Basic Microbiology

Cell membrane structures and functions

Cell membrane (also called"the plasma membrane (PM) or the cytoplasmic membrane and has been traditionally known as"the plasmalemma) is a membrane in...

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Cell membrane structures and functions
Cell membrane structures and functions

Cell membrane or Plasma Membrane

  • Cell membrane (also called”the plasma membrane (PM) or the cytoplasmic membrane and has been traditionally known as”the plasmalemma) is a membrane in the body which separates the inner part and exterior of cells. It also separates them from their external environment (the extracellular spaces) and shields cells from the elements.
  • The cell membrane comprises the lipid bilayer. It is composed by two layers of polyphospholipids that have cholesterols (a lipid component) interspersed between them, ensuring proper membrane fluidity at different temperatures.
  • The membrane also has membrane proteins, such as integral proteins that cover the membrane and function as transporters for the membrane, as well as peripheral proteins that attach on the exterior (peripheral) part that is the membrane of cells. They are serving as enzymes that help the cell interact with its surroundings.
  • Glycolipids that are embedded within the outer layer of lipid serve the same purpose.
  • The cell membrane regulates the flow of chemicals into and out of organelles and cells by allowing selective permeation to organic molecules and ions.
  • Cell membranes are involved in range of cell-related processes like cell adhesion cell signalling and ion conductivity and act as an connection surface for a variety of extracellular structures like the cell wall as well as the carbohydrate layer, also known as the glycocalyx, aswell being the intracellular network of protein fibers, referred to as the cytoskeleton.
  • In the area of synthetic biology, cell membranes can be made artificially rebuilt.
  • Lipids and proteins are the most important components of the cell’s membrane. The precise mix of lipids and proteins can differ based on the purpose of a particular cell.
  • Phospholipids are essential components in cell membranes. They can spontaneously create a bilayer of lipids which is semi-permeable, meaning it is only certain compounds are able to move through the membrane and into inside the cell.
  • Like the membrane of cell, certain organelles of cells are covered by membranes. The mitochondria and nucleus are two of them.
Cell membrane structures and functions
Cell membrane structures and functions | Image Source: biologydictionary.net

Components Plasma Membrane

It is made up of the following elements:

  • Phospholipids: Phospholipids are the primary membrane’s material
  • Peripheral proteins: Peripheral proteins are found on the inner or outer bilayer of phospholipids, but not incorporated into the hydrophobic core
  • Cholesterol: Cholesterol is folded between the hydrophobic tails on the membrane of phospholipids
  • Carbohydrates: Carbohydrates have been found to be linked to the proteins or lipids that are on the extracellular surface of the membrane. This leads to the creation of glycolipids and glycoproteins
  • Integral proteins: Integral proteins have been discovered to be present within the bilayer of phospholipids
Components Plasma Membrane
Components Plasma Membrane | Image Source: https://www.britannica.com/science/cell-membrane

Structure of Cell Membrane or Plasma Membrane

Cell membranes hold a wide range in biological substances, most notably proteins and lipids.

Structure of Cell Membrane or Plasma Membrane
Structure of Cell Membrane or Plasma Membrane | Image Source: biologydictionary.net

1. Lipids

  • The cell membrane is composed of three types of amphipathic lipids that include glycolipids, phospholipids and the sterols.
  • The quantity of each is contingent on the kind of cell However, most of the time they are the most abundant with a majority of them accounting for more than 50% of the plasma membrane lipids.
  • Glycolipids make up only a tiny amount of 22%, while the rest are sterols.
  • For the vast majority of eukaryotic cells, Plasma membranes contain around half proteins and half lipids in weight.

