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Cell signalling

In multicellular organisms, there is a need for the cells to communicate with one another in order to coordinate their growth and metabolism. The principal way by which cells communicates with each other is by means of extracellular signaling molecules or hormones. These molecules are synthesized and secreted by signaling cells and produce a specific response in target cells that have specific receptors for the signaling molecule. Different cells can respond differently to the same signaling molecule depending on the type of receptor and the intracellular reactions initiated. 

            Signaling refer to the process by which cell release, receive, propagate or to spread and respond to information from their environment and from each other. There are hundreds of kinds of molecules that act as signals and almost an infinite number of potential cellular responses. Signal transduction is an important part of the cell signaling process and refers to the conversion of a signal from one form to another. The signal sent by one cell is received by receptor (proteins) in the target cell. The receptor transmits the signal across the membrane, converting the extracellular signal to an intracellular signal. The signal is propagated through a series of intracellular molecules. Ultimately, the signal receives its intracellular destination to draw out a cellular response.

Importance of Signaling

Signaling is essential to the organism for their survival and also provides an opportunity to adopt suitably e. g. they maintain homeostasis, control of cell division, development, growth, and death. Adaptation to environmental conditions and release as well as the production of hormones and other regulatory molecules.

a.      Categories of signaling

Signaling molecules are those which are released from one cell and traveled to another as opposed to the molecules involved in propagating message inside the cells. These include proteins, hormones, small peptides, amino acids, fatty acids derivatives, nucleotides, dissolved gases like Nitric oxides and carbon monoxide, etc. The signaling molecules and their pathway can be classified into distinct types based on distance over which the signaling molecule acts i.e. reach the target cell. Following are the main types of signaling molecules (fig. 4.1).

Fig. --Categories of signaling: a) Endocrine b) Paracrine and c) Autocrine signaling.

i.                    Endocrine signals: These chemical messengers are hormones and act as endocrine signals. They are produced at great distances from their target tissues.  The endocrine cells secrete the signaling molecule into the bloodstream which carries it to the target cells elsewhere in the organism. This type of signaling relies on diffusion and blood flow and hence a relatively slow process.

ii.                  Paracrine signals: Some signals, such as growth factors are released locally, where they diffuse to acts as short-range on nearby tissues. Such signals are referred to as paracrine signals. The conduction by a neurotransmitter of a signal from one nerve cell to another or from a nerve cell to a muscle cell occurs via paracrine signaling. Many growth factors regulating development in multicellular organisms also acts at short range.

iii.                Autocrine signals: Some local mediators act on the same cell that produces them; such signals are called autocrine signals. Some growth factors acts in this fashion. Cultured cells often secrete growth factors that stimulate their own growth and proliferation. Another important example of autocrine signaling is; in the immune system and also in uncontrolled growth of cancer cells. In this situation cancer cells produce a factor to which they respond, during their own unregulated proliferation. In short, the cell responds to a molecule that it has produced itself.

iv.                Juxtacrine signals: Unlike the above types of signaling, juxtacrine signaling requires physical contact between cells involved. Notch signaling and classical cadherin signaling are examples of juxtacrine signaling. 

Question Bank

Q. 1 A) Multiple choice questions (1 mark each)

1.  Cell signaling refers to cell …… and respond to the environment.

      a. Release                  b. Receive                  c. Propogate                 d. All of them

2.   Signaling is essential to organisms for their ……

      a. Survival                 b. Death                     c. Both a and b              d. None

3.      ……. Signals are chemical messengers, produced at great distance from the target tissues.

a. Paracrine                 b. Autocrine                c. Endocrine                 d.  Juxtacrine

4.      ……. Signals are growth factors and release locally.

a. Paracrine                b. Autocrine                c. Endocrine                   d.  Juxtacrine

5.      ……. Signals requires physical contact between cells

a. Paracrine                 b. Autocrine                c. Endocrine                   d.  Juxtacrine

Q. 1 B) Answer in one sentences (1 mark each)

        i.         What do you mean by cell signaling?

ii What do you mean by endocrine signals?

iii What do you mean by paracrine signals?

iv What do you mean by autocrine signals?

v What do you mean by juxtacrine signals?

Q.2. Define/ Explain/ Comments (2 mark each)

      i.      Cell signaling                ii.   Endocrine signals

      iii.    Paracrine signals           iv.  Autocrine signals

      v.     Juxtacrine signals          

Q. 3. Attempt the following (3 mark each)

a.            Explain briefly endocrine signals.

b.            Explain briefly paracrine signals

c.             Explain briefly autocrine signals

d.            Explain briefly juxtacrine signals           

f.              Sketch and label autocrine signaling

g.            Sketch and label paracrine signaling

Q. 4. Attempt the following (4 mark each)

a.            Describe any two types of cell signaling

b.             Sketch and label endocrine signaling.

Q. 5. Attempt the following (6 mark each)

1.            What is cell signaling? Describe categories of cell signaling.