AP Biology · Unit 4 · Topic 4.1
A signal goes out to everything nearby. Only the cells carrying the right receptor hear it — and two cells that hear the same signal can do completely different things with it.
Learning objective 4.1.A Explain how cell communication is critical for multicellular organisms.
Pick how the signal is sent and what the signal is. A cell lights up only if the signal reaches it and it has the receptor. Watch for the cell that fails each test on its own — those two are the point of the panel.
The field is wider than your screen — scroll it sideways to reach the cell in another organ.
Send epinephrine by the endocrine route and look at the cell with no matching receptor. It is right next to the sender and soaked in hormone, and nothing happens to it. Being reached is not receiving.
Now send epinephrine by the paracrine route and look at the cell in another organ. It has exactly the right receptor, and it never gets the chance to use it. Having the receptor is not enough either.
A synapse skips three cells that are closer to the sender and delivers to a muscle fiber further away. That is not a mistake in the diagram. A neuron can run its axon a meter down your leg and still release neurotransmitter onto one target at the far end.
So “local” means two different things in this topic. Paracrine signaling is local because diffusion runs out. Synaptic signaling is local because the cell is wired to one place. Both are short-range in effect, and only one of them is short-range in distance.
Epinephrine goes everywhere in the bloodstream, so every target gets the same molecule at the same concentration. What each one does about it is not the same at all.
This is what makes a hormone useful for coordination. One release from one gland produces a whole set of matched changes across the body — sugar released here, heart rate up there, airways opened somewhere else — without the gland needing to address each organ separately.
The neighboring cell in the first panel carries three different receptors and gives three different responses, one per signal. So a cell is not simply “a responder”; it is a specific set of receptors wired to a specific set of responses.
Every bacterium in this population secretes the same signaling molecule at the same rate. None of them can count. Between them, they can.
Nothing in the system measures the population directly. Each cell secretes at a fixed rate and each cell measures the concentration around it, and because those two things are linked, the concentration is the count.
Set the slider to 1. That bacterium secretes signal continuously and never reaches the threshold, because the signal diffuses away as fast as one cell can make it. The behavior is not something the cell is unable to perform — it is something it is unable to trigger alone.
This matters for pathogens. A few bacteria launching an attack on a host would be destroyed. The same bacteria waiting until there are enough of them, and then all attacking at once, are a different proposition — and quorum sensing is how they tell the difference.
Answer these in your own words before you open the answers.
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| Mode | How far it goes | What limits it | Example |
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Read the third column down. Three different things limit the range of these four modes, and only one of them is distance.