A. Phospholipids

  • The membrane bilayer is made up of numerous phospholipid molecules, each with a different sizes of heads and tails.
  • These are made up of a head-molecule, the phosphate molecule Glycerol, and two chains of fatty acids.
  • Head group – This is one of the polar groups e.g. the sugar or choline which means that the head end that is phospholipid hydrophilic.
  • Tail of two chains of fatty acids, typically comprised of 14-24 carbons (but the most commonly used lengths of carbons range from 16 to 18). If the chain is made up of an cis double bond, the chain is kinked , which reduces the packing tightness of the membrane and expanding its motion. Because the tail is made of fat acids, it can not make hydrogen bonding with water, and thus is non-polar and non-polar.
  • The molecules of phospholipids are thus amphipathic. They are both hydrophilic as well as hydrophobic.
  • They create bilayers on the water, with their head group facing out , and those facing the tail group into.
  • The bilayer there is van der Waal forces between the acids fatty tails of phospholipids, as well as electrostatic and hydrogen bonds between hydrophilic groups as well as water.

b. Cholesterol

  • In the animal cell, cholesterol is usually found dispersed to different amounts across cell membranes. It is also found in the irregular spaces between hydrophobic tails of membrane lipids. This provides a stiffening and strengthening effect to the membrane.
  • Furthermore, the quantity of cholesterol found in biological membranes is different between organisms or cell types, and even in the individual cells.
  • Cholesterol is the most important component of plasma membranes in animals regulates the flow of the entire membrane. This means cholesterol regulates the quantity of movement of cells’ membrane components based upon the concentrations.
  • At higher temperatures, cholesterol hinders the movement of phospholipids’ chains of fatty acids, which results in an increase in permeability for small molecules and a decrease in membrane fluidity. This is not the case regarding the function of cholesterol at lower temperatures.
  • Cholesterol production, and consequently concentration, is increased (increased) as a result of cold temperatures.
  • In cold temperatures cholesterol can interfere with the fatty acid chain interactions. As an antifreeze, cholesterol keeps the membrane’s fluidity.
  • Cholesterol is much more abundant in warm-weather animals than cold-weather animals.
  • In plants that do not have cholesterol, compounds that are related to cholesterol called Sterols serve the same purpose as cholesterol.

2. Carbohydrates

  • The plasma membranes contain also carbohydrates. mostly glycoproteins. However, they also contain a few glycolipids (cerebrosides and Gangliosides).
  • Carbohydrates play a crucial role of cell-cell recognition within eukaryotes. They’re located on the outside of cells where they identify host cells and transmit information to viruses that attach to cells via these receptors can cause infection. Most of the time there is no glycosylation on the membranes inside the cell however, it is more common for glycosylation to occur on the outer surface that is the plasma membrane.
  • The glycocalyx is an essential element in all cells, particularly epithelia that have microvilli.
  • Glycocalyx plays a role in lymphocyte adhesion, cell adhesion Homing, and many more.
  • The final sugar is galactose, and the final syrup is sialic acid because the sugar backbone gets altered in the Golgi apparatus.
  • Sialic acid is charged with the negative charge and acts as an external barrier for charged particles.

3. Proteins

  • The cell membrane has two kinds of proteins that are associated with it that are known as Peripheral membrane proteins as well as Membrane Integral proteins.
  • Membrane proteins from the peripheral membrane are external to and linked to the membrane through interactions with other proteins.
  • Membrane proteins that are integrally incorporated introduced into the membrane. They are the majority of the proteins that are able to pass through the membrane. The transmembrane proteins are visible on both sides of membrane.
  • Structural proteins aid in giving the cells support and shape.
  • The cell membrane receptors assist cell membrane receptor proteins communicate with the environment by using neurotransmitters, hormones, and various other signaling molecules.
  • Transport proteins, like globular proteins, move cells’ membranes via the process of diffusion.
  • Glycoproteins are carbohydrate chains connected to them. They are embedded within the cell membrane and assist in cell-to-cell communication and also transporting molecules through the membrane.

Functions of Cell membrane

1. Selective Permeability

Membrane membranes can be selectively permeable (or semi-permeable) this means they allow only specific molecules are able to traverse them. The oxygen, water carbon dioxide, and water can effortlessly pass across the membrane. In general the ions (e.g. sodium or potassium) and polar molecules are unable to traverse the membrane. they must pass through certain channels or pores within the membrane, instead of dispersing through. In this way, the membrane is able to control the speed that certain molecules be allowed to enter and leave the cell.

2. Physical Barrier

The plasma membrane covers every cell and physically divides the cytoplasm which is the cell’s material, is the body of the cell and the fluid that is that is outside the cell. It shields the various components inside the cell against outside surroundings and allows for separate functions to take place within and outside of the cell.

