Cell Biology: Membranes Answers 103the gangliosides, terminate in sialic acid residues with a strong negative
charge. Cholesterol (answer c)alters membrane fluidity (see figure below and
question 34) and is amphipathic (hydrophilic and hydrophobic properties). It
reduces the packing of lipid acyl groups through its steroid ring structure and
hydrocarbon tail and cements hydrophilic regions of the membrane through
interactions with its hydroxyl (OH-) region. Peripheral membrane proteins
(answer d)are found primarily on the cytosolic leaflet of the membrane
bilayer. Integrins (answer e)are heterodimeric receptors that bind with extra-
cellular matrix (ECM) molecules such as laminin and fibronectin.
32.The answer is b.(Alberts, p 567. Ross and Pawlina, p 30.)The P face of
a cell-membrane freeze fracture is labeled with the asterisks and faces away
from the cytoplasm. Freeze fracture is a procedure in which the tissue is
rapidly frozen and fractured with a knife. The fracture plane occurs
through the hydrophobic central plane of membranes, which is the plane
of least resistance to the cleavage force. The two faces are essentially the
two interior faces of the membrane. They are described as the extracellular
face (E face) and the protoplasmic face (P face). The cytoplasm is the back-
ing for the P face, which in general contains numerous intramembranous
particles (mostly protein). The E face is backed by the extracellular space
A = Integral membrane protein, B = Glycoprotein, C = Peripheral membrane protein
(more abundant on cytosolic surface), D = sugar, E = cholesterol, F = hydrophobic
fatty acid chains (hydrophilic polar head groups are not labeled), G = glycolipid