Lecture 7 - Membrane Structure and Transport Flashcards
General features of the Fluid Mosaic Model: thermodynamic considerations for bilayers, favorability/unfavorability of lateral vs transverse diffusion of phospholipids
Fluid: lipids and proteins in membrane can move laterally
Mosaic: comprised of different lipids and proteins
Thermodynamic considerations: phospholipids are amphipathic (polar head, nonpolar tails) => polar heads face water, nonpolar tails face each other
Lateral > transverse diffusion of phospholipids b/c eliminates need for amphipathic parts to flip (otherwise need an enzyme)
Membrane function: Compartmentalization
Membranes form continuous sheets that enclose intracellular compartments
Membrane function: Scaffold for biochemical activities
Membranes provide a framework (resulting from physical positioning of protein complexes) that organizes enzymes for effective interaction - also creates protective/”reductive” environment that protects proteins + DNA from oxidative stress
Membrane function: Selectively permeable barrier
Membranes allow regulated exchange of substances b/t compartments
Membrane function: Transporting solutes
Membrane proteins facilitate the movement of substances b/t compartments
Membrane function: Responding to external signals
Membrane receptors transduce signals from outside the cell in response to specific ligands
Membrane function: Intracellular interaction
Membranes mediate recognition and interaction b/t adjacent cells => important for formation of larger cell-based structures like tissues and organs
Membrane function: Energy transduction
Membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy
Ex: mitochondria and chloroplasts are double membrane
Membrane lipid function and examples
For formation of membrane; Ex: Phospholipids, specifically glycerophospholipids
Storage lipid example
For energy storage; Ex: Triglycerides (dietary lipids)
Phospholipid structure:
Glycerol backbone + (phosphate + head group) + fatty acid tails (saturated/unsaturated)
Head group in glycerophospholipids is alcohol
Fatty acid structure:
Carboxyl group attached to hydrocarbon tail (can be saturated or unsaturated)
Effects of temperature on membrane fluidity; compensation mechanism for unicellular organisms
- Less heat => ordered => lower fluidity
- More heat => disordered => more fluidity
More important for unicellular organisms than multicellular b/c latter have thermal regulation
Unicellular compensation mechanism via enzymes:
If hot -> increase saturated fatty acids, decrease unsaturated fatty acids
If cold -> decrease saturated fatty acids, increase unsaturated fatty acids
Effects of fatty acid saturation on membrane fluidity
- Saturated => straight => ordered => lower fluidity
- Unsaturated -=> bent/kinks => disordered => more fluidity
Effects of sterols (cholesterol) on membrane fluidity
- More cholesterol insertion => more fluidity
- Less cholesterol insertion => lower fluidity
Triglyceride structure:
Glycerol backbone + 3 fatty acids
Steroid/sterol structure:
Polar head + 4 carbon ring steroid nucleus + alkyl side chain
Asymmetry of membranes
The differences in lipid composition on each layer of the cell membrane bilayer
3 membrane proteins:
- Peripheral proteins
- Integral/transmembrane proteins
- Lipid anchor proteins (GPI-linked protein)
What are lipidation rxns of proteins?
- Specific AA on protein is covalently modified through attachment of various kinds of lipids
- Attached lipid can insert into bilayer => draws protein to cell membrane => protein localize to membranes via lipidation (or also GPI anchors)
What are isoprenylation rxns of proteins?
- Attachment of different sized isoprenyl group
Structural features of peripheral proteins
- Can be lipidated
- Can interact directly w/ phospholipid head group
- Can interact w/ other proteins (integral proteins)
- Can be covalently linked to specific lipids (GPI Anchor)
Structural features of integral/transmembrane proteins
- Can be alpha helices or beta-sheets/barrels
- Alpha helices can cross bilayer either once or multiple times
=> if once, usually act as receptor, R groups must have stretches of nonpolar AA (amphipathic alpha-helix)
=> if more than once, usually act as transporter, outer alpha helices must be nonpolar and inner alpha helices must be polar for hydrophilic molecule transport (amphipathic alpha-helix)
What is a lipid raft?
