The Hawaiian bobtail squid hunts at night with a light organ on its underside. The light doesn't come from the squid. It comes from bacteria, Vibrio fischeri, living inside the organ. One bacterium alone doesn't glow. Each one releases a small signaling molecule, and only when the bacteria are packed together does that molecule build up enough to switch on their light-making genes. This is quorum sensing, and it's one of the examples of cell communication the AP course lists. The light is thought to help hide the squid's shadow from predators below on moonlit nights. Let's look closer at how a signal outside a cell changes what happens inside it. This is AP Biology Unit 4.
How far a signal travels
Cells communicate in two broad ways:
- Direct contact. Cells touch or are joined. An antigen-presenting cell shows an antigen to a helper T cell by contact, and gap junctions in animals let small molecules pass straight between neighbors.
- Chemical signals over a distance. Local regulators act on cells close by, like neurotransmitters or morphogens in an embryo. Hormones such as insulin, thyroid hormones and estrogen travel in the blood to target cells of a different type.
Reception: the receptor decides
A signaling molecule is a ligand. Only cells with a matching receptor respond, because the receptor's ligand-binding domain fits one specific ligand. Ligands can be peptides (proteins) or small molecules.
Receptors sit in two places:
- On the surface, for ligands that can't cross the membrane. G protein-coupled receptors (GPCRs) are the example the AP course names. Ligand-gated ion channels are another: acetylcholine at a muscle opens a channel that lets ions in.
- Inside the cell, in the cytoplasm or nucleus, for small nonpolar ligands. Steroid hormones like testosterone cross the membrane, bind their receptor, and the complex changes gene expression.
Transduction: relay and amplify
When the ligand binds, the receptor's intracellular domain changes shape. That shape change is the start of transduction. In the classic epinephrine pathway in a liver cell:
- Epinephrine binds a GPCR.
- The receptor activates a G protein, which swaps GDP for GTP.
- The G protein switches on adenylyl cyclase, which turns ATP into cyclic AMP (cAMP), a second messenger.
- cAMP activates a protein kinase, starting a phosphorylation cascade: each kinase adds phosphate to the next protein.
- The last enzyme in the chain breaks glycogen down, and the cell releases glucose.
Each step activates many molecules at the next, so the signal is amplified. Pathways also need an off switch: the G protein hydrolyzes its GTP, phosphodiesterase breaks down cAMP, and protein phosphatases remove the phosphates.
Response: what the cell does
A pathway can end in a change in gene expression, a change in cell function (an enzyme switched on, a molecule secreted, growth), or apoptosis. Ethylene changes which enzymes fruit cells make, so the fruit ripens. Mating pheromones in yeast switch on mating genes. Different cells carry different receptors and relay proteins, so the same ligand can trigger different responses in different tissues.
When a pathway changes
A mutation in any part of the receptor, or in any relay protein, can change everything downstream. A protein stuck "on" keeps signaling with no ligand present; one stuck "off" blocks the response. Chemicals can do the same. Cholera toxin keeps a G protein active, so intestinal cells make cAMP nonstop and secrete chloride ions, and sodium and water follow. Many medicines work by blocking or activating a receptor. Feedback loops (topic 4.4) have their own deck.
Common mistakes students make
- "The hormone goes into the cell to do its job." Only some do. Epinephrine and insulin bind receptors on the cell surface, and relay molecules carry the message inside.
- Calling cAMP an enzyme. It's a small molecule. Adenylyl cyclase is the enzyme that makes it.
- Mixing up kinases and phosphatases. Kinases add phosphate; phosphatases remove it.
- Saying a mutation only affects its own step. Predict the effect on every step downstream too.
Frequently asked questions
What are the steps of signal transduction in AP Biology?
Reception (a ligand binds a receptor), transduction (relay molecules pass the signal on, often through second messengers and a phosphorylation cascade) and response (a change in the cell).
What is a second messenger?
A small molecule inside the cell, such as cAMP, that relays and amplifies a signal after the receptor is activated.
Why do some hormones have receptors inside the cell?
Small nonpolar hormones like steroids can cross the phospholipid bilayer, so their receptors can be in the cytoplasm or nucleus.
How does a mutation affect a signaling pathway?
It can block the signal or keep the pathway switched on, and either way every step downstream of the faulty protein is affected.
Try it
Say each answer before you flip the card. Then predict: what would you expect to see if a liver cell had a mutant protein phosphatase that could no longer remove phosphates from the cascade's enzymes?
AP is a trademark registered by the College Board, which is not affiliated with Braineos and doesn't endorse it.