Items 52-57

9. 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 Ca²+

53. All of the following substances are transported by active transport EXCEPT:

(A) Na+
(B) K+
(C) Glucose
(D) Ca²+
(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 + concentration
(C) ATP hydrolysis
(D) maintenance of cell volume
(E) maintenance of cell surface charge

55. All of the following statements concerning membrane transport of Ca²+ 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 Ca²+
(D) Under normal physiological conditions, the intracellular Ca²+
Concentration much greater than the extracellular Ca²+ concentration.
(F) The Ca²+ 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 Ca²+ 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 Ca²+ concentration is extremely low. The extracellular Ca²+ concentration is relatively high. This gradient is maintained by a Ca²+ 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 Ca²+ is 10¯7 M. In contrast, there is a large store of Ca²+ 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 Ca²+ permeability of the sarcoplasmic reticulum. This results in a rush of Ca²+ ions into the region of the myofibrils, increasing the cytosolic free Ca²+ 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