The plasma membrane offers structural support for the cell. It is the anchor for the cytoskeleton which is a system of protein filaments within the cell. They holds all the components of the cell. The cell’s cytoskeleton gives it its shape. Certain organisms , like mushrooms and plants have cells that have walls along with the membrane. The cell wall is made of cellulose-based molecules. It is a source of support for cells and is the reason plants don’t explode as animal cells do when there is too much water leaking into the cell wall.

3. Endocytosis

Endocytosis refers to the process by the cells absorb molecules by taking them and engulfing them. The plasma membrane causes an inward-facing small deformation, known as an invagination, where the substance being transported is taken in. The invagination is caused by proteins located outside the cell membrane acting as receptors, and forming clusters into depressions, which eventually lead to the an accumulation of liquids and proteins on the cytosolic portion of the membrane. The deformation is able to pinch off the membrane within the cell, forming an elongated vesicle that contains the captured substance. Endocytosis can be described as a method of taking in solid particles (“cell eating” or the process of phagocytosis) as well as small molecules and ions (“cell drinking” or pinocytosis) and macromolecules. Endocytosis is a process that requires energy, and thus is an active form of transport.

Functions of Cell membrane
Large molecules can be taken into the cell through the process of endocytosis. | Image Source: biologydictionary.net

4. Exocytosis

As material is introduced into cells through invagination and the formation of a vesicle membrane of a cell can be joined to the plasma membrane and extrude its contents into the medium surrounding it. This is known as exocytosis. Exocytosis is a process that occurs in a variety of cells to clear the undigested residues of the substances brought in through endocytosis, and to release substances such as hormones or enzymes, and also to move substances completely across a cell barrier. When exocytosis occurs the waste that is accumulated in the food vacuole, or secretory vesicle, which is budded from the Golgi apparatus is initially transported by the cytoskeleton from cell’s interior towards the outer surface. The vesicle membrane is brought into close contact with plasma membrane. The lipid molecules in the two bilayers reorganize themselves in a way that both membranes become and are thereby joined. A passage is formed within the membrane that has been fused and the vesicles expel their contents to the outside of the cell.

5. Cell Signaling

Another crucial purpose that the membrane serves is to aid in communication and signals between cells. It accomplishes this through the use of different proteins and carbohydrates within the membrane. The proteins on the cell “mark” this cell, so that other cells are able to recognize it. The membrane is also equipped with receptors that allow it to perform specific functions when substances like hormones or other hormones are bound to the receptors.

Organelle Membranes

Organelles in cells are covered by protective membranes. The nucleus, endoplasmic-reticulum vacuoles, lysosomes and the Golgi apparatus are all instances of membrane-bound organelles. Mitochondria and chloroplasts , on the other hand, are bonded by two membranes. The membranes of different organelles differ in molecular structure and are designed for the tasks they play. Organelle membranes are essential to many vital cell functions like the synthesis of proteins, lipid production and cell respiration.

Models of plasma membrane

Fluid Mosaic Model (1972)

Description of plasma membranes is possible through an fluid model in terms of mosaic cholesterol, carbohydrates, proteins , and phospholipids.

In 1972, the model was first suggested in 1972 by Garth L. Nicolson and S.J. Singer The model was developed by Garth L. Nicolson and S.J. Singer to what the plasma membranes’ structure was. The model has evolved over time but is able to explain the function and structure of plasma membranes in the most effective method. The model defines the structure of plasma membranes as a mosaic of elements that includes cholesterol, proteins, phospholipids, carbohydrates, and proteins It also gives a fluid appearance to the membrane.

The thickness of the membrane is between 5-10nm. The percentage of constituency of plasma membrane i.e. the proteins, carbohydrates, and lipids differs from cell to. As an example, outer membrane of mitochondria is composed of 24 percent lipid and 76 percent protein. In myelin 76% of lipid is detected and 18 percent protein.

The principal fabric of this membrane is made up of phospholipid molecules which are amphiphilic. The hydrophilic regions of these molecules are in contact with the aqueous fluid on the outside as well as inside. The hydrophobic , or water hateful molecules on the contrary on the other hand, are not polar in the natural world. A phospholipid molecule consists of three carbons of glycerol’s backbone, two fat acid molecules that are connected with carbons 1, 2 and a phosphate-containing group that is connected by the 3rd carbon.

