Membrane structure
The plasma membrane is not simply a bag wrapped around the cell: it is a dynamic structure, with components that move sideways while still keeping a precise organization. Think about a swimmer in a lake at Yellowstone: the water around her is always shifting, yet she stays in one piece. Understanding the membrane at the molecular level explains why some substances cross it easily and others need help to do so.
The fluid mosaic model
When the cell surface is observed with fluorescence microscopy, the points of light do not stay fixed: they spread out and mix continually across the whole cell. How can a barrier hold in the contents of the cell if its components are not rigidly welded together? Keep this question in mind; we answer it at the end of the page.
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…
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.
Observe the diagram of the plasma membrane and identify the structures marked with numbers 1, 2, 3 and 4.
The phospholipid bilayer
This is why a drop of cooking oil in a glass of water clumps together instead of spreading out.
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,…
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 explain why this arrangement forms spontaneously.
The lipid composition of the plasma membranes of two organisms is analyzed in the lab: sample A contains $68\,\%$ short-chain polyunsaturated fatty acids and scarce…
The lipid composition of the plasma membranes of two organisms is analyzed in the lab: 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.
Proteins, cholesterol and the glycocalyx
A few more details you need: integral proteins (also called transmembrane proteins) cross the whole bilayer, while peripheral proteins sit on one surface. The glycocalyx is made of glycoproteins and glycolipids, and cholesterol is a lipid (a steroid) found between the phospholipids of animal cell membranes.
Explain the function played in the plasma membrane by the following components: cholesterol, the glycocalyx, and transport proteins.
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…
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.
The fluidity of the membrane
If a cell cools down, like a lizard on a 35 °F morning in the Texas Hill Country, its membrane risks becoming stiff like solid wax; if it heats up too much, it risks becoming too runny. What molecular mechanisms allow the cell to keep its membrane at just the right fluidity when the temperature changes?
The fatty acid tails matter too. Unsaturated fatty acids have kinks (double bonds) that stop the tails from packing tightly, so membranes rich in them stay fluid when it is cold. Saturated fatty acids are straight and pack closely, making the membrane more rigid. Longer tails have more contact with each other and also make the membrane less fluid.
Membrane fluidity changes with temperature. Explain what role cholesterol plays when temperature is high and when it is low to maintain fluidity within appropriate…
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…
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…
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 explain your reasoning.
Summary
Back to the start: the membrane holds in the cell's contents because the phospholipid bilayer is held together by the hydrophobic effect, not by rigid bonds, so its parts can move and the barrier stays intact.