Cell Transport

Transporters allow solutes to cross a membrane at much faster rates than do channels.

• True

• False

Answer explanation

Transporters are slower. They have enzyme like properties, i.e., they bind solutes and need to undergo conformational changes during their functional cycle. This limits the maximal rate of transport, whereas channels are much faster.

The plasma membrane of many animal cells contains open K+ channels, yet the K+ concentration in the cytosol is much higher than outside the cell.

• True

• False

Answer explanation

Most animal cells contain K+ leak channels in their plasma membrane that are predominantly open. The K+ concentration inside the cell still remains higher than outside, because the membrane potential is negative and therefore inhibits the positively charged K+ from leaking out. K+ is also continually pumped into the cell by the Na+ pump.

The membrane potential of an axon temporarily becomes more negative when an action potential excites it.

• True

• False

Answer explanation

The peak of an action potential corresponds

to a transientshiftofthemembranepotentialfrom a negative to a positive value. Theinfluxof Na+ causes themembranepotentialfirst to move toward zero andthen to reverse, renderingthecellpositivelychargedonitsinside. Eventually, therestingpotentialisrestoredbyaneffluxof K+ throughvoltage-gated K+ channelsand K+ leakchannels

The interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other in which of the following ways?

• By small vesicles that bud off of one organelle and fuse with another

• By open pores that allow ions to exit and enter the organelles

• They do not communicate with one another.

• By excreting hormones and other small signaling molecules

The movement of materials from the plasma membrane, through endosomes, and then to lysosomes describes which type of pathway?

• Secretory pathway

• Endocytic pathway

• None of the above

What protein can assemble into a basket-like network that gives budding vesicles their shape?

• Dynamin

• Clathrin

• Actin

• Myosin

The regulated exocytosis pathway of the Golgi apparatus operates continually in all eucaryotic cells.

• True

• False

Which of the following is NOT true of receptor-mediated endocytosis?

• In receptor-mediated endocytosis, extracellular substances are internalized in clathrin-coated vesicles.

• . Low-density lipoproteins (LDLs) are taken into cells by receptor mediated endocytosis.

• . In receptor-mediated endocytosis, internalized vesicles fuse with lysosomes, which then mature into endosomes.

• Defective LDL receptor proteins may contribute to the development of atherosclerosis.

Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?

• Transport vesicles

• The Golgi apparatus

• Endosomes

• Lysosomes

Lipid bilayers are highly impermeable to:

• carbon dioxide.

• water.

• Na+ and Cl-

• ethanol.