# Biochemistry Chapter 8-MCAT

1. A student is trying to determine the type of membrane transport occurring in a cell. She finds that the molecule to be transported is very large and polar, and when transported across the membrane, no energy is required. Which of the following is the most likely mechanism of transport?
A. Active transport
B. Simple diffusion
C. Facilitated diffusion
D. Exocytosis
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1. A student is trying to determine the type of membrane transport occurring in a cell. She finds that the molecule to be transported is very large and polar, and when transported across the membrane, no energy is required. Which of the following is the most likely mechanism of transport?
A. Active transport
B. Simple diffusion
C. Facilitated diffusion
D. Exocytosis
C. Facilitated diffusion

We are asked to identify the type of transport that would allow a large, polar molecule to cross the membrane without any energy expenditure. This scenario describes facilitated diffusion, which uses a transport protein (or channel) to facilitate the movement of large, polar molecules across the nonpolar, hydrophobic membrane. Facilitated diffusion, like simple diffusion, does not require energy, which explains why no ATP was consumed during this transport process.
2. A researcher treats a solution containing animal cells with ouabain, a poisonous substance that interferes with the sodium-potassium ATPase embedded in the cell membrane, and the cell lyses as a result. Which of the following statements best describes ouabain's effects?
A. Treatment with ouabain results in high levels of extracellular calcium.
B. Treatment with ouabain results in high levels of extracellular potassium and sodium.
C. Treatment with ouabain increases intracellular concentrations of sodium.
D. Treatment with ouabain decreases intracellular concentrations of sodium
C. Treatment with ouabain increases intracellular concentrations of sodium.

This question requires an understanding of osmosis and the action of the sodium-potassium pump. When a cell is placed in a hypertonic solution (a solution having a higher solute concentration than the cell), fluid will diffuse out of the cell and result in cell shrinkage. When a cell is placed in hypotonic solution (a solution having a lower solute concentration than the cell), fluid will diffuse from the solution into the cell, causing the cell to expand and possibly lyse. The sodium-potassium pump moves three sodium ions out of the cell for every two potassium ions it lets into the cell. Therefore, inhibition of the sodium-potassium pump by ouabain will cause a net increase in the sodium concentration inside the cell and water will diffuse in, causing the cell to swell and then lyse.
D. I, II, and III

The polarization of the membrane at rest is the result of an uneven distribution of ions between the inside and outside of the cell. This difference is achieved through the active pumping of ions (predominantly sodium and potassium) into and out of the cell and the selective permeability of the membrane, which allows only certain ions to cross.
B. Protein synthesis

Ribosomes are the site of protein synthesis within a cell and are not coupled to the cell membrane. The cell membrane functions as a site for cytoskeletal attachment, choice (A), through proteins and lipid rafts. Transport regulation, choice (C), is accomplished through channels, transporters, and selective permeability, while the phospholipids act as a reagent for second messenger formation, choice (D).
A. phospholipids moving within the plane of the membrane.

Movement of individual molecules in the cell membrane will be affected by size and polarity, just as with diffusion. Lipids are much smaller than proteins in the plasma membrane and will move more quickly. Lipids will move fastest within the plane of the cell membrane because the polar head group does not need to pass through the hydrophobic tail region in the same way that it would if it were moving between the membrane planes.
B. trans glycerophospholipids

Compounds that contribute to membrane fluidity will lower the melting point or disrupt the crystal structure. Cholesterol, choice (C), and unsaturated lipids, choices (A) and (D), are known for these functions. trans glycerophospholipids tend to increase the melting point of the membrane and therefore decrease membrane fluidity.
D. transmembrane protein with catalytic activity.

Membrane receptors must have both an extracellular and intracellular domain; therefore, they are considered transmembrane proteins. In order to initiate a second messenger cascade, they typically display enzymatic activity, though some may act strictly as channels.
9. Which of the following is true of diffusion and osmosis?
A. Diffusion and osmosis rely on the electrochemical gradient of only the compound of interest.
B. Diffusion and osmosis rely on the electrochemical gradient of all compounds in a cell.
C. Diffusion and osmosis will proceed in the same direction if there is only one solute.
D. Diffusion and osmosis cannot occur simultaneously.