Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Communication needs a signalling cell, a signal, a target cell with a receptor, and a response.
- Direct contact, junctions: gap junctions (animals) and plasmodesmata (plants) join the cytoplasm of neighbouring cells, so ions and small molecules pass straight through.
- Direct contact, surface molecules: a molecule fixed on one cell binds a receptor on a touching cell. An antigen-presenting cell shows antigen fragments to a helper T cell; a killer T cell recognises fragments shown by an infected cell.
- Local regulators are released into the fluid around a cell and act on cells nearby: neurotransmitters at synapses, morphogens in embryos, quorum-sensing signals in bacteria, and defence signals in plants.
- Hormones travel in the blood to distant targets: insulin, human growth hormone, thyroid hormones, testosterone, estrogen.
Key relationships
| Route | Distance | Example | Why it suits the job |
|---|---|---|---|
| Junctions | touching cells | gap junctions in heart muscle | fast spread through a tissue |
| Surface contact | touching cells | APC → helper T cell | precise: only the cell touched |
| Local regulator | nearby cells | neurotransmitter, morphogen | fast; concentration falls with distance |
| Quorum sensing | cells in one population | bacterial glow, biofilms | response only once the population is dense |
| Hormone | whole body | insulin, thyroid hormones | reaches distant organs; long-lasting |
| Idea | What to remember |
|---|---|
| Morphogen gradient | high near source, low far away; different concentrations switch on different genes |
| Quorum threshold | signal concentration rises with cell density; genes switch on together above a threshold |
| Specificity | delivery is wide; receptors decide which cells are targets |
| Different responses | the same signal can cause different responses in different target cells |
Assumptions behind the models
- Model gradients assume the signal spreads evenly from one source and is removed at a steady rate.
- In quorum-sensing experiments, filtered medium may differ from fresh medium in food and waste, not only in signal.
Mistakes to avoid
- Saying hormones are carried only to their target organs.
- Calling neurotransmitter signalling long-distance because the neuron is long.
- Treating all direct contact as channels between cells.
- Saying bacteria do not communicate.
- Assuming every target cell responds to a signal in the same way.
- Describing a gradient boundary as the place where the signal “stops”.
Quick self-check
- Why do muscle cells respond to insulin but some other cells do not? (Only cells with insulin receptors are targets.)
- A morphogen halves every 20 µm from a source of 100 units. Concentration at 40 µm? (100 × 0.5 × 0.5 = 25 units)
- Why is quorum sensing useful to bacteria? (Jobs such as glowing or forming a biofilm work only when many cells do them together.)
Next: practice questions.