Cell Communication: Study Guide (Biology 4.1)

Why cells need to communicate

Your body has trillions of cells. They must work together: a cut must heal, blood sugar must stay steady, and an embryo must grow fingers in the right places. None of this can happen unless cells pass information to each other. Single-celled organisms communicate too: bacteria can sense how many of their own kind are nearby.

Every act of communication has the same basic parts:

• a signalling cell that sends the message;
• a signal, usually a chemical (a molecule or an ion), sometimes a molecule fixed on the surface of the sending cell;
• a target cell with a receptor, a protein that the signal fits and binds to;
• a response, such as a change in an enzyme’s activity, in which genes are switched on, or in whether the cell divides.

This topic is about how the signal gets from one cell to the other. Topic 4.2 picks up the story once the signal reaches the receptor.

There are two broad routes. Cells can communicate by direct contact, or from a distance by releasing a chemical signal. Distance signalling is then split again into short-distance (local) signalling and long-distance signalling.

Communication by direct contact

Some signals never leave the cell. They work only when two cells touch. There are two kinds.

Junctions that join the cytoplasm

In animal tissues, neighbouring cells can be linked by gap junctions: protein channels that cross both plasma membranes. Ions and small molecules pass straight from one cell’s cytoplasm into the next. In heart muscle, for example, ions flowing through gap junctions let a wave of electrical activity spread quickly from cell to cell, so the cells contract together.

Plant cells have walls, so they cannot touch membrane to membrane in the same way. Instead, they are joined by plasmodesmata: narrow channels through the cell walls, lined by plasma membrane, that connect the cytoplasm of neighbouring cells. Small signal molecules can move through them.

Surface molecules that touch

In the second kind, the signal is a molecule fixed to the outside of one cell. It binds to a receptor on another cell only when the two cells touch.

The immune system gives the clearest example:

1. An antigen-presenting cell (APC), such as a macrophage, takes in a pathogen and breaks it down.
2. It displays fragments of the pathogen’s molecules (antigens) on its surface, held by special presenting proteins.
3. A helper T cell with a matching receptor touches the APC and binds the displayed fragment. This contact activates the helper T cell, which then helps to direct the immune response.
4. A killer (cytotoxic) T cell recognises an infected body cell in the same way: the infected cell displays fragments of the pathogen on its surface. When the T cell’s receptor binds them, the T cell kills the infected cell.

Because the signal is attached to a cell, only cells in contact receive it. This makes contact signalling very precise: the immune system can act on one infected cell and leave its healthy neighbours alone.

Communication over short distances: local regulators

A local regulator is a chemical signal that a cell releases into the fluid around it. It reaches only cells close to the cell that released it. Why only nearby cells? The signal spreads out by diffusion, so its concentration falls quickly with distance. It is also taken up by nearby cells or broken down by enzymes. Only cells near the source receive enough of it to respond (Figure 1, panel C).

Neurotransmitters

A neuron passes its message to the next cell at a synapse. When an impulse arrives at the end of the neuron, the neuron releases a neurotransmitter, such as acetylcholine, into the synaptic gap. This gap is only tens of nanometres wide. The neurotransmitter diffuses across in a tiny fraction of a second and binds receptors on the next cell, which may be another neuron, a muscle cell or a gland cell. Enzymes or uptake proteins then clear the neurotransmitter, so each signal is short.

The neuron itself may be long, but the chemical signal crosses only the narrow gap. That is why neurotransmitter signalling counts as short-distance communication.

Morphogens in embryonic development

In an embryo, a small group of cells can release a signal called a morphogen. The morphogen diffuses away from its source, forming a concentration gradient: high near the source and lower further away. Cells respond differently depending on how much morphogen they receive. A high concentration switches on one set of genes; a medium concentration, another set; a low concentration, a third. In this way, a single signal tells cells where they are, and each region develops into the right structure. For example, in a developing vertebrate limb, a morphogen called Sonic hedgehog, made at the back edge of the limb bud, helps to decide which digit forms where. Worked example 1 shows how a gradient sets these boundaries.

