Sites+of+membrane+synthesus


 * Background**

The topic of the presentation is sites of membrane synthesis. Cells possess an extensive set of intracellular membranes which are called the endomembrane system. The endomembrane system consists of the endoplasmic reticulum, the golgi apparatus, and the lysosomes. These three organelles operate collaboratively in order to synthesize the lipid bilayers, primarily for the endoplasmic reticulum itself as well as for the plasma membrane surrounding the cell. The endoplasmic reticulum consists of two types of components which either lack ribosomes (smooth) or are studded with ribosomes (rough). Much of the protein and lipid biosynthesis required for membrane synthesis occurs in the endoplasmic reticulum. This includes the crucial components of the lipid bilayer, which are phospholipids and cholesterol.

Lecithin is one of the phospholipids utilized in order to synthesize new membranes in the endoplasmic reticulum membrane. It is synthesized on the part of the endoplasmic reticulum facing the cytosol, essentially meaning it is only present on one side of the bilayer. To facilitate symmetric growth of the lipid bilayer, enzymes known as flippases quickly transport the phospholipid on one side of the bilayer, essentially forming equilibrium between phospholipid concentrations on both sides of the bilayer 6. Synthesis of the plasma membrane is fundamentally different. The endoplasmic reticulum synthesizes phospholipids which are then transported by vesicles to the plasma membrane of the cell in order to synthesize more membrane. However, different types of phospholipids are not present in even amounts of both faces of the bilayer. Certain types of phospholipids will be facing the cytosol while other types will be facing the cell exterior. This results in asymmetric growth of the plasma membrane, in direct contrast to the symmetric growth found in the endoplasmic reticulum membrane 6. Figures 3 and 4 illustrate this distinction.



The golgi apparatus and lysosomes form pathways with the endoplasmic reticulum in order to carry out membrane synthesis. Products from the endoplasmic reticulum are modified by the golgi and are trannsported to the plasma membrane by endosomes in order to be added to the membrane. The plasma membrane can also undergo an endolytic cycle in which endosomes engulf compartments from the plasma membrane and bring them to the lysosomes where they are degraded. The degraded components can then be recycle for incorporation into the plasma membrane once again.


 * Discovery of plasma membrane**

Plasma membrane synthesis is important to cellular physiology because of how crucial lipid bilayers are to the compartmentalization of the cell that separates it from its outer environment.With regard to endoplasmic reticulum membrane synthesis, it is important because of how crucial membrane-enclosed organelles were for the formation of eukaryotic organisms (as opposed to prokaryotes which do not have membrane-enclosed organelles). Biological membranes such as the plasma membrane were discovered and explored in detail by several different techniques performed by several key individuals. In 1899, Overton had discovered that the cell membrane was a lipid by observing that facts that other lipids could cross it. In 1917, Irving langmuir developed a technique capable of measuring the thickness of lipid membranes as they exerted pressure 7. The langmuir trough was a simple experiment that involved spreading oil over a trough filled with water 7. Moving a barrier across the surface caused the oil film to exert surface tension which could be use to calculate area per lipid volume. This technique was then used by Gorter and Grendel to calculate the thickness of the plasma membrane within erythrocytes. This expriment with erythrocytes showed that the lipids measured were enough to cover twice the area needed to surround the cells which suggested that the membrane was a bilayer.


 * Modern day uses of Langmuir trough**

There are several modern day experiments that still utilize Langmuir troughs. One such experiment is described in “Study of the monolayer structure and wettability properties of silica nanoparticles and CTAB using the Langmuir trough technique.” The experiment involved observing the different solid-like structures at the water/air interface that forms when silica nanoparticles were combined with a cationic surfactant 8. The Langmuir trough was used in this experiment in order to measure phase transitions that occurred between these solid-like particles by determining the area based on surface pressure exerted. The Langmuir trough was also used in “Properties of long alkyl-chained resorcin [4] arenes in bilayers and on the Langmuir trough” in order to study two resorcinarenes and one cavitand having 10-(decylthio)decyl side chains. The Langmuir trough was used in order to study the aforementioned amphiphiles by being able to measure their aggregation behavior as a monolayer. The Langmuir trough helped show that higher polarity head groups dominated the monolayer organization and while lower polarity head groups did not 9. Lastly, the technique was used in “Prospects for using native and recombinant rhamnolipid producers for microbially enhanced oil recovery” in order to measure surface pressure of the lipids. The rhamnolipid films were reconstituted into 7:3 chloroform:methanol solutions and added to the Langmuir trough along with a Teflon barrier. The surface pressure and area of the rhamnolipid films were measured in the trough. The three research articles had revealed that the Langmuir trough could have been used to make other determinations during the discovery of the lipid bilayer. The first research article showed that the technique can also reveal phase transitions that occur in the plasma membrane (Such as when the membrane becomes more solid-like under lower temperatures 6 ). Regarding the second article, the trough could have also been used to measure how different phospholipids aggregate on an aqueous surface depending on their polarity. The final research did not reveal any additional pertinent details regarding the topic of membrane synthesis, although it was the only research article of the three that involved a biological system.


 * References**

6. Molecular Biology of the Cell, 5th edition Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

7. Edidin, M. Lipids on the frontier: a century of cell-membrane lipids Nature Reviews: Molecular Cell Biology 4: 414–418 (2003).

8. Santini, E., Krägel, J., Ravera, F., Liggieri, L., & Miller, R. (2011). Study of the monolayer structure and wettability properties of silica nanoparticles and CTAB using the Langmuir trough technique. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 382(1), 186-191.

9. Ogirala, P., Negin, S., Agena, C., Schäfer, C., Geisler, T., Mattay, J., & Gokel, G. W. (2013). Properties of long alkyl-chained resorcin [4] arenes in bilayers and on the Langmuir trough. New Journal of Chemistry, 37(1), 105-111.