Membranes/Membrane transport Flashcards
What enzymes enable lipid transfer between leaflets?
Lipid transfer between leaflets
• Outer to Inner – flippase
• Inner to Outer – floppase
• Bidirectional – scramblase
What molecules can move across membranes?
Things that can move across
• Lipophilic molecules
• Very small, uncharged polar molecules
• Water (somewhat)
What molecules can’t move across membranes?
Things that cannot move across
• Ions
• Large molecules
• Charged, polar molecules
What is the difference between integral and peripheral proteins?
Peripheral vs. Integral
• Peripheral - localized near the cell membrane
• Non-covalent, prenylation, GPI anchor etc.
• Integral - traverse the membrane or are embed
How does cholesterol affect membrane fluidity?
Acts as a “buffer” to fluidity
• high temperature – stabilizes neighboring
phospholipids
• low temperature – spaces out phospholipids to
prevent solidification
• Generally speaking, more cholesterol = more stability (i.e. less fluidity)
What are lipid rafts?
Segments of the plasma membrane that contain higher proportion of
cholesterol and sphingolipids (saturated F.A. tails)
Pack more tightly together than unsaturated F.A. tails of neighboring
phospholipids
Help to stabilize and localize certain groups of membrane proteins
together
What is passive transport? What are the two types, and how are they different?
Do not require energy input directly
• Simple or facilitated diffusion
• Simple – no transport protein needed
• Facilitated – transport protein required
• “channels” or “carriers”
• Solutes move down concentration gradient
What factors determine the rate of simple diffusion?
Rate is determined by
• concentration gradient
• lipid solubility
• molecular weight
Gases diffuse only according to
concentration (partial pressure)
gradient
What is the difference between channel and carrier proteins?
Channels allow passage through a pore
Carriers usually transport through a conformational change
How are channel and carrier proteins regulated?
Channels or carriers can be regulated
• Ligand binding–e.g. nicotinic receptor
• Physical stimuli
• Phosphorylation–e.g. CFTR channel
• Trafficking–e.g. GLUT4 (translocation)
What is primary active transport?
Primary active transport – ATP hydrolysis directly couple to solute
movement
e.g. Na/K pump
What is secondary active transport?
Secondary active transport – Concentration gradient established by primary transport drives movement of a separate solute
e.g. Na/Glucose transport–uses gradient established by Na/K pump
Describe the importance of aquaporins
Important for cell volume regulation and water transport across
epithelial tissues
In kidney epithelium…
ADH docks at GPCR
Adenylate cyclase/cAMP/PKA pathway activated
Aquaporins translocated to urine facing membrane
Water enters cell and is taken back into blood
What is CFTR?
Chloride ion channel found in various epithelial tissues
• Important role in electrolyte and water transport
• Chloride secretion to the surface of a tissue draws sodium and water
along, keeping outer mucus layer hydrated
• Important in lung, intestine, sweat gland and pancreas
- Regulated by phosphorylation of regulatory (R) domain
- cAMP-dependent protein kinase A
ABC transporter (ABC = ATP binding cassette)
Binds and hydrolyzes ATP
Causes release of chloride ions
Cholera toxin leads to massive activation of CFTR channels–dehydration
People with CF are semi-resistant to effects of cholera b/c non functional CFTR channel
What are LDLs?
Low density lipoproteins carry lipids and cholesterol in the blood
