1Communication by direct contact

Joined cytoplasm. Neighbouring animal cells can be linked by gap junctions, channels made of protein that run through both membranes. Plant cells are linked by plasmodesmata, narrow channels through the cell walls. Small molecules and ions pass straight through, cytoplasm to cytoplasm. In the heart, ions flow through gap junctions from one muscle cell to the next, so the heart contracts as one coordinated wave.

Surface molecules that meet. Two cells can also communicate by pressing their membranes together, so that a protein on one binds a protein on the other. The immune system relies on this. An antigen-presenting cell displays fragments of a pathogen it has taken in on its surface. A helper T cell whose receptor matches that fragment binds to it, and only that contact switches the helper T cell on. Killer T cells also recognise infected cells by contact before destroying them. The signal never leaves the surface of the cell that carries it, so contact signalling is precise and works only between neighbours.

2Chemical signals over short distances: local regulators

A local regulator is a chemical released by one cell that acts on cells close by. It is taken up, broken down or diluted before it gets far, so its effect stays local.

Four examples, all with the same pattern (release, short trip, receptor on a nearby cell):

• Neurotransmitters. At a synapse, a neuron releases a neurotransmitter into a gap about 20 nm wide. It diffuses across in well under a millisecond and binds receptors on the next cell only (Figure 1a).
• Quorum sensing in bacteria. Each bacterium releases small amounts of a signalling molecule (an autoinducer). When there are few cells, the molecule diffuses away and its concentration stays low. When the population is dense, the concentration rises past a threshold, the molecule binds receptors in or on the cells, and the whole population switches on a set of genes together, for light or a biofilm.
• Morphogens in embryos. A group of cells in an embryo releases a morphogen, which spreads into the surrounding tissue and forms a concentration gradient: high near the source, low further away. Cells develop according to how much they receive.
• Plant immune responses. A leaf cell attacked by a pathogen releases signals that reach the cells around it, which respond by strengthening their walls and making antimicrobial compounds.

A worked reading of quorum-sensing data. An invented data set: a culture of a marine bacterium was sampled as it grew. The table shows cell density, autoinducer concentration and light output.

Describe it in two steps. The autoinducer concentration is directly proportional to cell density: it doubles each time the density doubles (3, 6, 12, 24, 48). (every cell releases the same amount). Light output is not proportional: it stays at about 1 to 2 units up to a density of $2 \times 10^{8}$ cells $\text{mL}^{-1}$, then jumps thirtyfold to 60 units when the autoinducer reaches 24 nmol $\text{L}^{-1}$. The claim: the light genes switch on when the autoinducer passes a threshold between 12 and 24 nmol $\text{L}^{-1}$. The evidence is the jump. The reasoning: below the threshold too few receptors are bound to switch on the genes; above it, enough are bound in every cell at once.

3Chemical signals over long distances: hormones

A hormone is a signalling molecule released by cells of one type, usually in a gland, that travels in the blood to target cells of a different type elsewhere in the body (Figure 1b). The blood carries it everywhere, so the hormone reaches almost every cell. Only cells with the matching receptor respond. This is how one small gland can change the behaviour of tissues all over the body.

Examples you may meet:

Hormones act more slowly than neurotransmitters (the blood takes seconds to a minute to deliver them) and their effects often last longer. In plants, hormones move in the vascular tissue, and ethylene, a gas, can pass between fruits through the air.

4Why only the target cell responds

Every kind of signalling in this topic depends on the same fact: a cell responds to a signal only if it has a receptor for it. A receptor is a protein with a binding site whose shape and chemistry fit one signal molecule, its ligand, and not others (topic 4.2 goes into receptors). Insulin reaches brain, skin and liver alike within a minute; liver cells respond strongly because they carry many insulin receptors, and a cell with none does nothing.

So the same signal can do different things in different cells (epinephrine speeds the heart and makes the liver release glucose), and a cell that loses its receptor stops responding however much signal is present.

A signal can reach a cell without being received: only a cell with the matching receptor responds.

5Classifying a signal you have never met

A question may ask you to classify a signal you have never heard of. Read the scenario, not the name. Does the signal leave the cell? If not, it is contact. If it does, does it enter the blood and reach distant cells? If yes, it is a hormone. If it acts only nearby, it is a local regulator.