Mastering Biology Chapter 7 Pretest Answers & Notes

Which of the following statements about a typical plasma membrane is correct?

a. Phospholipids are the primary component that determines which solutes can cross the plasma membrane.

b. The plasma membrane is a covalently linked network of phospholipids and proteins that controls the movement of solutes into and out of a cell.

c. Carbohydrates on the membrane surface are important in determining the overall bilayer structure.

d. The two sides of the plasma membrane have different lipid and protein composition.

e. The hydrophilic interior of the membrane is composed primarily of the fatty acid tails of the phospholipids.

Answer:  d. The two sides of the plasma membrane have different lipid and protein composition. Because the membrane serves different functions on the cytoplasmic and exterior surfaces, the structure and composition of the surfaces must be different.

Which of the following best describes the structure of a biological membrane?

a. two layers of phospholipids with proteins either crossing the layers or on the surface of the layers

b. two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins

c. two layers of phospholipids with proteins embedded between the two layers

d. a fluid structure in which phospholipids and proteins move freely between sides of the membrane

e. a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot

Answer:  a. two layers of phospholipids with proteins either crossing the layers or on the surface of the layers *The membrane proteins can be found either embedded in or attached to the surface of the phospholipid bilayer.

The permeability of a biological membrane to a specific polar solute may depend on which of the following?

Answer:  the types of transport proteins in the membrane

Which of the following is least likely to be important in holding the components of a biological membrane together?

Answer:  covalent interactions between the phospholipid and protein components of the membrane

Which of the following statements about osmosis is correct?

Answer:  The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis.  *Aquaporins facilitate water movement across membranes and thus speed up the process of osmosis.

Which of the following pairs correctly matches a membrane transport process to its primary function?

Answer:  pinocytosis: the uptake of water and small solutes into the cell by the formation of vesicles at the plasma membrane

A nursing infant is able to obtain disease-fighting antibodies, which are large protein molecules, from its mother’s milk. These molecules probably enter the cells lining the baby’s digestive tract via which process?

Answer:  endocytosis

Consider the transport of protons and sucrose into a plant cell by the sucrose-proton cotransport protein. Plant cells continuously produce a proton gradient by using the energy of ATP hydrolysis to pump protons out of the cell. Why, in the absence of sucrose, do protons not move back into the cell through the sucrose-proton cotransport protein?

Answer:  The movement of protons through the cotransport protein cannot occur unless sucrose moves at the same time.

A cell has a membrane potential of -100 mV (more negative inside than outside) and has 1,000 times more calcium ions outside the cell than inside. Which of the following best describes a mechanism by which Ca2+ enters the cell?

Answer:  facilitated diffusion of Ca2+ into the cell down its electrochemical gradient

Glucose can be moved into cells via an active transport mechanism when the concentration of glucose inside the cell is higher than the concentration of glucose outside of the cell. This active transport mechanism moves glucose and sodium into the cell at the same time. The glucose moves up its gradient and the sodium moves down its gradient. Which of the following statements about this mechanism is accurate?

Answer:  To pump glucose up its concentration gradient, sodium moves down its concentration gradient, and the distribution of sodium ions across the membrane forms an electrochemical gradient that drives this mechanism.