Biosignaling
Biosignaling is how cells receive and respond to environmental signals through ion channels, enzyme-linked receptors, and G protein-coupled receptors.
Biosignaling is how cells receive and respond to signals from their environment — essential for communication between cells and for coordinating physiology throughout the body. Three membrane protein categories carry out this job: ion channels, enzyme-linked receptors, and G protein-coupled receptors.
Key Takeaways
Ion channels move ions by facilitated diffusion; they're ungated (always open), voltage-gated, or ligand-gated.
Enzyme-linked receptors have three domains — membrane-spanning, ligand-binding, and catalytic — with receptor tyrosine kinases as the classic example.
GPCRs activate G proteins via a GDP-to-GTP switch on the alpha subunit.
Gs increases cAMP (via adenylate cyclase); Gi decreases cAMP (same enzyme, opposite effect); Gq uses a separate pathway — phospholipase C, PIP2, DAG/IP3, and calcium release.
cAMP's next step is activating protein kinase A (PKA).
Ion Channels
Ion channels are membrane proteins that form specific pathways for charged molecules — like sodium or potassium ions — to cross the membrane. They allow facilitated diffusion: ions move down their concentration gradient without the cell spending energy.
Type | Trigger | Notes |
|---|---|---|
Ungated | Always open | No gate; movement depends only on the concentration gradient |
Voltage-gated | Change in membrane potential | Especially important in neurons and muscle cells |
Ligand-gated | Binding of a specific ligand (neurotransmitter, hormone, or other signaling molecule) | Opens/closes on binding |
Enzyme-Linked Receptors
Some membrane proteins act as enzyme-linked receptors: ligand binding triggers a cascade of reactions inside the cell. Each enzyme-linked receptor has three key parts:
A membrane-spanning domain, which anchors the receptor in the cell membrane.
A ligand-binding domain, where the signaling molecule binds.
A catalytic domain, which becomes activated and carries out an enzymatic function inside the cell.
The classic example of this receptor category is the receptor tyrosine kinase (RTK) — its catalytic domain phosphorylates tyrosine residues once a ligand (such as a growth factor or insulin) binds. Enzyme-linked receptors are central to signaling pathways involved in growth, metabolism, and immune responses.
G Protein-Coupled Receptors (GPCRs)
G protein-coupled receptors (GPCRs) are a large family of membrane proteins involved in signal transduction. When a ligand binds a GPCR, it activates an associated G protein, which then triggers a cascade of events inside the cell.
Every G protein is built from three subunits: alpha, beta, and gamma. In the inactive state, the alpha subunit is bound to GDP. Once the receptor is activated by ligand binding, GDP is exchanged for GTP, and the alpha subunit separates from the beta-gamma pair to go activate (or inhibit) a target enzyme.
The Three G Proteins: Gs, Gi, and Gq
G Protein | Target Enzyme | Second Messenger Effect | Net Result |
|---|---|---|---|
Gs | Adenylate cyclase (stimulates) | Increases cAMP | Activates cAMP-dependent signaling |
Gi | Adenylate cyclase (inhibits) | Decreases cAMP | Suppresses cAMP-dependent signaling |
Gq | Phospholipase C (activates) | Cleaves PIP2 into DAG and IP3 | IP3 opens ER calcium channels, raising intracellular calcium |
Gs stimulates adenylate cyclase, raising cyclic AMP (cAMP) levels inside the cell. The rise in cAMP goes on to activate protein kinase A (PKA), which carries the signal forward by phosphorylating downstream targets.
Gi does the opposite: it inhibits adenylate cyclase, lowering cAMP — the same target enzyme as Gs, but the reverse effect. This Gs/Gi contrast is one of the most commonly tested distinctions in GPCR signaling.
Gq works through an entirely different enzyme and messenger system. It activates phospholipase C, which cleaves the membrane phospholipid PIP2 into two products: DAG and IP3. IP3 then opens calcium channels on the endoplasmic reticulum, raising intracellular calcium levels.
This signaling framework is widely used across hormone signaling, neurotransmission, and sensory pathways like vision and smell.
Common MCAT Mistakes
Mixing up Gs and Gi effects on cAMP. Both target the same enzyme, adenylate cyclase — Gs stimulates it (raises cAMP), Gi inhibits it (lowers cAMP). A question that names the wrong direction for either is testing this exact mix-up.
Forgetting Gq uses a different pathway entirely. Gq doesn't touch adenylate cyclase or cAMP — it activates phospholipase C, which cleaves PIP2 into DAG and IP3, with IP3 raising intracellular calcium. Don't fold Gq into the Gs/Gi cAMP framework.
Treating all membrane receptors as one category. Ion channels, enzyme-linked receptors, and GPCRs are structurally and mechanistically distinct — an ion channel physically gates ion flow, an enzyme-linked receptor has its own built-in catalytic domain, and a GPCR works indirectly through a separate G protein.
Missing the GDP-to-GTP switch as the activation step for GPCRs. The G protein alpha subunit is inactive when bound to GDP; ligand binding drives GDP-to-GTP exchange, which is what causes the alpha subunit to separate from beta-gamma and act on its target enzyme.
MCAT-Style Concept Check
Question: A drug increases intracellular calcium levels in a target cell by activating a G protein-coupled receptor pathway. Which G protein and target enzyme combination best explains this effect?
A) Gs activating adenylate cyclase
B) Gi inhibiting adenylate cyclase
C) Gq activating phospholipase C
D) Gq activating adenylate cyclase
Answer: C
Explanation: Gq activates phospholipase C, which cleaves PIP2 into DAG and IP3; IP3 then opens calcium channels on the endoplasmic reticulum, raising intracellular calcium — matching the effect in the question. Option A is wrong because Gs/adenylate cyclase raises cAMP, not calcium. Option B is wrong because Gi/adenylate cyclase lowers cAMP and also doesn't affect calcium. Option D is wrong because Gq does not target adenylate cyclase — that enzyme is regulated by Gs and Gi, not Gq.
FAQ
What's the difference between Gs and Gi?
Both act on the same target enzyme, adenylate cyclase, but with opposite effects: Gs stimulates it, raising cAMP; Gi inhibits it, lowering cAMP.
How does Gq raise intracellular calcium?
Gq activates phospholipase C, which cleaves the membrane phospholipid PIP2 into DAG and IP3. IP3 then opens calcium channels on the endoplasmic reticulum, releasing calcium into the cytoplasm.
What activates a G protein?
Ligand binding to a GPCR causes the G protein's alpha subunit to exchange GDP for GTP. Once bound to GTP, the alpha subunit separates from the beta-gamma pair and goes on to activate or inhibit its target enzyme.
What's the difference between an ion channel and an enzyme-linked receptor?
An ion channel is a passageway that lets ions cross the membrane by facilitated diffusion, gated by voltage, ligand binding, or nothing at all. An enzyme-linked receptor has its own catalytic domain that activates inside the cell once a ligand binds — the classic example being the receptor tyrosine kinase.