Quorum sensing in bacteria

Many bacteria release small signal molecules all the time. When there are few cells, the signal diffuses away and stays at a low concentration. As the population grows, the signal builds up. Once it passes a threshold concentration, it binds receptors in enough cells to switch on a set of genes in the whole population at once. This is quorum sensing: the bacteria are, in effect, counting themselves.

Quorum sensing makes sense when a job only works if many cells do it together. A well-studied example is the marine bacterium Aliivibrio fischeri (formerly Vibrio fischeri), which glows only when crowded, as it is inside the light organ of the Hawaiian bobtail squid. Many bacteria also use quorum sensing to form biofilms, dense layers of cells stuck to a surface. Worked example 2 analyses quorum-sensing data.

Local signalling in plants

When a pathogen attacks a plant, cells at the infection site release chemical signals. Neighbouring cells respond by strengthening their walls and making antimicrobial compounds, and some infected cells die on purpose, which walls off the pathogen. Signals involving salicylic acid can also spread through the plant and prepare tissues further away for attack.

Communication over long distances: hormones

Some signals must reach cells far from where they are made. In animals, these are hormones. Endocrine cells, often grouped into glands, release hormones into the blood. The blood carries them around the whole body in about a minute, and they reach target cells in distant organs.

Hormone	Made by	Main targets	Example of response
Insulin	β cells of the pancreas	liver, muscle and fat cells	cells take up more glucose; the liver stores it as glycogen, so blood glucose falls
Human growth hormone	anterior pituitary gland	many tissues, including bone and muscle	growth and cell division
Thyroid hormones	thyroid gland	almost every cell	a higher metabolic rate
Testosterone	testes	many tissues	development of male characteristics, such as larger muscles
Estrogen	ovaries	many tissues, including the uterus	development of female characteristics; control of the menstrual cycle

Hormone signals are slower to arrive than neurotransmitters, but they spread to the whole body and their effects often last longer: minutes, hours or even years in the case of growth.

Specificity comes from receptors

A hormone in the blood reaches nearly every cell in the body. So why do only some cells respond? Because a cell can respond only if it has a receptor that the hormone fits. A target cell is simply a cell with the right receptor (Figure 1, panel D). A cell without the receptor ignores the hormone, however much of it there is.

This also explains why one hormone can cause different responses in different cells. Liver cells and fat cells both respond to insulin, but the liver stores glucose as glycogen while fat cells turn it into fat. The receptor is the same; what happens inside the cell afterwards differs. You will see how that inside part works in Topic 4.2.

Comparing the routes

Route	Distance	Signal	Speed and reach	Examples
Junctions	cell to touching cell	ions, small molecules through channels	very fast; only joined cells	gap junctions in heart muscle; plasmodesmata
Surface contact	cell to touching cell	molecule fixed on the surface	precise; only the cell touched	APC and helper T cell; killer T cell and infected cell
Local regulator	nearby cells	molecule released into fluid	fast; falls with distance	neurotransmitters; morphogens; quorum sensing
Hormone	distant cells	molecule carried in blood	slower; whole body; long-lasting	insulin; growth hormone; thyroid hormones; testosterone; estrogen

Worked example 1: reading a morphogen gradient

Question. In a fictional embryo, a line of cells lies next to a source that releases morphogen M. In this model, the concentration of M (arbitrary units) at the source is 100 and halves every 20 µm away from it. Cells exposed to more than 60 units become type A; 30 to 60 units, type B; less than 30 units, type C.

(a) Calculate the concentration of M at 30 µm from the source. (b) Find where the A/B and B/C boundaries lie, and state the fate of cells at 0, 10, 20, … 80 µm. (c) A mutation makes the source release twice as much M (200 units at the source). Predict the effect on the pattern.

(a) 30 µm is 30 ÷ 20 = 1.5 halving distances. Concentration = 100 × 0.5^1.5 = 100 × 0.354 = 35.4 units.

