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.
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 StandM
Items 52-57
The following set of numbered items relate to solute transport phenomena. For each, choose the most appropriate lettered answer.
52. Which of the following membrane transport mechanisms requires the expenditure of metabolic energy in the form of hydrolysis of ATP?
(A) simple diffusion of steroids
(B) simple diffusion of water
(C) facilitated diffusion of valine
(D) facilitated diffusion of glucose
(E) active transport of CaČ+
53. All of the following substances are transported by active transport EXCEPT:
(A) Na+
(B) K+
(C) Glucose
(D) CaČ+
(E) H+
54. The carrier protein for the Na+ -K+ pump has hall of the following characteristics EXCEPT:
(A) 3 Na + binding sites on the interior of the cell
(B) 2 K + binding sites on the exterior of the cell
(C) a site with ATPase activity closer to the K + binding sites than to the Na + binding sites
(D) two subunits with MW = 55,000 and MW = 100,000
(E) found on all cells of the body
55. The Na + -K + pumps has with most crucial function for all cells?
(A) maintains a low extracellular Na + concentration
(B) maintains a high extracellular K + concentrationv
(C) ATP hydrolysis
(D) maintenance of cell volume
(E) maintenance of cell surface charge
55. All of the following statements concerning membrane transport of CaČ+ are true EXCEPT:
(A) Different carrier proteins exist in plasma membranes and mitochondrial membranes.
(B) Different carrier proteins exists in plasma membranes and endoplasmic reticulum.
(C) The sarcoplasmic reticulum of muscle cells acitively sequesters CaČ+
(D) Under normal physiological conditions, the intracellular CaČ+ Concentration much greater than the extracellular CaČ+ concentration.
(F) The CaČ+ pump is an ATPase.
57. All of the following statements concerning ion transport across epithelial sheets are true EXCEPT:
(A) It occurs in the proximal convoluted renal tubeles.
(B) Na+ is actively pumped into cells apically and actively pumped out of cell basally.
(C) It is an ATP consuming process.
(D) Ions are actively pumped out of the lateral and basal surfaces of epithelial cells
.
(E) Apical junctional complexes prevent ions from diffusing from the latera compartment into the lumen.
ANSWERS AND TUTORIAL ON ITEMS 52-57
The answers are: 52-E;53-C;54-C;55-D;56-D;57-B. There are three basic mechanisms for transport of solutes across semi-permeable cell membrane:simple diffusion, facilitated diffusion and active transport. Simple diffusion can occur either by solutes passing through membranes, e.g., lipids solube steroids; or, by solutes passing through aqueous pores in membrane. Facilitated diffusion requires that there be a carrier protein present. Glucose, other sugars and many amino acids cross cell membranes by facilitated diffusion. Active transport involves movement of substances up an electrochemical gradient. Na+,K+H+, and CaČ+ are all moved by active transport. It is mediated by intergral membrane transport proteins called pumps and requires the hydolysis of ATP to drive it.
The Na+-K+ pump is an important integral membrane carrier protein which spans the plasma membrane. It is found in the cell membrane of all cells. It consists of two different protein subunits. The larger subunits has a MW = 1000,000 with 3 Na+ binding sites and an ATP binding site on its intracellular side and 2 K+ binding sites on its extracellular side. When Na+ and ATP bind on the inside, the ATPase becomes activated ant the protein becomes phosphprylated. This causes a conformational change in the carrier protein which expels Na+ on the extracellular side of the membrane. In this conformation, the Na+-K+ ATPase binds extracellular K+. The binding of K+ dephoshorylates the ATPase, returning it to its original conformation and expelling K+ at the intracellular side of the membrane. The function of the smaller subunit with a MW = 55,000 is uncle at present.
The most essential role of this ion pump is to maintain the volume of cells. The cytosol contains many negatively charged proteins and low molecular weight solutes. These binds cations. If the ion pumps were not active in expelling more cations than they look into cells, then water would diffuse into cells (via osmosis), eventually causing them to awell to bursting. The Na + -K+ pump also establishes grasdients of Na+ and K+ across the plasma membrane since it expels 3 Na+ ions for every 2 K+ taken in. Since these ion gradients cause resting membrane potential, the Na+-K+ pump is called electrogenic.
Under normal circumstances, the intracellular CaČ+ concentration is extremely low. The extracellular CaČ+ concentration is relatively high. This gradient is maintained by a CaČ+ pump which is quite similar to the Na+-K+ pump in that it has a carrier protein with ion binding sites and an ATPase activity. The carrier proteins in mitochondria and the endoplasmic reticulum are distinct from those in the plasma membrane. In resting muscle cells, the cytosolic concentration of free CaČ+ is 10Ż7 M. In contrast, there is a large store of CaČ+ in the sarcoplasmic reticulum. When a nerve action potential reaches the motor end plate of a muscle cell, it causes a depolarization of the sarcolemma which in turn causes an increase in the CaČ+ permeability of the sarcoplasmic reticulum. This results in a rush of CaČ+ ions into the region of the myofibrils, increasing the cytosolic free CaČ+ concentration to 10-5 M, a crucial event fro initiating the sliding of thick and thin filaments relative to one another. These sliding filaments ultimately result in forceful contraction of the entire cell.
Epithelial sheets in many locations in the human body are capable of polarizes transport of ions between different compartments. Examples of these transporting epithelia are found in the gastronintestinal track, kidney tubules, exocrine glands, the cilialry body of the eye and the choroid plexus in the ventricles of the brain. This polarized transport, fro example as seen with Na+ transport out of the proximal convoluted tubules ( PCTs) in the kidney, involves active transport of Na+ out of cells on the lateral and basal surfaces. The apical surface of PCT epithelial cells is freely permeable to Na+ and water by diffusion. Thus, when Na+ is actively pumped out or cells at the lateral and basal surfaces, water exits these cells by osmosis at the same locations. Individual epithelial cells are joined apically by an extensive network of tight junctions that are impermeable to Na+ ions, to prevent leakage of extracellular Na+ into the lumen. boom-mounted microscopes, articulating microscope
