Cell Signaling & Communication

Receptors • Ligands • Hormones • Pathways • Second Messengers • Yeast vs. Human Signaling

Cells constantly communicate with each other and with their environment.
They sense signals, respond to stress, adjust metabolism, activate genes, and coordinate behavior — even simple organisms like yeast.

This page introduces how cells detect and respond to signals, the types of receptors, major signaling pathways, and what can (and cannot) be seen under optical microscopy.

1. What Is Cell Signaling?

Cell signaling is how cells: - detect information
- respond to external cues
- coordinate actions
- adapt to changes

Signals can be:

• chemical (hormones, nutrients, ions)
• physical (heat, pressure)
• environmental (glucose, oxygen levels)

All living cells, from yeast to humans, rely on the same basic signaling logic.

2. Types of Signals

2.1. Autocrine signaling

A cell sends signals to itself.
Example: immune cells reinforcing activation.

2.2. Paracrine signaling

Signals travel short distances to nearby cells.
Example: neurotransmitters between neurons.

2.3. Endocrine signaling

Signals travel through the bloodstream.
Example: insulin, adrenaline, growth hormone.

2.4. Direct contact signaling

Cells communicate via physical contact.
Example: immune cell activation, development.

2.5. Environmental sensing (microbes & yeast)

Yeast detect:

• glucose
• nitrogen
• stress
• cell density (quorum-like sensing)
• mating pheromones

3. Receptors: How Cells Detect Signals

Receptors are proteins that detect specific signals (ligands).

3.1. Types of receptors

1) Membrane receptors

Found on the cell surface. Detect extracellular signals.

Major families:

• GPCRs (G-protein–coupled receptors)
• Receptor tyrosine kinases
• Ion channels
• Cytokine receptors

Yeast also have GPCRs — especially for mating pheromones.

2) Intracellular receptors

Found inside the cytoplasm or nucleus.
They detect signals that can cross the membrane:

• steroid hormones
• nitric oxide
• small lipophilic molecules

3) Nuclear receptors

Bind directly to DNA to change gene expression.

Examples:

• estrogen receptor
• glucocorticoid receptor

(Yeast have transcription factors but not human-style hormones.)

4. Ligands: The “Messages” Cells Read

A ligand is the molecule that binds a receptor.

Types include:

• hormones (insulin, testosterone, cortisol)
• neurotransmitters (dopamine, serotonin)
• growth factors (EGF, FGF)
• ions (Ca²⁺)
• gases (NO)
• nutrients (glucose, amino acids)
• yeast pheromones (a-factor, α-factor)

Cells only respond if they have the appropriate receptor.

5. Signal Transduction Pathways

Once a receptor detects a signal, it activates a cascade of molecular events.

5.1. Example pathway: GPCR → G-protein → second messenger

1. Ligand binds receptor
2. Receptor activates G-protein
3. G-protein activates enzymes
4. Enzymes produce second messengers

5.2. Common second messengers

• cAMP
• Calcium ions (Ca²⁺)
• IP₃
• DAG
• cGMP

Second messengers amplify signals inside the cell.

5.3. MAP kinase pathways

Final step often activates transcription factors, altering gene expression.

These pathways are conserved in:

• humans
• yeast
• plants
• animals

6. Yeast-Specific Signaling

Yeast have surprisingly complex signaling systems.

6.1. Nutrient sensing

Yeast detect:

• glucose
• nitrogen
• amino acids
• osmotic stress

Pathways include:

• TOR (nutrient sensing)
• PKA (glucose sensing)
• HOG pathway (osmotic stress)

6.2. Mating pheromone signaling

Yeast have mating types:

• MATa
• MATα

Each secretes a pheromone:

• a-factor
• α-factor

Detection triggers:

• growth arrest
• morphological changes (“shmooing”)
• gene expression for mating

6.3. Stress responses

Yeast activate pathways for:

• heat shock
• oxidative stress
• starvation
• mitochondrial dysfunction

These pathways help researchers study aging and proteostasis.

7. Signaling and Aging

In all organisms, aging is linked to changes in signaling pathways.

Examples:

• Insulin/IGF1 signaling → longevity in animals
• TOR inhibition (rapamycin) → lifespan extension across species
• AMPK activation → improves metabolic health
• Sirtuins (SIR2 in yeast) → require NAD⁺

Yeast are a ideal tool to study:

• nutrient sensing
• DNA repair signaling
• stress response pathways
• mitochondrial signaling

These pathways are highly conserved in humans.

8. What Can You See Under a Light Microscope?

Cell signaling happens at the molecular level — far smaller than the resolution of optical microscopes.

Not visible:

• receptors
• ligands
• signaling pathways
• second messengers
• phosphorylation events
• kinase cascades

Visible only indirectly:

• yeast mating shmoos (morphological change)
• growth arrest or stress phenotypes
• budding patterns
• vacuole enlargement
• cell shape changes

These are the consequences of signaling, not the molecules themselves.

9. Why Cell Signaling Matters

Understanding signaling helps explain:

• how hormones work
• how drugs function
• how cells respond to stress
• how yeast detect nutrients
• how aging pathways (TOR, AMPK, sirtuins) operate
• how cell identity and development work
• how cancer begins (misregulated signaling)
• why cells sometimes fail to repair themselves

All complex biology depends on signaling networks.

10. Quick Beginner Summary

• Cells communicate using signals (ligands) and receptors.
• Signals activate pathways inside the cell.
• Pathways change gene expression, metabolism, or behavior.
• Yeast use signaling for nutrients, stress response, and mating.
• Human cells use signaling for hormones, growth, immunity, and more.
• Signaling molecules are invisible under optical microscopes,
  but their effects can be seen in yeast behavior and cell morphology.