The organization provides a specific region known as the head to the molecule as a total. The head of the group that is phosphate-rich, has the polarity or negative charge whereas the tail, which is a separate area that is a source of fatty acids has no charge. They are more likely to interact with non-polar molecules during reactions, but are not able to connect with non-polar molecules.

When hydrophobic molecules are introduced into water, show the potential to form clusters. However, hydrophilic phospholipids are more likely to create hydrogen bonds with water , along the other polymers inside and outside of the cell. Thus, the membrane’s surface which interacts with the outside and inside of cells is classified as hydrophilic. However, the center of cell membranes is hydrophobic, and is not in contact with water. Thus, phospholipids are able to create a wonderful bilayer of lipids in the cell, which separates liquid inside the cell from the fluid that is on the outside that surrounds the cell.

The second component is created by the the plasma membrane. Integrins or integral proteins are integrated completely into the membrane’s structure together with their hydrophobic membrane. They range from hydrophobic regions to areas of the phospholipid bilayer. In general, single-pass integral membrane proteins have a transmembrane that is hydrophobic and consists of between 20 and 25 amino acids. A few of them traverse one portion of the membrane, connecting to a single layer, while others are able to span from one to the other aspect of membrane which exposes the reverse side.

Some complex proteins are composed of 12 segments of a single protein, which is extremely convoluted for placement in the membrane. This kind of protein is hydrophilic and has a region or two together with some mildly hydrophobic zones. The organization of these regions of proteins is likely to bring the proteins with phospholipids. The hydrophobic portion of the protein adjacent to the tails of hydrophilic regions and phospholipids of protein extends through the membrane and is in contact with the extracellular fluid, or cytosol.

The third major part of plasma membrane is the presence of carbohydrates. They’re typically located on the outside of cells and connected with lipids, forming glycolipids, or proteins to create glycoproteins. This carbohydrate chain could consist of two to sixty monosaccharide units, which can be straight or branched.

Carbohydrates in conjunction with peripheral proteins lead to the formation of areas on the surface of cells that recognize each with one another. This recognition is vital for cells because they allow your immune system differentiate between cells in the body and those of tissues and cells of foreign origin. Glycoproteins and glycoproteins are also present on the surface of viruses. They may differ, which can hinder immune cells from recognizing them and draw them.

On the outer surface that cells reside, sugars as well as their components of glycoproteins and glycolipids are named glycocalyx. This is extremely hydrophilic , and attracts huge amounts of water to the cell’s surface. This allows the cell to connect with the fluid surroundings and further enhances the ability of cells to absorb substances that dissolve in water.

Fluid Mosaic Model
Fluid Mosaic Model | Image Source: https://www.brainkart.com/article/The-Fluid-Mosaic-Model-of-Membrane-Structure_27522/

Davson-Danielli model

  • An observation that is consistent by plasma membranes that could not explained by the bimolecular model of lipid leaflets was the extremely minimal surface tension on the membrane.
  • The year 1935 was the time when J. F. Danielli and E. N. Harvey suggested that oil droplets and other lipid inclusions inside cells were bound at their surface by a layered structure of lipids, as well as the protein layer.
  • It was proposed that the proteins, composed of a monomolecular layered of molecules that were hydrated, faced the aqueous cell wall and connected with the polar parts that comprised the lipid layer.
  • The nonpolar parts that comprised the lipid layers sat against the hydrophobic oil layer of the droplet’s inside.
  • In this case it is believed that the normal surface activity of the protein will be the reason for the low interfacial pressure that forms the membrane around the droplet. Then, Danielli and H. Davson proposed that plasma could be made up of two bilayers of lipid-protein – one that faces the interior of the cell, and another facing outwards to the external environment.
  • In this setup it is possible that the connection between surfaces proteins as well as the bimolecular lipid leaflet could be maintained by electrical interactions that occur between the two polar end of each lipid molecule as well as the amino acid charged side chains of polypeptide layers.
  • Electrostatic or van der Waals bonds can bind other groups to the outer surface. Danielli and Davson suggested that such membranes would show specific permeability and be capable of distinguishing between molecules with different sizes and properties of solubility as well as between ions with different charges.
  • In the 1950s, various modifications were made to the model of the Danielli-Davson membrane. For instance it was believed that glycoproteins could be absorbed by the membrane’s surface, thus explaining the antigenic characteristics that cell membranes exhibit. The pores through which certain substances are exchanged between cells and its surroundings were believed to exist. The channels created by regular continuousities (bridges) in the protein layers that are between the inner and outer layers.
Davson-Danielli model
Davson-Danielli model | Image Source: https://sebiology.weebly.com/blog/plasma-membrane-the-davson-danielli-model-vs-the-singer-nicolson-model

