Question
A study was conducted to understand the factors controlling the rate at which molecules or ions travel across cell membranes. An artificial membrane was created that was composed of a phospholipid bilayer only. The speed at which various substances crossed this membrane was measured.
Some substances can pass through an actual cell membrane much faster than they passed through the artificial membrane in this study. Which of the following statements best explains this finding?
A. Actual cell membranes have a much thicker phospholipid bilayer than the artificial membrane does.
B. Actual cell membranes have a variety of proteins embedded in the membrane that are absent in the artificial membrane.
C. Hydrophobic substances spend more time between the two layers of phospholipid in the artificial membrane than they do between the layers in an actual membrane.
D. Hydrophilic substances spend more time attached to the polar region of the phospholipids in the artificial membrane than they do attached to the polar region of the phospholipids in an actual membrane.
▶️Answer/Explanation
Ans: B
Actual cell membranes have a variety of embedded proteins that increase the movement of substances across the membrane by both active and passive transport.
Question
A magnesium sulfate solution taken orally can cause a net movement of water into the large intestine, which results from water molecules diffusing through aquaporins embedded in the cells of the intestinal lining.
By which of the following mechanisms do the water molecules most likely move into the large intestine?
A. By passive transport from an area of low osmolarity to an area of high osmolarity
B. By passive transport from an area of high osmolarity to an area of low osmolarity
C. By active transport from an area of low osmolarity to an area of high osmolarity
D. By active transport from an area of high osmolarity to an area of low osmolarity
▶️Answer/Explanation
Ans: A
Based on the information presented, the water molecules move through aquaporins by diffusing from an area of low osmolarity to an area of high osmolarity, which is an example of passive transport.
Question
Aldosterone (a steroid hormone) is a small, nonpolar, hydrophobic molecule that enters a target cell by moving across the plasma membrane, down a concentration gradient.
Based on the information presented, how does aldosterone most likely enter target cells?
A. By simple diffusion because aldersterone can enter a cell by moving across the plasma membrane and moving down the concentration gradient.
B. By facilitated diffusion because aldersterone would need the assistance of a protein channel to move across the plasma membrane.
C. By active transport because aldersterone would need energy to move across the plasma membrane.
D. By endocytosis because aldersterone would need to be transported by vesicle across the plasma membrane
▶️Answer/Explanation
Ans: A
Small, nonpolar, hydrophobic molecules can enter a cell by moving across the plasma membrane and down a concentration gradient by simple diffusion.
Question
Cholesterol is an important component of animal cell membranes. Cholesterol molecules are often delivered to body cells by the blood, which transports the molecules in the form of cholesterol-protein complexes. The complexes must be moved into the body cells before the cholesterol molecules can be incorporated into the phospholipid bilayers of cell membranes.
Based on the information presented, which of the following is the most likely explanation for a buildup of cholesterol molecules in the blood of an animal?
A. The animal’s body cells are defective in exocytosis.
B. The animal’s body cells are defective in endocytosis.
C. The animal’s body cells are defective in cholesterol synthesis.
D. The animal’s body cells are defective in phospholipid synthesis.
▶️Answer/Explanation
Ans: B
The cholesterol-protein complexes are most likely moved into the body cells by endocytosis. A defect in endocytosis is likely to result in a buildup of cholesterol in the blood.
Question
Which of the following statements best explains the processes of passive and active transport?
A. Passive transport is the net movement of substances down a concentration gradient that requires metabolic energy. Active transport is the movement of substances up a concentration gradient that does not require energy.
B. Passive transport is the net movement of substances down a concentration gradient that does not require metabolic energy. Active transport is the movement of substances up a concentration gradient that requires energy.
C. Passive transport is the net movement of substances up a concentration gradient that requires metabolic energy. Active transport is the movement of substances down a concentration gradient that does not require metabolic energy.
D. Passive transport is the net movement of substances up a concentration gradient that does not require metabolic energy. Active transport is the movement of substances down a concentration gradient that requires energy.
▶️Answer/Explanation
Ans: B
Passive transport is the net movement of substances down (with) a concentration gradient that does not require metabolic energy and active transport is the movement of substances up (against) a concentration gradient that requires energy.