Questions: Plasma Membrane Organization and Dynamics

Question 1

A researcher uses FRAP (fluorescence recovery after photobleaching) and finds that some labeled membrane proteins recover slowly and in a patchy pattern, while others recover rapidly and uniformly. The slow-recovering proteins are most likely:

• A. Damaged by the photobleaching laser and unable to diffuse normally
• B. Confined within lipid raft domains or organized protein complexes that restrict free lateral diffusion
• C. Too large to diffuse through the fluid bilayer at detectable rates
• D. Peripheral membrane proteins that have partially detached from the bilayer

Answer: (B)

If all membrane proteins diffused freely (as the simple fluid mosaic model predicts), FRAP recovery would be rapid and uniform everywhere. Patchy, slow recovery indicates that some proteins are confined to organized domains — lipid rafts or protein scaffolds — that restrict their diffusion. This is precisely the kind of evidence that revealed the membrane is not a homogeneous fluid but contains structured, dynamic microdomains. The fluid mosaic model was correct that lipids and proteins can diffuse, but wrong that they do so randomly throughout the bilayer.

Question 2

Phosphatidylserine is normally confined to the inner leaflet of the plasma membrane. When it is detected on the outer leaflet of a cell, this most likely indicates:

• A. Lipid raft formation is being initiated on the outer surface
• B. The cell is rapidly growing and needs additional membrane area on the outer leaflet
• C. The cell is undergoing apoptosis and is being flagged for recognition by phagocytes
• D. Normal membrane turnover has briefly disrupted leaflet asymmetry

Answer: (C)

Membrane asymmetry is actively maintained by flippases and floppases that consume ATP to keep phosphatidylserine on the inner leaflet. During apoptosis, this asymmetry is deliberately broken: scramblase enzymes mix the two leaflets, allowing phosphatidylserine to appear on the outer surface. Phagocytes recognize this 'eat me' signal through receptors like Annexin V binding sites. This is not incidental turnover but a specific, programmed signal. The appearance of PS on the outer leaflet is so reliably a marker of apoptosis that it is used as a diagnostic assay.

Question 3

The plasma membrane is best described as a uniformly fluid bilayer in which most membrane lipids and proteins diffuse freely without spatial restriction.

• T. True
• F. False

Answer: False

This describes the original fluid mosaic model (Singer and Nicolson, 1972), which was an important advance but is now known to be an oversimplification. The real plasma membrane contains lipid rafts — domains enriched in cholesterol and sphingolipids that are more ordered and less fluid than surrounding regions. Proteins preferentially partition into or out of these rafts, creating functional neighborhoods. Cytoskeletal attachments, protein scaffolds, and tight junctions further restrict diffusion. Single-particle tracking and FRAP experiments both demonstrate that diffusion is heterogeneous, not uniform.

Question 4

The entire surface area of a typical cell's plasma membrane is turned over — internalized and replaced — approximately every 30 to 60 minutes through the combined action of endocytosis and exocytosis, yet barrier function is maintained throughout.

• T. True
• F. False

Answer: True

This is one of the most striking facts about membrane dynamics. The membrane is not a static boundary but a continuously self-renewing structure. Endocytosis removes patches of membrane along with surface receptors and extracellular cargo; exocytosis from intracellular vesicles replenishes it. In epithelial cells, tight junctions between adjacent cells maintain the barrier function even as individual membranes are replaced, because the tight junction seals are maintained independently of lipid bilayer continuity. This continuous turnover enables rapid remodeling of receptor composition and cell surface identity.

Question 5

What are lipid rafts, and why does their existence challenge the simple fluid mosaic model of membrane organization?

Model answer: Lipid rafts are small, dynamic membrane domains enriched in cholesterol and sphingolipids. Because cholesterol fills the gaps between tightly packed sphingolipid tails, raft regions are more ordered and thicker than the surrounding glycerophospholipid-rich membrane. Certain proteins — especially GPI-anchored proteins on the outer leaflet and Src-family kinases on the inner leaflet — preferentially partition into these domains, concentrating signaling components together. The simple fluid mosaic model predicted a homogeneous, randomly mixed lipid bilayer in which proteins diffuse freely. Lipid rafts contradict this by showing that the membrane has lateral organization — specific lipid phases that partition proteins non-randomly and create functional signaling platforms.

The functional significance is that signal transduction is more efficient when receptors and downstream signaling molecules are co-localized in rafts rather than randomly distributed. Disrupting rafts (e.g., by extracting cholesterol) impairs signaling pathways that depend on this co-localization. Lipid rafts thus represent the membrane's way of organizing biochemistry spatially without requiring gene expression or protein synthesis — a rapid, tunable mechanism for controlling which molecules interact.