Robertson’s Model

  • According to Robertson the unit membrane was bimolecular lipid leaflets sandwiched between the inner and outer layers of protein arranged with pleated sheets.
  • The arrangement was thought to be identical across every cell membrane.
  • In the 1950s electron microscopy was used to provide additional information on what the shape of plasma membranes.
  • J. D. Robertson was one of the pioneers in this field, demonstrating that membranes fixed using the osmium tetroxide had a distinct tri-laminar appearance , consisting of two distinct layers, the outer darker (osmiophilic) layers as well as an inner lighter (osmiophobic) layers.
  • The osmiophilic layer typically measured between 20 and 25 A (2.0-2.5nm) on average, while the osmiophobic layers measured between 25 and 35 A (2.5-3.5 millimeters) with a final amount of about 65 to 85 A (6.5-8.5 millimeters). This number is comparable to the thickness that was predicted by chemical studies.
  • Robertson and his colleagues demonstrated that the tri-laminar structure was common to many other cellular membranes that included the endoplasmic Reticulum. With regard to the fundamental uniformity in the appearance of cell membranes examined, Robertson proposed his now well-known unit membrane model of the unit membrane. According to Robertson the unit membrane was bimolecular lipid leaves sandwiched between the inner and outer layers of protein that were organized in the pleated sheet arrangement. The arrangement was thought to be the same across the cell membranes of all cells.
  • Although Robertson acknowledged the distinct distinctions in the chemical composition of membranes (i.e. that the specific molecular species that comprise the membranes differ) Robertson believed the molecular pattern was fundamentally similar. While there is no doubt that the same electron microscopic appearance of a majority of membranes, an absolute chemical explanation to explain the uniformity of appearance is not viable anymore.
  • Robertson has extended his model of the unit membrane to incorporate the idea that there is continuity between the layers of nuclear membranes as well as the plasma membrane through the endoplasmic retina. The existence of such continuity has been proven in studies using electron microscopy of a variety of tissues and cells.
  • In addition, Robertson suggested that vesicular organelles could originate from this membrane system, and can be later pinched to form distinct structures. There is evidence to the direction of this idea with regard to micro-bodies and lysosomes.
Robertson's Model
Robertson’s Model | Image Source: http://cytochemistry.net/cell-biology/membrane.htm

Henderson as well as Unwin’s theory of the membrane

Henderson Unwin and Henderson Unwin have examined the purple membrane using electron microscopy using an approach to identify the projected structures of uncolored crystal specimens. Applying the technique to specimens that were tilted, and applying the concepts proposed by DeRosier and Klug to combine these two-dimensional images and 3D views, they have created a image of membrane with seven A resolution. The map shows the position of the lipid and protein components, the order of polypeptide chains inside each protein molecule, as well as the relationships between the proteins in the lattice.

High-resolution micrographs of crystal patterns of membrane proteins which were taken with a small amount of electrons in order to limit radiation damage, can be used to establish the 3-dimensional structure of the protein by using an Fourier transform. Recent studies of negatively stained rat liver cells Gap(tm) junctions that were subjected to three-dimensional Fourier reconstructions (of low-dose electron micrographs) reveal that the six sub-units of protein are placed in a cylinder tilted to the tangential, creating the channel to be 2 nanometers in width within the extracellular region. Dimensions of the channels inside the membrane were smaller, but were not able to be determined (Unwin and Zampighi 1980). A slight movement of the sub-units near the cytoplasmic end could decrease the tangential inclination of sub-units to a six-fold and thus end the channel.

