biotest1

How does cholesterol extend the functional temperature range of a lipid bilayer?

At high temperatures cholesterol's ring structure inhibits the movement of phospholipids in the bilayer, thereby reducing membrane fluidity and increasing stability. At low temperatures cholesterol interferes with fatty acid chain interactions and maintains membrane fluidity.

How did the cell fusion experiments of Frye and Edidin support the fluid mosaic model of membrane structure? What results would they have obtained if they had incubated fused cells at 2 degrees C?

Frye and Edinin fused mouse and human cells and examined the distribution of membrane proteins after staining with anti-mouse and anti-human antibodies labeled with different fluorescent dyes. Immediately after fusion, the proteins were located in different halves of the fused cell surface, but after a brief incubation at 37 degrees C the proteins were intermixed. This demonstrated that the proteins could diffuse laterally in a fluid membrane.If incubated at 2 degrees C the proteins would remain separated because the membrane is not fluid at this temperature.

The concentration of K+ is about 20 times higher inside squid axons than in extracellular fluids, generating an equilibrium membrane potential of -75mV. What would be the expected equilibrium membrane potential if the K+ concentration were only 10 times higher inside than outside the cell? Why does the actual resting membrane potential (-60mV) differ from the K+ equilibrium potential of -75mV

Given Co/Ci= 10, the K+ equilibrium potential calculated from the Nernst equation is about -59 mV. The actual resting membrane potential differs from the K+ equilibrium potential because resting squid axons are more permeable to K+ than the other ions.

What are lipid rafts and what are the cellular processes they are involved in?

Lipid rafts are discrete membrane domains that are enriched in cholesterol and sphingolipids. Lipid rafts are believed to play important roles in cell movement, endocytosis, and cell signaling.

How are peripheral membrane proteins distinguished from integral membrane proteins?

Peripheral membrane proteins can be removed from a membrane by a high-salt wash or by solutions of extreme pH that do not disrupt the phospholipid bilayer. Integral membrane proteins can only be extracted from membranes by detergents that disrupt the phospholipid bilayer.

How does the selectivity filter of K+ channels differentiate between K+ and Na+ ions?

The K+ channel contains a selectivity filter lined with carbonyl oxygen atoms. The pore is just wide enough to allow passage of dehydrated K+ ion from which all water molecules have been displaced as aresult of association with the carbonyl oxygen atoms. Hydrated Na+ ions are too small to interact with the carbonyl oxygen atoms and remain associated with water molecules. This complex is too large to pass through the channel pore.

How did Brown and Goldstein determine that LDL binds to a limited number of specific binding sites on the surface of normal cells?

The addition of excess unlabeled LDL reduced the binding of labeled LDL to the surface of normal cells. This indicated that the labeled and unlabeled LDL were competing for a limited number of specific binding sites on the surface of normal cells?

What are the main functions of the glycocalyx?

The glycocalyx protects the cell surface and is involved in cell-cell interactions.

How does the mdr gene confer drug resistance upon cancer cells?

The mdr gene encodes an ABC transporter that is frequently overexpressed in cancer cells. The transporter can recognize a variety of drugs and pump them out of the cell, conferring resistance to chemotherapeutic drugs.

Curare binds to nicotinic acetylcholine receptors and prevents them from opening. How would it affect the contraction of muscles?

The opening of nicotinic acetylcholine receptors is required for membrane depolarization in muscle cells. Thus curare blocks the contraction of muscle cells in response to acetylcholine.

How can glucose be transported against its concentration gradient without the direct expenditure of ATP in intestinal epithelial cells?

The uptake of glucose against its concentration gradient is coupled to the transport of Na+ ions in the energetically favorable direction.

What have studies on cells from children with familial hypercholesterolemia told us about the mechanisms of receptor-mediated endocytosis?

Two types of mutations in the LDL receptor resulting in the inability to take up LDL were identified in FH patients failed to bind LDL, demonstrating that a specific receptor was required for LDL uptake. Other mutant receptors bound LDL, but failed to cluster in coated pits, demonstrating the role of coated pits, in receptor-mediated endocytosis.