4.1: Cell Communication

In Unit 3, we learned about how individual cells get their energy and function. However, all multicellular organisms, including ourselves, need their cells to work together as one being. Topic 4.1 begins to explain how cells talk to one another to convey the information necessary for day-to-day operations.

Vocab List

Written Explanation

The Signal Transduction Pathway:

The signal transduction pathway is a broad framework for understanding cellular signaling. It involves three basic steps: reception, transduction, and response. Reception involves the cell detecting a signal molecule. Transduction is the process by which the signal is moved from its area of reception to where it really does things. Lastly, the response is the final action a cell takes in reaction to that initial signal molecule.

Diagram of a simple signal transduction pathway

Reception:

The first part of the signal transduction pathway, reception, can take many different forms. What all these forms share is a signaling molecule (also known as a ligand) and a receptor protein. The receptor protein is what "detects" the presence of a ligand by binding to it. The cell that the receptor protein is a part of is called the target cell (it is the target of the ligand).

Diagram of a receptor protein in action

Transduction:

Likely the most complex part of signal transduction is, well, transduction, or the way in which the signal reaches from start to its destination. In some cases, transduction may be a short and direct chain of actions, but in many other cases, transduction is a chain reaction that amplifies a signal (more on that in 4.2).

Diagram of a simple transduction process

Response:

The last part of the pathway is the part that matters most: the actual action being taken. Some signals may also lead to the activation of multiple cellular responses, and some may be part of a larger signaling system (like the nervous system). In many cases, the response is carried out by the nucleus and results in the production of certain mRNA, which become proteins.

An example of a signal transduction response

Autocrine signaling:

In autocrine signaling, a chemical signal affects the cell it was sent from. This essentially acts as a positive feedback loop, reinforcing the role of the cell during its initial development. Autocrine signaling also plays a role in metastasis which is when cancer tumors spread.

Diagram of autocrine signaling

Juxtacrine signaling:

In juxtacrine signaling, signals pass between cells through direct cell-to-cell contact. For example, plant cells conduct juxtacrine signaling using plasmodesmata. Animal cells use gap junctions, which function very similarly to plasmodesmata.

Diagrams of plasmodesmata and gap junctions

Paracrine signaling:

Paracrine signaling is communication across short distances, and as such has very local effects. Some prominent examples of paracrine signaling include:

Synaptic signaling

The release of ligands (neurotransmitters) between different neurons, with the response being a change in electric potential

Diagram of synaptic signaling

Quorum sensing

Bacteria (unicellular prokaryotes) release autoinducers (ligand) to determine the population density of their species in a local area. They use this information to "decide" how much they should reproduce

Diagram of quorum sensing

Morphogens

Signaling molecules (ligands) that influence the cell behavior and development of an embryo

Diagram of morphogen behavior
Table with the three examples of paracrine signaling

Endocrine signaling:

Endocrine signaling is communication along much longer distances, sometimes going throughout an entire organism. The ligands in use are called hormones, and they travel through the bloodstream. Some examples of endocrine signaling are:

Insulin

Released by the pancreas into the bloodstream, insulin regulates glucose levels in blood (you may have heard of this being called "blood sugar"). Insulin promotes the storage of glucose as glycogen in the liver and the removal of glucose from the bloodstream.

Human growth hormone

This hormone grows bones and cartilage, and promotes cell division.

Thyroid hormones

These two hormones regulate your body's metabolism. They are both released by the thyroid gland, which is where they get their name from.

Estrogen & Testosterone

These two hormones regulate the reproductive processes of the cell, and are important in the process of puberty in humans and other mammalian creatures.
Table with the four examples of endocrine signaling