Cell Biology and Signaling

Why do phospholipids form bilayers?

They are amphipathic: hydrophilic heads interact with water while hydrophobic tails avoid it.

Hydrophilic means...

Water-loving.

Hydrophobic means...

Water-avoiding.

Fluid mosaic model

Lipids and proteins form a dynamic membrane and can move laterally.

What does freeze-fracture show?

Proteins are embedded within membrane leaflets.

FRAP

Fluorescence recovery after photobleaching; measures membrane molecule mobility.

What does fluorescence recovery in FRAP mean?

Unbleached molecules moved into the bleached region.

Frye & Edidin conclusion

Membrane proteins can move laterally.

Simple diffusion direction

High → low concentration.

What crosses a bilayer easily?

Small nonpolar/noncharged molecules such as O₂.

Channel

Allows molecules/ions to flow down an electrochemical gradient.

Transporter

Binds cargo and changes conformation; can saturate.

Pump

Uses energy such as ATP to move molecules against gradients.

Channel flux graph

Approximately linear with concentration.

Transporter/pump graph

Saturates/plateaus.

GLUT2

Glucose transporter using glucose's gradient.

SGLT-1

Na⁺/glucose symporter using the Na⁺ gradient.

Na⁺/K⁺ pump

Uses ATP to maintain Na⁺ and K⁺ gradients.

Resting membrane potential

About −70 mV.

Na⁺ movement during depolarization

Into the neuron.

K⁺ movement during repolarization

Out of the neuron.

Why hyperpolarization?

K⁺ channels close slowly, so extra K⁺ temporarily leaves.

First event at chemical synapse

Voltage-gated Ca²⁺ channels open after action potential arrives.

Ca²⁺ at synapse

Triggers synaptic vesicle fusion/neurotransmitter release.

V-ATPase

Pumps H⁺ into synaptic vesicles using ATP.

VGAT

Uses H⁺ gradient to load GABA into vesicles.

COPI/COPII

Coat proteins associated with ER/Golgi trafficking.

Clathrin

Coat protein used for several Golgi/plasma-membrane/endocytic routes.

Rab proteins

Help direct vesicles to correct destinations.

v-SNARE + t-SNARE

Promote vesicle-target membrane fusion.

Early endosome

Sorting station after endocytosis.

Recycling endosome

Returns proteins/membrane to plasma membrane.

Late endosome → lysosome

Degradation pathway.

CFTR

ATP-gated Cl⁻/HCO₃⁻ channel.

CFTR Class I

No CFTR protein produced.

CFTR Class II

Misfolded CFTR trapped in ER.

CFTR Class III

CFTR reaches membrane but cannot open correctly.

CFTR Class IV

Channel conducts ions poorly.

CFTR Class V

Too little CFTR is produced.

ΔF508

Major Class II CFTR mutation.

Tezacaftor/Elexacaftor

Help CFTR fold/traffic; especially Class II.

Ivacaftor

Helps CFTR channel opening; especially Class III.

Ligand

Extracellular signaling molecule/first messenger.

Second messenger example

cAMP.

Signal amplification

One signal activates many downstream molecules.

Negative feedback

Pathway output reduces its own activity.

Positive feedback

Pathway output increases its own activity.

GTPase ON

GTP-bound.

GTPase OFF

GDP-bound.

GEF

Promotes GDP release/GTP binding → ON.

GAP

Promotes GTP hydrolysis → OFF.

GPCR → Gs → ?

Adenylyl cyclase.

Adenylyl cyclase produces...

cAMP.

cAMP activates...

PKA.

PKA

Protein kinase that phosphorylates downstream proteins.

Why receptor kinase dimerization?

Brings kinase domains together so they can phosphorylate one another.

IP

Pulls a specific protein and interacting proteins from a mixture.

Western blot

Detects particular proteins or phosphorylation states.

Signaling reporter

Fluorescent system used to observe pathway activity in living cells.

Quorum sensing

Bacterial communication based on population density.

Autoinducer

Quorum-sensing signaling molecule.

LuxI

Produces autoinducer.

LuxR

Autoinducer-binding receptor/transcription factor.

Why is Lux quorum sensing positive feedback?

Autoinducer activates LuxI expression, producing even more autoinducer.

Positive-feedback graph shape

Sigmoidal/S-shaped.