Modes of cell communication

Signals, receptors and target cells

Every cell message has two parts. A signal is the thing that carries the message, usually a molecule. A receptor is a protein that recognises the signal and binds it. A signal molecule that binds a receptor is often called a ligand.

Only cells that make the right receptor can respond. These are the target cells. Secretin floods the whole bloodstream, but only cells with secretin receptors, like those in the pancreas, react to it. So whether a cell "hears" a signal depends on the receptors it has, not on whether the signal reaches it.

Biologists sort cell signalling in two ways: by how far the signal travels, and by where its receptor sits. We'll take distance first.

Direct contact: cells that touch

The shortest route is no gap at all. Gap junctions are protein channels that join the cytoplasm of two neighbouring animal cells. Ions such as Ca²⁺ and small molecules can pass straight through, but big molecules like proteins and DNA can't. Heart muscle cells use gap junctions to pass electrical signals along, so they contract together.

Plant cells have walls, so they use plasmodesmata instead: narrow channels through the cell walls that link the cytoplasm of neighbouring cells.

Cells can also talk through molecules fixed on their surfaces. One cell's surface protein binds a receptor on the cell it's touching, and nothing is released into the fluid between them. Immune cells often recognise each other this way.

Local signals: paracrine, synaptic and autocrine

In paracrine signalling, a cell releases a signal that diffuses a short distance to nearby cells. These local regulators act quickly, but they don't last: they are broken down, taken back up or diluted before they can travel far. Growth factors that tell nearby cells to divide often work like this.

Synaptic signalling is a special, very targeted kind of local signalling between a nerve cell and the cell it contacts. The nerve cell releases a neurotransmitter into the synapse, a gap only about 20–40 nanometres wide. It crosses in a fraction of a second, binds receptors on the other side, and is then quickly removed.

In autocrine signalling, a cell responds to a signal that it released itself, or that its identical neighbours released. It's common in development, where it helps a group of cells commit to the same fate together.

Long distance: endocrine signalling

In endocrine signalling, specialised cells release hormones into the blood, which carries them all over the body. Secretin, insulin, adrenaline and oestrogen are all hormones.

Because hormones are diluted in the whole volume of the blood, they work at very low concentrations. Endocrine responses are usually slower to start than paracrine or synaptic ones, but they tend to last longer.

Plants have hormones too. Some move through the plant's vascular tissue, and one, the gas ethylene, can even travel through the air from one fruit to another.

Where is the receptor? Let the chemistry decide

The plasma membrane has a hydrophobic (water-fearing) core, so it lets small non-polar molecules through but blocks charged and polar ones. That single fact tells you where a signal's receptor must be.

• Hydrophilic (water-loving) signals, such as peptide and protein hormones (insulin, secretin), most neurotransmitters and adrenaline, can't cross the membrane. Their receptors are cell-surface receptors, proteins that span the membrane with a binding site facing outside.
• Small hydrophobic signals, such as steroid hormones (oestrogen, testosterone, cortisol) and thyroid hormone, get across the membrane. Their receptors are intracellular receptors, in the cytoplasm or nucleus.

Most intracellular receptors are also transcription factors: once the hormone binds, the receptor–hormone complex attaches to DNA and switches particular genes on or off. That makes steroid responses fairly slow (it takes time to make new proteins) but long-lasting.

There's one odd case worth knowing: nitric oxide (NO) is a gas, small enough to slip straight through membranes. It's made by one cell and diffuses into neighbours, such as the smooth muscle around blood vessels, where it binds an enzyme inside the cell and makes the muscle relax. It breaks down within seconds, so it only acts locally.

Bacteria talk too: quorum sensing

Single-celled organisms signal to each other as well. Yeast cells release a mating factor that tells nearby yeast of the opposite mating type that a partner is close. Bacteria go further: they can count themselves.

In 1970, Kenneth Nealson, Terry Platt and Woodland Hastings noticed that the glowing marine bacterium now called Aliivibrio (formerly Vibrio) fischeri didn't glow in a freshly started culture. It lit up only once the population had grown dense. The bacteria were changing their own surroundings, and later work showed how. Each cell constantly releases a small signal molecule, an autoinducer. When there are only a few cells, it diffuses away. When there are many, it builds up, and once it passes a threshold it binds a receptor inside the cells that switches on genes, including the genes for light.

This is quorum sensing: bacteria using the concentration of a shared signal to detect their population density and act together. The name was introduced in 1994. It makes sense for jobs that are pointless alone: one glowing bacterium is invisible, but billions of them are bright. In the Hawaiian bobtail squid, packed V. fischeri light up an organ that hides the squid's shadow from below, and the squid feeds the bacteria in return.

Bonnie Bassler's work around 1998–2001 showed that quorum sensing is not a quirk of a few glowing species but is widespread among bacteria. Many use it to coordinate forming a biofilm (a slimy, protected community stuck to a surface) or releasing toxins only once there are enough of them to overwhelm a host.

Key terms

Ligand / receptor

A signal molecule that binds a receptor / the protein that recognises and binds it.

Gap junction / plasmodesma

Channels linking the cytoplasm of neighbouring animal cells / plant cells.

Paracrine signalling

A signal that diffuses a short distance to act on nearby cells.

Synaptic signalling

A nerve cell releases a neurotransmitter across a synapse to one target cell.

Autocrine signalling

A cell responds to a signal it released itself.

Endocrine signalling

Hormones travel in the blood to act on distant target cells.

Intracellular receptor

A receptor inside the cell for a hydrophobic signal; often a transcription factor.

Quorum sensing

Bacteria detect their population density from the concentration of an autoinducer and change gene expression together.

Check yourself

Try answering in your head before you open each answer.

• 1.A new hormone is discovered. When it's attached to a large bead that can't enter cells, it still triggers its normal response. What does this tell you about the hormone's receptor?Show answerHide

  The receptor must be on the cell surface. The hormone produces its effect without ever getting inside, so the message must be relayed inward by the receptor and proteins in the cytoplasm. That's typical of a hydrophilic signal.

• 2.A mutant strain of V. fischeri can't make its autoinducer. Predict whether a dense culture of it will glow, and what happens if you add culture fluid from a dense wild-type culture.Show answerHide

  Alone, the dense mutant culture stays dark, because no autoinducer builds up, however many cells there are. Adding fluid from a dense wild-type culture supplies the autoinducer, so the mutants (which still have the receptor) should light up.

• 3.Adrenaline released into the blood makes the heart beat faster and the liver release glucose, yet it has no effect on many other cells it reaches. Explain why.Show answerHide

  Adrenaline reaches almost every cell, but only target cells that make adrenaline receptors can respond. Heart and liver cells have them; cells without them don't react. And the response differs between heart and liver because their receptors and internal relay proteins differ.