Membranes and Membrane Transport

The fluid mosaic model proposed by Singer and Nicolson in 1972 describes cell membranes as having which key characteristic that allows endocytosis and exocytosis to occur?

• Static arrangement of proteins that never move
• Fluidity that allows the membrane to change shape, break, and reform
• Impermeable barrier that prevents all molecular movement
• Rigid structure that maintains cell shape

Why do phospholipids naturally form bilayers in aqueous environments rather than remaining as individual molecules?

• The fatty acid chains are attracted to water molecules
• The phosphate groups repel each other strongly
• They are forced together by external pressure
• The amphipathic nature causes hydrophobic tails to cluster together while hydrophilic heads face the water

A researcher observes that oxygen and carbon dioxide can cross a cell membrane without the assistance of transport proteins, while glucose of similar size cannot. What property of these gas molecules best explains this difference?

• They are nonpolar and lipid-soluble
• They are smaller than glucose molecules
• They carry electrical charges
• They bind to specific receptor proteins

Which statement best explains why the sodium-potassium pump is classified as active transport rather than facilitated diffusion?

• It requires ATP and moves substances against their concentration gradient
• It only works in nerve cells
• It uses channel proteins
• It moves ions across the membrane

A white blood cell engulfs a bacterium through phagocytosis. What happens to the bacterium after it is enclosed in a phagosome?

• It is immediately expelled from the cell
• It remains unchanged within the cell
• It is transported to the nucleus for analysis
• The phagosome fuses with a lysosome where digestive enzymes break down the bacterium

Aquaporins are described as bidirectional and tetrameric proteins. What does this mean for water transport across membranes?

• Four water molecules must bind together before transport
• Water can only flow in one direction through four separate channels
• Water transport requires four ATP molecules
• Water flows in single file through four channels, and can move in either direction depending on concentration gradients

Why is it incorrect to say 'this solution is hypertonic' without additional context?

• The term only applies to plant cells
• Tonicity is always a comparison between two solutions
• Only isotonic solutions can be properly identified
• Hypertonic solutions don't exist in nature

How does cholesterol affect membrane properties differently from phospholipids?

• It only affects the outer layer of the membrane
• It reduces membrane fluidity and restricts entry of hydrophilic ions like sodium and hydrogen
• It creates channels for protein transport
• It increases membrane fluidity and permeability to all ions

A student observes that diffusion occurs faster at higher temperatures. Which explanation best accounts for this observation?

• Heat breaks down the cell membrane
• Increased kinetic energy causes molecules to move more rapidly
• Temperature changes the size of molecules
• Higher temperatures create more concentration gradients

What is the primary difference between channel proteins and carrier proteins in facilitated diffusion?

• Channel proteins require ATP while carrier proteins do not
• Channel proteins are peripheral while carrier proteins are integral
• Channel proteins have pores for passage while carrier proteins change shape to transport substances
• Channel proteins only transport water while carrier proteins transport ions