Contact-dependent signaling, as the name suggests, requires that communicating cells be in direct contact with each other. This is achieved either through receptor-ligand interactions or by specialized cytoplasmic channels that allow the flow of small molecules between cells. In animal cells, channels called gap junctions facilitate contact-dependent signaling in certain tissues, whereas, plasmodesmata perform a similar function in plants.
Gap Junctions
In animal cells, gap junctions are formed...

The movement of ions like sodium, potassium, and calcium into and out of the cell is essential to maintain the electrochemical gradient in living cells. The ion channels—a class of membrane transport proteins—help maintain this ionic gradient for the smooth functioning of physiological activities such as maintaining cell size and volume, conducting nerve impulses, and gas and nutrient exchange.
Ion channels are specialized integral membrane proteins on the plasma membrane that allow specific...

Multicellular organisms employ a variety of ways for cells to communicate with each other. Gap junctions are specialized proteins that form pores between neighboring cells in animals, connecting the cytoplasm between the two, and allowing for the exchange of molecules and ions. They are found in a wide range of invertebrate and vertebrate species, mediate numerous functions including cell differentiation and development, and are associated with numerous human diseases, including cardiac and...

The chemical and physical properties of plasma membranes cause them to be selectively permeable. Since plasma membranes have both hydrophobic and hydrophilic regions, substances need to be able to transverse both regions. The hydrophobic area of membranes repels substances such as charged ions. Therefore, such substances need special membrane proteins to cross a membrane successfully. In  facilitated transport, also known as facilitated diffusion, molecules and ions travel across a membrane via...

The complex three-dimensional arrangement of cells in any multicellular organism is defined and maintained by interactions of cells with each other and the extracellular matrix. Cell-cell junctions are specialized structures where the multi-protein complexes on one cell interact with the multi-protein complexes on another  cell. These cell junctions are classified  into three main types based on their function — occluding, anchoring, and gap junctions.
Occluding or Tight Junctions
Tight...

The cytoplasm of adjacent animal cells can exchange small molecules, ions, and secondary messengers via the communication channels which form the gap junctions. These junctions comprise a few hundred to thousands of molecular channels, each made of two halves, called the connexon hemichannel. A connexon is a hexamer of six transmembrane connexin proteins, which assemble radially, thus forming a pore or channel in the center. One connexon hemichannel docks with a corresponding connexon on the...