Question

List and describe four types of cell signalling induced by soluble extracellular molecules. (16 marks)

Answer 1

There are four categories of chemical signaling found in multicellular organisms: paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions. The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell. We should note here that not all cells are affected by the same signals.

Now We can briefly understand each Signaling One by One.

1. In endocrine signaling, signaling molecules, called hormones, act on target cells distant from their site of synthesis by cells of endocrine organs. In animals, an endocrine hormone usually is carried by the blood from its site of release to its target.

2.In paracrine signaling, the signaling molecules released by a cell only affect target cells in close proximity to it. The conduction of an electric impulse from one nerve cell to another or from a nerve cell to a muscle cell (inducing or inhibiting muscle contraction) occurs via paracrine signaling.

3.In autocrine signaling, cells respond to substances that they themselves release. Many growth factors act in this fashion, and cultured cells often secrete growth factors that stimulate their own growth and proliferation. This type of signaling is particularly common in tumor cells, many of which overproduce and release growth factors that stimulate inappropriate, unregulated proliferation of themselves as well as adjacent nontumor cells; this process may lead to formation of tumor mass.

4.Direct signaling across gap junctions, involves signaling molecules moving directly between adjacent cells.Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighboring cells. These fluid-filled channels allow small signaling molecules, called intracellular mediators, to diffuse between the two cells.The specificity of the channels ensures that the cells remain independent but can quickly and easily transmit signals. The transfer of signaling molecules communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received.