(b) Work out each cell’s concentration, halving every 20 µm:

Distance / µm	0	10	20	30	40	50	60	70	80
M (normal)	100	70.7	50.0	35.4	25.0	17.7	12.5	8.8	6.3
Fate	A	A	B	B	C	C	C	C	C

The curve crosses 60 units at about 14.7 µm and 30 units at about 34.7 µm. Cells at 0 and 10 µm become A; at 20 and 30 µm, B; at 40 µm and beyond, C.

(c) With 200 units at the source, each concentration doubles: 141 at 10 µm, 100 at 20 µm, 70.7 at 30 µm, 50.0 at 40 µm, 35.4 at 50 µm and 25.0 at 60 µm. The boundaries move to about 34.7 µm (A/B) and 54.7 µm (B/C). Cells at 0–30 µm become A, at 40–50 µm B, and at 60 µm and beyond C. Both boundaries shift 20 µm away from the source, exactly one halving distance, because doubling the source cancels one halving. The A region grows and the C region shrinks.

Interpretation. A local signal carries information in its concentration, not just in whether it is present. Real gradients are less smooth, but the idea holds: change the source and you shift where structures form.

Worked example 2: is a signal behind the glow?

Question. A fictional glowing marine bacterium is grown in a flask. Every 2 hours a student measures cell density and light output:

Time / h	0	2	4	6	8	10
Density / 10⁷ cells mL⁻¹	1	2	4	8	16	32
Light / arbitrary units	2	4	8	16	800	3200

(a) Calculate the light per 10⁷ cells at each time and describe the pattern. (b) The student proposes that the cells release a signal and glow only when it is concentrated enough. In a second experiment, she adds cell-free medium, filtered from a dense culture, to a dilute culture (1 × 10⁷ cells mL⁻¹). A matched dilute culture gets fresh medium. After 1 hour, light per 10⁷ cells is 48 with the filtered medium and 2 with fresh medium. Evaluate the proposal.

(a) Divide light by density: 2, 2, 2, 2, 50 and 100 units per 10⁷ cells. Up to 6 hours, each cell gives out the same small amount of light, so total light simply rises with the number of cells. Between 6 and 8 hours the density doubles (8 → 16), but total light rises 50-fold (16 → 800). Each cell now glows 25 times as brightly, and by 10 hours 50 times as brightly. The cells have switched on light production together, once the population passed a threshold between 8 and 16 × 10⁷ cells mL⁻¹.

(b) The filtered medium contains no cells, yet it makes dilute cells glow 24 times as brightly as cells given fresh medium (48 ÷ 2 = 24). So something released into the medium by the dense culture triggers the response. This supports the signal hypothesis and fits quorum sensing: a crowded culture has a high concentration of the signal, so even a few cells respond when given it.

How confident? The fresh-medium culture is a good control: same cells, same density, only the medium differs. But the filtered medium also differs in other ways, for example it has less food and more waste. A stronger test would show that a purified signal molecule, added on its own, has the same effect, or that a mutant unable to make the signal never glows unless the signal is supplied.

Common misconceptions

• “Hormones go only to their target organs.” Blood carries a hormone to almost every cell. Only cells with the matching receptor respond.
• “Only multicellular organisms communicate.” Bacteria communicate by quorum sensing.
• “Direct contact always means a channel between cells.” Junctions join the cytoplasm, but immune cells communicate by surface molecules binding receptors, with no channel.
• “A nerve signal is long-distance chemical signalling.” The impulse runs along the neuron; the chemical signal, the neurotransmitter, crosses only a tiny gap.
• “A signal has the same effect on every cell that responds.” Different target cells can respond differently to the same signal, as liver and fat cells do to insulin.
• “A local regulator stops at a fixed distance.” Its concentration falls gradually with distance. Cells respond only where it is above their threshold.
• “Quorum sensing means the bacteria count each other by touching.” They sense the concentration of a released signal, which rises as the population grows.

Where this leads

So far the signal has reached its target. Topic 4.2, introduction to signal transduction, explains what happens next: how a receptor recognises the signal and how the message is relayed and amplified inside the cell. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.