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.

Membranes are semi-permeable, meaning that they only allow certain compounds to cross without protein assistance. The two factors that determine permeability across a membrane are size and polarity. Polar and charged compounds very rarely cross the membrane by simple diffusion, and often require a carrier protein or pump in order to cross. Glucose and amino acids are polar, and a sodium ion carries a positive charge. These compounds will be blocked by the hydrophobic interior of the membrane, formed by the fatty acids tails of the phospholipids bilayer.

Nonpolar compounds, on the other hand, can very easily cross the cell membrane. Even larger nonpolar molecules, like steroid hormones, can easily cross the membrane via simple diffusion.

As the name implies, gap junctions are literally a gap in the plasma membranes of two adjacent cells that connect their cytoplasms. These junctions allow for electrical synapses and rapid cell signaling. While most notably present in cardiac muscle, gap junctions are also present in some neural cells and receptors.

Adherens junctions anchor cells through interactions with the actin cytoskeleton. Desmosomes use cadherin proteins to anchor cells via interactions with intermediate filaments. Tight junctions are used to create barriers that selectively allow molecules through layers of epithelial cells. 

All of the choices describe functions of different types of cell junctions. Anchoring junctions (such as adherens junctions and desmosomes) use the cytoskeletons of each cell, as well as certain transmembrane proteins, to anchor the cells together. Gap junctions create gaps in the plasma membrane between two adjacent cells that connect their cytoplasms. Tight junctions are capable of forming barriers that are nearly impermeable to fluid flow. 

In order to have the action potential spread evenly and completely throughout the cardiac muscle cells, the cells need junctions that allow for ions to move between them. This action is accomplished by gap junctions between the cells. The intercalated discs between cardiac muscle cells are composed of gap junctions, and allow for electrical stimulation to travel from one cell to the next. This feature means that cardiac muscle can be depolarized and contract simultaneously.

Tight junctions are used to prevent fluid flow between cells. Desmosomes help with force transduction by linking the cytoskeletons of adjacent cells. Valves are not a type of cellular junction, and are macrostructures that prevent the backflow of fluids in vessels of the body.

Cellular junctions allow cells to block materials from moving between cells, or for providing communication between cells. Tight junctions are junctions between cells that form a tight seal. This prevents water and other fluids from moving past the cells. Tight junctions are essential for maintaining concentration gradients, preventing osmosis from equilibrating ion concentrations between two regions.

Desmosomes serve to anchor the cytoskeletons of two adjacent cells, helping with force transduction. Gap junctions allow for cellular communication by creating perforations between cells through which ions and small molecules can flow. Villi are not a type of cell junction, and are structures that serve to increase cell surface area.