Which means of communication between cell is mostly used by multicellular organisms and why give any two reason?

As with people, it is vital for individual cells to be able to interact with their environment. This is true whether a cell is growing by itself in a pond or is one of many cells that form a larger organism. In order to properly respond to external stimuli, cells have developed complex mechanisms of communication that can receive a message, transfer the information across the plasma membrane, and then produce changes within the cell in response to the message.

In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of distant organs, tissues, and cells. The ability to send messages quickly and efficiently enables cells to coordinate and fine-tune their functions.

While the necessity for cellular communication in larger organisms seems obvious, even single-celled organisms communicate with each other. Yeast cells signal each other to aid mating. Some forms of bacteria coordinate their actions in order to form large complexes called biofilms or to organize the production of toxins to remove competing organisms. The ability of cells to communicate through chemical signals originated in single cells and was essential for the evolution of multicellular organisms. The efficient and error-free function of communication systems is vital for all life as we know it.

Learning Outcomes

• Differentiate between different types of signals
• Describe how a cell propagates a signal
• Describe how a cell responds to a signal
• Discuss the process of signaling in single-celled organisms

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Matthew Johnston is a Phd student in the Dr Christine Faulkner lab.

His research focuses on cell-to-cell communication in plants, so we asked Matt, how and why do cells communicate?

“Roughly 600 million years ago, the first multicellular organism evolved from single-celled life.

The rise of multicellular life gave multiple benefits to those organisms over their single-celled compatriots, with perhaps the most important advantage being specialisation.

Specialisation allows one part of an organism to become extremely good at doing one thing, such as forming teeth for biting, or leaves for photosynthesising; better than a single-celled organism could do it alone.

However, for multicellularity to be successful there must be transport and communication between cells.

Transport is required for the movement of resources around a multi-cellular organism. For example, our brains are great for thinking, but require oxygen obtained in the lungs, so there needs to be a way to get to from one to the other.

Communication is required for sharing information, in two senses. First, to alert the rest of the organism about one part’s requirements, “I need sugar over here” and to inform neighbouring tissues of each other’s state, “I’m a root, so you don’t have to be” or “Help, I’m under attack…”.

Much like their animal counterparts plant cells communicate in a multitude of ways, from chemical to electrical.

Communication in its most basic form occurs when cells detect any change of their environment.

A common form of communication between cells is the release of chemicals. These chemicals can be noxious, so as to directly elicit a defence response in neighbouring cells. Alternatively, they can be benign but are perceived by unique receptors within cells. An example of this, in animals, is oestrogen which is produced in the ovaries and is circulated in the blood and then diffuses into cells throughout the body, where it is perceived.

Not all hormones can diffuse directly into the cell and some require a specialised protein which can transport the chemical into (and out of) the cell. A classic example is auxin, the hormone responsible for driving plant growth.

However, not all communication has to be done by chemicals moving across membranes and back again.

Plants have evolved tunnels which link cells directly through the cell wall. These are called plasmodesmata and they enable direct cell-to-cell communication.

Plasmodesmata can be used for both transport and communication and often these are same process. For example, transcription factors (proteins which modify what genes the cell is expressing) can be transported cell to cell via the plasmodesmata. Once transported to a new cell, the transcription factor then directly reprograms the cell to express different genes. These instructions for what genes to express, have been communicated by its neighbour.

Interestingly, movement through plasmodesmata can be coupled with movement by transporters, simultaneously as has recently been shown to be the case with auxin.

In the Faulkner lab, we explore how plasmodesmata function in plant defence.

When a plant cell comes under attack the plasmodesmata shut, closing the channels between plant cells. This has a knock-on effect, changing the communication between cells.

We have recently published a paper exploring what proteins are involved in this process, and how the plant becomes more susceptible to pathogens when this process is interrupted.

Lastly, not all communication has to be by small molecules and over short distances.

Plants, like animals, also use electrical signalling to transmit information over long distances very rapidly. For example, when a plant is wounded, say by a snail, rapid waves of calcium and electricity are sent along the plant to alert distant leaves that they may soon be eaten too. This, in turn elicits defence compounds to be made in leaves far away from the invader in preparation in an attempt to stave off further attack.

