How Membrane Systems of Eukaryotic Cells Work

Cell Membranes External and Internal are Important to Cell Survival

Sep 1, 2009

Eukaryotic Cell Diagram, SEER Cancer NCI U.S.

Cell membranes once seemed to be simple cell structures. Today, membranes are recognized as complex, highly-organized structures that still require intensive research.

Cells survive because they get needed nutrients and can dispose of their wastes. Every cell that lives must do this, and they must do it well to survive. This brief review shows the organization of the outer and the inner membranes of cells, and explains how these membranes work.

How Cell Membranes are Organized and Structured

Membranes have been likened to large plates of jelly that are semi-solid and contain embedded proteins that serve as membrane transporters or structured channels. This metaphor can be carried further by imagining the membrane as a jellied or gelatinous layer with embedded raisins, grapes, pieces of pineapple and banana (all figurehead protein examples). These proteins are positioned critically in the membrane to perform essential cell functions. In some cases they form channels, in other cases they actively carry molecules inside the cell, or flip other molecules outside in a manner similar to a law officer escorting an unruly, unwanted, customer out of a gathering at a restaurant.

All membranes in cells are selectively or differentially permeable barriers. All membranes have water-impermeable lipid bilayers. This cell membrane model of Singer and Nicholson is known as the fluid mosaic model. An outer membrane figure is shown below.

Lipid and protein membrane components contribute to the overall characteristics and functions of each organelle within eukaryotic cells. Extended membrane systems and folded membrane complexes are common in eukaryotic cells.

Cell membranes are large, expansive surface areas for cell activities. Membrane plasticity permits expansion and subtraction of membrane structure by synthesis and vesicle release. Vesicles carry material to intracellular locations, or to the outer cell membrane for export.

Outer Membranes and Inner Membranes Compared and Membrane Names and Functions

The major membrane double-membrane systems of eukaryotic cells are:

outer cell membrane; very complex organization; permits free flow of water by osmosis and transports and exports chemicals by simple diffusion, passive and active transport.
nuclear membrane; separates eukaryotic DNA from the cytoplasm and communicates by pores with the cytoplasm. Nuclear DNA makes and exports several types of RNA through pores: mRNA (messenger RNA), rRNA (ribosomal) and tRNA (transfer RNA).
endoplasmic reticulum – rough and smooth forms; major site of protein synthesis and channel for conveyance to Golgi apparatus for final export.
Golgi apparatus; produces vesicles for export (e.g., hormones, enzymes).

Single-layered membranes include:

vesicles, packaged hormones, enzymes or proteins for export.
endosomes and lysosomes, packaged enzymes and proteins for intracellular use.

Research on Activities of Cell Membranes, Lipid Rafts and Yeast Vesicles

"Lipid rafts" are specific clusters of cholesterol and glycosphingolipids in cell membranes. These lipid rafts tend to cluster in definite ways and proteins show distinct affinities or preferences for association within, or outside the raft areas. Lipid raft experiments can be synthesized in the laboratory and may reflect real, transient lipid rafts that are formed in living cell membranes. Researchers hypothesize that lipid rafts are assembly or recruiting sites for various proteins prior to transport. Understanding lipid rafts and membrane functions may help scientists' understanding of certain diseases such as Alzheimer's, Parkinson's and atherosclerosis.

The Lasker Foundation 2002 Award acknowledged two researchers who uncovered the detailed attributes of secretory vesicles. Drs. James Rothman (Sloan-Kettering Cancer Center in New York) and Randy Schekman (University of California, Berkeley) each demonstrated how vesicles internally organize synthesized materials for secretion. Genetically-altered and mutant yeast cells promote an understanding of how cells secrete insulin, organize and develop tissues and organs, and accomplish nerve communication.

These same yeast cells are used as genetically-altered protein factories to produce significant amounts of commercial insulin, and antigen for hepatitis B vaccines.

Sources

Lodish, H. et al. 2000. Molecular Cell Biology. Fourth Ed., W. H. Freeman and Co., New York, N.Y.

The copyright of the article How Membrane Systems of Eukaryotic Cells Work in Scientific Inquiry is owned by Donald Reinhardt. Permission to republish How Membrane Systems of Eukaryotic Cells Work in print or online must be granted by the author in writing.