Membrane: Definition & Overview

A close-up of different hand-drawn cells with varying shades of greens and yellows

Definition: Membrane

A cell membrane is a thin layer present around living cells (also known as the biological membrane or plasma membrane) and some organelles such as mitochondria, nucleus, and chloroplast. 

The cell membrane is semipermeable and allows the transport of only a few specific ions and molecules. However, the nuclear membrane is highly specialized and selective in terms of molecular transport.

Membranes are structurally composed of lipid bilayers embedded with proteins, along with carbohydrates on the membrane’s surface. A bilayer consists of phospholipids, cholesterol, and glycolipids, all of which contain chains of fatty acids. This membrane structure was first proposed by the fluid mosaic model (describes the fluidity and elasticity of the membrane).

The structure and arrangement of membrane proteins and lipids in the membrane play varied roles including: 

  • Imparting selective permeability of the membrane
  • Determine whether the membrane will have a vesicle or a flat structure
  • Maintaining membrane fluidity at different temperatures
  • Active and passive transport of molecules across the membrane through transmembrane proteins.

While the arrangement and distribution of membrane lipids play a major role in membrane fluidity, membrane structure is more determined by the fences formed by the actin-based cytoskeleton, membrane protein, protein-protein complexes, and lipid rafts.

Lipid rafts are glycoprotein lipid microdomains consisting of cholesterol, glycosphingolipids, and protein receptors.

Figure: An illustrative detailed diagram of plasma or biological membrane. 

In animal cells, the plasma membrane is the only barrier between the internal and external cell environment. However, in plants and bacteria, the membrane is layered by a strong cell wall.

The selectivity and semi-permeability of the membranes are utilized in labs and industries in many assays to achieve specific results. Many membrane processes and technologies have also been developed to serve this purpose.

In this article, we will review the structure and function of natural membranes and how labs and industries leverage their properties to design synthetic membranes for a variety of lab assays.

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Membrane Overview

Cellular membranes, nuclear membranes, and the membranes present around other organelles are some membrane types naturally found in organisms. They are mainly composed of lipids, proteins, carbohydrates, and sterols.

The arrangement, distribution, and population of these components in the membrane play a crucial role in determining the membrane’s properties. Furthermore, they also determine the integrity and physical structure of the membrane. 

The phospholipid bilayer involved in the cell membrane structure is amphipathic—having both hydrophilic (the head group) and hydrophobic (the tail group) properties. The arrangement of both groups in the membrane is essential as both impact the transport of molecules from one side of the membrane to the other.

Membrane Functions

Cell membranes play numerous roles within organisms. Some of which are:

  • Transport of ions and water-soluble molecules in and out of the cell either through diffusion or active transport via membrane transport proteins
  • Interaction with other cells through glycoprotein or lipid-protein, present on the membrane surface
  • Synthesis of new molecules
  • Uptake and conversion of nutrients
  • Regulation of metabolic processes
  • Energy production
  • Cell signaling

The membrane present around organelles also plays some specific roles, such as:

  • The membrane present around the nucleus facilitates the exchange of proteins and other molecules from the cytoplasm.
  • The outer membrane of the nucleus extends to form the endoplasmic reticulum where protein and lipid synthesis takes place.
  • In mitochondria, the outer membrane is permeable to small molecules and some other substances. However, the inner membrane is less permeable and has more control over the transport of molecules in the mitochondria.

Membrane Product and Technologies

Apart from the naturally found membranes, there are also synthetic membranes designed by scientists in labs for research or industrial applications of extraction, separation, and purification.

Synthetic membranes have major applications in enhancing the efficiency of the separation process for the following molecules:

Enzymes

Membrane filtration is an effective technique to purify, concentrate, and clarify enzymes that are susceptible to harsh environments such as too hot conditions.

The membrane is also used to remove impurities during enzymatic preparations. 

Organic Acids

Membranes are used in membrane technologies to remove impurities from organic acids and amino acids during manufacturing and produce products of high quality and value. 

Fermentation

Membranes are used to optimize the separation process during fermentation. To enhance the yield and product quality, microorganisms should be eliminated from the target molecule.

Nutraceuticals

Membrane technology is an efficient technique used to manufacture nutraceutical products. They are used in the extraction and fermentation steps of the process to clarify and purify biobased molecules.

Membrane Applications

Synthetic membrane or membrane filters are used in a spectrum of lab procedures. A few of them are given below: 

Microfiltration

Microfiltration is a filtration technique that removes microorganisms and residual suspended particles through a membrane. It can separate particles ranging from 0.08 µm to 2 µm. 

The technique also is used as pre-treatment for processes like ultrafiltration and granular media filtration.

A membrane bioreactor is now also available for the separation process, designed by combining microfiltration/ultrafiltration and a bioreactor. This is commonly used in municipal and industrial wastewater treatment.

Ultrafiltration

In this technique, suspended solids and solutes of high molecular weights are separated through a semipermeable membrane based on forces like concentration gradient and pressure. It can separate macromolecules of weight ranging from 103 to 106 Da and size 0.005-2 µm.

Membrane Distillation

Membrane distillation is an alternative membrane technology for energy-intensive processes such as reverse osmosis and distillation. The technique works on the principle of vapor difference across the two sides of the hydrophobic membrane, induced by temperature difference.

Reverse Osmosis

This is a water purification technique. It’s used to remove ions, molecules, and some larger particles from a water sample by using pressure as the driving force. It’s mainly used to treat wastewater.  

Membrane Industries

Membrane filters or synthetic membranes are used for the separation and purification of ions and molecules from a variety of samples. It’s most extensively used in industries like metallurgy, water treatment, and food and beverages. 

Figure: A schematic representation of the applications of membrane products and processes.

Food and Beverage

Membrane products are used in the food and beverage industry to remove bacteria and other microbes and increase the product yield. It’s used to produce bottled water, milk, cheese, juices, beer, and other alcoholic beverages.

Pharmaceutical 

Membrane products are efficient in harvesting cells and recovering biomass for the processing of small molecule products, like antibiotics and enzymes. It’s a cost-effective approach to concentrate these products and boost their production.

Life Sciences

The membrane technology plays an important role in the production of biomolecules and in sterilizing heat-labile solutions, such as hormones. Additionally, it can also desalinate and purify products like dyes and pigments. 

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Membranes are a thin layer surrounding cells and organelles, separating their inner environment from the outside. They are composed of lipid bilayers with embedded proteins and cholesterol. Carbohydrates are found on the surface of the membrane and helps in cellular interaction.

Membranes can be semi-permeable or selectively permeable, allowing molecules to pass from one side of the membrane to another. Based on this structural property, scientists have developed synthetic membrane products and technologies for lab and industrial uses. They are used for the separation and purification of solute-solute, particle-solute, solute-solvent, and particle-solvent from a range of samples. 

Some applications of membrane technologies include ultrafiltration, membrane distillation, and reverse osmosis. They are extensively used in industries like food and beverage, mining, pharma, and life sciences.

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