BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 10. INTRODUCTION TO BIOLOGICAL MEMBRANES

QUESTIONS AND PROBLEMS

1. How many phospholipid molecules are contained in 1 μm2 of a phospholipid bilayer membrane? Assume that a single phospholipid molecule occupies an area of 70 Å2.

2. The Fatty acids of bacterial Phospholipids contain a cyclopropane ring.

What effect would the cyclopropane ring have on the packing of hydrocarbon chains within the bilayer? Would such cyclopropane fatty acids increase or decrease membrane fluidity?

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3. What is the average distance traveled by Membrane Lipids in 1 µs, 1 ms, and 1 s? The diffusion coefficient can be assumed to be 10-8cm2 • s-1.

4. The diffusion coefficient D of a rigid spherical molecule is given by the equation

D = kT / (6πηr),

where η is the viscosity of the solvent, r is the radius of the sphere, k is the Boltzmann constant (1.38 • 10- 6 erg/deg), T is the absolute Temperature. What is the diffusion coefficient at 37°C of a 100 kDa protein in a membrane with an effective viscosity of 1 poise (1 poise = 1 erg • s/cm3)? What is the average distance traveled by this protein in 1 µs, 1 ms, and 1 s? Assume that the protein is a dehydrated, rigid spherical particle with a density of 1.35 g/cm3.

5. Kornberg and McConnell (R.D. Kornberg, H.M. McConnell, 1971) investigated the transverse diffusion of phospholipids in a bilayer membrane using a paramagnetic analog of phosphatidylcholine, the so-called spin-labeled phosphatidylcholine:

The nitroxide group (NO) in spin-labeled phosphatidylcholine yields a sharp paramagnetic Resonance spectrum. If, in the presence of reducing agents such as ascorbate, the nitroxide compounds are reduced to amines, this spectrum disappears.

By sonication followed by purification using Gel filtration, lipid vesicles consisting of phosphatidylcholine (95%) and the spin-labeled analog (5%) were prepared. The outer diameter of these Liposomes was about 250 Å. A few minutes after The addition of ascorbate, the amplitude of the paramagnetic resonance spectrum decreased to 35% of its initial level. After the addition of a second aliquot of ascorbate, no significant Changes in the spectrum were observed for several minutes; however, the amplitude of this residual spectrum subsequently decayed exponentially with a half-life of 6.5 h. How can these changes in the amplitude of the paramagnetic spectrum be explained?



Last update: 06/08/2026

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