Day 15: Model of Energy & Matter Transformations

What is cellular communication?

The process by which cells detect, interpret, and respond to signals in their environment or from other cells to coordinate activities.

Why is cell signaling essential for multicellular organisms?

It enables coordination of growth, development, metabolism, immune responses, and homeostasis among different tissues.

What are the three general stages of cell signaling?

1. Reception (signal detection), 2. Transduction (signal relay and amplification), 3. Response (cellular activity change).

What is a signaling molecule (ligand)?

A chemical messenger that binds to a specific receptor to trigger a cellular response.

What types of molecules can act as ligands?

Hormones, neurotransmitters, growth factors, cytokines, gases, and even small peptides.

What is the difference between local and long-distance signaling?

Local signaling acts near the source (paracrine, synaptic), whereas long-distance signaling involves hormones traveling through the bloodstream.

What is paracrine signaling?

Short-range communication where a cell secretes molecules that act on nearby target cells.

What is autocrine signaling?

A cell produces signaling molecules that bind to receptors on its own surface, affecting itself.

What is endocrine signaling?

Long-distance communication through hormones that travel via the bloodstream to reach target cells.

What is synaptic signaling?

A type of local signaling in which neurotransmitters are released from nerve cells to target adjacent cells across synapses.

What is direct cell-to-cell signaling?

Communication through physical contact, often via gap junctions in animals or plasmodesmata in plants.

What is a receptor protein?

A molecule, usually on the cell surface or inside the cell, that specifically binds a signaling molecule and initiates a cellular response.

Where are cell-surface receptors located?

In the plasma membrane, facing the extracellular environment to detect hydrophilic signals.

Where are intracellular receptors located?

In the cytoplasm or nucleus; they bind hydrophobic ligands that cross the membrane, like steroid hormones.

What happens when a ligand binds to a receptor?

The receptor undergoes a conformational change that activates its intracellular signaling function.

What are the main classes of cell-surface receptors?

G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ion channel-linked receptors.

What is a G protein-coupled receptor (GPCR)?

A receptor that activates an intracellular G protein when bound by a ligand, triggering downstream signaling cascades.

Describe the basic GPCR mechanism.

Ligand binding activates the receptor, causing it to exchange GDP for GTP on the G protein, which then activates target enzymes or channels.

What is a second messenger?

A small intracellular molecule that transmits and amplifies signals from receptors to target molecules inside the cell.

Give examples of second messengers.

cAMP, Ca²⁺ ions, IP₃, DAG, and cyclic GMP.

What enzyme produces cAMP?

Adenylyl cyclase, which converts ATP to cyclic AMP upon G protein activation.

How is cAMP signaling terminated?

The enzyme phosphodiesterase converts cAMP back to AMP, reducing its concentration.

What is a receptor tyrosine kinase (RTK)?

A receptor that dimerizes and autophosphorylates upon ligand binding, activating signaling pathways controlling cell growth and metabolism.

What ligands typically activate RTKs?

Growth factors such as insulin, epidermal growth factor (EGF), and platelet-derived growth factor (PDGF).

How does phosphorylation regulate protein activity?

Addition of a phosphate group by a kinase changes the protein's shape or function, activating or deactivating it.

What enzymes remove phosphate groups?

Phosphatases, which reverse phosphorylation to turn off signaling pathways.

What is a phosphorylation cascade?

A series of sequential kinase activations that amplify and distribute the signal within the cell.

What is the advantage of signaling cascades?

Amplification of the original signal and multiple points of regulation or integration.

What is the MAP kinase pathway?

A conserved phosphorylation cascade that controls cell growth, division, and differentiation.

What are ion channel-linked receptors?

Receptors that open or close ion channels in response to ligand binding, altering membrane potential or ion concentration.

Give an example of an ion channel receptor.

Nicotinic acetylcholine receptor at neuromuscular junctions, which opens to allow Na⁺ influx when acetylcholine binds.

What are intracellular receptors?

Receptors located in the cytoplasm or nucleus that bind lipid-soluble molecules (e.g., steroid hormones) and regulate gene transcription.

Give an example of an intracellular receptor.

The estrogen receptor, which binds estrogen and acts as a transcription factor in the nucleus.

How does signal transduction amplify responses?

Each activated molecule (e.g., enzyme) can activate many downstream targets, multiplying the effect of one ligand.

Why do cells use multiple signaling pathways?

To integrate diverse signals and produce coordinated responses appropriate for changing conditions.

What determines a cell's specific response to a signal?

The receptor type, the combination of signaling proteins expressed, and the intracellular context.

What are scaffolding proteins?

Large molecules that organize groups of signaling components, improving speed and accuracy of signal transduction.

What is cross-talk between signaling pathways?

Interaction between different signaling pathways that allows integration and coordination of cellular responses.

How do calcium ions (Ca²⁺) act as second messengers?

They are released from intracellular stores (like the ER) via IP₃ signaling and trigger processes like muscle contraction or secretion.

What is the IP₃/DAG pathway?

A GPCR-activated pathway where phospholipase C cleaves PIP₂ into IP₃ and DAG, which increase Ca²⁺ release and activate protein kinase C.

How does nitric oxide (NO) act as a signaling molecule?

It diffuses rapidly across membranes and activates guanylyl cyclase to produce cGMP, causing smooth muscle relaxation and vasodilation.

How do cells terminate signaling?

By ligand degradation, receptor internalization, GTP hydrolysis, or dephosphorylation to reset the pathway.

What happens if signaling is not properly regulated?

It can lead to diseases such as cancer, diabetes, or neurological disorders.

How can mutations in RTKs cause cancer?

They can produce receptors that signal continuously without ligand binding, driving uncontrolled cell division.

What is apoptosis?

Programmed cell death, a controlled process that eliminates damaged or unnecessary cells without inflammation.

Why is apoptosis important?

It shapes development, removes damaged cells, and prevents cancer by eliminating abnormal cells.

Which signaling pathways commonly trigger apoptosis?

Intrinsic (mitochondrial) and extrinsic (death receptor) pathways.

How is the intrinsic pathway of apoptosis initiated?

Internal stress signals (DNA damage, oxidative stress) activate mitochondrial release of cytochrome c.

What role does cytochrome c play in apoptosis?

It binds Apaf-1 to form the apoptosome, activating caspases that execute cell death.

What are caspases?

Proteases that cleave key cellular proteins, dismantling the cell during apoptosis.

How do survival factors prevent apoptosis?

They activate signaling pathways (e.g., PI3K/Akt) that inhibit pro-apoptotic proteins.

How can defects in apoptosis contribute to disease?

Failure of apoptosis can cause cancer; excessive apoptosis contributes to neurodegenerative diseases.

What is signal specificity in cell communication?

Only target cells with matching receptors respond to a given ligand, ensuring accurate communication.

Why is desensitization important in signaling?

It prevents overstimulation by temporarily reducing receptor sensitivity after prolonged exposure to a ligand.

How do cells adapt to persistent signals?

By internalizing or degrading receptors or inhibiting downstream signaling proteins.

What are examples of therapeutic drugs that target signaling pathways?

Beta-blockers (GPCRs), tyrosine kinase inhibitors (RTKs), and PDE inhibitors (cAMP signaling).

How does signaling control gene expression?

By activating transcription factors that bind DNA and regulate mRNA synthesis for specific proteins.

What is the ultimate outcome of most signaling pathways? A change in gene expression, metabolism, cytoskeletal organization, or cell behavior.