Exercises: Membrane ultrastructure

Which two types of components are the most abundant in the plasma membrane according to the fluid mosaic model? Explain what the "mosaic" represents and what the term "fluid" refers to.

Observe the diagram of the plasma membrane and identify the structures marked with numbers 1, 2, 3 and 4.

Membrane phospholipids are arranged to form a bilayer. Explain how the heads and tails of the phospholipids are oriented relative to water inside and outside the cell, and reason why this arrangement forms spontaneously.

Explain the function played in the plasma membrane by the following components: cholesterol, the glycocalyx and transport proteins.

Why is the plasma membrane said to have selective permeability? Give one example of a substance that crosses it freely and another of a substance that requires assistance to cross it, and explain the difference.

Membrane fluidity changes with temperature. Explain what role cholesterol plays when temperature is high and when it is low to maintain fluidity within appropriate limits.

In a replication of the Frye and Edidin experiment, human and mouse cells labeled with fluorophores of different colors were fused under two conditions: batch A was incubated at $37\ \text{°C}$ and batch B at $4\ \text{°C}$. At 40 minutes, the fluorophores had mixed throughout the hybrid membrane only in batch A; in batch B they remained confined to their respective hemispheres. Furthermore, when a metabolic poison that completely abolished ATP synthesis was added to batch A, the mixing of fluorophores occurred with identical speed. From these results, interpret which property of the membrane is demonstrated and justify whether the movement of proteins directly depends on the supply of metabolic energy or is a passive process determined by the physical state of the lipids.

An experimental drug breaks the linkages between the cytoskeleton and membrane proteins. Predict what effects it could have on the shape of the cell and on the distribution of proteins across the membrane, and reason your answer.

The lipid composition of the plasma membranes of two organisms is analyzed in the laboratory: sample A contains $68\,\%$ short-chain polyunsaturated fatty acids and scarce cholesterol; sample B exhibits $22\,\%$ polyunsaturated fatty acids, longer hydrocarbon tails, and abundant cholesterol. When both are subjected to gradual cooling from $37\ \text{°C}$ down to $2\ \text{°C}$, sample B undergoes drastic gelation of its bilayer at $16\ \text{°C}$ with blockage of transmembrane transport, whereas sample A maintains functional fluidity at $2\ \text{°C}$. Justify which of the two samples corresponds to an Antarctic fish and which to a terrestrial mammal, and predict with reasoning what consequences a sudden temperature increase to $38\ \text{°C}$ would have for sample A if it could not modify its lipid composition.