1.6 - structure + function of membranes Flashcards
tonicity
is the osmotic force exerted by a solution
term used to describe the osmolarity of a solution compared with the osmolarity of a blood (isotonic, hypertonic e.g. severe dehydration, hypotonic)
what does the direction of diffusion depend on for ions
determined by electrical gradient AND chemical gradient (opposing)
The balance is determined by the Nernst potential
e.g K+ tend to diffuse out of cells via chemical gradient but are drawn back in due to electrical gradient (negative membrane potential left behind)
What is ficks law
Why can gases like O₂ and CO₂ freely pass through the lipid bilayer?
Gases are uncharged and lipid-soluble, enabling them to freely pass through the lipid bilayer.
Why is water theoretically unable to easily pass through the lipid bilayer?
Water is polar and has low lipid solubility, which should prevent it from passing easily through the membrane.
How does water actually move across the lipid bilayer?
Despite low lipid solubility, water moves rapidly and in an organized manner across the membrane, indicating specialized transport mechanisms beyond simple leakage. = aquaporins
What is an artificial lipid membrane?
An artificial lipid membrane is a model membrane composed purely of a lipid bilayer, used to study permeability.
Why is the permeability of ions and molecules compared between artificial and real membranes?
This comparison helps identify additional components, like proteins, in real membranes that affect permeability.
What does the comparison between artificial lipid bilayers and real membranes reveal?
shows that real membranes have other components enabling functions that wouldn’t be possible with just a lipid bilayer.
types of membrane transport
porins
channels
carriers
types of channels
voltage gated
ligand gated
stretch activated
types of carriers
primary active transport
secondary active transport
facilitated diffusion
What role do aquaporins play in the membrane?
Aquaporins are water channels that facilitate the rapid and organized transport of water across the membrane.
How does perforin function in cytotoxic T-lymphocytes?
Cytotoxic T-lymphocytes release perforin monomers, which form large channels to kill target cells.
Perforin monomers polymerize and assemble like staves in a barrel, forming doughnut-like channels that disrupt the target cell membrane.
what are porins
Transmembrane pores that are ALWAYS open
What are channels in the context of cell membranes?
Channels are aqueous-filled gated pores that can switch between open and closed states to regulate ion flow across the membrane.
what do channels allow the passage of
ions and some organic osmolytes e.g. taurine via passive diffusion
What are leak channels, and how do they function?
Leak channels are constitutively open channels that flicker rapidly between open and closed states without biological regulation, such as K⁺ leak channels that help control membrane potential.
What types of gating mechanisms can control channel opening?
Voltage-gated: changes in membrane potential
Ligand-gated: extracellular binding of a ligand to the channel or to an associated receptor e.g. G-protein coupled receptor (e.g., nAChR or GPCR)
Secondary messenger-gated: intracellular binding of a secondary messenger (e.g., cGMP)
Stretch-activated: membrane deformation
What are gap junctions, and what is their function?
Gap junctions are channels connecting the cytoplasm of adjacent cells, allowing diffusion of small molecules and ions to mediate cell-cell communication.
non-selective channel for electrical and chemical communication = e.g in heart. cardiac myocytes
How are gap junctions structured?
Each gap junction is formed by two connexon pores, each made of six connexin proteins, which align to create a non-selective channel for electrical and chemical communication.
Why can K⁺ ions pass through K⁺ channels, but Na⁺ ions cannot?
K⁺ channels have pore linings that efficiently replace the water molecules that shield K⁺ ions. Na⁺ ions are too small to interact effectively with these pore linings, preventing their passage.
Describe the signaling cascade of a G protein-coupled receptor (GPCR) from activation to signal termination.
An external ligand (e.g., hormone, neurotransmitter) binds to the GPCR’s extracellular domain, causing a conformational change.
The GPCR activates an associated G protein by promoting the exchange of GDP for GTP on the Gα subunit, causing the Gα and Gβγ subunits to dissociate.
The activated Gα and Gβγ subunits interact with target proteins, initiating different signaling pathways:
Gαs stimulates adenylyl cyclase, increasing cAMP and activating protein kinase A (PKA).
Gαi inhibits adenylyl cyclase, reducing cAMP levels.
Gαq activates phospholipase C (PLC), generating IP₃ and DAG, which release Ca²⁺ and activate protein kinase C (PKC).
The Gβγ subunits can also activate ion channels, influence other signaling molecules, and add diversity to the cellular response.
The Gα subunit hydrolyzes GTP to GDP, allowing Gα and Gβγ to reassociate, resetting the G protein. GPCR phosphorylation by GRKs and binding of arrestins can desensitize the receptor, stopping further activation.
How do carrier proteins facilitate transport across the membrane
Carrier proteins binds tosolute + traps it. Its undergoes a reversible conformational change that alternates exposure of the solute-binding site from one side of the membrane to the other. Then releases it.
while channels are often open to both sides.
