is called active transport.