Membranes and membrane transport practice questions.

In the fluid mosaic model, which property of membrane phospholipids allows the bilayer to remain fluid at physiological temperatures?

• The hydrophilic phosphate heads face inward, away from water
• Cholesterol covalently bonds adjacent phospholipids together
• Unsaturated fatty acid tails have kinks that prevent tight packing
• Integral proteins lock the phospholipids into a fixed lattice

Answer: Unsaturated fatty acid tails have kinks that prevent tight packing

Double bonds in unsaturated fatty acid tails create kinks, so the phospholipids cannot pack tightly, increasing membrane fluidity.

A membrane protein spans the bilayer and hydrolyses ATP to move Na+ and K+ against their concentration gradients. This protein is best classified as a

• channel protein for facilitated diffusion
• pump carrying out primary active transport
• glycoprotein used for cell recognition
• carrier protein for facilitated diffusion

Answer: pump carrying out primary active transport

Direct use of ATP to move ions against their gradients defines a pump performing primary active transport.

Which role does cholesterol perform in an animal cell membrane?

• It acts as a receptor for peptide hormones
• It forms the aqueous channels through which ions diffuse
• It modulates membrane fluidity across temperatures
• It provides the energy for active transport

Answer: It modulates membrane fluidity across temperatures

Cholesterol reduces fluidity at high temperatures and prevents solidification at low temperatures, buffering membrane fluidity.

Molecules that cross a membrane by simple diffusion, rather than requiring a transport protein, are typically

• large and polar
• small and nonpolar
• charged ions
• large polysaccharides

Answer: small and nonpolar

Small, nonpolar molecules such as O2 and CO2 dissolve in the hydrophobic core and diffuse directly through the bilayer.

A plant cell is placed in a concentrated sugar solution and its plasma membrane pulls away from the cell wall. The cell has undergone

• cytolysis (bursting)
• plasmolysis
• turgor
• haemolysis

Answer: plasmolysis

In a hypertonic solution water leaves the cell by osmosis, the protoplast shrinks and detaches from the wall: plasmolysis.

During endocytosis, material is brought into a cell by

• diffusion of solutes down their gradient
• vesicles forming as the plasma membrane folds inward
• proteins pumping ions across the bilayer
• fusion of internal vesicles with the plasma membrane

Answer: vesicles forming as the plasma membrane folds inward

In endocytosis the membrane invaginates and pinches off a vesicle enclosing the material; fusion of vesicles outward is exocytosis.