- Microdomains in plasma membrane that diffuse laterally
- Contain clusters of specialized proteins (ex: signal transduction), high cholesterol content, and lipidated proteins
- Structural and functional unit of various proteins important for certain things
- Idea of localized signal transduction modules
What are hydrophobicity/hydropathy plots?
Predict location of transmembrane domains in proteins
What is simple diffusion? Effect of polarity and size? Why does this effect exist?
Solute diffuses through bilayer down its concentration gradient depending on polarity and size
Polarity and size in increasing diffusion rate (ions < large uncharged polar < small uncharged polar < nonpolar molecules)
Polar/charged molecules often form hydration shell, and removal of such shell is endergonic => high Ea for diffusion
What are the types of membrane transport proteins?
Transporters (conformational change after solute binds) and channels (aqueous pore)
Movement of electrically neutral solutes vs charged solutes
Neutral: down concentration gradient until equilibrium reached
Charged: combo of electrical and chemical potential difference
Equilibrium w/o electrical potential across membrane has equal particles and charge on both sides
Equilibrium w/ electrical potential across membrane has unequal number of particles and charges on each side
What is passive transport? How do proteins deal with hydration shell?
AKA facilitated diffusion; transport of molecules down concentration gradient via transporters or channels w/o ATP
Hydration shell: protein form noncovalent bonds w/ dehydrated solute to replace H-bonding with water or by creating a hydrophilic transmembrane passageway/pore
What is active transport? What are the associated forms of energy?
Transport of molecules via transport proteins that use a form of energy
Energies: ATP, light, or coupled/cotransport
How does coupled/cotransport work?
Relies on electrochemical gradient established by a pump
3 types of transporter-mediated movement
- Uniport
- Symport
- Antiport
Symport and antiport are cotransport
3 examples of transporters
- Glucose transporters
- Na+/K+ ATPase
- ATP synthase
Elaborate on glucose transporter (Glut1)
- Glut1 uniporter
- Facilitated diffusion (works w/ [glucose] gradient) or cotransport
- Multi transmembrane domains
- Amphipathic alpha helices
What regulates Glut1 expression?
insulin hormone
How do amphipathic alpha helices work in transporters?
Some residue stretches of alpha helix are polar, others nonpolar
When clustered to higher order, polar residues face transported molecule and nonpolar residues face hydrophobic membrane
Elaborate on Na+/K+ ATPase
- Electrogenic pump, primary active transport
- 3 Na+ pumped out, 2 K+ pumped in => net negative charge inside cell
- Phosphorylated during transport
- Kind of like antiport, except both Na and K are pumped against gradients
How does Na+/K+ ATPase pump drive glucose uptake into bloodstream? What are the sources of energy for glucose to be transported into bloodstream?
- Cell has negative charge, low sodium, and high glucose
- Na+-glucose symporter in intestinal lumen is a symporter => sodium enters cell down gradient, glucose against gradient
- Glucose uniporter (ex: Glut2) facilitates glucose export down gradient into blood
Two sources of energy:
1. [Na+] outside»_space;» [Na+] inside
2. Transmembrane potential (inside negative, draws Na+ inward
=> glucose inside cell»_space;» glucose outside cell
(Refer to slides 58 and 59 lec 7)
Elaborate on ATP Synthase
- How eukaryotes make ATP in mitochondria
- Last step in ETC
- Fo unit is water insoluble transmembrane proton pore
-F1 unit is water soluble peripheral membrane protein complex - Couples ATP synthesis (endergonic) with passive diffusion of protons through inner mitochondrial membrane (exergonic)
Uncoupling of ETC from ATP synthesis generates ________ via pore protein ______ which allows protons to flow down gradient like the ________ of ATP synthase
Heat; thermogenin; Fo unit