We may not hear it, but cells are constantly communicating to keep the plant alive”.

The study of cell communication focuses on how a cell gives and receives messages with its environment and with itself. Indeed, cells do not live in isolation. Their survival depends on receiving and processing information from the outside environment, whether that information pertains to the availability of nutrients, changes in temperature, or variations in light levels. Cells can also communicate directly with one another — and change their own internal workings in response — by way of a variety of chemical and mechanical signals. In multicellular organisms, cell signaling allows for specialization of groups of cells. Multiple cell types can then join together to form tissues such as muscle, blood, and brain tissue. In single-celled organisms, signaling allows populations of cells to coordinate with one another and work like a team to accomplish tasks no single cell could carry out on its own.

The study of cell signaling touches multiple biological disciplines, such as developmental biology, neurobiology, and endocrinology. Consequently, the relevance of cell communication is quite vast, but major areas of fundamental research are often divided between the study of signals at the cell membrane and the study of signals within and between intracellular compartments. Membrane signaling involves proteins shaped into receptors embedded in the cell's membrane that biophysically connect the triggers in the external environment to the ongoing dynamic chemistry inside a cell. Signaling at the membrane also involves ion channels, which allow the direct passage of molecules between external and internal compartments of the cell. Scientists ask: What is the receptor structure that enables it to react to an external signal (such as a ligand or even a mechanical force)? Others ask: Once triggered, how is the signal processed inside the cell?

Cells have evolved a variety of signaling mechanisms to accomplish the transmission of important biological information. Some examples of this variety are receptors that allow ion currents to flow in response to photons, which effectively translates light into chemical messengers inside the cone and rod cells of the retina; growth factors that interact with the cell membrane and can trigger receptors that powerfully affect chromatin structure and the modulation of gene expression; metabolites in the blood that can trigger a cell's receptors to cause the release of a hormone needed for glucose regulation; adhesion receptors that can convey tension-generated forces that direct a cell to stay put or change direction of movement; and developmentally regulated receptors that can strictly guide the path of a migrating cell, ultimately controlling how an entire organism is wired together.

How do scientists go about studying such an intricate meshwork of interactions at the crossroads of chemistry, physics, and biology? One method is reductionist, whereby cells are isolated and cultured in vitro so that specific signals can be carefully tested with chemicals and cellular responses can be measured. Another more holistic method involves measuring cellular signaling in an intact organism (in vivo) by applying specific chemical agents that block or activate receptors in a carefully chosen tissue region and then measuring the response via an electrode that relays the activity of ion currents or via fluid sampling of the activated area. For both approaches, response measurement is vitally important, and measuring the small cellular entities is indeed a challenge. Scientists use sophisticated time-lapse microscopy to track labeled molecules that travel between subcellular compartments after a signaling event or to track the conformation of a receptor that has gone from an inactive to an active state. Furthermore, mass spectrometry techniques permit measurements of picomolar amounts, enabling the tracking of intracellular second messenger molecules that are crucial in the regulation of signals in the intracellular milieu.

Despite technical advances, global understanding of signal transduction, its internal hierarchies, and its highly integrated and extremely dynamic nature remains largely mysterious. A potential breakthrough in the field arose recently when scientists realized that there are striking analogies between signaling networks in biological systems and electronic circuits; both of them involve hierarchies, switches, modularity, redundancy, and the existence of powerful feedback mechanisms. Such a realization gave impetus to the field of computational biology as applied to cellular signaling. Today, the study of cell signaling is not restricted to biologists; with the contribution of engineers and biophysicists, scientists can now create computational algorithms that model the structure of a signaling network based on biological measurements, and these models can be used to predict the outcome of otherwise physically impossible experimental conditions. As it turns out, we are just beginning to appreciate that many of the designs and strategies we have developed to manipulate information, particularly within the digital world, are actually present in biological networks, having already been invented over the course of a hundred million years of evolution.

Image: Jorge Barrios.