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.

非极性化合物,另一方面,可以很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.

All of the choices presented describe functions of cell junctions. Adherens junctions and desmosomes are responsible for anchoring neighboring cells to one another. Hemidesmosomes help anchor the extracellular matrix in place, binding the cell membrane to proteins in the basal lamina or matrix. Gap junctions are an example of junctions that connect the cytoplasm of two neighboring cells and allow for communication via ions and other small molecules.