More details on the molecular structure will emerge as more preparation methods are made available, ensuring that high-resolution 3-dimensional images similar to those of the membranes in purple are created. Utilizing ingenuous methods to analyze periodic arrays of macromolecules in biological systems that combine data from low-dose electron pictures along with diffraction patterns merged, Henderson and Unwin (1975) created a 3D model of the purple membranes with 0.7 millimeter resolution. The embedding of glucose was used to reduce dehydration-related damage, and low doses (less than 0.5 A*/e) to minimize radiation damage. The electron micrographs of stained membranes were recorded so that the sole reason for contrast is a small phase contrast that was induced by defocusing.

In their research, Unwin and Henderson found that the protein extends to each side of bi-layer of lipids and is comprised of seven a-helices arranged 1-1.2 millimeters apart, 3.5-4.0 inches in length and parallel to the membrane’s plane. The molecules are organised around a three-fold axis, with an area of 2 nm in the center, which is full of the lipids. This remarkable work is the most significant advancement so far, because it is the first time, provided researchers with the structural details of a membrane protein that is integral in the presence of. The amino acid sequence in conjunction with the details about electron scattering density derived from the research by Henderson and Unwin and others, has prompted models-building efforts (Engleman and colleagues. 1980) to integrate the sequence of bacteriorhodopsin into a set of A-helical segments.

Henderson as well as Unwin's theory of the membrane
Henderson as well as Unwin’s theory of the membrane | Image Source: https://en.wikipedia.org/wiki/File:Fluid_Mosaic.svg

Gorter with Grendel’s Membrane Theory (1920)

Evert Gorter and Francois Grendel (Dutch scientists) were the first to come up with our current model of plasma membrane as a bi-layer of lipids. They simply speculated that the plasma membrane could be bi-layered and the thickness of the monolayer is twice that of the surface area of plasma membrane. To test their hypothesis they conducted an experiment where the researchers extracted lipids out of an established amount of blood vessels (erythrocytes) from a variety of mammalian sourceslike sheep, goats and humans and so on. and then spread the lipids in a mono-layer inside a Langmuir-Blodgett tube. They examined the size of the plasma membranes that is composed of blood red cells and then using Langmuir’s method they measured the size in the monolayer the lipids. When comparing both, they came up with an approximate ratio of 2: mono-layer of lipids: Plasma membrane. This confirmed their theory that resulted in concluding that membranes of cells are made up of two distinct molecular layers. One of the scientists suggested a model of this bi-layer with the hydrophilic heads of the polar polar facing outwards toward the aqueous environment , and those with hydrophobic heads facing inside away from the surrounding aqueous environment to both the sides. Although they reached the right conclusion, certain aspects of the data were not correct, such as the incorrect estimation of the size and the pressure within the mono-layer of lipids and the lack of completeness in the extraction of lipids. They also did not adequately describe the function of membranes, and made incorrect assumptions, such as those of plasma membranes made up of mainly the lipids. On the other hand, in general this conception of bi-layer structure of lipids became the fundamental assumption that underlies each subsequent refinement in our knowledge of the function of membranes.

Gorter with Grendel's Membrane Theory (1920)
Gorter with Grendel’s Membrane Theory (1920) | Image Source: https://en.wikipedia.org/wiki/File:Trpv1_pip2_bilayer.png

References

  • https://www.genome.gov/genetics-glossary/Cell-Membrane#:~:text=The%20cell%20membrane%2C%20also%20called,entering%20and%20exiting%20the%20cell.
  • https://www.thoughtco.com/cell-membrane-373364
  • https://en.wikipedia.org/wiki/Cell_membrane
  • https://www.slideshare.net/harshrajshinde1/cell-membrane-53215807
  • https://courses.lumenlearning.com/boundless-biology/chapter/components-and-structure/
  • https://biologydictionary.net/plasma-membrane/
  • https://teachmephysiology.com/histology/cell-structures/cell-membrane/
  • https://www.khanacademy.org/science/ap-biology/cell-structure-and-function/plasma-membranes/a/structure-of-the-plasma-membrane
  • https://byjus.com/neet/plasma-membrane-structure/
  • https://opentextbc.ca/biology/chapter/3-4-the-cell-membrane/
  • https://www.biologydiscussion.com/cell-biology/the-danielli-davson-membrane-model-with-diagram/3789
  • https://www.biologydiscussion.com/notes/useful-notes-on-robertsons-unit-membrane/3781
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