Gap junctions allow the flow of water and ions. They are seen in the heart muscle cells, to allow quick electrical conduction from one cell to another, which is observed as a wavelike contraction of the heart. Tight junctions are networks of spot attachments that are impermeable to fluids. Desmosomes are random, spot attachments that are responsible for cell-cell adhesion.

Gap junctions form pores connecting neighboring cells and allowing the mixture of cytoplasm and small solutes including ions. Desmosomes are specialized for cell-cell adhesion, and hemidesmosomes are specialized for cell-extracellular matrix adhesion. Tight junctions (occluding junctions) form a seal across a layer of cells and is virtually impermeable. 

Gap junctions are protein channels that span the intercellular space that connect two cells. They allow cytoplasmic exchange in animal cells. The diameter of gap junctions limits what is able to travel though them from one cell to another. Ions, amino acids, and small molecules can flow through gap junctions; however, proteins, polysaccharides, and nucleic acids cannot. Gap junctions allow the transfer of small molecules to direct communication and cellular activities.

Plasmodesmata are the plant structures that are analogous to gap junctions in animal cells. Plasmodesmata are protein channels between the cell walls of plant cells. They facilitate communication and the transport of solutes and small proteins between plant cells.

Gap junctions are cellular junctions that attach two or more cells together but also allow the exchange of products through an opening. Tight junctions, desmosomes, and hemidesmosomes do not allow direct communication among cells.

The answer to this question is facilitated diffusion.

Facilitated diffusion involves the movement down a concentration across a plasma membrane without ATP. Bulk flow is a completely different process and active transport is the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy. Active transport needs an input of energy, unlike facilitated diffusion. Osmosis is the simple diffusion of water.

Endothelial cells line the insides of blood vessels and lymphatic vessels and have many important functions, including but not limited to those described in the question. One additional function of endothelial cells is involvement in blood clotting.

Endothelium generally lines fully internal pathways (such as the vascular system), while epithelium generally lines pathways that are open to the external environment (such as the respiratory and digestive systems). Nerve cells are specialized for signaling, and red blood cells are specialized for oxygen transport.

Epithelial cells compose the outside of the body, namely skin and the lining of systems that connect to the outside of the body, such as the respiratory, excretory, and digestive tracts. The stomach is part of the digestive tract, as are the intestines. Knowing that the digestive tract is lined with epithelial cells allows us to eliminate the lining of the intestine from the answer choices. Similarly, the alveoli are a part of the respiratory system and nephrons are part of the excretory system.

The lining of the heart's chambers is part of the vascular lining, which is made of endothelial cells and is not exposed to the outside environment.

Epithilial tissue is usually classified in two ways: by the shape of the cells and by their organization. Cell shape can be flat and polygonal (squamous), elongated and rectangular (columnar), or short and rectangular (cuboidal). Cells can be found in a single layer (simple) or multiple layers (stratified). Pseudostratified epithelium is usually columnar in shape and consists of a single cell layer that has the appearance of multiple layers.

Sprilli is a classification of spiral-shaped bacteria.

Light enters the eye through the pupil and is focused at the back of the eye to form an image on the retina. The retina contains rods and cones that can convert the image to nerual signals for the brain to interpret.

The iris is the muscle around the pupil that allows it to dilate or constrict. Changing the size of the pupil will alter the amount of light entering the eye.

The ciliary muscles attach the lens to the scelera (the outer white part of the eye). Contracting or relaxing these muscles will change the focal point of the lens, allowing the eye to properly focus the image on the retina. The ciliary muscles and lens do not affect the amount of light to enter the eye.

The ciliary body is adjacent to the ciliary muscles and produces aqueous humor, a liquid that fills the space between the cornea and the lens.