Items 58-60
Examine the high power electron micrograph below in Tissue-culture microscope 1.2 and then choose the best answer in the items below. The arrow marks the phospholipids bilayer of the plasma membrane and the cytoplasm of the cell is in the bottom of the picture.
Tissue-culture microscope 1.2
58. The electron dense material attached to the outer leaflet of the plasma Membrane is best described as the
(A) cell cortex
(B) contractile ring
(C) clathrin-coating
(D) glycocalyx
(E) nuclear pore complex
59. The most abundant constituent of this layer is
(A) actin
(B) myosin
(C) spectrin
(D) glycoprotein
(E) phospholipid
60. All of the following characteristic might be attributed to this structure EXCEPT:
(A) required for cell movement
(B) involved in protein of cell from proteolytic digestion
(C) involved in immunological recognition
(D) involved in cell-extracellular matrix adhesion
(E) involve in cell-cell adhesion
ANSWERS AND TUTORIAL ON ITEMS 58-60
The answers are: 58-D;59-D;60-A. Tissue-culture microscope 1.2 shows the external surface of a cell. Many cells in the human body have a thick coating applied to the outer leaflet of the plasma membrane. This coat is composed of the carbohydrate-rich portions of integral membrane glycoproteins.It known as the glycocalyx. The glycocalyx has several important functions. For example, it is well developed on the apical surface of luminal intestinal epithelial cells. Here, it is thought to prevent the noxious lytic digestive enzymes from autodigesting mucosal epithelial cells. Disaccharidase activity is also part of the intestinal glycocalyx. In other locations, the gycocalyx functions in cell-cell and cell-maatrix adhesion. The glcoproteins of the glycocalyx are also involved in immunological recognition phenomena. Boom Mounted Microscopes, Dual Arm type of microscope, Boom Mounted Microscopes with Mobile Floor Stand
11. Items 61-69
(A) Integral membrane proteins
(B) Peripheral membrane proteins
(C) Both
(D) Neither
61. Associated with membrane lipids by hydrophilic but not hydrophobic interactions.
62. Often span the entire thickness of the phospholipids bilayer.
63. Would be numerous in the P-face of a freeze-fracture electron micrograph.
64. Are often intimately associated with cytoskeletal proteins.
65. Can have a-helical segments embedded in the hydrophobic of the membrane lipid bilayer.
66. Can often be dissociated from membrane preparations by buffers without detergents.
67. Abundant in erythrocytes.
68. Transport proteins in many cell membranes are part of this class.
69. Have little functional significance for the inactive erythrocyte membrane.
ANSWERS AND TUTORIAL ON ITEMS 61-69
The answers are: 61-B; 62-A;63-A;64-C; 65-A; 66-B; 67-C; 68-A;69-D. Cell membranes are composed of lipids, which are the structural basis fro the bilayer of the membrane, and proteins, which are essentially responsible for most membrane functions including ion transport, signal transduction, and cell recognition. Membrane proteins fall into two broad categories: integral membrane proteins and peripheral membrane proteins.
Integral membrane proteins are characteristically rich in hydrophobic domains that interact strongly with the hydrophobic portions of membrane lipids. Thus, they cannot be extracted from membranes easily but require the use of strong detergents that disrupt llipid-protein hydrophobic interactions and solubilize the integral membrane proteins. Some integral membrane proteins do not span the entire thickness of the lipid bilayer but many others are transmembrane proteins that have one or more region rich in hydrophobic domains, often arrange in a-helices. There may be a single a-helical domain that makes a single pass trough the bilayer (e.g., erythrocyte glycophorin A) or there may be many such domains making multiple passes through the membrane (e.ge., erythrocyte band 3 which froms
HCO 3- and CI- transport channels. In some instances, integral transmembrane proteins are anchored to cytoskeletal proteins. When cell membranes are cleaved in the hydrophobic plane by freeze-fracture, they often leave intramembranous particles exposed in the P-face. Darkfield Microscopes, Embryo Transplant Microscopes, ENT Surgical Microscopes Peripheral membrane proteins can be extracted from membranes by nondisruptive treatments because they are associated with lipids or other membrane proteins by relatively weak ionic interactions. Thus, shifts in the ionic strength or pH of extraction buffers will often solubize them. Thus, for example, in the erythrocyte membrane, the peripheral protein spectrin is associated with another peripheral ankyrin and a complex between both is ionically associated with hydrophibic groups in the erythrocyte membrane.
