Principles of Cell Signaling

What is a signal in cell signaling?

A change or stimulus; often an extracellular signaling molecule (ligand or agonist).

What is a ligand or agonist?

An extracellular signal molecule.

What is the function of a receptor in cell signaling?

It senses a specific signal and relays it into the cell.

What do intracellular signaling molecules do?

Interpret and/or amplify the signal inside the cell.

What is the role of effectors?

They convert the signal into a cellular response or behavior change.

What kinds of molecules are hydrophilic signals?

Peptides, proteins, and charged molecules.

Can hydrophilic signals cross the plasma membrane freely?

No, they are impermeable and must use cell-surface receptors.

What characteristics allow small, hydrophobic molecules to cross the plasma membrane?

Their small size and nonpolarity (membrane permeable).

What types of receptors do small, hydrophobic signals use?

Intracellular receptors or cell-surface receptors.

Can gases cross the plasma membrane?

Yes, gases rapidly diffuse across and act through intracellular receptors.

What types of molecules bind intracellular receptors?

Small, hydrophobic, plasma-membrane-permeable molecules.

What are examples of hydrophobic signaling molecules?

Steroid hormones (from cholesterol), thyroid hormones, retinoids, vitamin D.

How do hydrophobic signaling molecules travel in the bloodstream?

Bound to protein carriers because they are water-insoluble.

What do many intracellular receptors do after binding their ligand?

They bind DNA and regulate transcription.

Why are intracellular receptors also considered effectors?

They directly activate transcription upon ligand binding.

What is an example of a gas used in cell signaling?

Nitric oxide (NO).

What is the function of nitric oxide (NO) in smooth muscle?

It signals relaxation of smooth muscle cells.

Where is NO produced?

In endothelial cells under regulatory control.

How does NO move to neighboring cells?

It diffuses freely across membranes.

What are the four types of intercellular signaling?

Contact-dependent, paracrine, synaptic, endocrine.

Which two types of signaling act over short distances?

Contact-dependent and paracrine signaling.

Which two types of signaling act over long distances?

Synaptic and endocrine signaling.

What characterizes contact-dependent signaling?

Direct membrane-to-membrane contact.

What characterizes endocrine signaling?

Hormones traveling through the bloodstream to distant targets.

Why is synaptic signaling fast and precise?

Because signaling molecules are delivered at high concentrations directly to the target cell.

What is a chemical synapse?

A specialized site where a neuron transmits signals to a target cell.

What triggers neurotransmitter release at a synapse?

An action potential reaching the synaptic terminal.

How do neurotransmitters act on target cells?

They bind to cell-surface receptors.

Why is endocrine signaling considered broad?

Signaling molecules are diluted in the bloodstream and can reach many targets.

How do endocrine cells deliver signals?

They secrete hormones into the bloodstream.

How do endocrine hormones reach distant cells?

They travel through the blood to act on target cells throughout the body.

What molecule can cause different responses in different cell types?

Acetylcholine.

What determines acetylcholine's effect on a particular cell?

Differences in intracellular signaling proteins, effector proteins, and activated genes.

What is acetylcholine?

A neurotransmitter produced by acetylation of choline.

Why do cells need multiple extracellular signals to function?

They need distinct signals for survival, growth, division, and differentiation.

What happens to a cell deprived of survival signals?

It undergoes apoptosis (programmed cell death).

Why does each cell respond differently to external signals?

Each cell type displays its own unique set of receptors.

What is an example of signal integration?

Signals A and B activate different pathways that phosphorylate different sites on protein Y, which activates only when both are phosphorylated.

What is the difference between fast and slow cellular responses to signals?

Fast responses alter protein function; slow responses involve gene expression and take longer.

What are examples of fast cellular responses to signals?

Changes in movement, secretion, metabolism, or synaptic activity.

What are examples of slow cellular responses to signals?

Increased cell growth, division, or long-term changes in gene expression.

Can cells respond abruptly even if a signal increases gradually?

Yes, cells can produce abrupt responses to gradually increasing signals.

What are the three types of signal response curves?

All-or-none (switch-like), hyperbolic, and sigmoidal.

What characterizes an all-or-none response?

A switch-like behavior—either fully on or fully off.

What characterizes a hyperbolic response?

Gradual increase that eventually reaches a plateau.

What characterizes a sigmoidal response?

Three phases: slow start, rapid increase, then plateau.

What is valency in cell signaling?

The number of ligand molecules required simultaneously to activate a target protein.

How does valency affect response profiles?

Low valency (1-2 ligands) → hyperbolic response• High valency (multiple ligands) → sigmoidal or switch-like response

What do positive and negative feedback loops do?

They regulate, sustain, or modulate signaling responses.

What is the effect of positive feedback in signaling?

Sustains or amplifies signal output.

Example: How does positive feedback amplify kinase activity?

Activated E kinase phosphorylates more E kinase, promoting an all-or-none response.

What is the role of negative feedback in signaling?

It inhibits or limits signaling depending on the delay before inhibition.

What happens with short delays in negative feedback?

Quick damping of the response.

What happens with long delays in negative feedback?

A strong response builds to a maximum, then shuts off; removal of inhibition can cause repeated pulses.

What is the basic sequence of cell signaling?

Signal → receptor → intracellular signaling molecules → effectors → cellular response.

What determines how extracellular molecules interact with the cell?

Their size and chemical properties (hydrophilic vs hydrophobic).

What are the four major distances over which signals act?

Contact-dependent, paracrine, synaptic, endocrine.

Why do different cells respond differently to the same signal?

They express different receptors and intracellular signaling machinery.

What kinds of response times can signaling trigger?

Rapid (protein modification) or slow (gene expression changes).

How do cells regulate their signaling pathways?

Through positive and negative feedback loops.